Fundamentals of Wastewater Treatment and …...treatment of municipal wastewater. Filling the need for a textbook focused on wastewater, it first covers history, current practices,

ISBN: 978-0-415-66958-0

9 780415 669580

90000

Environmental Engineering

As the world’s population has increased, sources of clean water have decreased, shifting the focus toward pollution reduction and control. Disposal of wastes and wastewater without treatment is no longer an option. Fundamentals of Wastewater Treatment and Engineering introduces readers to the essential concepts of wastewater treatment, as well as the engineering design of unit processes for the sustainable treatment of municipal wastewater.

Filling the need for a textbook focused on wastewater, it first covers history, current practices, emerging concerns, and pertinent regulations and then examines the basic principles of reaction kinetics, reactor design, and environmental microbiology, along with natural purification processes. The text also details the design of unit processes for primary, secondary, and advanced treatment as well as solids processing and removal. Using detailed calculations, it discusses energy production from wastewater.

Comprehensive and accessible, the book addresses each design concept with the help of an underlying theory, followed by a mathematical model or formulation. Worked-out problems demonstrate how the mathematical formulations are applied in design. Throughout, the text incorporates recent advances in treatment technologies.

Based on a course taught by the author for the past 18 years, the book is designed for undergraduate and graduate students who have some knowledge of environmental chemistry and fluid mechanics. Readers will get a strong grounding in the principles and learn how to design the unit processes used in municipal wastewater treatment operations. Profes-sionals in the wastewater industry will also find this a handy reference.

Dr. Rumana Riffat is a professor in the Civil and Environmental Engineer-ing Department at George Washington University in Washington, D.C. Her research interests are in wastewater treatment, specifically anaerobic treatment of wastewater and biosolids, as well as nutrient removal.

FUN

DA

MEN

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STEWATER

TREATM

ENT A

ND

ENG

INEER

ING

RIFFAT FUNDAMENTALS OF

WASTEWATERTREATMENT ANDENGINEERING

RUMANA RIFFAT

w w w . s p o n p r e s s . c o m

Y117901

A S P O N P R E S S B O O K

Y117901 cvr mech.indd 1 7/24/12 10:20 AM

FUNDAMENTALS OF

WASTEWATERTREATMENT AND

ENGINEERING

A SPON PRESS BOOK

FUNDAMENTALS OF

WASTEWATERTREATMENT AND

ENGINEERING

RUMANA RIFFAT

Co-published by IWA Publishing

Alliance House, 12 Caxton Street, London SW1H 0QS, UK

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v

This book is dedicated to my husband, Wahid Sajjad, who has been my best friend forever;to my children, Roshan and Mehran, who I hope will use this book someday;and to my parents, Salma and Muhammad Chishty, who have taught me the two most important things in life—compassion and humility.

vii

Contents

Preface xvAcknowledgments xviiAbout the author xixList of symbols xxiList of abbreviations xxiii

1 Sustainablewastewatertreatmentandengineering 1

1.1 Introduction and history 11.2 Current practice 31.3 Emerging issues 41.4 Future directions 41.5 Regulatory requirements 7

1.5.1 U.S. regulations 71.5.2 European Union regulations 91.5.3 United Kingdom regulations 10

References 11

2 Reactionkineticsandchemicalreactors 13

2.1 Reaction kinetics 132.2 How to find the order of a reaction 142.3 Zero order reaction 162.4 First order reaction 182.5 Second order reaction 192.6 Reactors 19

2.6.1 Conversion of a reactant 202.6.2 Detention time in reactor 20

2.7 Batch reactor 212.7.1 Design equation 21

2.8 Plug flow reactor (PFR) 232.8.1 Design equation 25

viii Contents

2.9 Continuous-flow stirred tank reactor 262.9.1 Design equation 28

2.10 Reactors in series 292.11 Semibatch or semiflow reactors 32Problems 32References 34

3 Wastewatermicrobiology 353.1 Introduction 353.2 Bacteria 36

3.2.1 Cell composition and structure 373.2.2 Bacterial growth curve 383.2.3 Classification by carbon and energy requirement 403.2.4 Classification by oxygen requirement 413.2.5 Classification by temperature 413.2.6 Bacteria of significance 41

3.3 Archaea 433.4 Protozoa 433.5 Algae 453.6 Fungi 463.7 Virus 46Problems 49References 49

4 Naturalpurificationprocesses 514.1 Impurities in water 514.2 Dilution 514.3 Sedimentation 534.4 Microbial degradation 534.5 Measurement of organic matter 53

4.5.1 Biochemical oxygen demand (BOD) 544.5.1.1 BOD kinetics 544.5.1.2 Laboratory measurement 584.5.1.3 Unseeded BOD test 584.5.1.4 Seeded BOD test 604.5.1.5 Determination of k and Lo 614.5.1.6 Thomas’s graphical method 61

4.5.2 Theoretical oxygen demand 624.6 Dissolved oxygen balance 63

4.6.1 Dissolved oxygen sag curve 64

Contents ix

4.6.1.1 Critical points 664.6.1.2 Limitations of the oxygen

sag curve model 70Problems 70References 73

5 Wastewatertreatmentfundamentals 75

5.1 Introduction 755.2 Sources of wastewater 765.3 Wastewater constituents 765.4 Wastewater treatment methods 78

5.4.1 Physical treatment 785.4.2 Chemical treatment 785.4.3 Biological treatment 78

5.5 Levels of wastewater treatment 795.5.1 Preliminary treatment 795.5.2 Primary treatment 795.5.3 Enhanced primary treatment 795.5.4 Conventional secondary treatment 795.5.5 Secondary treatment with nutrient removal 805.5.6 Tertiary treatment 805.5.7 Advanced treatment 80

5.6 Residuals and biosolids management 805.7 Flow diagrams of treatment options 815.8 Types of biological treatment processes 83Problems 84References 84

6 Preliminarytreatment 85

6.1 Introduction 856.2 Screens 85

6.2.1 Trash racks 856.2.2 Coarse screens or bar screens 86

6.2.2.1 Design of coarse screens 876.2.3 Fine screens 90

6.2.3.1 Design of fine screens 906.2.4 Microscreens 92

6.3 Shredder/grinder 92

x Contents

6.4 Grit chamber 93Problems 97References 98

7 Primarytreatment 99

7.1 Introduction 997.2 Types of settling/sedimentation 997.3 Type I sedimentation 100

7.3.1 Theory of discrete particle settling 1007.3.1.1 Stokes equation 102

7.3.2 Design of ideal sedimentation tank 1047.4 Type II sedimentation 1077.5 Primary sedimentation 109

7.5.1 Rectangular sedimentation tank 1107.5.2 Circular sedimentation tank 112

7.6 Chemically enhanced primary treatment 116Problems 116References 118

8 Secondarytreatment:Suspendedgrowthprocesses 119

8.1 Introduction 1198.2 Microbial growth kinetics 120

8.2.1 Biomass yield 1208.2.2 Logarithmic growth phase 1228.2.3 Monod model 1228.2.4 Biomass growth and substrate utilization 1238.2.5 Other rate expressions for rsu 1248.2.6 Endogenous metabolism 1248.2.7 Net rate of growth 1258.2.8 Rate of oxygen uptake 1258.2.9 Effect of temperature 126

8.3 Activated sludge process (for BOD removal) 1268.3.1 Design and operational parameters 1278.3.2 Factors affecting microbial growth 1318.3.3 Stoichiometry of aerobic oxidation 132

8.4 Modeling suspended growth processes 1328.4.1 CSTR without recycle 1328.4.2 Activated sludge reactor (CSTR with recycle) 135

8.4.2.1 Other useful relationships 138

Contents xi

8.4.3 Activated sludge reactor (plug flow reactor with recycle) 139

8.4.4 Limitations of the models 1418.4.5 Aeration requirements 145

8.4.5.1 Types of aerators 1478.5 Types of suspended growth processes 150

8.5.1 Conventional activated sludge 1508.5.2 Step aeration or step feed process 1508.5.3 Tapered aeration process 1508.5.4 Contact stabilization process 1518.5.5 Staged activated sludge process 1528.5.6 Extended aeration process 1528.5.7 Oxidation ditch 1538.5.8 Sequencing batch reactor (SBR) 1538.5.9 Membrane biological reactor (MBR) 154

8.6 Stabilization ponds and lagoons 1558.6.1 Process microbiology 1558.6.2 Design of pond or lagoon system 1578.6.3 Design practice 158

Problems 161References 163

9 Secondarytreatment:Attachedgrowthandcombinedprocesses 165

9.1 Introduction 1659.2 System microbiology and biofilms 1669.3 Important media characteristics 1679.4 Loading rates 1689.5 Stone media trickling filter 170

9.5.1 Design equations for stone media 1719.6 Biotower 175

9.6.1 Design equations for plastic media 1759.7 Rotating biological contactor 1799.8 Hybrid processes 181

9.8.1 Moving bed biofilm reactor (MBBR) 1819.8.2 Integrated fixed-film activated sludge (IFAS) 1829.8.3 Fluidized bed bioreactor (FBBR) 183

9.9 Combined processes 184Problems 184References 187

xii Contents

10 SecondaryClarification 189

10.1 Introduction 18910.2 Secondary clarifier for attached growth process 18910.3 Secondary clarifier for suspended growth process 191

10.3.1 Settling column test 19210.3.2 Solids flux analysis 194

10.3.2.1 Theory 19510.3.2.2 Determination of area

required for thickening 19710.3.3 Secondary clarifier design 199


11 Anaerobicwastewatertreatment 209

11.1 Introduction 20911.2 Process chemistry and microbiology 211

11.2.1 Syntrophic relationships 21311.3 Methanogenic bacteria 21411.4 Sulfate-reducing bacteria 21611.5 Environmental requirements and toxicity 21611.6 Methane gas production 217

11.6.1 Stoichiometry 21711.6.2 Biochemical methane potential assay 22011.6.3 Anaerobic toxicity assay 221

11.7 Anaerobic growth kinetics 22111.8 Anaerobic suspended growth processes 222

11.8.1 Anaerobic contact process 22211.8.2 Upflow anaerobic sludge blanket process 224

11.8.2.1 Design equations 22511.8.3 Expanded granular sludge bed 22611.8.4 Anaerobic sequencing batch reactor 22811.8.5 Anaerobic migrating blanket reactor 231

11.9 Anaerobic attached growth processes 23111.9.1 Anaerobic filter 23111.9.2 Anaerobic expanded bed reactor 233

11.10 Hybrid processes 23311.10.1 Anaerobic fluidized bed reactor 23311.10.2 Anaerobic membrane bioreactor 234


Contents xiii

12 Solidsprocessinganddisposal 239

12.1 Introduction 23912.2 Characteristics of municipal sludge 24012.3 Sludge quantification 24012.4 Sludge thickening 246

12.4.1 Gravity thickener 24612.4.2 Dissolved air flotation 24912.4.3 Centrifugation 251

12.5 Sludge stabilization 25112.5.1 Alkaline stabilization 252

12.5.1.1 Chemical reactions 25212.5.1.2 Lime pretreatment 25312.5.1.3 Lime posttreatment 253

12.5.2 Anaerobic digestion 25312.5.2.1 Single-stage mesophilic digestion 25512.5.2.2 Two-stage mesophilic digestion 25912.5.2.3 Thermophilic anaerobic digestion 26612.5.2.4 Temperature-phased anaerobic

digestion (TPAD) 26712.5.2.5 Acid-gas phased digestion 26812.5.2.6 Enhanced enzymic hydrolysisTM 26812.5.2.7 CambiTM process 269

12.5.3 Aerobic digestion 27012.5.3.1 Autothermal thermophilic

aerobic digestion 27112.5.3.2 Dual digestion 271

12.5.4 Composting 27312.6 Conditioning of biosolids 27412.7 Biosolids dewatering 275

12.7.1 Centrifugation 27512.7.1.1 High-solids centrifuge 275

12.7.2 Belt-filter press 27612.7.3 Drying beds 276

12.8 Disposal of biosolids 27712.8.1 Incineration 27712.8.2 Land disposal methods 277

12.9 Biosolids disposal regulations in the United States 27812.9.1 Class A biosolids 279

12.9.1.1 Processes to further reduce pathogens 28012.9.2 Class B biosolids 280

xiv Contents

12.9.2.1 Processes to significantly reduce pathogens 281


13 Advancedtreatmentprocesses 287

13.1 Introduction 28713.2 Nitrogen removal 287

13.2.1 Biological nitrogen removal 28813.2.1.1 Nitrification–denitrification 28813.2.1.2 Nitritation–denitritation 30113.2.1.3 Deammonification 302

13.2.2 Physicochemical process for nitrogen removal 30413.2.2.1 Air stripping 304

13.3 Phosphorus removal 30413.3.1 Chemical precipitation 30513.3.2 Biological phosphorus removal 306

13.3.2.1 Selected processes for BPR 30713.3.2.2 Phoredox 30713.3.2.3 A2OTM process 30713.3.2.4 Modified BardenphoTM (five stage) 30713.3.2.5 UCT process 307

13.4 Solids removal 30913.4.1 Granular media filtration 30913.4.2 Activated carbon adsorption 31113.4.3 Membrane filtration 312

13.4.3.1 Fundamental equations 31313.4.3.2 Membrane fouling 31513.4.3.3 Membrane configurations 315

13.4.4 Process flow diagrams 316Problems 316References 318

Appendix 323

xv

Preface

Thisbookisdesignedforacourseonwastewatertreatmentandengineer-ingforseniorlevelorearlygraduatelevelstudents.Asthenamesuggests,the book covers the fundamental concepts of wastewater treatment fol-lowedbyengineeringdesignofunitprocessesfortreatmentofmunicipalwastewater.Thestudentsshouldhavebackgroundknowledgeofenviron-mentalchemistryandfluidmechanics.Oneimportantcharacteristicofthisbookisthateachdesignconceptisexplainedwiththehelpofanunderlyingfundamental theory, followed by a mathematical model or formulation.Problems are presented and solved to demonstrate the use of the math-ematicalformulationsandapplythemindesign.

Chapter1startswithahistoryofwastewatertreatment,followedbycur-rentpractices,emergingconcerns,futuredirections,andpertinentregula-tionsthathaveshapedtheobjectivesanddirectionsofthisimportantareaofengineeringandresearch.Chapters2and3describe the fundamentalconceptsofreactionkinetics,reactordesign,andwastewatermicrobiology.Biochemicaloxygendemand(BOD) ispresented indetail,as it isoneofthemostimportantmeasurementsforwastewatercharacteristics.Chapter4 introducesnaturalpurificationprocessesandthedissolvedoxygensagcurve.Theconceptofsimplemassbalancesisintroducedinthischapter.Chapters5through10describeindetailtheunitprocessesinprimaryandsecondarytreatment.Massbalanceisusedtodevelopdesignequationsforbiologicaltreatmentprocesses.Aseparatechapter,Chapter11,isprovidedforanaerobictreatment,whichisbecomingmoreandmoreimportantduetotheenergyproductionpotentialfrommethanegasgeneration.Chapter12describessolidsprocessinganddisposal,togetherwithpertinentregula-tions.Anumberofproblemsandtheirsolutionsaregiventodemonstratecalculationofmassandvolumeofsludge,performsolidsbalance,andcal-culatetheefficiency.Thefinalchapter,Chapter13,describesadvancedandtertiary treatmentprocesses.Removalofnutrients, suchasnitrogenandphosphorus,ispresentedindetail,followedbyprocessesforsolidsremoval.Recentadvancesinnitrogenandphosphorusremovalareprovided.Ihave

xvi Preface

incorporated recent research advances in various sections of the book,whereverapplicable.

ThelayoutofthebookissimilartothemannerinwhichIhavetaughtthiscourseatGeorgeWashingtonUniversity(GWU)forthelast18years.AtGWU, I teach this course asEnvironmentalEngineering II,which istakenbyseniorlevelstudentsintheCivilandEnvironmentalEngineeringDepartment. The material is covered in one semester consisting of 14weeks.Attheendofthecourse,thestudentshouldhaveanunderstandingofthefundamentalconceptsofwastewatertreatmentandbeabletodesigntheunitprocessesfortreatmentofmunicipalwastewater.

xvii

Acknowledgments

First,IwouldliketoexpressmysincerethankstomydoctoralstudentTaqsimHusnain.Hehashelpedwiththisbook incountlessways.Hehas diligently and beautifully prepared all the diagrams and illustra-tionsforthisbook.Iamdeeplyindebtedtohimforallhishelpandforhis assistance in reviewing the manuscript and making corrections. Iwould also like to thank my former doctoral students Sebnem Aynurand Kannitha Krongthamchat for their contributions to Chapters 11and12.

IwouldliketothankmydearfriendFerhatZerinforherhelpindesign-ingthebookcover.Finally,Iwouldliketothankmyfamilyforalltheirpatienceandsupportduringthislongoneyear,whichmadeitpossibleformetowritethisbook.

xix

About the author

Dr.RumanaRiffatisprofessoroftheCivilandEnvironmentalEngineeringDepartment at George Washington University in Washington, D.C. Sheobtained her bachelor’s degree in civil engineering from BangladeshUniversityofEngineeringandTechnologyinDhaka,Bangladesh,andhergraduatedegrees incivilandenvironmentalengineeringfromIowaStateUniversityinAmes,Iowa.Shehasbeeninvolvedinteachingandresearchforthelast18years.

Dr. Riffat’s research interests are in wastewater treatment, specifi-callyanaerobictreatmentofwastewaterandbiosolids,aswellasnutrientremoval.Sheandherresearchgrouphaveconductedextensiveresearchonprocessestofurtherreducepathogens,suchasdualdigestion,temperature-phaseddigestion,andvariouspretreatmentoptions.Hernutrientremovalresearchhasfocusedondeterminationofkineticsandevaluationofvariousexternalcarbonsourcesfordenitrification.Dr.RiffatiscurrentlyinvolvedinanumberofresearchprojectswiththeDistrictofColumbiaWaterandSewerAuthorityattheBluePlainsAdvancedWastewaterTreatmentPlant,amongothers.

Dr.RiffatreceivedtheDistinguishedTeacherAwardfromtheSchoolofEngineeringandAppliedScienceofGeorgeWashingtonUniversityin2011.She received theGeorgeBradleyGascoigneWastewaterTreatmentPlantOperational Improvement Medal of the Water Environment Federation(WEF) in 2010. She is a member of several professional organizations,includingWEFandtheAmericanSocietyofCivilEngineers.Sheisaregis-teredprofessionalengineeroftheDistrictofColumbia.

xxi

List of symbols

α oxygentransfercorrectionfactorβ salinity–surfacetensioncorrectionfactorC concentrationCd dragcoefficientD dissolvedoxygenconcentrationdp diameterofparticleF foulingfactorFg forceduetogravityFb forceduetobuoyancyFD dragforceΦ shapefactorofparticleg accelerationduetogravityKs halfsaturationcoefficientk1 BODrateconstantk2 re-aerationrateconstantkd endogenousdecaycoefficientkt reactionratecoefficientLt oxygenequivalentoforganicmatterremainingattimetMo massofoxygenµ specificgrowthrateofbiomassµmax maximumspecificgrowthrateofbiomassµw dynamicviscosityofwaterρp densityofparticleρw densityofwaterPx biomasswastedQ flowrateR recycleratioRe Reynoldsnumberrd rateofdecayrg growthrateofbiomassrmax maximumbiomassproductionratero rateofoxygenuptake

xxii List of symbols

rsu rateofsubstrateutilizationS substrateconcentrationSt substrateconcentrationattimett timeT temperatureθ hydraulicretentiontimeθc solidsretentiontimeV volumeofreactorVL volumetricloadingratevt terminalsettlingvelocityX biomassconcentrationY biomassyieldcoefficient

xxiii

List of abbreviations

AEBR anaerobicexpandedbedreactorAMBR anaerobicmigratingblanketreactorAOTR actualoxygentransferrateAPD acidphasedigestionAS activatedsludgeASBR anaerobicsequencingbatchreactorATA anaerobictoxicityassayATAD autothermalthermophilicaerobicdigestionAWTP advancedwastewatertreatmentplantbCOD biodegradablechemicaloxygendemandBOD biochemicaloxygendemandBOD5 5dbiochemicaloxygendemandBMP biochemicalmethanepotentialBNR biologicalnutrientremovalBPR biologicalphosphorusremovalbsCOD biodegradablesolubleCODCEPT chemicallyenhancedprimarytreatmentCOD chemicaloxygendemandCSTR continuous-flowstirredtankreactorCWA CleanWaterActDAF dissolvedairflotationDD dualdigestionDNA deoxyribonucleicacidDO dissolvedoxygenEC EuropeanCommissionE. coli Escherichia coliEDC endocrinedisruptingcompoundEEH enhancedenzymichydrolysisEGSB expandedgranularsludgebedEPA EnvironmentalProtectionAgencyEU EuropeanUnionFBBR fluidizedbedbio-reactor

xxiv List of abbreviations

FC fecalcoliformFWPCA FederalWaterPollutionControlActGAC granularactivatedcarbonHRT hydraulicretentiontimeIFAS integratedfixed-filmactivatedsludgeMAD mesophilicanaerobicdigestionMBBR movingbedbiofilmreactorMBR membranebiologicalreactorMF microfiltrationMGD milliongallonsperdayMLE modifiedLutzack–EttingerMLSS mixedliquorsuspendedsolidsMLVSS mixedliquorvolatilesuspendedsolidsMPN mostprobablenumberN nitrogenNH3 ammoniaNF nanofiltrationNPDES NationalPollutantDischargeEliminationSystemNOD nitrogenousoxygendemandOLR organicloadingrateOUR oxygenutilizationratePAC powderedactivatedcarbonPAO phosphorus-accumulatingorganismsPES polyethersulfonePFR plugflowreactorPFRP processestofurtherreducepathogensPHB poly-hydroxy-butyratePOTW publiclyownedtreatmentworksPS polysulfonePSRP processestosignificantlyreducepathogensPVC polyvinylchloridePVDF polyvinylidenedifluorideRAS returnactivatedsludgeRBC rotatingbiologicalcontactorRNA ribonucleicacidRO reverseosmosisSBR sequencingbatchreactorsCOD solublechemicaloxygendemandSDNR specificdenitrificationrateSES sandequivalentsizeSHARON singlereactorsystemforhighammoniumremovalovernitriteSOC syntheticorganiccompoundSOR surfaceoverflowratesSOTR oxygentransferrateatstandardtemperatureandpressure

List of abbreviations xxv

SRT solidsretentiontimeSTP standardtemperatureandpressureTC totalcoliformTF tricklingfilterThOD theoreticaloxygendemandTKN totalKjeldahlnitrogenTMDL totalmaximumdailyloadTOC totalorganiccarbonTPAD temperature-phasedanaerobicdigestionTS totalsolidsTSS totalsuspendedsolidsUASB upflowanaerobicsludgeblanketUF ultrafiltrationUK UnitedKingdomUV ultravioletVFA volatilefattyacidVS volatilesolidsVSS volatilesuspendedsolidsWAS waste-activatedsludge

1

Chapter 1

Sustainable wastewater treatment and engineering

1.1 INTRODUCTION AND HISTORY

Thescienceandengineeringofwastewatertreatmenthasevolvedsignifi-cantlyoverthelastcentury.Asthepopulationoftheworldhasincreased,our sources of clean water have decreased. This has shifted our focustowardpollutionreductionandcontrol.Disposalofwastesandwastewaterwithout treatment in landsandwaterbodies isno longeranoption.Anincreasingbodyof scientificknowledge relatingwaterbornemicroorgan-ismsandconstituentstothehealthofthepopulationandtheenvironmenthasspurredthedevelopmentofnewengineeredtechnologiesfortreatmentofwastewaterandpotentialreuse.

Thetermwastewater includesliquidwastesandwastestransportedinwaterfromhouseholds,commercialestablishments,andindustries,aswellasstormwaterandothersurfacerunoff.Wastewatermaycontainhighcon-centrationsoforganic and inorganicpollutants, pathogenicmicroorgan-isms,aswellas toxicchemicals. If thewastewater isdischargedwithouttreatmenttoastreamorriver,itwillresultinseverepollutionoftheaquaticenvironment. The decline in water quality will render the stream waterunusableforfuturedrinkingwaterpurposes.Sustainable wastewater engi-neering involves application of the principles of science and engineeringforthetreatmentofwastewatertoremovepollutantsorreducethemtoanacceptablelevelpriortodischargetoawaterbodyorotherenvironment,withoutcompromisingtheself-purificationcapacityofthatenvironment.Thetreatmentanddisposalofthegeneratedsolidsandotherby-productsisanintegralpartofthetotalprocess.

If we look back in time, wastewater engineering has progressed fromcollectionandopendumping,tocollectionanddisposalwithouttreatment,tocollectionandtreatmentbeforedisposal,allthewaytocollectionandtreatmentpriortoreuse.EvidenceofwastecollectioninthestreetsandthenuseofwatertowashthewastethroughopensewershasbeenfoundintheancientRomanempire.Intheearly1800s,theconstructionofsewerswasstartedinLondon.In1843,thefirstsewersystem,inHamburg,Germany,

2 Fundamentals of wastewater treatment and engineering

wasofficiallydesignedbyaBritishengineer,Lindley(Anon,2011).Insev-enteenth century Colonial America, household wastewater managementconsistedofaprivy(toilet)withanoutletconstructedatgroundlevelthatdischargedoutsidetoacesspoolorasewer.Withlowpopulationdensities,priviesandcesspoolsconstructedinthiswaydidnotcausemanyproblems(Duffy,1968).Butasthepopulationincreased,theneedforanengineeredsystem forwastewatermanagement in large citiesbecamemore evident.Scientists and public health officials started to understand the relation-shipbetweendiseaseoutbreaksandcontaminationofdrinkingwaterfromwastewater.Nuisancecausedbyodors,outbreakofdiseases,e.g.cholera,andotherpublichealthconcernspromptedthedesignofacomprehensivesewersysteminChicagointhe1850s.Atthattime,thesewersystemwasusedtotransporttheuntreatedwastewateroutsideoftheresidentialcom-munity toa streamor river.Dilutionof thewastewaterwith the streamwater was the primary means of pollutant reduction. These were calledwater-carriage sewer systems.

Publichealthconcerninthe1850salsoresultedintheplanninganddevel-opmentof awater-carriage sewer system for the city ofLondon.A chol-eraepidemicstruckLondonin1848andagainin1854,causingmorethan25,000 deaths (Burian et al., 2000). Dr. John Snow was the first doctoratthattimetoestablishaconnectionbetweenthecholeraoutbreakandacontaminatedwatersupplyattheBroadStreetpublicwell.Inaddition,heshowedstatisticallythatcholeravictimshaddrawntheirdrinkingwaterfromasewage-contaminatedpartoftheriverThames,whilethosewhoremainedhealthydrewwaterfromanuncontaminatedpartoftheriver.Thesefind-ings,togetherwiththediscoveriesbyPasteurandKoch,promptedtheBritishParliamenttopassanactin1855toimproveLondon’swastemanagementsystem.Thisledtothedevelopmentofacomprehensivewater-carriagesewersystemforLondon,designedbyJosephBazalgette(HeyandWaggy,1979).

Towardthebeginningofthetwentiethcentury,sewagetreatmentplantsmainlyusedsettlingtanks(primarytreatment)toremovesuspendedpar-ticles fromthewastewaterbeforedischargetostreamsandrivers. Intheearly1900s,aboutonemillionpeopleintheUnitedStateswereservedby60suchtreatmentplants.Intheearly1900s,thefirsttricklingfilterwasconstructedinMadison,Wisconsin,toprovidebiological(secondary)treat-menttowastewater.TheImhofftankwasdevelopedbyGermanengineerKarlImhoffin1906forsolidsseparationandfurthertreatment.Thefirstactivated sludgeprocesswasconstructed inSanMarcos,Texas, in1916(Burianetal.,2000).Advancesinsludgedigestionandgasproductionwerealsobeingaccomplishedbyresearchersandutilities.Fromthemid-1900stothepresenttime,wehaveseendevelopmentofvarioustypesofbiologi-calandbiochemicalprocesses for the removalofpollutants fromwaste-water. The earlier objectives were mainly to reduce the total suspendedsolids(TSS),biochemicaloxygendemand(BOD),andpathogens.Primary

Sustainable wastewater treatment and engineering 3

andsecondarybiologicaltreatmentwasconsideredsufficientforproduc-tionoftreatedwastewaterofacceptablestandards.Withindustrializationandscientificadvances,chemicalandtoxiccompoundshavebeendetectedinmunicipalwastewatertreatmentplantinfluents.Thishasresultedintheneedforadditionaltreatmentbeyondthesecondary,givingrisetotertiarytreatment.Tertiaryor advanced treatment canbephysical, chemical, orbiological,oracombinationoftheseprocesses.

1.2 CURRENT PRACTICE

Primarytreatmentinmostmunicipalwastewatertreatmentplantsconsistsofpreliminaryandprimarystages.Ittypicallyincludesscreens,gritchambers,comminutors, andprimary clarifiers, dependingon the flow rates. Largerplantsusechemicallyenhancedprimaryclarificationforhighersolids-removalefficiency.Primarytreatmentisfollowedbysecondarytreatment.Secondarytreatmentconsistsofabiologicalprocessfollowedbyasecondaryclarifier.IfthesecondaryeffluentmeetstheregulatorystandardsforBODandTSS,thenitisdischargedtoreceivingwatersfollowingdisinfection.Thesolidsandsludgecollectedfromthevariousunitsundergofurtherprocessingandtreat-mentbeforedisposal.Variousoptionsareavailableforsludgeprocessing.AconventionalwastewatertreatmentplantisillustratedinFigure 1.1.

More than half of the municipal wastewater treatment plants in theUnitedStatesarecapableofprovidingatleastsecondarytreatment.About92%of the totalflowis treatedbyplantswithacapacityof0.044m3/s(1milliongallonsperdayorMgal/d)orlarger(MetcalfandEddy,2003).In the last twodecades, nutrient removalhasbecome increasinglymore

Screens Comminutor GritChamber

Primaryclari�er

Biologicalreactor

Secondaryclari�er

Gravity thickener

Primarysludge

Secondarysludge

Grit toland�ll

Screens toland�ll

Sludge recycle

Disinfectant

Anaerobicdigester Centrifuge

Biosolidsfor landapplication

E�uent recycledthru secondary

reactor

Preliminary and Primary Treatment Secondary Treatment

Sludge treatment

Wastewater �ow Sludge �ow

E�uent Wastewater

in�uent

Figure 1.1 Flow diagram of a conventional wastewater treatment plant.


important in parts of the United States, as well as in Europe and Asia.Eutrophication caused by excessive nitrogen and phosphorus in waste-waterdischargeshasdisruptedtheaquaticlifeinreceivingwaterbodies,withasubsequentdeclineinwaterquality.Wastewatertreatmentplantsinaffectedareasandwatershedshavetoprovideadditionalnutrientremovalpriortodischarge.Biologicalnutrientremoval is incorporatedaspartofthesecondarytreatmentoras tertiarytreatment.Nutrientremoval isnolongerconsideredanadvancedtreatmentoption.AnexampleofthisistheChesapeakeBaywatershedintheeasternUnitedStatesandthemunicipalwastewatertreatmentplantswithinthewatershed.Mostoftheplantsusebiologicalnitrification–denitrificationtogetherwithBODremoval,and/orchemicalprecipitationforremovalofphosphorus.Useofgranularmediafiltration as tertiary treatment for reduction of total suspended solids isalsoquitecommon.Table 1.1presentsthepollutantscommonlyfoundinmunicipalwastewaterandthephysical,chemical,andbiologicalprocessesusedtoremoveorreducetheirconcentrations.

1.3 EMERGING ISSUES

Thefollowingareareasofimportanceandconcernformunicipalwastewa-tertreatmentplants:

• Risingenergycostsforoperationoftreatmentplants• Disposalofbiosolidsinasustainablemanner• Performanceandreliabilityofplantsinthedigitalage• Presenceofendocrinedisruptingcompounds(EDC)inwastewater• Presenceoftoxicchemicalsinwastewaterfromhouseholdproducts• More stringent discharge limits due to continued degradation of

waterbodies• Scarcityoffreshwatersources• Theneedtoupgradeaginginfrastructureandtreatmentplants• Theneedforadequatemathematicalmodelsandsoftwareforprocess

analysisandcontrol

1.4 FUTURE DIRECTIONS

Basedontheemergingissues,wastewaterengineeringandresearchshouldbefocusedonthefollowingareasinthefuture:

• Energy generation—Typically, wastewater treatment plants havehigh energy requirements for plant operation. They are big con-sumers of energy or electricity. Wastewater plants can generate


significantamountsofmethanegasfromanaerobicdigestionofthesludge.Thegas can in turnbeused toheat thedigesters, aswellasgeneratepowerthatcanbeusedbytheplantorsoldtonearbyindustries.Withrisingenergycosts,thisshouldbethefuturedirec-tion of operation of wastewater treatment plants. For sustainableoperation, treatment plants need to evolve into energy producersfromenergyconsumers.

OneexampleofsuchenergyproductionusinganaerobicdigestionistheEastBayMunicipalUtilityDistrict,California,whichcogen-erateselectricityandthermalenergyonsitefromwastemethane.Thisresulted in an annual reduction in energy costs from $4.6 millionto$2.9million(EastBayMunicipalDistrict,2011).Otherexamplesare the Encina Wastewater Authority and Point Loma Wastewater

Table 1.1 Common wastewater pollutants and the processes used to reduce/remove them

Pollutant Unit process

Suspended solids Coarse screens, fine screensGrit chamberClarificationFiltrationChemically enhanced clarification

Colloidal and dissolved solids Chemical precipitationMembrane filtrationIon exchangeActivated carbon adsorption

Biodegradable organics Suspended growth processes (aerobic and anaerobic)Attached growth processes (aerobic and anaerobic)Ponds and lagoonsMembrane bioreactors

Pathogens ChlorinationOzonationUltraviolet disinfection

NutrientsNitrogen Biological nitrification–denitrification

(suspended and fixed-film variations)Air strippingBreakpoint chlorination

Phosphorus Biological phosphorus removalChemical precipitation

Volatile organic compounds Activated carbon adsorptionAir stripping

Source: Adapted from Metcalf and Eddy (2003) and Peavy et al. (1985).


Treatment Plant in California, among others. The West PointTreatmentPlantinKingsCounty,Washington,usesthemethanegasgenerated from anaerobic digestion to run generators that produceelectricity. They are able to produce 1.5 to 2.0 megawatts of elec-tricity,whichissoldtothelocalutilitycompanyaftermeetingplantdemands(WestPointTreatmentPlant,2011).

• Beneficial reuse of biosolids—Thecostofprocessinganddisposalofthebiosolidsproducedatawastewatertreatmentplantcanamounttoalmost50%ofthetotalcapitalandoperationcosts.Futuredirectionshouldbemoretowardproducingaproductthatcanbereusedinabeneficialmanner,suchasfuelorfertilizer.AnexampleistheEncinaWastewaterAuthority,whichproducesbiosolidspelletsthataresoldto a cement manufacturing facility as an alternative fuel (EncinaWastewaterAuthority,2011).

• Wastewater reuse—Asfreshwaterresourcesbecomemorescarce,theneedforrecycledwastewaterwillincrease.Futuredirectionsshouldinclude increased research on water quality and safety of recycledwastewater,aswellaspubliceducationfordirectpotablereuse.TheislandnationofSingaporeusesreclaimedwaterthatisproducedfromamultiplebarrierwastewater treatmentprocess.Thewastewater isfirst treatedbyconventionaltreatment, followedbymicrofiltration/ultrafiltration,membranefilters,andfinallyultravioletdisinfection.NEWateristhebrandnamegiventothereclaimedwaterinSingapore(NEWater,2011).

• Fundamental research—With an increasing aging population andan increase in the use of pharmaceutical products, a vast num-berofnewandemerging contaminantshave found theirway intowastewatertreatmentplants.Ofconcernaretheendocrinedisrupt-ing compounds (EDCs),whichhave caused feminizationoffish inthe waters of Maryland, among others. A number of these com-pounds pass through the treatment plant unchanged and end upinstreamsandrivers,havingvariousconsequencesonaquaticlife.Fundamentalresearch isnecessary todetermine thecharacteristicsofthesecompoundsofconcernandtodevelopmethodsfortreatmentandremoval.

• Mathematical modeling—Wastewaterengineeringisstillinitsinfancywhencomparedwithotherengineeringdisciplines,withregardtothedevelopmentandavailabilityofprocessmodelsfordesignandcontrolof treatmentoperations.A fewmodelshavebeendevelopedby theInternationalWaterAssociationandBiowin®,amongothers.Inorderforthisbodyofscienceandengineeringtohaveasignificantpositiveimpactontheplanet’sscarcewaterresources,futuredirectionshouldbeinattractingbrilliantscientificmindstodevelopadequateandver-satileprocessmodels.


1.5 REGULATORY REQUIREMENTS

Regulatoryrequirementshaveplayedasignificantroleinthedevelopmentandapplicationofwastewatertreatmentprocesses.Emergingresearchandsubsequent regulationshave shiftedcurrentgoalsoraddednewgoals tothetreatmentprocessfromtimetotime.Thishasresultedin innovationofnewengineeredprocesses.Inthefollowingsections,thedevelopmentofregulationsandstandardspertainingtowastewater intheUnitedStates,EuropeanUnion,andtheUnitedKingdomwillbediscussedindetail.

1.5.1 U.S. regulations

TheFederalWaterPollutionControlAct(FWPCA)of1948wasthefirstlegislationenactedbythefederalgovernmenttoaddressurbanwastewatermanagementissues(PublicLaw,1948).Theactprovidedforcomprehen-siveprogramsforeliminatingorreducingthepollutionofinterstatewatersand tributaries, and for research and technical assistance for improvingthesanitaryconditionofsurfaceandgroundwaters.MajoramendmentstotheFWPCAwereenactedin1961,1966,1970,1972,1977,and1987.The1966amendments, titled theCleanWaterRestorationAct, stronglyaddressed the issueofprotectingwaterquality (PublicLaw,1966).This1966actprovidedforauthorizationofacomprehensivestudyoftheeffectsofpollution,includingsedimentation,inU.S.estuariesandestuarinezonesonfishandwildlife,sportandcommercialfishing,recreation,watersupplyandpower, andother specifieduses.The legislation established a set ofwaterqualitystandards.Protectingpublichealthwastheprimarygoal,butadditionalgoalsofprotectingaquaticlifeandaestheticsofwaterresourceswereincluded.

TheFWPCAamendmentsof1972stipulatedbroadnationalobjectivestorestoreandmaintainthechemical,physical,andbiologicalintegrityofthenation’swaters(PublicLaw,1972).ThisbecameknownastheCleanWater Act (CWA) together with subsequent amendments in 1977. TheCWAestablishedthebasicstructureforregulatingdischargesofpollutantsintothenation’swatersandregulatingqualitystandardsforsurfacewaters.Newregulationswereestablishedforindustrialandagriculturalpolluters.TheCWAauthorizedtheEnvironmentalProtectionAgency(EPA)toestab-lish the National Pollutant Discharge Elimination System (NPDES) per-mitprogram.Allmunicipal,industrial,andotherfacilitiesthatdischargedtheirwastewatertosurfacewaterswererequiredtoobtainanNPDESper-mitfromEPA,whichspecifiedtechnology-basedeffluentstandardsforspe-cificpollutants.TheCWAalsoauthorizedsignificantfederalfundingforresearchandconstructiongrants,withtheambitiousgoalofeliminatingallwaterpollutionby1985.Allpubliclyownedtreatmentworks(POTWs)wererequiredtomeettheminimumstandardsforsecondarytreatment.


In 1973, the U.S. EPA published its definition of minimum standardsfor secondary treatment. This was amended in 1985 to include percentremoval requirements for treatmentplants servedby separate sewer sys-tems.Thestandardswereamendedagainin1989toclarifypercentremovalrequirementsduringdryperiodsfortreatmentfacilitiesservedbycombinedsewers.ThesecondarytreatmentstandardsareprovidedinTable 1.2.ThestandardsarepublishedintheCodeofFederalRegulations(40CFR,Part133.102). Three important effluent parameters are included: BOD5 (5 dBOD),TSS,andpH.CBOD5orcarbonaceousBOD5maybesubstitutedforBOD5at theoptionof thepermittingauthority.Special interpretationofthedefinitionofsecondarytreatmentispermittedforPOTWsthatreceiveindustrialflowsorusewastestabilizationpondsandtricklingfilters.

TheCWAwasamendedin1987toemphasizeidentificationandregula-tionoftoxiccompoundsinsludge,aswellastoauthorizepenaltiesforper-mitviolations.ThisamendmentwasknownastheWaterQualityAct.Theactestablishedfundingforstatestodevelopandimplement,onawatershedbasis, nonpoint sourcemanagement and controlprograms.A significantamendmentoftheCWAwasmadein2000(Section303(d)oftheCWA),whichrequiredtheestablishmentofatotalmaximumdailyload(TMDL)oramountofapollutantthatawaterbodycouldreceivewithoutcompro-misingwaterqualitystandards.

Theuseanddisposaloftreatedsludgeorbiosolidsareregulatedunder40CFR,Part503(U.S.EPA,1994).Theregulationwaspromulgatedin1993to regulate theuseanddisposalofbiosolids frommunicipalwastewatertreatment plants and to establish limits for contaminants (e.g. metals),pathogens, and vector attraction. The regulations are applicable to alltreatmentplants thatuse landapplication forfinaldisposalofbiosolids.Theregulationsareself-implementing,i.e.permitsarenotrequiredbytheplants.Butfailuretoconformtotheregulationsisconsideredtobeaviola-tionofthe law.Frequencyofmonitoringandreportingrequirementsareprovidedindetail.ThePart503ruledefinestwotypesofbiosolids,Class

Table 1.2 Secondary treatment standards as defined by U.S. EPA (2012)

Effluent Parameter Average 30-d concentration Average 7-d concentration

BOD5 30 mg/L 45 mg/LTSS 30 mg/L 45 mg/LRemoval 85% BOD5 and TSSpH Within range of 6.0 to 9.0 at all timesCBOD5 25 mg/L 40 mg/L

Note: Treatment facilities using stabilization ponds and trickling filters are allowed to have higher average 30-d and average 7-d concentrations of 45 mg/L and 65 mg/L of BOD5 and TSS, as long as the water quality of the receiving body is not adversely affected. Exceptions are also permitted for facili-ties with combined sewers, etc. The CBOD5 may be substituted for BOD5 at the discretion of the permitting authority. (Source: U.S. EPA 2012)


AandClassB,basedonthelevelofpathogenreduction,metalconcentra-tions,andvectorattractionreduction.ClassAbiosolidscanbeappliedtolandwithoutanyrestrictions.Sludgestabilizationrequirementsandpatho-genreductionalternativesarespecifiedinthelaw.AdditionaldetailsofthePart503ruleareprovidedinChapter12(Section12.9).

1.5.2 European Union regulations

TheEuropeanUnion(EU)hasestablishedanumberofpoliciesordirec-tives that address the quality of surface and ground waters. Water sup-plyandsanitationistheresponsibilityofeachmembernationintheEU.However, theEUdirectives serve as a baseline for individual nations toformtheirownlegislation.

TherearethreemajorEUdirectives:

• TheUrbanWasteWaterTreatmentDirective(91/271/EEC)of1991pertainingtodischargesofmunicipalandsomeindustrialwastewaters

• TheDrinkingWaterDirective(98/83/EC)of1998pertainingtopota-blewater

• TheWaterFrameworkDirective(2000/60/EC)of2000pertainingtomanagementofsurfaceandgroundwaterresources

TheUrbanWasteWaterTreatmentDirectivewasaimedatprotectingthe environment fromadverse effectsdue to collection, treatment, anddischargeofwastewater frommunicipalandsome industrial treatmentfacilities (Europa,2012).Thetwomajorelementsof thedirectivewereas follows: (1) Depending on the population size and designated loca-tion,allbuilt-upareaswererequiredtohaveurbanwastewatercollectionandtreatmentsystemsbytheyear1998,2000,or2005(newmembersby 2015). (2) The level of treatment had to be primary, secondary, ortertiary,dependingonthesensitivityofthereceivingwater(vanRiesen,2004).Member states had to establish lists of sensitive areas. Primarytreatment was deemed sufficient for less-sensitive areas. The directivewasamendedbytheCommissionDirective98/15/ECin1998.Thedis-chargestandardsfornormalareasareprovidedinTable 1.3.DischargerequirementsfornitrogenandphosphorusinsensitiveareasareprovidedinTable 1.4.

The European Commission (EC) has published three reports on theimplementation of the directive.The last reportwaspublished in 2004.Thereportnoted that thewastewater treatmentsituation inEuropewasstillquiteunsatisfactory,andthatnoneofthedeadlineshasbeenmetbyallmembercountries.OnlyAustria,Denmark,andGermanyhadfullycom-plied with the directive. BOD levels had been reduced by 20%–30% inEUrivers,butotherpollutionparameterssuchasnitrogenlevelsremained


high.Outof556 cities in theEU,25hadnowastewater treatment sys-tematall.AccordingtotheEC’s2004report,thedirectiverepresentsthemostcost-intensiveEuropeanlegislationintheenvironmentalsector.TheECestimatedthat152billioneuroswereinvestedinwastewatertreatmentfrom1990to2010.TheEUprovidedsupportofaboutfivebillioneurosperyearfortheimplementationofthedirective.

AccesstowatersupplyandwastewatertreatmentvariesacrossEurope.Averageconnectionratesarebetween80%and90%fornorthern,south-ern,andcentralEurope.EasternEuropehasmuch lower ratesof40%–65%ofthepopulationconnectedtoatleastprimarywastewatertreatment.However, conditions are slowly improving. Large numbers of treatmentplantshavebeenupgradedfromprimarytosecondarytreatmentorfromsecondarytotertiarytreatment.

1.5.3 United Kingdom regulations

AnexampleoftheadoptionoftheUrbanWasteWaterTreatmentDirectiveisdiscussedintermsoftheUnitedKingdom(UK).IntheUK,theUrbanWasteWaterTreatmentRegulationsof1994wereenactedbasedontheUrban Waste Water Treatment Directive of the EU. These were lateramended in 2003. The regulations set the standards for collection andtreatment of wastewater. The law stipulated that a sewerage system beprovidedforallurbanareasaboveaspecifiedsizeandthatthecollectedsewageshouldreceiveatleastsecondary(biological)treatmentbeforeitisdischargedtotheenvironment.Uncontrolleddischargesfromthesewerage

Table 1.3 EU standards for effluent discharge in normal areas

Parameters Concentration, mg/L Minimum reduction, %

BOD5 25 70–90COD 125 75TSS 35 90

Source: Adapted from van Riesen (2004).

Table 1.4 EU standards for nutrient discharge in sensitive areas

Parameter Concentration, max. annual mean Minimum reduction, %

Total Phosphorus 2 mg/L as P (10,000–100,000 P.E.) 801 mg/L as P ( >100,000 P.E.) 80

Total Nitrogen 15 mg/L as N (10,000–100,000 P.E.) 70–8010 mg/L as N ( >100,000 P.E.) 70–80

Source: Adapted from van Riesen (2004).

Note: P.E. indicates population equivalent.


systemsareallowedonlyunderstormconditions.Thelawidentifiedsensi-tiveareas,e.g.eutrophicwaters.Largertreatmentplantshavetoreducetheirnutrientloadspriortodischargetoeutrophicwaters.Theregulationalsobanned thedisposalof sludge to seaby the endof1998 (DEFRA,2012).

TheDepartmentofEnvironment,Food,andRuralAffairs(DEFRA)isresponsible for policy on implementation of the regulations in England,NorthernIreland,theScottishgovernmentinScotland,andtheWelshgov-ernmentinWales.Theirenvironmentalregulators(theEnvironmentAgencyforEngland,NorthernIrelandEnvironmentAgency,ScottishEnvironmentProtection Agency, and Environment Agency Wales) are responsible formonitoringdischargesfromtreatmentplantsforcompliancewiththelegis-lation’streatmentstandards(DEFRA,2012).

REFERENCES

Anon(2011)“TheHistoryofWastewaterTreatment.”http://www.cityoflewisville.com/wcmsite/publishing.nsf/AttachmentsByTitle/Wastewater+Treatment+History/$FILE/The+History+of+Wastewater+Treatment3.pdf.

Burian,S.J.,Nix,S.J.,Pitt,R.E.,andDurrans,S.R. (2000)“UrbanWastewaterManagementintheUnitedStates:Past,PresentandFuture.”Journal of Urban Technology,vol.7,no.3,p.33–62.

DEFRA(2012)http://www.defra.gov.uk/environment/quality/water/sewage/sewage-treatment/.

Duffy,J.(1968)A History of Public Health in New York City: 1625–1866.RussellSageFoundation,NewYork.

EastBayMunicipalDistrict(2011)http://www.ebmud.com/about-ebmud.Encina Wastewater Authority (2011) http://www.encinajpa.com/Home/Public

Information.aspx.Europa(2012)“SummariesofEULegislation.”http://europa.eu/legislation_summaries/

environment/water_protection_management/l28008_en.htm.EuropeanCommission(2004)“EuropeanCommissionReport”p.108.http://eur-lex.

europa.eu/LexUriServ/site/en/com/2004/com2004_0248en01.pdf.Hey,D.L.,andWaggy,W.H.(1979)“PlanningforWaterQuality:1776to1976.”

ASCE Journal of the Water Resources Planning and Management Division,105(March),p.121–131.

Metcalf and Eddy, Inc. (2003) Wastewater Engineering: Treatment and Reuse.Fourthedition.McGraw-Hill,Inc.,NewYork.

NEWater(2011)http://en.wikipedia.org/wiki/NEWater.Peavy,H.S.,Rowe,D.R.,andTchobanoglous,G.(1985)Environmental Engineering.

McGraw-Hill,Inc.,NewYork.PublicLaw(1948)P.L.845,Ch758,33U.S.C.1251-1376.PublicLaw(1966)P.L.89-753,33U.S.C.466.PublicLaw(1972)P.L.92-500,33U.S.C.1251,1311-1317.


U.S. EPA (1994)“A Plain English Guide to the EPA Part 503, Biosolids Rule.”EPA/832/R-93/003.Washington,D.C.

U.S. EPA (2012)“EPA NPDES PermitWriters’ Manual.” Chapter 5, Section 5.2.http://www.epa.gov/npdes/pubs/chapt_05.pdf.

vanRiesen,S.(2004)“EuropeanWastewaterStandards.”PresentedattheWastewaterForum of IFAT China 2004: International Trade Fair for EnvironmentalProtection,Jun29-Jul2,Shanghai,China.

West Point Treatment Plant (2011) http://www.kingcounty.gov/environment/wtd/About/System/West.aspx.

13

Chapter 2

Reaction kinetics and chemical reactors

2.1 REACTION KINETICS

A variety of chemical and biochemical reactions take place in the envi-ronmentthatareofimportancetoenvironmentalengineersandscientists.Theseincludereactionsbetweenvariouselementsoftheair,water,andsoil,aswellaswithmicroorganisms.Anumberofthesereactionsaredepen-dent on time, temperature, pressure, and/or concentration: for example,biodegradation of organic matter, bacterial growth and decay, chemicaldisinfection.

Reaction kineticscanbedefinedasthestudyoftheeffectsoftempera-ture, pressure, and concentrationof reactants andproducts on the rateofachemical reaction(HenryandHeinke,1996).Reactions thatoccurwithinasinglephase(solid, liquid,orgaseous)arecalledhomogeneous reactions,e.g.nitrification inwastewater.Reactionsthat involvetwoormore phases are called heterogeneous reactions, e.g. gas adsorption onactivatedcarbon.

The rate of reaction, ri, isused todescribe the rateof formationof aproduct, or rate of disappearance of a reactant. For homogeneous reac-tions,riiscalculatedasthemolesormassproducedorconsumedperunitvolumeperunittime.

Letusconsiderthefollowinghomogeneousreaction:

aA+bB→cC (2.1)

where:C=productA,B=reactantsa,b,c=stoichiometriccoefficients

Therateequationfortheabovereactionis

rA=–k[A]α[B]β=k[C]γ (2.2)


where:α,β,γ=empiricallydeterminedexponents[A],[B],[C]=molarconcentrationsofA,B,andCk=reactionrateconstant

Theorder of a reactionisthesumoftheempiricallydeterminedexponents,e.g.theorderis(α+β)withrespecttothereactantsAandB,whiletheorderisγwithrespecttotheproductC.Theorderofareactioncanbeawholenumber(e.g.0,1,2)orafraction.Figure 2.1illustratesthevariationofreactionraterAwithtimeforzero,first,andsecondorderreactions.Fora homogeneous, irreversible, elementary reaction that occurs in a singlestep,theempiricallydeterminedexponentsareequaltothestoichiometriccoefficients.Inthatcase,equation(2.2)becomes

rA=–k[A]a[B]b=k[C]c (2.3)

2.2 HOW TO FIND THE ORDER OF A REACTION

ConsiderthefollowingirreversibleelementaryreactionwherereactantAisconvertedtoaproductC:

A→C (2.4)

Therateequationcanbewrittenasfollows:

rA=–k[A]α

Time, t

Reac

tion

Rate

, rA

Zero order

First order

Second order

Figure 2.1 Variation of reaction rate with time.

Reaction kinetics and chemical reactors 15

or

loge(–rA)=loge(k)+αloge[A] (2.5)

where:α=orderofthereaction(e.g.0,1,2,etc.)k=reactionrateconstant

Anexperimentisconductedwheretheabovereactionisallowedtoproceed.TheconcentrationofA([A])atvarioustimeintervals(t)ismeasured.Plot[A]versust,asshowninFigure 2.2(a).Calculateslope(rA)ofthetangentatvariouspointsalongthecurve.Plotloge(–rA)versusloge[A],asshowninFigure 2.2(b).Abestfitlineisdrawntorepresentequation(2.5).Slopeofthebestfitlineisequaltotheorderofreaction.

EXAMPLE2.1The following data were obtained from a batch experiment for thereactionA→P.Determinetheorderofthereaction.

Time (min) 0 10 20 40 60 80 100

A (mg/L) 100 74 55 30 17 9 5

SOLUTION

AnExcelspreadsheetisusedtocalculatethevalues.

Time, t

Conc

entr

atio

n of

Reac

tant

, [A]

Slope oftangent, rA

[A0]

loge [A]

log e (–

r A)

loge (k)

Slope = α

Figure 2.2 (a) Concentration of A versus time plot, (b) logarithmic plot of reaction rate versus concentration of A.


t, min A, mg/L Ln(A) rA Ln(–rA)

0 100 4.61 10 74 4.31 –2.59 0.95 20 55 4.01 –1.92 0.65 40 30 3.41 –1.24 0.21 60 17 2.81 –0.68 –0.39 80 9 2.21 –0.37 –0.99100 5 1.61 –0.20 –1.59

Figure (a)isaplotofconcentrationversustime.Thesectionofthecurvebetween each time interval is assumed tobe a straight line,andtheratesarecalculatedfromtheslopeofthatsection.So,rA=dA/dt=(100–74)/(0–10)=–2.59forthefirstintervalandsoon.Figure (b)isaplotofln(–rA)versusln(A).Theslopeofthebestfitlineis0.935,whichcanberoundedto1.Sothereactionisfirstorder.

(a) (b)

020406080

1 0 01 2 0

0 50 1 0 0 1 5 0t, min

A, m

g/L

-2-1 .5

-1-0 .5

00 .5

11 .5

0 2 4 6

Ln(A)

y = 0 .9 3 5x -3 .0 4 7R2 = 0 .9 9 7

Ln(-

r A)

2.3 ZERO ORDER REACTION

Azeroorder reactionproceeds at a rate that is independentof the con-centrationofthereactantsorproducts.ConsiderthefollowingirreversibleelementaryreactionwherereactantAisconvertedtoproductC:

A→C (2.6)

Ifthisreactioniszeroorder,therateexpressioncanbewrittenas:

rA=–k (2.7)

or

d A

dtk

[ ]= − (2.8)


where:

d A

dt

[ ]= rateofchangeofconcentrationofAwithtime

k=reactionrateconstant,time–1

Integrateequation(2.8)betweeninitialvaluesandvaluesaftertimet

[ ]

[ ]

[ ]

A

A t

o

t

d A k dt∫ ∫= −0

or

[At]–[Ao]=–kt

or

[At]=[Ao]–kt (2.9)

where:[Ao]=initialconcentrationofreactantAattimezero,mg/L[At]=concentrationofAaftertimet,mg/L

To determine the rate constant k for zero order kinetics (equation 2.9),anexperimentisconductedwheretheconcentrationofAismeasuredatregularintervalsoftime.ConcentrationofAversustimeisplotted.AbestfitlineisdrawnthroughthedatapointsasshowninFigure 2.3.Thesloperepresentstherateconstantk,andtheinterceptrepresents[Ao].

Time, t

Conc

entr

atio

n of

Reac

tant

, [A] Slope = – k

[A0]

Figure 2.3 Concentration versus time plot for zero order reaction.


2.4 FIRST ORDER REACTION

Considertheirreversibleelementaryreactionrepresentedbyequation(2.6).If the reaction isfirstorderwith respect to concentrationofA, the rateexpressionbecomes

d A

dtk A

[ ][ ]= − (2.10)

Integrateequation(2.10)betweeninitialvaluesandvaluesaftertimet,

[ ]

[ ]

[ ]

[ ]A

A t

o

t

d A

Ak dt∫ ∫= −

0

or

logA

Akte

o

t

[ ]

[ ]=

or

[At]=[Ao]e–kt (2.11)

Anexperimentalproceduresimilartothepreviousoneisfollowedtodeter-minetherateconstantkforfirstorderkinetics.ConcentrationofAversustime isplotted.Forafirstorder reactiona curve isobtained, similar toFigure 2.4(a).Theslopeofthetangentatanypointonthecurverepresentsequation(2.10).Aplotofloge[A]versustimeshouldyieldastraightline,asshowninFigure 2.4(b).Theslopeofthebestfitlineisequaltotherateconstantk.

Time, t

Conc

entr

atio

n of

Reac

tant

, [A]

Slope oftangent, rA

[A0]

Time, t

Log

Conc

entr

atio

nof

Rea

ctan

t, lo

g [A

]

loge [A0]

Slope = – k

Figure 2.4 Plots of concentration versus time for a first order reaction on (a) arithmetic scale and (b) semilogarithmic scale.


2.5 SECOND ORDER REACTION

Letusconsidertheirreversibleelementaryreactionrepresentedbyequation(2.6).IfthereactionissecondorderwithrespecttoconcentrationofA,therateexpressionbecomes

d A

dtk A

[ ][ ]= − 2 (2.12)

Integrateequation(2.12)betweeninitialvaluesandvaluesaftertimet,

[ ]

[ ]

[ ]

[ ]A

A t

o

t

d A

Ak dt∫ ∫= −

2

0

or

1 1

[ ] [ ]A Akt

t o

− = (2.13)

Anexperimentalproceduresimilartothepreviousoneisfollowedtodeter-minetherateconstantkforsecondorderkinetics.Valuesof1/[A]versustimeareplotted,asshowninFigure 2.5.Theslopeofthebestfitlinepro-videsthevalueofk.

2.6 REACTORS

A reactor is a tankor vesselwhere chemical, biological, or biochemicalreactionstakeplace,usuallyinaliquidmedium.Reactionscanalsotakeplaceinsolidorgaseousmediumorinacombination.Chemicalreactorsareusedinawatertreatmentplantincoagulation–flocculation,limesoftening,

Time, t

Slope = k

1[A0]

Reci

proc

al o

f Con

c.of

Rea

ctan

t, 1/

[A]

Figure 2.5 Plot of 1/[A] versus time for a second order reaction.


tasteandodorcontrol,disinfection,andotherunitprocessesthatinvolvechemicalreactions.Reactorsusedinwastewatertreatmentplantsinvolvemostlybiochemicalandbiologicalreactions,e.g.activatedsludgereactor,membranebioreactor.

Therearethreetypesof idealreactors: (1)batchreactor, (2)plugflowreactor (PFR),and (3)continuous-flowstirred tankreactor (CSTR).Thehydraulicsandconversionefficienciesofthesereactorscanbedeterminedusing mathematical models. Models developed for ideal reactors can befurther modified to represent real-life processes and flow conditions forreactorsusedat treatmentplants. In the followingsections,basicdesignequationsforidealreactorswillbediscussed.

2.6.1 Conversion of a reactant

Theconversionorremovalofareactantiscalculatedasfollows:

fA A

A

A

Ao t

o

t

o

= − = −[ ] [ ]

[ ]

[ ]

[ ]1 (2.14)

where:f=conversionorremovalefficiency[Ao]=initialconcentrationofreactantAattimezero,mg/L[At]=concentrationofAaftertimet,mg/L

2.6.2 Detention time in reactor

Thetheoreticaldetentiontimeorresidencetimeofthefluidparticlesinareactorisgivenby

tV

Q= (2.15)

where:t=detentiontimeinreactorV=volumeofreactorQ=volumetricflowrate,volume/time

Theactualdetentiontimeinareactorcanbedeterminedbyaddingatracerordyetotheinfluentduringsteadystateflowandthenmeasuringthecon-centrationof the tracer in the effluentover aperiodof time.The tracerconcentration in the effluent is plotted versus time on graph paper, andthecentroidoftheresultingcurveislocatedastheactualdetentiontime


(Figure 2.6).Theactualdetentiontimeisusuallylessthanthetheoreticaldetentiontimecalculatedusingequation(2.15).Thiscanbeduetobackmixingandshortcircuitingoffluidinthereactor.

2.7 BATCH REACTOR

Inabatchreactor,reactantsareaddedtothereactorandmixedforareq-uisiteamountoftimeforthereactionstooccur(Figure 2.7a).Attheendofthereactiontime,thecontentsareremovedfromthereactor.Onecharac-teristicofthebatchreactoristhatallfluidparticleshavethesameresidencetimeinthereactor.Homogeneousmixingisassumed,sothatthecomposi-tionofthemixtureisthesamethroughoutthereactor.Theconcentrationvarieswithtimeasthereactionproceeds.Figure 2.7(b)illustratesthevaria-tionofreactantconcentrationwithtime.

Batch reactors are generally used for bench scale experiments andliquid phase reactions. They are useful in determining the effects ofvariablesonareactionprocess.Anumberofexperimentscanbecon-ducted at the same time in batch reactors, thus facilitating the studyofprocessvariables.Theyareusedextensively inpharmaceuticalandotherindustries.

Batch reactors are not suitable for gas phase reactions or large-scalecommercialapplications.Laborcostsandmaterialshandlingcostscanrunhigh,duetothetimeandeffortinvolvedinfilling,emptying,andcleaningthereactors.

2.7.1 Design equation

Considerthefollowingmass/materialbalanceofareactantAconvertedtoaproductCinareactor:

Time

E�ue

nt T

race

r Con

cent

ratio

n

QVtTracer

Centroid

Figure 2.6 Effluent tracer profile for calculation of detention time in a reactor.


(Rateofinput)=(Rateofoutput)+(Rateofaccumulation) –(Rateofconsumption) (2.16)

Forabatchreactor,thetimeperiodforreactionbeginsjustafterthereactorisfilledandendsjustbeforecontentsareemptied.So,rateofinput=0,andrateofoutput=0.Equation(2.16)becomes

Rateofconsumption=Rateofaccumulation

So,thedesignequationiswrittenasfollows:

rd A

dtA = [ ]

(2.17)

where:

(a)

(b)Time

Conc

entr

atio

n of

Rea

ctan

t

Figure 2.7 (a) Batch reactor, (b) concentration profile for a batch reactor with time.


rA=rateofconsumptionoflimitingreactantA,concentration/time[A]=concentrationoflimitingreactantA

Whentheorderofthereactionisknown,anexpressionforrAcanbesub-stitutedintotheleftsideofequation(2.17),andtheresultingdifferentialequationcanbeintegratedtoobtainthedesignexpression.Table 2.1pres-ents thedesignequations for zero,first,and secondorder reactions inabatchreactor.

EXAMPLE2.2Considerafirstorderreactiontakingplaceinabatchreactor.Developanexpressionforthedetentiontimeinthereactor.

SOLUTION

Forafirstorderreaction,rA=–k[A]SubstitutetheexpressionforrAinequation(2.17):

− =k Ad A

dt[ ]

[ ]

Thisissimilartoequation(2.10).Uponintegrationbetweenlimitsweobtain,

log

A

Akte

o

t

[ ]

[ ]=

or

t

klog

A

Ae

o

t

= 1 [ ]

[ ]

2.8 PLUG FLOW REACTOR (PFR)

Inaplugflowreactor,fluidparticlesflowthroughthetankandaredis-charged in the same sequence as they entered. The fluid particles movethrough the reactor tube as plugs moving parallel to the tube axis(Figure 2.8a).Thereisnolongitudinalmixingoffluid,thoughtheremaybesomelateralmixing.Allfluidelementshavethesameresidencetimeinthereactor.Figure 2.8(b)presentstheconcentrationgradientfromreactorinlettooutlet.Thisisduetotheconversionofreactantasitflowsthroughthereactor.Thevelocityprofileatanygivencross-sectionisflat,asthereisnobackmixingoraxialdiffusion.Asaresult,theconcentrationofreactantacrossanyverticalcross-sectionisthesame,asillustratedinFigure 2.8(c).


Theplugflowreactorissuitableforgasphasereactionsthattakeplaceathighpressureandtemperature.Aninsulatingjacketcanbeplacedaroundthereactortomaintainthedesiredtemperature.Therearenomovingpartsinsidethereactor.TheaveragereactionrateisusuallyhigherinaPFRascomparedwithaCSTRofsimilarvolume,forthesamefeedcompositionandreactiontemperature.ThePFRmakesmoreefficientuseofreactorvol-ume,whichmakesitsuitableforprocessesthatrequirelargevolumes.With

(a)

(b)

(c)Section a-a

Conc

entr

atio

n of

Rea

ctan

t

a a

Distance from Inlet

Conc

entr

atio

n of

Rea

ctan

t

Inlet Outlet

In�uent E�uent

Inlet Outletdx

a

a

Figure 2.8 (a) Flow through a PFR, (b) variation of reactant concentration in a PFR, (c) longitudinal distribution of reactant concentration for section a–a in the PFR.


sufficientlyhighrecyclerates,thebehaviorofthePFRbecomessimilartothatofaCSTR.


Considerthedifferentialsectiondx(Figure 2.8a)withadifferentialvolumedVinthereactor.Amass/materialbalanceonlimitingreactantAinthedifferentialvolumeisasfollows:

(Rateofinput)=(Rateofoutput)+(Rateofaccumulation) –(Rateofconsumption)

Forsteadystateconditions,thedesignequationiswrittenas

rd A

dtA = [ ]

(2.18)

which is the sameas thedesign equation for abatch reactor.When theorderofthereactionisknown,anexpressionforrAcanbesubstitutedintotheleftsideoftheaboveequation,andtheresultingdifferentialequationcanbe integratedtoobtainthedesignexpression.Table 2.1presentsthedesignequationsforzero,first,andsecondorderreactionsinaPFR.

EXAMPLE2.3AreactiontakesplaceinaPFR,wherereactantAisconvertedtoprod-uctP.Therateequationis

rA=–0.38[A]mol/L·s

DeterminethevolumeofPFRrequiredfor95%conversionofA.Theini-tialconcentrationofAis0.25mol/L,andvolumetricflowrateis5m3/s.

Table 2.1 Design equations for batch, PFR, and CSTR

Order Rate of reaction Batch reactor expression CSTR or PFR

0 –k kt = [Ao] – [At] kt = [Ao] – [At]

1 –k[A] ktAAo

t

=

ln[ ][ ]

ktAAo

t

=

−[ ][ ]

1

2 –k[A]2 ktA

AAo

o

t

= −

11

[ ][ ][ ]

ktA

AAt

o

t

= −

11

[ ][ ][ ]


SOLUTION

Thegivenreactionisfirstorderwithk=0.38s–1,[Ao]=0.25mol/L.With95%conversion,[A]=(1–0.95)[Ao]=0.05×0.25=0.0125

mol/L.FromTable 2.1,firstorderdesignequationforaPFRis

ktA

Ao

t

=

ln[ ]

[ ]

or

0.38t=ln(0.25/0.0125)

or

t=7.88s

VolumeofPFR,V=Qt=(5m3/s)(7.88s)=39.42m3.

2.9 CONTINUOUS-FLOW STIRRED TANK REACTOR

Continuous-flow stirred tank reactors (CSTRs) are used mainly for liq-uidphase reactionsat loworatmosphericpressures. In this reactor, thereactantflowscontinuouslyintothereactor,theproducteffluentflowsoutcontinuously, and the reactor contents are mixed on a continuous basis(Figure 2.9a).Thistypeofreactorisalsocalledbackmixreactororcom-pletelymixedreactor.

ThebasicassumptionforanidealCSTRisthatthereactorcontentsarecompletely mixed and homogeneous throughout. When a reactant [Ao]enters the reactor, it is subjected to instantaneousandcompletemixing,resulting in immediate reduction to the final effluent concentration [At].Theeffluentcompositionandtemperaturearethesameasthoseofthereac-torcontents.Thisremainsthesameovertime,asshowninFigure 2.9(b).

Atracermoleculeintheinfluenthasequalprobabilityofbeinglocatedanywhereinthereactorafterasmalltimeinterval,withinthelimitofcom-pletemixing(Hill,1977).Thusallfluidelementsinthereactorhaveequalprobabilityofleavingthereactorwiththeeffluentinthenexttimeincre-ment.Asaresult,thereisabroaddistributionofresidencetimesforvariousfluidparticlesasillustratedinFigure 2.9(c).

LowerconversionofreactantisachievedinaCSTRascomparedwithaPFR,at thesameoperating temperatureandfeedcomposition.This ismainlyduetothevariationofparticleresidencetimeswithinthereactorandtheinabilitytoachievecompletemixing.Asaresult,aCSTRoflargervolumeisrequiredtoachievethesameconversionasaPFR.


(a)

(b)Time

E�ue

nt R

eact

ant C

once

ntra

tion

In�uent E�uent [A0] [A]

[A]

(c)Residence Time

Perc

ent o

f Par

ticle

s

0 t

100

0

Figure 2.9 (a) CSTR, (b) effluent concentration variation for a CSTR, (c) residence time distribution of fluid particles in a CSTR.



Amass/materialbalancecanbewrittenforthelimitingreactantA,assum-inghomogeneousconditionsthroughoutthereactor:

(Rateofinput)=(Rateofoutput)+(Rateofaccumulation) –(Rateofconsumption)

Atsteadystateconditions,rateofaccumulation=0.Sothedesignequationcanbewrittenas

rA A

tA

t o= −[ [ ]] (2.19)

whereallthetermshavethesamemeaningsasdefinedintheprevioussec-tions.Whentheorderofthereactionisknown,anexpressionforrAcanbesubstitutedintotheleftsideoftheaboveequationtoobtainthedesignexpression.Table 2.1presentsthedesignequationsforzero,first,andsec-ondorderreactionsinaCSTR.

EXAMPLE2.4AchemicalreactiontakesplaceinaCSTR,whereAisconvertedtoproductP.TheinitialconcentrationofAis45mg/L.After5min,con-centrationofAismeasuredas36mg/L.

a.Calculatetheratecoefficientassumingthatthereactionisfirstorder.

b.Calculatetheratecoefficientassumingthatthereactionissec-ondorder.

SOLUTION

a.Forfirstorderreaction,usethedesignequationfromTable 2.1

ktA

Ao

t

=

−[ ]

[ ]1

Therefore, km in

m g L

m g L= −

1

5

45

361

/

/=0.05min–1.

b.Forsecondorderreaction,usethedesignequationfromTable 2.1

ktA

A

At

o

t

= −

11

[ ]

[ ]

[ ]


Therefore, km g L

m g L

m g L=

×−

1

5 36

45

361

m in /

/

/=0.0014(mg/l·min)–1.

2.10 REACTORS IN SERIES

Onemethodof increasingtheremovalefficiencyofaprocess is tousea number of reactors in series. This is usually applicable for CSTRs,thoughacombinationofCSTRandPFRcanalsobeused.WhenaseriesorcascadeofCSTRsareused, theeffluent fromone reactor servesastheinfluenttothenextreactor,asshowninFigure 2.10.Thereisastep-wisedecreaseinthecompositionofreactantandtemperatureastheflowtravelsfromonereactortothenextone.Assumingthattheconditionsinanyindividualreactorintheseriesarenotinfluencedbydownstreamconditions, and conditions of the inlet streamand those prevailing inthe reactor are the only variables that influence reactor performance(Hill, 1977), the following design equation can be written for steadystateconditions:

rA A

tAi

i i

i

=− −[ ] [ ]( )1 (2.20)

where:rAi=rateofconsumptionofAinithreactorti=detentiontimeinithreactor[A]i=concentrationofAineffluentfromithreactor[A](i–1)=concentrationofAineffluentfrom(i–1)threactor =concentrationofAininfluenttoithreactor

V1, t1 V(i–1), t(i–1) Vi, ti Vn, tn

[A]1 [A](i–1) [A]i

[A]n

[A0]

Q

Figure 2.10 Series of CSTRs.


Thedetentiontimeintheithreactorisgivenby

tV

Qi

i= (2.21)

where:Vi=volumeofithreactorQ=volumetricflowrateintoreactor

Inaseriesofnreactors,theoverallconversionisgivenby

fA A

Ao n

o

= −[ ] [ ]

[ ] (2.22)

where:[Ao]=concentrationofAininfluentto1streactor[An]=concentrationofAineffluentfromnthreactor

Conversioninindividualreactorscanbecalculatedfrominfluentandefflu-entreactantconcentrationsofthatreactor.

EXAMPLE2.5Consider the same first order chemical reaction from Example 2.4.Tworeactorsareusedinseries,aCSTRfollowedbyaPFRforproductformation.Thedetentiontimeinthefirstreactor(CSTR)is5min.Thetworeactorsareoperatedatthesametemperatureandhavethesamevolume.WhatwillbetheeffluentconcentrationofAfromthePFR?Whatistheconversionefficiency?

SOLUTION

Thetworeactorsareoperatedatthesametemperature:

Therefore,kCSTR=kPFR

Thetworeactorshavethesamevolume,andiftheflowrateisthesame:

detentiontime,tCSTR=tPFR

A1 A2AoQ

PFRCSTR

FromExample2.4,Ao=45mg/L,A1=36mg/L,kCSTR=0.05min–1.

Therefore,kPFR=0.05min–1


UsethedesignequationforPFRforfirstorderreactionfromTable 2.1:

kt lnA

A=

1

2

or

0 05 5361

2

. m in/

m in lnm g L

A− × =

or

A2=28.04mg/L

Overallconversionefficiency=45 28 04

45100

− ×.% =37.7%.

EXAMPLE2.6In Example 2.5, if another CSTR was used as the second reactorinsteadof thePFR,whatwouldbe the effluent concentrationofA?Calculate the conversion efficiency. Determine the concentration ofreactantAinthefirstandsecondreactors.

SOLUTION

UsedesignequationforCSTRforfirstorderreactionforreactor#2:

A1=36mg/L,kCSTR=0.05min–1,t=5min

ktA

A=

−1

2

1

or

0 05 536

11

2

./

m in m inm g L

A− × = −

or

A2=28.8mg/L

Overallconversionefficiency=45 28 8

45100

− ×.% =36%.

ForaCSTR,concentrationineffluent=concentrationinreactorConcentrationofAinreactor#1=A1=36mg/LConcentrationofAinreactor#2=A2=28.8mg/L


2.11 SEMIBATCH OR SEMIFLOW REACTORS

Reactorsused inactual treatmentplantsandprocessesmaybeoperatedsomewhereinbetweenidealreactormodes.Reactoroperationcanbesemi-batchorsemiflow.Afewexamplesaregivenbelow:

1.A reactor where all the reactants are added at the same time as abatch,buttheproductsaredischargedcontinuously

2.Areactorwherethereactantsareaddedatdifferenttimeintervals 3.Areactorwheretheproductsareremovedatdifferenttimeintervals 4.Abatch reactorpartially filledwithone reactant,withprogressive

additionofotherreactantsuntilthereactioniscompleted.

PROBLEMS

2.1 Itwasobservedfromanexperimentalstudythattherateofachemi-cal reaction did not depend on the concentration of the reactantbutwasinfluencedbytheconcentrationoftheproduct.Whatistheorderofthereactionwithrespecttothereactant?

2.2 Drawthecurvesforreactionrateversustimeforzero,first,andsec-ondorderreactions.Writedowntherateexpressionsforeachcurve.

2.3 AdenitrificationexperimentwasconductedbyagraduatestudentintheenvironmentalengineeringlaboratoryatGeorgeWashingtonUniversity,wherenitrate(NO3)wasconvertedtonitriteandnitro-gengas.Theconcentrationofnitratewasmeasuredatregulartimeintervals. The data are given below. Determine the order of thereaction.

Time, h NO3, mg/L

0.0 30.00.5 23.31.0 19.01.5 15.32.0 11.02.5 8.33.0 7.03.5 6.34.0 5.74.5 5.37.75 4.7


2.4 Wastewateristreatedinareactorvessel.Afirstorderreactiontakesplacewithrespecttotheorganicmatterinthewastewater.Therateconstantisdeterminedtobe0.23d–1.Theinitialconcentrationoforganicmatteris150mg/L,anditisdesiredtoachieve90%conver-sion.Theflowrateofthewastewateris500m3/d.

a. Calculate thedetention time and volumeof PFR required toachievethisconversion.

b. Calculate the volume of CSTR required to achieve the sameconversion.

c. Whichoptionseemsbettertoyouandwhy? 2.5 A laboratoryanalysis is carriedout inbatch reactors. Initial con-

centration of reactant was 0.25 mol/L, and 85% conversion wasachievedin20min.Itwasassumedthatthereactionwaszeroorderwithrespecttoreactant.

a. Calculatethezeroorderratecoefficient. b. Afterfurtherexperimentation,itwasdiscoveredthattherate

wasfirstorderandnotzeroorder.Calculate thecorrect ratecoefficient.

2.6 Theammoniainwastewateristobeconvertedtonitrateinabiore-actor.Initialconcentrationofammoniais145mg/L.Itisdesiredtoachieve90%conversionina50m3reactor.ThedesignengineeristryingtoselectbetweenaPFRandCSTRmodeforoperationofthereactor.Whichmodeofoperationwillallowtheengineertoprocessalargervolumeofwastewaterwithinashorterperiodoftime?

2.7 IndustrialwastewateristreatedinaCSTR.Theconversionofreac-tantAtoproductCisgovernedbythefollowingrateequation:

rA=–1.2[A]mg/L·h

a. The volume of the reactor is 60 m3. What is the volumetricflowrateofthewastewater,correspondingtoaconversioneffi-ciencyof95%?

b. If only 90% conversion efficiency is desired, can we use asmaller reactor volume to handle the same flow rate? Whatwouldbethevolume?

2.8 DairywastewateristreatedinaseriesofCSTRs.Theinitialcon-centrationofcomplexorganicsinthewastewateris1500mg/L.Thefirstorderratecoefficientis0.45d–1.Detentiontimeineachreactor is1.5d. If tworeactorsareused inseries,calculatethefinal effluent concentration of the organic matter. What is theconversionefficiency?

2.9 UsingthedatafromProblem2.8,calculatetheoverallefficiencyifthreereactorsareused inseries.Would itbe feasible touse threereactors?


2.10 Whatisaplugflowreactor?Whataretheadvantagesanddisadvan-tagesofusingaPFR?

2.11 Illustrate graphically the variation of reactant concentration withtimein(1)aPFRand(2)aCSTR.

2.12 WhatisaCSTR?MentiontwoadvantagesandtwodisadvantagesofaCSTR.

2.13 Itisdesiredtoincreasetheconversionefficiencyofachemicalpro-cess. Would you use multiple reactors in series or in parallel toachievethis?Why?

REFERENCES

Henry, J. G., and Heinke, G.W. (1996) Environmental Science and Engineering.Secondedition.PrenticeHall,NewYork.

Hill,C.G.Jr.(1977)An Introduction to Chemical Engineering Kinetics and Reactor Design.JohnWiley&Sons,NewYork.

35

Chapter 3

Wastewater microbiology

3.1 INTRODUCTION

Wastewatercontainsawidevarietyofmicroorganisms,someofwhicharepathogens,whileothersplaya significant role indegradationoforganicmatter.Bacteria,protozoa,andothermicroorganismsplayanactiverolein the conversion of biodegradable organic matter to simpler end prod-uctsthatresultinstabilizationofthewaste.Thisisacontinuousprocessoccurring instreamsandriversasnaturalpurificationprocesses.This isdescribedinmoredetailinChapter4.Thesenaturalpurificationprocessesareenhancedandacceleratedinengineeredbiologicaltreatmentsystemsatwastewatertreatmentplants.Forefficientremovaloforganicmatterandother pollutants, it is essential tohave a thoroughunderstanding of thenature,growthkinetics,andprocessrequirementsofthemicroorganismsinvolvedandutilized in thebiological treatmentprocesses.This chapterwillprovideanoverviewofthemajorgroupsofmicroorganismsusedinbiologicaltreatmentofwastewater.

The three major domains of living organisms are the Bacteria, theArchaea,andtheEukarya.ThisisaccordingtotheUniversalPhylogenetic(Evolutionary) Tree, which was derived from comparative sequencing of16Sor18SribosomalRNA(ribonucleicacid)(Madiganetal.,2010).Basedoncellstructure,alllivingorganismsaredividedintotwotypes:prokary-otic and eukaryotic.Themajorstructuraldifferencebetweenprokaryotesandeukaryotes is theirnuclear structure.Theeukaryoticnucleus is sur-rounded by a nuclear membrane, contains DNA (deoxyribonucleic acid)molecules,andundergoesdivisionbymitosis.Ontheotherhand,thepro-karyotic nuclear region is not surrounded by a membrane and containsa single DNA molecule whose division is nonmitotic. The prokaryotesincludebacteria,blue-greenalgae (cyanobacter), andarchaea.Figure 3.1shows typical cell structure of (a) prokaryotes and (b) eukaryotes. Thearchaea are separated frombacteria due to theirDNAcomposition anduniquecellularchemistry.Examplesofarchaeaare themethaneproduc-ers,e.g.methanococcus, methanosarcina.Theeukaryotesaremuchmore


complexandincludeplantsandanimals,aswellasprotozoa,fungi,andalgae.Table 3.1presentstheclassifications.MacroscopicanimalsincludeRotifers, Crustaceans,etc.Rotifersactaspolishersofeffluentfromwaste-watertreatmentplantsbyconsumingorganiccolloids,bacteria,andalgae.Themicroorganismsarediscussedinmoredetailinthefollowingsections.

3.2 BACTERIA

Bacteriaareunicellularprokaryoticmicroorganisms.Theyusesolublefood.Bacteriausuallyreproducebybinaryfission,althoughsomespe-ciesreproducesexuallyorbybudding.Theyaregenerallycharacterized

(a) (b)

Flagellum

Plasmid

Nuclear area

Cytoplasm

Nucleus

Centrioles

Mitochondrion

Lysosome

Microtubules

Endoplasmic reticulum

Cilium

Figure 3.1 Typical cell structure of microorganisms: (a) prokaryotic cell, (b) eukaryotic cell.

Table 3.1 General classification of organisms

Organisms Eukaryotes Prokaryotes

Macroorganisms Animals None knownPlants

Microorganisms Algae ArchaeaFungi BacteriaProtozoa

Wastewater microbiology 37

by the shape, size, and structureof their cells. Bacteria canhaveoneof three general shapes: spherical (coccus), cylindrical or rod shaped(bacillus),orspiralshaped(spirillum).Bacteriacanrange insizefrom0.5to5.0μmlongand0.3to1.5μmwide.Cocciareabout0.1μmindiameter(HenryandHeinke,1996).Figure 3.2illustratesthedifferentshapesofbacteria.

3.2.1 Cell composition and structure

A bacterial cell has about 80% water and 20% dry matter. Of the drymatter,90%isorganicand10%isinorganic.Abacterialcellisgenerallyexpressedbythefollowingsimplechemicalformula:C5H7O2N.Thiscanbeexpandedtoincludesulfurandphosphorus.Figure 3.3illustratesatypi-calbacterialcell.Thecell wallisarigidstructurethatprovidesshapetothecellandprotectsitfromosmoticpressure.Thewallisusually0.2to0.3μmthickandaccountsfor10%to50%ofthedryweightofthecell.Insidethecellwallisthecytoplasmic membrane,acriticalpermeabilitybarrierthatregulatesthetransportoffoodintothecellandofwasteproductsoutofthecell.Theinteriorofthecellcontainsthecytoplasm,thenucleararea,andthepolyribosomes.Thecytoplasmisacolloidalsuspensionofproteins,carbohydrates, and other complex organic compounds. The cytoplasmcontainsRNA,whichcausesbiosynthesisofproteins.TheRNA,togetherwithproteins,formsdenselypackedparticlescalledpolyribosomes,whichmanufactureenzymesforeachspecificbiochemicalreaction.ThenuclearareacontainsDNA,whichcontainsallthegeneticinformationnecessaryfor reproduction and is considered to be the blueprint of the cell. Somebacteriaoccasionallyhaveinclusionsconsistingofexcessnutrientsthatare

(a)

(b)

(c)

Figure 3.2 Bacteria of different shapes: (a) coccus, (b) rod, (c) spirillum.


storedforfutureuse.Thethicknessoftheinclusionorslimelayerdependsontheageofthecell.

3.2.2 Bacterial growth curve

A number of factors affect the growth and death of bacteria. Theseincludetypeoffoodorcarbonsource,abundanceoffood,nutrients,pH,temperature,presenceorabsenceofoxygen,andtoxicsubstances.Giventhepresenceofoptimalconditions,bacteriacangrowinlogarithmicpro-portions.Abatchexperimentwithalimitedamountoffoodorsubstratecan produce a bacterial growth curve similar to the one illustrated inFigure 3.4.

Time

Log

Num

ber o

f Bac

teria

Lagphase

Exponentialphase Stationary phase Death phase

Figure 3.4 Typical bacterial growth curve from a batch study.

Flagella

Nuclear area

CytoplasmCytoplasmicmembrane

Cell wallSlime layer of

organic polymer

Figure 3.3 Diagram of a typical bacterial cell.


Thebacterial growth curve exhibits fourdistinctphases, as shown inFigure 3.4.Theyarethefollowing:

1.Thefirstphaseiscalledthelag phase.Thisrepresentsthetimeneededbythebacteriatoadjusttothenewenvironmentandstartproducingenzymesnecessarytodegradethesubstratesurroundingthem.Ifthesubstrateisreadilydegradable,thenthelagphaseisshort.Ifthesub-strateisnotreadilybiodegradable,thenitmaytaketimeforthebac-teriatoproducethenecessaryenzymes.Thismayresultinalonglagphase,asillustratedinFigure 3.5(a),untilthebacteriaareacclimatedtothesubstrateandthenstartreproducing.Ifthebacteriaarenotabletosynthesizethenecessaryenzymes,thesubstratemaybetoxicandeventuallyresultindeathofthecells(Figure 3.5b).

(a)

(b)Time

Log

Num

ber o

f Bac

teria

Lag phase Death phase

Time

Log

Num

ber o

f Bac

teria

Lag phaseExponential

phaseStationary

phase Death phase

Figure 3.5 Bacterial growth curves exhibiting (a) acclimation and (b) toxic response.


2.After the lag phase comes the logarithmic or exponential growth phase. In thisphase, thepopulationdoublesat regular intervalsoftimeduetoabundanceofsubstrateandoptimalgrowthconditions.

3.Eventuallyasthesubstrateconcentrationdecreases,thegrowthratestartsdecreasingand thestationary phase isobserved.During thisphase,thegrowthrateequalsthedeathrate,resultinginadynamicequilibriumatwhichthereisnofurtherincreaseinpopulation.Thisphasecorrespondstoverylowsubstrateconcentration.

4.Thelastphaseiscalledtheendogenous decay ordeath phase,whenoneormorenutrientsorthesubstrateiscompletelyexhausted.Celldeathandlysisreleasessomesolubleorganicsthatareusedbysurviv-ingbacteriaforawhile.Thedeathratekeepsonincreasinguntilallbacterialcellsdieoff.

Mostbiologicalwastewater treatmentprocessesareoperatedsomewhereinbetweenthestationaryphaseandthedeathphase.Thisistrueforbio-logicalreactorsoperatedascontinuous-flowstirredtankreactors(CSTRs),sincethiscorrespondstoverylowsubstrateorbiochemicaloxygendemand(BOD)concentrationsinthereactorandeffluent.

3.2.3 Classification by carbon and energy requirement

Allcellsneedasourceofcarbonandasourceofenergytocarryoutcellsynthesis.Oneofthegoalsofwastewatertreatmentistoconvertboththecarbonandenergyofthewastewaterintomicrobialcells,whichcanthenberemovedfromwaterbysettlingorfiltration.Bacterialcellscanbedividedintotwobroadgroupsaccordingtocarbonandenergysources:

1.Heterotrophic—uses organic compounds as both their carbon andenergysource.Alargenumberofwastewaterbacteriaareheterotrophic.

2.Autotrophic—usesinorganiccompoundsascarbonsource(e.g.CO2,HCO3

–)andsunlightorinorganiccompoundsforenergy.Twotypesofautotrophsareofinterest:(a)PhotoautotrophsthatobtainenergyfromsunlightandcarbonfromCO2,and(b)Chemoautotrophsthatobtain energy fromoxidationof inorganic compounds and carbonfromCO2,e.g.nitrifyingbacteria—nitrosomonasandnitrobacter.

Thenitrifyingbacteriaareofgreatsignificanceinwastewatertreat-mentfornitrogenremoval.Thenitrifyingbacteriacarryoutthetwo-stepprocessofnitrification,whichresultsinconversionofammoniatonitritesinthefirststep,followedbyconversionofnitritestonitratesinthesecondstep.Thenitratesareconvertedtonitrogengasinasub-sequentstepcalleddenitrification.Thenitrificationreactionisgivenbelow:


nitrosomonas NH3+O2

______________▶ NO2–+energy (3.1)

nitrobacter NO2

–+O2______________▶ NO3

–+energy (3.2)

3.2.4 Classification by oxygen requirement

Bacteriacanbedivided intothefollowinggroupsbasedontheiroxygenrequirements:

1.Aerobic—requiresoxygenforgrowthandsurvival. 2.Anaerobic—grows in absence of oxygen. They cannot survive in

presenceofoxygen. 3.Facultative—cangrowbothinpresenceorabsenceofoxygen.E.g.,

Denitrifying bacteria are facultative anaerobes that grow underanoxicconditions.

3.2.5 Classification by temperature

Certaingroupsofbacteriagrowatspecificrangesoftemperature:

1.Cryophilic or psychrophilic—grows at temperatures below 20°C,usuallybetween12°Cand18°C.

2.Mesophilic—growsbetween25°Cand40°C,optimumat35°C. 3.Thermophilic—growsbetween50°Cand75°C,optimumat55°C.

Growthisnotlimitedtothesetemperaturerangesonly.Bacteriawillgrowatslowerratesatothertemperaturesandcansurviveoverawiderangeoftemperatures.Somecansurviveattemperaturesas lowas0°C.Iffrozenrapidly,bacteriacanbestoredforalongtimewithinsignificantdeathrates.BacteriareproducebybinaryfissionasillustratedinFigure 3.6.

3.2.6 Bacteria of significance

Bacteriaarethemostimportantgroupofmicroorganismsintheenviron-ment. The largest population of microorganisms present in water andwastewaterisbacteria.Someofthemarepathogenicandcausediseasesinhumansandanimals.Othergroupsofbacteriaareimportantinbio-logical wastewater treatment processes, natural purification processesin lakes and streams, anddecompositionoforganicmatter in soil andlandfills.Table 3.2presentssomesignificantgroupsofbacteriaandtheirfunctions.


Bacterial cell

Cell elongation

Distribution of nuclear material

Formation of transverse cellwalls and cellular material

New cells are formed andeach cell repeats the process

Figure 3.6 Cell reproduction by binary fission (Source: Adapted from Henry and Heinke, 1996).

Table 3.2 Important Groups of Bacteria in Water and Wastewater

Bacteria Genus Importance

Nitrifying bacteria Nitrosomonas Oxidizes ammonia to nitritesNitrobacter Oxidizes nitrites to nitrates

Denitrifying bacteria Pseudomonas, Bacillus Reduces nitrite and nitrate to nitrogen gas

Iron bacteria Leptothrix, Crenothrix Oxidizes ferrous iron to ferric ironSulfur bacteria Thiobacillus Oxidizes sulfur and iron, causes

corrosion of iron sewer pipesPhotosynthetic bacteria Chromatium, Chlorobium Reduces sulfides to sulfurIndicator bacteria Escherichia,

EnterobacterIndicates fecal pollution

Pathogenic bacteria Salmonella Causes salmonellosisVibrio cholera Causes choleraSalmonella typhi Causes typhoid feverLegionella pneumophila Causes legionairres’ disease

Source: Adapted from Henry and Heinke (1996) and Metcalf and Eddy (2003).


3.3 ARCHAEA

At the molecular level, both archaea and bacteria are structurally pro-karyotic.But theyare evolutionarilydistinct fromoneanother.Archaeawaspreviouslyknownasthearchaebacteria(Madiganetal.,2010).Theircell wall, cell material, and RNA composition are different from bacte-ria. Some archaea are important in anaerobic processes, e.g. methano-gensthatproducemethanegasfromdegradationoforganicmatterunderanaerobicconditions.Theseincludemethanobacterium, methanosarcina, and methanothrix,whichareimportantinanaerobicdigestionofsludge.Somearchaeaexhibithighlyspecializedmetabolicpathwaysandarefoundunderextremeenvironmentalconditions.Onedistinctgroupisthehyper-thermophiles.Theyareobligateanaerobesandhaveatemperatureoptimaabove 80°C. Examples are thermoproteus, sulfolobus, and methanopy-rus.Extremelyhalophilicarchaeaareanotherdiversegroupthatinhabitshighlysalineenvironments,suchassolarsaltevaporationponds.Examplesarehalobacteriumandhalococcus.

3.4 PROTOZOA

Protozoaaremostlyunicellulareukaryotesthatlackcellwalls.Theycanbefreelivingorparasitic.Mostareaerobicheterotrophs,someareaero-tolerantanaerobes,andafewareobligateanaerobes.Theyreproducebybinaryfission.Theycanrangeinsizefromafewtoseveralhundredµm.Theyareanorderofmagnitudelargerthanbacteria.Protozoaactaspol-ishersofeffluent frombiological treatmentprocessesby feedingonbac-teria,algae,andparticulateorganicmatter.Someprotozoahavehairlikestrands calledflagella,whichprovidemotilitybyawhiplikeaction, e.g.Giardia.Someflagellatedspeciesfeedonsolubleorganics.Free-swimmingprotozoahavecilia,whichareusedforpropulsionandgatheringoforganicmatter,e.g.Paramecium.

Anumberofprotozoaareimportantinwaterandwastewater,astheycauseentericdiseasesinhumansandanimals:

1.Amoeba—Theymovebyextendingtheircytoplasminsearchoffood.These extensions are called pseudopods or false feet (Figure 3.7).Theyarepathogenicandcauseameobicdysenteryinhumans.

2.Giardia lamblia—These are parasitic protozoa. They range in sizefrom8to18µmlongand5to15µmwide(HammerandHammer,2012).Insideahostbody,theGiardiacystreleasesatrophozoitethatfeeds,growsandreproduces,causingagastrointestinaldiseasecalledgiardiasis,whichcausescramps,diarrhea,andfatigueandcanbecome


severe. In a drinking water treatment plant, coagulation–floccula-tionfollowedbyfiltrationanddisinfectionisrequiredtokill them.DrinkingwatertreatmentplantsintheUnitedStateshavetoachieve99.9%removalofGiardia.

3.Cryptosporidium—Thisformsathick-walledoocystintheenviron-mentandcansurviveforlongperiodsoftime.Theoocystissphericalwithadiameterof4to6µm.Cryptosporidiumoocystsarepresentinsmallnumbersinsurfacewaters.Whenhumansingestitwithdrink-ingwater,theoocystopensinthesmallintestine,releasessporozoitesthatattachthemselvestothewallsoftheintestine,anddisruptsintes-tinal functions causing cryptosporidiosis. Cryptosporidiosis causesseverediarrheaandcanbecome life threatening.Chlorinationcan-notkilltheoocysts.Basedonthesize,theycanberemovedbyusingenhancedcoagulation–flocculationprocesses,andozonedisinfectionindrinkingwatertreatmentprocesses.

In1993,therewasadevastatingoutbreakofcryptosporidiosisinMilwaukee,Wisconsin, in the United States. This resulted in about 400,000 peoplebecomingsickfromtheprotozoa,andanumberofdeaths.Theoutbreakbecametheimpetusforatremendousamountofresearchonthesurvivaloftheprotozoaanditsefficientmonitoringandremovaltechniquesfromdrinkingwatersystems.Theconceptofmultiplebarriersystemsintreat-mentplantsalsogainedmoreimportance.

Contractile vacuole

Nucleus

Food vacuole

Pseudopod

Figure 3.7 Amoeba.


3.5 ALGAE

Algaeareautotrophic,photosyntheticeukaryoticplants.Oneexceptionis the blue-green algae or cyanobacter, which is prokaryotic and pro-ducestoxinsthatareharmfultofishandbirds,e.g.Anabena.Algaecanbeunicellularormulticellular.Theirsizerangesfrom5μmto100μmormore,whentheyarevisibleasagreenslimeonwatersurfaces,e.g.Pediastrum.Theyhavenoroots,stems,orleaves.Multicellularcoloniescangrowinfilamentsorsimplemassesofsinglecellsthatclumptogether.Allalgal cellsarecapableofphotosynthesis.The simplified reaction isgivenbelow: sunlight CO2+PO4

3–+NH3+H2O ________▶ newcells+O2+H2O (3.3)

Algaeareautotrophic.Theyusesunlightastheirenergysource,andcar-bon dioxide or bicarbonates as their carbon source. The oxygen that isproducedduringphotosynthesisreplenishesthedissolvedoxygencontentofthewater.Theyareoftenusedinaerobicoxidationponds,sincetheycanproduce the oxygen necessary for aerobic bacteria. Algae are importantprimaryproducersintheaquaticfoodchain.

Excessivealgaegrowthscancausetasteandodorproblems,clogwaterintakes at treatment plants, and shorten filter runs. Algae grows veryquickly, producing algae blooms when high concentrations of nutrientssuchasnitrogenandphosphorusareavailable.This leadstoaconditioncalledeutrophicationof lakes, streams,andestuaries.Thealgaebloomsformagreen-coloredmatonthewatersurfaceblockingthepenetrationofsunlight.Thisadverselyaffectsotheraquaticplants.Atnightduringrespi-ration,algaeusesupoxygenfromthewaterandproducescarbondioxideaccordingtothefollowingsimplifiedreaction:

Algalcells+O2________▶ CO2+H2O (3.4)

Thiscausessignificantdepletionofdissolvedoxygeninthelakeandcanaffectthefishpopulation.Mostgamefishrequireatleast4mg/Ldissolvedoxygen (DO) for survival. Other aquatic species are adversely affectedbelow2mg/LDO.Theexcessdissolvedoxygenproducedduringphoto-synthesiscannotbestoredandisreleasedintotheatmosphere.Thuseutro-phic lakes are characterized by unsightly green polluted waters, loss ofspeciesdiversity,verylowdissolvedoxygen,andabsenceofgamefish.OneexampleisthedeteriorationandimpairedwatersoftheChesapeakeBayintheeasternUnitedStates.Agriculturalrunoffandothersourcescontributenutrientsthatleadtoeutrophication.Controlofthesesourcesisnecessarytolimitalgalgrowths.


3.6 FUNGI

Fungi are multicellular, nonphotosynthetic, heterotrophic eukaryotes.Mostareobligateorfacultativeaerobes.Theycanreproducesexuallyorasexuallybyfission,budding,orsporeformation.Fungicangrowunderlownitrogen,lowmoisture,andlowpHconditions.OptimumpHisabout5.6,buttherangeisbetween2to9.Theycanalsodegradecellulose,whichmakesthemusefulincompostingprocesses.Therearemainlythreegroupsoffungi:molds,yeasts,andmushrooms.Yeastsareusedinbaking,distill-ing,andbrewingoperations.FungiareillustratedinFigure3.9.

3.7 VIRUS

Avirusisanoncellulargeneticelementthatusesahostcellforitsreplica-tionandalsohasanextracellularstate.Intheextracellularstate,itiscalledavirion.Avirionismetabolicallyinertanddoesnotcarryoutrespiratoryorbiosynthesisfunctions.Virusesareobligateintracellularparasites.TheyarecomposedofanucleicacidcorethatcontainseitherDNAorRNA,sur-roundedbyaproteinshellcalledcapsid.Accordingtoshapeandstructure,

(a)

(b) (c)

Figure 3.8 Different types of algae: (a) anabaena, (b) euglena, (c) lepocinclis.


they can be polyhedral, helical, or combination T-even as illustrated inFigure 3.10. Viruses are usually very small, ranging in size from a fewnanometerstoabout100nm.Virusesareclassifiedbasedonthehostthattheyinfect,e.g.animalviruses,plantviruses,bacterialviruses,orbacte-riophages.Virusesofconcerninwastewateraretheonesexcretedinlargenumbersinhumanfeces.Theseincludepoliovirus,hepatitisAvirus,andenterovirusesthatcausediarrhea,amongothers.Drinkingwatertreatmentplantshavetoachieve99.99%removalofviruses.

(a) (b)

Figure 3.9 Types of fungi: (a) mold, (b) mushroom.

(a) (b)

Figure 3.10 Viruses of different shapes: (a) helical, (b) combination T-even.


Aviruscannotreproduceorreplicateonitsown.Itcanonlyreplicateinside a host body. The various phases of the replication process of abacteriophage are given below (Madigan et al., 2010) and illustrated inFigure 3.11:

1.Attachment—adsorptionoftheviriontoasusceptiblehostcell. 2.Penetration—injectionofthevirionoritsnucleicacidintothecell. 3.Replication—of the virus nucleic acid. The virus alters the cell’s

metabolismtosynthesizenewvirusnucleicacids. 4.Synthesis—ofproteinsubunitsoftheviruscoat.

Attachment

Viral DNA enters cell

Nucleic acid replication

Synthesis of protein coats

Assembly and packaging

Release (lysis)

Figure 3.11 Cell replication of a virus in a bacterial cell (Source: Adapted from Madigan and Martinko, 2006).


5.Assemblyandpackaging—ofproteinsubunitsandnucleicacidintonewvirusparticles.

6.Release—ofmaturenewvirusesfromthecellbylysisasthecellbreaksopen.

Foravirusinfectingbacteria,thewholereplicationprocesscanbecom-pletedin30to40minutes.

PROBLEMS

3.1 What are the differences between eukaryotes and prokaryotes?Explainwithdiagramsoftheircells.

3.2 Drawatypicalbacterialcellandlabelthedifferentparts.Mentionthefunctionsofthecytoplasmicmembrane,cytoplasm,andDNA.

3.3 Whatisthelogarithmicgrowthphase?Developamodeltocalculatethenumberofbacteria inthelogarithmicgrowthphaseassumingfirstorderkinetics.

3.4 Explaintheprocessofnitrificationwiththehelpofequations.Whattypesofbacteriaareinvolvedintheprocess?Namethem.

3.5 Writeshortnotesonalgae,protozoa,andvirus. 3.6 Conducta literature review tofindoutabout the lastwaterborne

diseaseoutbreakinyourcityorcountry.Whattypeofmicroorgan-ismwasresponsible,whatwere thereasonsfor theoutbreak,andwhatmeasuresweretakenforcontrolandfutureprevention?

3.7 What is cryptosporidium? Why is chlorine disinfection unable toremoveitfromdrinkingwatersupply?

3.8 Whatiseutrophication? 3.9 Whatare the steps in the replicationofabacteriophage?Explain

withthehelpofadiagram. 3.10 Arrangethefollowingmicroorganismsaccordingtosizeandpreda-

tionfromlargesttosmallest:bacteria,virus,protozoa,crustaceans.

REFERENCES

Hammer,M.J.,andHammerM.J.Jr.(2012)Water and Wastewater Technology.Seventhedition.Pearson-PrenticeHall,Inc.,NewJersey.

Henry, J. G., and Heinke, G.W. (1996) Environmental Science and Engineering.Secondedition.PrenticeHall,NewYork.

Madigan,M.,andMartinko,J.(2006)Brock Biology of Microorganisms.Eleventhedition.PrenticeHall,NewYork.

Madigan, M., Martinko, J., Stahl, D., and Clark, D. (2010) Brock Biology of Microorganisms.Thirteenthedition.PrenticeHall,NewYork.

Metcalf and Eddy, Inc. (2003) Wastewater Engineering: Treatment and Reuse.Fourthedition.McGraw-Hill,NewYork.

51

Chapter 4

Natural purification processes

4.1 IMPURITIES IN WATER

Awidevarietyofpollutantsarepresentinnaturalwaters.Theseincludesand,silt,clay,organicmatterfromdecayingvegetation,andproductsofchemical conversions, among others. Natural purification processes arecontinuallyactiveinstreamsandriverstoreducethelevelsofthepollutantstoacceptableornegligibleconcentrations.Theseprocessesincludedilution,sedimentation,filtration,heattransfer,andchemicalandbiologicalconver-sions.Thesenaturalpurificationmechanismsareslowandcanrestorethehealthofwaterbodiesoveraperiodoftime,dependingontheconcentra-tionofthepollutants.

Ashumanand industrialactivityhas increased, sohas theamountofpollutants discharged into the water bodies. Various types of industrialchemicals,fertilizers,andpesticidesendupinwater.Inmostcases,naturalpurificationprocessesbecomeinsufficienttoreducethelevelsofpollutants.Asaresult,thehealthofthewaterbodybecomesimpaired.Environmentalregulationsareintroducedandenforcedinanefforttoreducethepollutionofnaturalstreamsandrivers.

Engineeredsystemsareusedinwastewatertreatmentplantstoreducethepollutantconcentrationstoacceptablelevelspriortodischargetostreamsandrivers.Thesesystemsaredesignedbasedontheprinciplesofnaturalpurificationprocesses.Thedifferenceliesintheratesofreactionandcon-version. In treatmentplants,unitprocesses aredesigned toachieve con-versionswithin a shortperiodof time.For this reason, it is essential tounderstand the basic principles and kinetics of natural purification pro-cesses,aswellasquantificationofthepollutionstrengthofwastewater.

4.2 DILUTION

Dilutionisaprocesswherebytheconcentrationofpollutantsisreducedduetomixingofasmallvolumeofpollutedwaterwithalargebodyofwater,


e.g.astreamorriver.Thisusuallyhappenswhenwastewaterisdischargedintoastream.Ifthestreamhasalowornegligibleamountofpollutants,anditsvolumeflowrateismuchgreaterthanthewastewater,dilutionwilltakeplaceandisreflectedindownstreamwatercharacteristics.Lowpol-lutantconcentrations,adequatemixing,temperature,andhydraulicchar-acteristicswilldictatethesuccessofdilution.

Theprinciplesofcontinuityandmassbalancecanbeusedtocalculatethedilutioncapacityofastream.Considerthefollowingexample,asillus-tratedinFigure4.1.

WastewaterisdischargedintoastreamataflowrateQwwithaconcentra-tionCwofapollutant.Priortodischarge,thestreamflowratewasQuswithaconcentrationCusofthepollutant.Assumingcompletemixingatthepointofdischargeandnoaccumulationorchemicalconversion,wecancalculatethedownstreamflowrateQdsandconcentrationCdsofthemixafterdischarge.

Fromtheprincipleofcontinuity,

Qw+Qus=Qds (4.1)

Fromtheprincipleofmassbalance,

(Massflowrateofpollutants)in=(Massflowrateofpollutants)out (4.2)

Qw·Cw+Qus·Cus=Qds·Cds (4.3)

EXAMPLE4.1Atanningindustrydischargeswastewaterwithammoniaintoastreamas illustrated in Figure 4.1. Prior to discharge, the flow rate of thestreamis30m3/swithanammoniaconcentrationof0.2mg/L.Theflowrateoftheindustrialdischargeis1.3m3/swithanammoniacon-centrationof50mg/L.Calculatetheresultantflowrateandammoniaconcentrationdownstreamfromthepointofdischarge.

us, us ds, ds

w, w

Figure 4.1 Stream flow with wastewater discharge.

Natural purification processes 53

SOLUTION

Calculateresultantflowrateusingequationofcontinuity(equation4.1).

Qds=30m3/s+1.3m3/s

Qds=31.3m3/s

Write a mass balance between upstream and downstream points(equation4.2).

(Massflowrateofammonia)in=(Massflowrateofammonia)out

(30m3/s×0.2mg/L)+(1.3m3/s×50mg/L)=31.3m3/s×Cds

Cds=2.27mg/L

4.3 SEDIMENTATION

Sedimentation isaprocess that involves theremovalof suspendedsolidsfromawaterbodybysettlingthemout.Thesizeofthesolidparticlesplaysamajorroleintheefficiencyofsedimentation.Largerparticlessettleoutquickly,whereassmallerparticlesmayremainsuspendedforlongerperiodsand eventually settleout. Streamcharacteristics, suchasflow rates, beddepth,androughness,alsoaffecttheratesofsedimentation.

Excessiveturbulenceorfloodingcancauseresuspensionofdepositedsol-ids.Thiscantransfersolidsdepositsfromonelocationtoanother.

4.4 MICROBIAL DEGRADATION

Wastewaterdischargedfrommunicipalsourcescontainsalargeamountoforganicmatter.Whenuntreatedwastewaterisdischargedintostreamsandrivers,theorganicmatterisusedasfoodbybacteria,protozoa,andothermicroorganismsinthewaterbodies.Aerobicmicroorganismsuseoxygenduringaerobicoxidationoforganicmatter.Thiscreatesasubstantialoxy-gendemandinthewaterbodyandcanlowerthedissolvedoxygenconcen-trationssignificantly.Theoxygeninwaterbodiesisreplenishedbytransferfromtheatmosphere.

4.5 MEASUREMENT OF ORGANIC MATTER

Anumberofdifferentmethodscanbeusedtomeasuretheorganiccon-tentofwastewater.Thecommonlyusedtechniquesinclude(1)biochemical


oxygendemand(BOD),(2)chemicaloxygendemand(COD),and(3)totalorganiccarbon(TOC).

BOD is themostwidelyusedparameter formeasuringtheamountofbiodegradableorganicmatterpresentinawastewater.StandardBODtestresultsareobtainedafterfivedays.Thisisdiscussedinmoredetailinthefollowingsection.

CODisdefinedastheoxygenequivalentoforganicmatterthatcanbeoxidizedbyastrongchemicaloxidizerinanacidicmedium.CODmeasuresbothbiodegradableandnonbiodegradableorganicmatter.Theresultscanbeobtainedinafewhours.

TheTOC testmeasuresthetotalorganiccarbonthatcanbeoxidizedtocarbondioxideinthepresenceofacatalyst.Thetestcanbeperformedrapidlyandresultsobtainedinashortperiodoftime.

4.5.1 Biochemical oxygen demand (BOD)

TheBODisusedasameasureofthepollutionpotentialofwastewater.Itgivesusanideaoftheamountofbiodegradableorganicmatterthatispres-entinawastewater.BODisdefinedastheamountofoxygenutilizedbyamixedpopulationofmicroorganismsduringaerobicoxidationoforganicmatteratacontrolledtemperatureof20°Cforaspecifiedtime.

Theoreticallyitwouldtakeaninfinitelylongtimeforthemicroorgan-isms to degrade all the organic matter present in the sample. The BODvalueistimedependent.Withina20dperiod,theoxidationofthecarbo-naceousorganicmatterisabout95%complete.Inthewastewaterindustry,theBOD5inmg/LofO2isusedasastandardvaluethatisobtainedfromaBODtestconductedforfivedays.About60%to70%oftheorganicmatterisoxidizedafterfivedays.AmeasureofthetotalamountoforganicmatterpresentinthesampleisobtainedfromtheultimateBOD,orBODult.

If thewastewater containsproteins andothernitrogenousmatter, thenitrifyingbacteriawillalsoexertameasurabledemandaftersixtosevendays.Thedelayinexhibitionofthenitrogenousoxygendemand(NOD)isduetotheslowgrowthrateofthenitrifyingbacteria,ascomparedwiththegrowthrateoftheheterotrophicbacteriaresponsibleforexertionofthecarbonaceousoxygendemandtypicallyknownascarbonaceousBODorsimplyBOD.Figure 4.2illustratestypicalBODandNODcurves.

4.5.1.1 BOD kinetics

The rate at which organic matter is utilized by microorganisms can beassumedtobeafirstorderreaction(Peavyetal.,1985).Inotherwords,therateatwhichorganicmatterisutilizedisproportionaltotheamountoforganicmatterremaining.Thiscanbeexpressedasfollows:


dL

dtkLt

t= − (4.4)

where:Lt=oxygenequivalentoforganicmatterremainingattimet,mg/Lk=reactionrateconstant,d–1

Equation4.4canberearrangedandintegratedas:

L

L

t

t

t

o

t

dL

Lk dt∫ ∫= −0

(4.5)

lnL

Lktt

0

= −

Lt=Loe–kt (4.6)

whereLo=oxygenequivalentoftotalorganicmatterattime0.Figure 4.3illustratestherelationshipoforganicmatterremainingtothe

exertionofBOD.Theamountoforganicmatterdecaysexponentiallywithtime.SinceLoistheoxygenequivalentofthetotalamountoforganicmat-ter,theamountofoxygenusedinthedegradationoforganicmatter,ortheBOD,canbedeterminedfromtheLtvalue.Therefore,

BODt=Lo–Lt (4.7)

Time, Days

Oxy

gen

Dem

and,

mg/

LNitrogenous oxygendemand or NOD

Carbonaceous oxygendemand or BOD

Figure 4.2 Typical BOD and NOD curves.


SubstitutingthevalueofLtfromequation(4.6)inequation(4.7)

BODt=Lo–Loe–kt (4.8)

The BODult of the wastewater approaches Lo in an asymptotic manner,indicating that the ultimateBOD is equal to the initial total amount oforganic matter present in the sample, as shown in Figure 4.3. Equation(4.8)canbewrittenas,

BODt=BODult(1–e–kt) (4.9)

AnotherformoftheBODequationcanbewrittenasfollows,whenequa-tion(4.5)issimplifiedusinglogbase10:

BODt=BODult(1–10–k ′t) (4.10)

wherek′=BODrateconstant(base10)correspondingtoequation(4.10).Inequations(4.9)and(4.10),BODultisaconstantforaparticularwaste-

water, regardless of timeor temperature, since it corresponds to the totalamountoforganicmatterinitiallypresentinthesample.TypicalvaluesofBODultformunicipalwastewatercanrangefrom100mg/Lto300mg/Lormore.ThevalueoftheBODrateconstantk(ork′)representstherateofthereactionandistemperaturedependent.Sincemicroorganismsaremoreactiveat higher temperatures, the k value increaseswith temperature.The van’tHoff-Arrheniusmodelcanbeusedtodeterminek,whenkat20°Cisknown.

kT=k20θ(T–20) (4.11)

whereθ=Arrheniuscoefficientvalue,of1.047oftenusedforBOD.

Time, Days

Oxy

gen

Dem

and,

mg/

LL0

BOD exerted

Organic matterremaining, Lt

BODt

L t(L

0 – L

t)

Figure 4.3 Organic matter remaining and BOD exertion curves.


Thevalueofkcanvaryfrom0.1to0.4ormore,dependingonthebio-degradabilityoftheorganicmatter.Sugarsandsimplecarbohydratesthatareeasilydegradedbymicroorganismshaveahigherkvalue,ascomparedwithcomplexcompoundsandfatsthataredifficulttodegradeandhavealowerkvalue.Figure 4.4illustratesBODcurvesforwastewaterswiththesameultimateBODbutwithdifferentrateconstants.

EXAMPLE4.2TheBOD5ofamunicipalwastewateris200mg/Lat20°C.Theamountoforganicmatterremaining in thesampleafter5d isequivalent to151.93mg/LofO2.CalculatetheBOD8ofthesampleat30°C.Useθ=1.047astheArrheniuscoefficientforBODrateconstant.

SOLUTION

Given,BOD5at20°C=200mg/L,andL5=151.93mg/L,calculateLousingequation(4.7).

BOD5=Lo–L5

200=Lo–151.93

Therefore,BODult=Lo=200+151.93=351.93mg/L.Calculatek20usingequation(4.9)witht=5dandk=k20.

BOD5=BODult(1–e–k.5)

200=351.93(1–e–k.5)

Therefore,k=k20=0.168d–1

Calculatek30usingequation(4.11)withT=30andθ=1.047.

K30=0.168×(1.047)30–20=0.266d–1

Time, Days

BOD

, mg/

L of

O2

L0

k 3 k 2

k3 > k2 > k1 k 1

Figure 4.4 Variation of BOD curves with different rate constants.


CalculateBOD8at30°Cusingequation(4.9)witht=8dandk=k30.

BOD8=BODult(1–e–k.8) =351.93(1–e–0.266×8) =310.02mg/L

4.5.1.2 Laboratory measurement

TherearetwotypesofteststhatareusedtodeterminetheBODofawaste-watersampleinthelaboratory:(1)unseededBODtestand(2)seededBODtest.Theunseededtestisusedforwastewaterthathasasufficientpopula-tionofmicroorganismsinittoexertameasurableoxygendemandforfivedaysormore.Theseededtest isusedforwastewater thatdoesnothaveenoughmicroorganismsinittoexertameasurabledemandduringthetest.Additionalseedmicroorganismsareaddedtothesample.

Two criteria have to be satisfied for a valid BOD test (AWWA et al.,2005):(1)atleast2mg/Lofdissolvedoxygenshouldbeconsumedbythemicroorganisms after 5 d, and (2) final dissolved oxygen of the sampleshouldnotbelessthan1mg/L.

4.5.1.3 Unseeded BOD test

TheBODtestiscarriedoutin300mlBODbottlesaccordingtoStandardMethods(AWWAetal.,2005).Ameasuredvolumeofwastewaterisaddedtothebottle,togetherwithdilutionwater.Thedilutionwaterispreparedbyaddingphosphatebuffer,magnesiumsulfate,calciumchloride,andfer-ricchloride.Thewateristhensaturatedwithoxygen.Thewastewatersup-pliestheorganicmatterandmicroorganisms,anddilutionwaterprovidestheoxygenandnutrients.Aninhibitorsuchas2-chloro-6(trichloromethyl)pyridinecanbeaddedtotheBODbottletopreventnitrificationreactions.Thebottleisincubatedat20°Cforaspecifictime.Depletionofdissolvedoxygeninthetestbottleismeasureddailytodeterminetheoxygenusedbythemicroorganismsindegradingtheorganicmatter.TheBODaftertimetdaysiscalculatedfromthefollowingequation:

BO DD D

Pt= −1 2 (4.12)

where:D1=InitialdissolvedoxygenconcentrationintheBODbottle,mg/LD2=Finaldissolvedoxygenconcentration in theBODbottleafter t

days,mg/L

P=volum eofwastewatersam ple

volum eofBO D botttle= x m l

m l300


Thevolumeofwastewater(xml)thatisaddedtotheBODbottledependsontheBODofthewastewater,whichisanunknownquantity.ArangeofBOD isassumed,basedonwhich testsare conductedwithanumberofdifferentxvalues.Usingthetwocriteriamentionedabove,andassumingtheinitialdissolvedoxygenconcentrationtobeclosetothesaturationcon-centrationof9.17mg/Lat20°C,equation(4.12)canbeusedtocalculatearangeofappropriatexvalues.Table 4.1presentswastewatersamplevol-umestobeusedfordifferentBODvalues.

EXAMPLE4.3In a laboratory BOD test, 8 ml of wastewater with no dissolvedoxygenismixedwith292mlofdilutionwatercontaining8.9mg/Ldissolved oxygen in a BOD bottle. After 5 d incubation, the dis-solvedoxygencontentofthemixtureis3.4mg/L.WhatistheBOD5ofthewastewater?

SOLUTION

CalculateinitialdissolvedoxygenD1ofthemixturefrommassbalance(equation4.3).

Dw·Vw+Dd·Vd=D1·V1

(0mg/L×8ml)+(8.9mg/L×292ml)=(D1×300ml)

D1=8.66mg/L

Table 4.1 Wastewater sample volumes for BOD tests

Wastewater sample(x ml)

Range of BOD(mg/L)

0.2 3000 – 12,0000.5 1200 – 48001.0 600 – 24002.0 300 – 12005.0 120 – 480

10.0 60 – 24020.0 30 – 12050.0 12 – 48

100.0 6 – 24300.0 2 – 8

Note: Values were calculated using equation (4.12) and D1 = 9.17 mg/L at 20°C.


CalculateBOD5usingequation(4.12).

BOD5=8 66 3 4

8

300

. . /−( )m g L=197.25mg/L

4.5.1.4 Seeded BOD test

Whenwastewaterdoesnothaveenoughmicrobialpopulationinittoexerta measurable oxygen depletion during the BOD test, seed microorgan-ismsareadded to themixture.Activatedsludge fromanaerationbasin,orwastewaterfromastabilizationpond,canbeusedtoprovidetheseed.Ageneralruleofthumbistoaddavolumeofseedwastewatersuchthat5%to10%ofthetotalBODofthemixtureresultsfromtheseedalone(HammerandHammer,2008).TheBODoftheseed iscalculatedsepa-ratelyandsubtractedfromthatofthemixture.ThefollowingequationisusedtocalculatetheBODofaseededwastewater:

BODt=D D B B f

P1 2 1 2−( ) − −( )

(4.13)

where:D1=Initialdissolvedoxygenconcentrationofdilutedseededwastewa-

termixture,mg/LD2=Finaldissolvedoxygenconcentrationofdilutedseededwastewa-

termixture,mg/LB1=Initialdissolvedoxygenconcentrationofseedmixturefromseed

BODtest,mg/LB2=Finaldissolvedoxygenconcentrationofseedmixturefromseed

BODtest,mg/Lf=Ratioofseedvolumeinseededwastewatermixturetoseedvol-

umeusedinseedBODtest=m lofseed in D

m lofseed in B1

1

= 0 05 0. .to 110

P=m lofwastewatersam plein D

m l1

300

EXAMPLE4.4DeterminetheBOD5ofafoodprocessingwastewater.ThedatafromtheseededBODtestareasfollows:

Volumeofwastewatersampleinseededmixture=15mlVolumeofseedinseededmixture=1mlInitialdissolvedoxygenofseededmixture=8.9mg/LFinaldissolvedoxygenofseededmixture=4.5mg/L


ForBODtestforseedconductedseparately:

Volumeofseed=10mlInitialdissolvedoxygen=8.7mg/LFinaldissolvedoxygen=5.1mg/L

SOLUTION

f=(1ml)/(10ml)=0.1

P= 15300

BOD5=8 9 4 5 8 7 5 1 0 1

15

300

. . . . .−( ) − −( ) ×=80.8mg/L

4.5.1.5 Determination of k and Lo

Thevaluesof theconstantskandLoorBODultcanbedeterminedfromaseriesofBODmeasurements.Anumberoftechniquescanbeused,e.g.(1)Thomas’s graphicalmethod (Droste, 1997;Thomas,1950); (2)Leastsquares method (Metcalf and Eddy, 2003; Moore et al., 1950); and (3)Fujimotomethod(Fujimoto,1964),amongothers.

4.5.1.6 Thomas’s graphical method

ABODtestisconductedfor7to10d,anddailymeasurementsaretaken.Foreachday,BODand(time/BOD)1/3arecalculated.Aplotof(time/BOD)1/3versustimeismade,andthebestfitlineisdrawn.Thebestfitlinehasaslope(S)andintercept(I).Theslopeandinterceptvaluesareusedtocalculatek(base10)andLofromthefollowingrelationships.Thederivationsforequa-tions(4.14)and(4.15)areprovidedelsewhere(Droste,1997;Thomas,1950).

kS

I= 2 61. (4.14)

Lo=1

2 3 3. kI (4.15)

Thisisanapproximatemethod.ItisnotvalidforBOD>0.9Lo,orafter90%oftheBODhasbeenexerted.


4.5.2 Theoretical oxygen demand

Thetheoreticaloxygendemand(ThOD)foracompoundorsubstancecanbedetermined fromthechemicaloxidationreactionsof thatcompound.Ifthesubstanceisacomplexofcarbohydratesandproteins,thenthetotalThODisthesumofthecarbonaceous ThODandthenitrogenous ThOD.Thecarbonaceous ThOD isequivalenttotheBODultofthesubstance.Thecalculationsareillustratedinthefollowingexample.

EXAMPLE4.5Awastewatercontains250mg/Lofglucose(C6H12O6)and60mg/LofNH3-N.CalculatethetotalThODforthewastewater.Atomicweights:C12,H1,O16.

Underanaerobicconditions,glucoseisconvertedtocarbondioxideandmethane.

C6H12O6→CO2+CH4

Methaneundergoesfurtheroxidationtocarbondioxideandwater.

CH4+O2→CO2+H2O

SOLUTION

Addingthetwochemicalreactionsandbalancingtheresultantreaction,

C6H12O6+6O2→6CO2+6H2O

Accordingtothisreaction,6molesofO2arerequiredtocompletelyoxidizeeachmoleofglucose.

MolecularwtofC6H12O6=(12×6)+(1×12)+(16×6)=180g/molMolecularwtofO2=16×2=32g/mol

Carbonaceous ThOD

= × ×2506 32

12 62

6 12 6

m g L H Om olO

m olC H O

g O/ C6

22

2

6 12 6

1000

1

180

m olO

m g

g

m ol C H O

g m ol

g

/

×

× ×11000 m g

=266.67mg/L=BODult

Nitrificationreaction(balanced): NH3–N+2O2→NO3––N+H++H2O


Accordingtothisreaction,2molesofO2arerequiredtocompletelyoxidizeeachmoleofNH3–N.Note,concentrationofNH3isgivenintermsofN.

Nitrogenous ThOD

= − ×−

×602 322

3

2m g L Nm olO

m olN H N

g O

m ol/ N H 3

OO

m g

g

m olN H N

g N m ol

g

2

3

1000

1

14 1000/

×

× − ×mm g

=274.29mg/L

Total ThOD=CarbonaceousThOD+NitrogenousThOD

=266.67mg/L+274.29mg/L=540.96mg/L

4.6 DISSOLVED OXYGEN BALANCE

Oneofthemostimportantparametersformaintainingahealthyecologyofnaturalstreamsandriversisthedissolvedoxygen(DO)concentration.Mostaquaticplantsandanimals requireaminimumconcentrationof2mg/LDOtosurvive.Gamefishandotherhigherlife-formsrequire4mg/Lormoreforsurvival(Peavyetal.,1985).

When wastewater with a high BOD is discharged into a stream, dis-solvedoxygenfromthewaterisusedupbythemicroorganismsindegrada-tionofBODororganicmatter.ThisresultsinadropinDOconcentrationof the stream. The amount of oxygen that can be dissolved in water atagiventemperatureisdefinedasitsequilibriumorsaturationconcentra-tion, or solubility. This can be calculated using Henry’s law (MihelcicandZimmerman,2010).Equilibrium concentrationsof oxygen inwateratvarioustemperaturesandsalinityvaluesareprovidedinTableA.2(intheAppendix).ThedifferencebetweenthesaturationDO(DOsat)andthemeasuredactualstreamDOconcentration(DOstream)iscalledthedissolved oxygen deficit(D).

D=DOsat–DOstream (4.16)

AtequilibriumDOsatisconstant,sotherateofchangeofdeficitdD

dt

is

proportionaltotherateofchangeofDOofthestream.TherateatwhichdissolvedoxygendecreasesisalsoproportionaltotherateatwhichBODisexerted.Thus,wecanobtainthefollowingrelationship(Peavyetal.,1985):


rD=k1Lt (4.17)

where:rD=rateofchangeofdeficitduetooxygenutilizationk1=BODrateconstantLt=organicmatterremainingaftertimet

The natural process of replenishment of the dissolved oxygen is calledreaeration,whichistherateatwhichoxygenisresuppliedfromtheatmo-sphere. The dissolved oxygen deficit is the driving force for reaeration.Therateofreaerationincreasesastheconcentrationofdissolvedoxygendecreases.Therateofreaeration(rR)isafirstorderreactionwithrespecttotheoxygendeficit(D).Thiscanbewrittenasfollows:

rR=–k2D (4.18)

where:k2=reaerationrateconstant.

Ifalgae ispresent in thewater, itcanreplenishthedissolvedoxygen inthe water in presence of sunlight, as it produces oxygen during photo-synthesis. Excessive algal growths sometimes outweigh these benefits,sinceitcanleadtoeutrophicationasexplainedinChapter3.Excessdis-solvedoxygencannotbe stored in thewater for futureuseandusuallyescapes to the atmospheredue to turbulence andwind action.Also, inabsenceofsunlightespeciallyatnight,algaeusedissolvedoxygenfromthestreamduringrespiration.Thiscanleadtosignificantoxygendeficitsinthestream.

4.6.1 Dissolved oxygen sag curve

When a wastewater with a significant amount of organic matter is dis-charged into a streamor river, thedissolvedoxygen level decreases anddropstoaminimumvalue.Asreaerationslowlyreplenishesthedissolvedoxygen,overtimeandwithdistance,thestreamDOlevelcomesbacktopredischargeconcentration.ThisisillustratedinFigure 4.5andisknownas thedissolvedoxygen sag curve. Streeter andPhelpsdevelopedoneoftheearliestmodelsofthedissolvedoxygensagcurvein1925.Theirbasicmodelwillbediscussedhere,whichpredictedchangesinthedeficitasafunctionofBODexertionandstreamreaeration.

According to the model, the rate of change in deficit is a function ofoxygendepletionduetoBODexertionandstreamreaeration.Thiscanbeexpressedmathematicallyasfollows:


dD

dtr rD R= + (4.19)

=k1Lt–k2D (4.20)

Equation(4.20)canbewrittenasafirstorderdifferentialequation,inte-grated and solved using boundary conditions to obtain the followingexpressionfordeficitatanytimet (Peavyetal.,1985):

Dk L

k ke e D et

o k t k to

k t=−

−( ) +− − −1

2 1

1 2 2 (4.21)

where:Do=initialdeficit,mg/L=DOsat–DOinitial

Lo=BODult,mg/Lt=timeoftravelinthestreamfromthepointofdischarge,d

Ifxisthedistancetraveledalongthestreamandvisthestreamvelocity,then:

tx

v= (4.22)

Wastedischarge Time, Days

Diss

olve

d O

xyge

n, m

g/L

DOc

DOt

t tc

Dc

Dt

DOsat

D0

Oxygen depletion curve due toBOD = rD

Stream reaerationcurve = rR

Dissolved oxygensag curve

Figure 4.5 Dissolved oxygen sag curve.


4.6.1.1 Critical points

Thelowestpointontheoxygensagcurve,wherethedeficit isthegreat-est, iscalledthecritical deficit Dc(Figure 4.5).Thispointrepresentsthemaximumimpactofthewastedischargeonthedissolvedoxygencontentofthestream.IftheBODofthewasteistoohigh,itmayresultinadeficitthatcausesanaerobicconditionsinthestream,i.e.DOlevelgoestozero.Thetimetakentoreachthecriticaldeficitiscalledcritical time tc,andthecorrespondingdistancecritical distance xc.Itisimperativetodeterminethedeficitatthecriticallocation.Ifstandardsaremetatthislocation,theywillbemetatotherlocationstoo.Equation(4.21)isdifferentiatedwithrespecttotime,settozerosinceDcismaximumattc,andsimplifiedtoobtain

tk k

k

kD

k k

k Lc o

o

=−

− −

1

12 1

2

1

2 1

1

ln

(4.23)

Anexpressionforcriticaldeficitcanbewrittenintermsofcriticaltimeasfollows:

Dk

kL ec o

k tc= −1

2

1 (4.24)

CriticalDOconcentration,DOc=DOsat–Dc (4.25)

DOConcentrationatanytimet,DOt=DOsat–Dt (4.26)

Equations (4.21) to (4.26)canbeused todetermine thedeficitsanddis-solvedoxygen concentrations along a stream followingwaste discharge,andtheoxygensagcurvecanbeproduced.Thisisillustratedinthefollow-ingexample.

EXAMPLE4.6Anindustrialprocessdischargesitseffluentintoastream.Itisdesiredtodeterminetheeffectsofthewastedischargeonthedissolvedoxygenconcentrationofthestream.k1at20°Cis0.23d–1,andk2at20°Cis0.43d–1.Thecharacteristicsofthestreamandindustrialwastewateraregivenbelow.

Characteristics Stream Industrial wastewater

Flow rate, m3/s 6.5 0.5Temperature, °C 19.2 25.0Dissolved oxygen, mg/L 8.2 0.5BOD5 at 20°C 3.0 200.0


a.Calculatethecriticalvaluesandthedistancefromthepointofdis-chargeatwhichthecriticalvalueswilloccur.Thestreamvelocityis0.2m/s.

b.Calculatethevaluesanddrawthedissolvedoxygensagcurvefora100kmreachofthestreamfromthepointofdischarge.

SOLUTION

Step1.Determinethecharacteristicsofthestream–wastewatermix-tureusingmassbalance.

MixFlowrate,Qm=6.5+0.5=7.0m3/s

Mixtemp, T Cm

. . . .

. ..= × + ×

+= °6 5 19 2 0 5 25 0

6 5 0 519 61

MixDO, DO m g Lm = × + ×+

=6 5 8 2 0 5 0 5

6 5 0 57 65

. . . .

. .. /

MixBOD5, BO D m g Lm5

6 5 3 0 0 5 200 0

6 5 0 517 07= × + ×

+=. . . .

. .. /

DOsatatTm=9.24mg/L(usingTableA.2andinterpolatingbetweentheDOsatvaluesfor19°Cand20°C)

Do=DOsat–DOm=9.24–7.65=1.59mg/L

Calculate Lo or BODult using equation (4.9). Note: BOD5 is alwaysmeasuredat20°C,unlessotherwisementioned.So,usek=k20inequa-tion(4.9).

17.07=Lo(1–e–0.23×5)

or Lo=24.98mg/L

Step2.Determinereactionrateconstantsformixtemperatureusingequation(4.11).

k1at19.61oC=0.23(1.047)(19.61–20)=0.226d–1

k2at19.61oC=0.43(1.016)(19.61–20)=0.427d–1

Step3.Determinecriticalvaluesusingequations(4.23)and(4.24).


tk k

k

kD

k k

k Lc o

o

=−

− −

1

12 1

2

1

2 1

1

ln

=−

− −1

0 427 0 226

0 427

0 2261 1 59

0 427 0 226

0. .ln

.

..

. .

.. .226 24 98×

=2.87d

Dk

kL ec o

k tc= −1

2

1

= −0 226

0 42724 98 0226 2 87.

.. ( . )( . )e

=6.91mg/L

DOc=DOsat–Dc=9.24–6.91=2.33mg/L

Criticaldistance,xc=tcv

=(2.87d)×(0.2m/s×86,400s/d×km/1000m)

=(2.87d)×(17.28km/d)

=49.59kmdownstreamfrompointofwastedischarge

Step4.Determinethedeficitatvariouspointsalongthestream,e.g.20,40,60,80,and100kmfromthepointofwastedischarge.First,calculatethetimescorrespondingtothesedistances.

Forx=20km, tx km

v km d

km

km dd20

20

17 281 16= = =

/ . /.

Forx=40km, tkm

km dd40

40

17 282 32= =

. /.

Forx=60km, tkm

km dd60

60

17 283 48= =

. /.

Forx=80km, tkm

km dd80

80

17 284 64= =

. /.

Forx=100km, tkm

km dd100

100

17 285 80= =

. /.


Next,calculatethedeficitsatthesetimesusingequation(4.21).

D

k L

k ke e D et

o k t k to

k t=−

−( ) +− − −1

2 1

1 2 2

D e e20

0 226 1 160 226 24 98

0 427 0 226= ×

−−− ×. .

. .( . . ) −− × − ×( ) +( . . ) ( . . ).0427 1 16 0 427 1 161 59 e

=5.46mg/L

D40=6.79mg/L

D60=6.80mg/L

D80=6.19mg/L

D100=5.35mg/L

CalculatetheDOlevelsinthestreamfromthedeficits.

DOx=DOsat–Dx

DO20=9.24–5.46=3.78mg/L

DO40=9.24–6.79=2.45mg/L

DO60=9.24–6.80=2.44mg/L

DO80=9.24–6.19=3.05mg/L

DO100=9.24–5.35=3.89mg/L

Usingthesevalues,thedissolvedoxygensagcurveisdrawnbelow.

0123456789

10

0 20 40 60 80 100 120

Diss

olve

d O

xyge

n, m

g/L

Distance from Point of Discharge, km

DOsat

Dc


4.6.1.2 Limitations of the oxygen sag curve model

1.ThemodelassumesonlyonesourceofBODdischargeintothestream.Iftherearemultiplewastedischargesalongthestream,theyhavetobetakenintoaccount.Thestreamcanbedividedintosegmentscon-sistingofasinglesourceofwastedischarge,andthemodelappliedsequentiallyfromthefirsttothelastsegment.

2.Oxygendemandduetonitrificationoralgalrespirationisnottakenintoaccount.

3.Contributionofalgaetoreaerationisnotconsidered. 4.Steadystateconditionsareassumedalongthestreamchannel,result-

ing in the use of a single value of k2. Stream bed characteristics,slopes,impoundments,etc.arenotconsidered.

ComputermodelshavebeendevelopedinrecentyearsbasedontheStreeter–Phelpsmodelthathaveincludedthenitrificationprocess,thediurnaleffectofalgalphotosynthesis and respiration, aswell as streamcharacteristicsaffectingreaerationrates.

PROBLEMS

4.1 AmunicipalwastewaterisdischargedintoastreamasillustratedinFigure 4.1.Priortodischarge,theflowrateofthestreamis45m3/swithaBOD5of1.5mg/L.Downstreamfromthepointofdischarge,thestreamflowrateis47.2m3/swithaBOD5of50mg/L.Calculatethecharacteristicsofthemunicipaldischarge.

4.2 Astreamflowsthroughasmalltownwhereanindustrydischargesitseffluentintothestream.Upstreamcharacteristicspriortoindus-trialdischargeareasfollows:flowrate1000m3/d,BOD52.5mg/L,nitrates2.0mg/L,andtemperature19°C.Theindustrydischargesat 50 m3/d. According to regulatory requirements, the maximumallowablevaluesinthestreamfollowinganydischargeareBOD530mg/L,nitrates4.0mg/L,andtemperaturedifferentialof4°Cfromupstreamconditions.Calculatethemaximumallowablevaluesfortheindustrialdischarge.

4.3 Whatsizeofsample,expressedasapercent,isrequiredifthe5dBODis650mg/LandtotaloxygenconsumedintheBODbottleislimitedto2mg/L?

4.4 TheBODvalueofawastewaterwasmeasuredat2dand8dandfoundtobe125and225mg/L,respectively.Determinethe5dBODvalueusingthefirstorderratemodel.

4.5 ForaBODanalysis, 30mlofwastewithaDOof zeromg/L, ismixedwith270mlofdilutionwaterwith aDOof9mg/L.The


sampleisthenputinanincubator.ThefinalDOismeasuredafter7d.ThefinalDOismeasuredat3.0mg/L.However,itisdiscoveredthattheincubatorwassetat30°C.Assumeak1of0.2d–1(basee)at20°Candθ=1.05.Determinethe5d,20°CBODofthesample.

4.6 InaBODtest,theamountoforganicmatterremaininginthewaste-water was measured at certain time intervals, instead of measur-ing the dissolved oxygen content. The amount of organic matterremainingafter4and9dwasmeasuredas52.86mg/Land8.11mg/L,respectively.CalculatetheultimateBODandBODratecon-stantkforthewastewater.

4.7 An unseeded BOD test was conducted on a raw domestic waste-watersample.Thewastewaterportionaddedtoeach300mlBODbottlewas8.0ml.Thedissolvedoxygenvaluesandincubationperi-odsarelistedbelow.PlotaBODversustimecurveanddeterminethe4dBODvalue.

Bottle Initial DO Incubation Final DOnumber mg/L days mg/L

1 8.4 0 8.42 8.4 0 8.43 8.4 1 6.24 8.4 1 5.95 8.4 2 5.26 8.4 2 5.27 8.4 3 4.48 8.4 3 4.69 8.4 5 0.8

10 8.4 5 3.5

4.8 Computetheultimatecarbonaceousoxygendemandofawasterepre-sentedbytheformulaC9N2H6O2,andusethereactionbelow.

C9N2H6O2+O2→CO2+NH3

4.9 A BOD test was run on wastewater taken from the Blue PlainsAdvancedWastewaterTreatmentPlant.Thetestwascontinuedfor12dat20°C,andthedissolvedoxygencontentwasmeasuredevery2d.Aplotof(t/BOD)1/3versustyieldedastraightline.Theequationofthestraightlineisasfollows:

(t/BOD)1/3=0.015t+0.18


a. CalculatetheBODratecoefficientkandtheultimateBODforthiswastewater.

b. CalculatetheBOD5at25°C. 4.10 IftheBOD5ofawateris350mg/L,whatarethemaximumand

minimumsamplevolumesthatcanbeusedforBODmeasurementinanunseededtest?Mentionanyassumptionsthatyoumake.

4.11 ThefollowingaretheresultsofaBODtestconductedonawaste-waterat20°C.CalculatetheultimateBODandtherateconstantk.(Answer:Lo=472.87mg/L,kbase10=0.075d–1)

Time, d 0 1 2 3 4 5 6 7 8

BOD, mg/L 0 75 138 191 236 274 305 332 354

4.12 Anicecreamplantdischargesitseffluentwastewaterintoastream.Itisdesiredtodeterminetheeffectsofthewastedischargeonthedissolved oxygen concentration of the stream. k1 at 20°C is 0.30d–1, and k2 at 20°C is 0.45 d–1. Calculate the critical values andthedistancefromthepointofdischargeatwhichthecriticalvalueswilloccur.Thestreamvelocityis0.2m/s.Thecharacteristicsofthestreamandwastewateraregivenbelow.

Characteristics Stream Wastewater

Flow rate, m3/d 16,000 1,200

Temperature, °C 12 40Dissolved oxygen, mg/L 8.2 0BOD5 at 20°C 3.0 910.0

4.13 Usingthedatafromproblem4.12,drawthedissolvedoxygensagcurvefora150kmreachofthestreamdownstreamfromthepointofdischarge.

4.14 Considertheicecreamplantdescribedinproblem4.12.TheBOD5ofthewastewateristoohighfordischargeintothestream.ItneedstobetreatedtoreducetheBOD5toacceptable levelspriortodis-charge.CalculatethemaximumBOD5thatcanbedischargedfromtheicecreamplant,ifaminimumof4.0mg/Ldissolvedoxygenhastobemaintainedinthestreamatalltimes.


REFERENCES

AWWA,WEF,andAPHA(2005)Standard Methods for the Examination of Water and Wastewater.Twenty-firstedition.EditedbyEaton,A.,andFranson,M.A.H.,AmericanWaterWorksAssociation.

Droste,R.L.(1997)Theory and Practice of Water and Wastewater Treatment.JohnWiley&Sons,Hoboken,NewJersey.

Fujimoto,Y.(1964)“GraphicalUseofFirst-StageBODEquation.”Journal of Water Pollution Control Federation,vol.36,no.1,p.69–71.

Hammer,M.J.,andHammer,M.J.Jr.(2008)Water and Wastewater Technology.Sixthedition.Pearson-PrenticeHall,Inc.,UpperSaddleRiver,NewJersey.


Mihelcic, J. R., and Zimmerman, J. B. (2010) Environmental Engineering: Fundamentals, Sustainability, Design.JohnWiley&Sons,Inc.,Hoboken,NewJersey.

Moore,E.W.,Thomas,H.A.,andSnow,W.B.(1950)“SimplifiedMethodforAnalysisofBODData.”Sewage and Industrial Wastes,vol.22,no.10,p. 1343–1353.

Peavy,H.S.,Rowe,D.R.,andTchobanoglous,G.(1985)Environmental Engineering.McGraw-Hill,Inc.,NewYork.

Thomas, H. A. Jr. (1950)“Graphical Determination of BOD Curve Constants.”Water & Sewage Works,vol.97,p.123–124.

75

Chapter 5

Wastewater treatment fundamentals

5.1 INTRODUCTION

Thescienceandengineeringofwastewatertreatmenthasprogressedtre-mendously over the last four or five decades. As knowledge and under-standing of the relationship between waterborne pathogens and publichealthhasincreased,sohastheimpetusforinnovationofnewtechnologiesfor treatmentofwastewater. In the last century,populationgrowthandindustrialization have resulted in significant degradation of the environ-ment.Disposalofuntreatedwastesandwastewateronlandorinstreamsandriversisnolongeranoption.Newerregulationsareaimedatprotect-ingtheenvironmentaswellaspublichealth.

Wastewaterengineeringhascomealongwayfromthetimewhencityresidentshadtoplacenight soil(fecalwaste)inbucketsalongthestreets,andworkerscollectedthewasteanddeliveredittoruralareasfordisposalonagriculturallands.Withtheinventionoftheflushtoilet,nightsoilwastransformedintowastewater. Itwasnotfeasible totransport these largeliquidvolumesfor landdisposal.Socitiesbegantousenaturaldrainagesystemsandstormsewerstotransportthewastewatertostreamsandriv-ers,whereitwasdischargedwithoutanytreatment.Thecommonnotionwas, “the solution to pollution is dilution.” However, with increasingurbanization, the self-purification capacity of the receiving waters wasexceeded,causingdegradationofthewaterbodiesandtheenvironment.Inthelate1800sandearly1900s,varioustreatmentprocesseswereappliedtowastewater (Peavy et al., 1985).By the1920s, treatmentplantsweredesignedandconstructedforpropertreatmentofwastewaterpriortodis-posal.Withnewerandmorestringentregulations,existingprocessesaremodifiedandinnovativetechnologiesareintroducedtoachieveenhancedremovalofpollutants.

Theobjectivesofwastewatertreatmentaretoreduce(1)thelevelofsol-ids,(2)thelevelofbiodegradableorganicmatter,(3)thelevelofpathogens,and(4)theleveloftoxiccompoundsinthewastewater,tomeetregulatorylimitsthatareprotectiveofpublichealthandtheenvironment.


5.2 SOURCES OF WASTEWATER

Thefollowingarecommonsourcesortypesofwastewater:

• Domestic or municipal wastewater: this includes wastewater dis-chargedfromresidences, institutionssuchasschoolsandhospitals,andcommercialfacilitiessuchasrestaurants,shoppingmalls,etc.

• Industrial wastewater: wastewater discharged from industrial pro-cesses,e.g.pharmaceuticalindustry,poultryprocessing.

• Infiltration and inflow:thisincludeswaterthateventuallyentersthesewer fromfoundationdrains, leakingpipes, submergedmanholes,andgroundwaterinfiltration,amongothers.

• Stormwater:rainfallrunoffandsnowmelt.

Municipal wastewater is usually collected in sanitary sewers and trans-portedtothewastewatertreatmentplant.Stormwatermaybecollectedinseparate sewer lines called storm sewers. In somecities, especiallyoldercities,stormwateriscollectedinthesamesewerlineasthedomesticwaste-water.Thistypeofsystemiscalledacombined sewersystem.Eachsystemhasadvantagesanddisadvantages. Industrialwastewatermaybe treatedon-site,orpretreatedandthendischargedtosanitarysewers,afterappro-priateremovalofpollutants.

5.3 WASTEWATER CONSTITUENTS

The major constituents of municipal wastewater are suspended solids,organicmatter,andpathogens.Nutrientssuchasnitrogenandphosphoruscancauseproblemswhenpresentinhighconcentrations.Inrecentyears,thepresenceofEDCs(endocrinedisruptingcompounds)hasbeenrecognizedasanareaofconcern.Industrialwastewatercancontaintheabove-mentionedcontaminants, aswell as heavymetals, toxic compounds, and refractoryorganics.Stormwatermaycontainpetroleumcompounds, silt,andpesti-cideswhenitincludesurbanrunoffandagriculturalrunoff.Table 5.1pro-videstheenvironmentalimpactsofthemajorconstituentsofwastewater.

Suspended solids consist of inert matter such as rags, silt, and paper,aswellasfoodwasteandhumanwaste.Biodegradableorganicmatteriscomposedof40%to60%proteins,25%to50%carbohydrates,andabout10%lipids(Peavyetal.,1985).Proteinsaremainlyaminoacidsandcontainnitrogen.Carbohydratesaresugars,starches,andcellulose.Lipidsincludefats,oils,andgrease.Alloftheseexertanoxygendemand.Table 5.2pres-entsthetypicalcharacteristicsofuntreatedmunicipalwastewater.

Theconstituentsofindustrialwastewatersvarywidelydependingonthetypeofindustryandtheprocessesusedinmanufacturingtheproduct.In

Wastewater treatment fundamentals 77

theUnitedStates,theEnvironmentalProtectionAgency(EPA)hasgroupedthepollutants intothreecategories:conventional pollutants suchaspH,BOD5 (biochemical oxygen demand), TSS (total suspended solids), oil,and grease; nonconventional pollutants such as COD (chemical oxygendemand),ammonia,hexavalentchromium,phenols,etc.;andpriority pol-lutantssuchasarsenic,cadmium,etc.ThecompletelistcanbefoundintheCodeofFederalRegulations (FederalRegister,2010).Table 5.3presentsselectedcharacteristicsofanumberofindustrialwastewaters.

Table 5.1 Environmental impacts of major wastewater pollutants

Pollutant Source Environmental impact on receiving waters

Suspended solids Municipal wastewater, stormwater

Scum layer on water surface, sludge deposits

Organic matter Municipal wastewater, possible industrial wastewater

Dissolved oxygen depletion, anaerobic conditions, fish kills

Nutrients Municipal wastewater, industrial wastewater

Eutrophication and impairment of water quality

Pathogens Municipal wastewater Transmission of diseasesHeavy metals Industrial wastewater Toxic to aquatic lifeRefractory organics Industrial wastewater May be toxic or carcinogenicEndocrine disrupting compounds

Municipal wastewater Feminization of fish, possible broader scope of impacts

Source: Adapted from Peavy et al. (1985).

Table 5.2 Typical characteristics of untreated municipal wastewater

Component Concentration range

Biochemical oxygen demand, BOD5 at 20°C 100–360 mg/LChemical oxygen demand, COD 250–1000 mg/LTotal organic carbon, TOC 80–300 mg/LTotal Kjeldahl nitrogen (TKN) 20–85 mg/L as NTotal phosphorus 5–15 mg/L as POil and grease 50–120 mg/LTotal solids (TS) 400–1200 mg/LTotal dissolved solids (TDS) 250–850 mg/LTotal suspended solids (TSS) 110–400 mg/LVolatile suspended solids (VSS) 90–320 mg/LFixed suspended solids (FSS) 20–80 mg/LSettleable solids 5–20 ml/LTotal coliform 106–1010 No./100 mlFecal coliform 103–108 No./100 ml

Source: Adapted from Metcalf and Eddy (2003).


5.4 WASTEWATER TREATMENT METHODS

Wastewater can be treatedusing anyor a combination of the followingtypesoftreatmentmethods,dependingonthenatureofpollutantsandthelevelofdesiredremoval.

5.4.1 Physical treatment

Physicaltreatmentinvolvestheremovalofpollutantsfromthewastewaterbysimplephysicalforces,e.g.sedimentation,screening,filtration.Physicaltreatmentprocessesareusedmainlyforremovalofsuspendedsolids.

5.4.2 Chemical treatment

Chemicaltreatmentinvolvestheadditionofchemicalstoachieveconversionordestructionofcontaminantsthroughchemicalreactions,e.g.coagula-tion–flocculationforsolidsremoval,disinfectionforpathogendestruction,chemicalprecipitationforphosphorusremoval.

5.4.3 Biological treatment

Biological treatment involves the conversion or destruction of contami-nantswiththehelpofmicroorganisms.Inmunicipalwastewatertreatmentplants,microorganisms indigenous towastewatersareused inbiologicaltreatmentoperations.Examplesofbiological treatment includeactivatedsludgeprocess,membranebioreactor,tricklingfilter,andothers.Thepri-marypurposeofbiologicaltreatmentistoremoveandreducethebiode-gradableorganicmatterfromwastewatertoanacceptablelevelaccordingtoregulatorylimits.Biologicaltreatmentisalsousedtoremovenutrientssuchasnitrogenandphosphorusfromwastewater.

Table 5.3 Typical characteristics of selected industrial wastewaters

Industry BOD, mg/L COD, mg/L TSS, mg/L

Milk processing 1300 3100 300Meat processing 1400 2500 950Pulp and paper (kraft) 300 550 250Tannery 4000 7500 15,000Slaughterhouse (cattle) 2000 3600 800Cheese production 3000 5500 950Pharmaceuticals 280 390 160

Source: Adapted from Hammer and Hammer (2012) and Davis (2011).


5.5 LEVELS OF WASTEWATER TREATMENT

Awastewatertreatmentsystemisacombinationofunitoperationsandunitprocessesdesignedtoreducecontaminantstoanacceptablelevel.Thetermunit operationreferstoprocessesthatusephysicaltreatmentmethods.Thetermunit processes referstoprocessesthatusebiologicaland/orchemicaltreatmentmethods.Unitoperationsandprocessesmaybegroupedtogethertoprovidethefollowinglevelsoftreatment(MetcalfandEddy,2003).

5.5.1 Preliminary treatment

Preliminarytreatmentinvolvesthephysicalremovalofpollutantsubstancessuch as rags, twigs, etc. that can cause operational problems in pumps,treatment processes, and other appurtenances. Examples of preliminarytreatmentarescreensforremovalof largedebris,comminutorforgrind-inglargeparticles,gritchamberforremovalofinertsuspendedsolids,andflotationforremovalofoilsandgrease.

5.5.2 Primary treatment

Primary treatment involves thephysical removalofaportionof thesus-pendedsolidsfromwastewater,usuallybysedimentation.Primaryclarifi-ersareusedforthispurpose.PrimaryclarifiereffluentcontainssignificantamountsofBODandrequiresfurthertreatment.Primarytreatmentoftenincludespreliminaryaswellasprimarytreatmentoperations.

5.5.3 Enhanced primary treatment

Enhanced primary treatment involves the use of chemical treatment toobtain additional solids removal in a sedimentation process. Chemicalcoagulantsareusedtopromotecoagulationandflocculationofsolidsinasedimentationtank,resultinginenhancedsuspendedsolidsremoval.BluePlainsAdvancedWastewaterTreatmentPlantinWashington,D.C.,usesanironcoagulanttogetherwithapolymertoachieveenhancedsolidsremovalintheirprimaryclarifiers(Neupaneetal.,2008).

5.5.4 Conventional secondary treatment

Conventional secondary treatment involvesbiological treatment fordeg-radation of organic matter and solids reduction. Efficiency is measuredmainlyintermsofBOD5andsuspendedsolidsremoval.Treatmentiscar-riedoutinabiologicalreactorfollowedbyasedimentationtankorsecond-aryclarifier.Examplesofsecondarytreatmentareactivatedsludgeprocess,tricklingfilter,etc.


5.5.5 Secondary treatment with nutrient removal

Whenremovalofnutrients,suchasnitrogenand/orphosphorus,isrequired,it may be combined with the secondary treatment for BOD removal.Additionalreactorsmayberequiredtoachievenitrogenremovalthroughthe nitrification–denitrification process. A combination of chemical andbiologicaltreatmentcanbeused.

5.5.6 Tertiary treatment

Tertiarytreatmentincludestreatmentprocessesusedafterthesecondary,e.g.granularmediafiltrationusedforremovalofresidualsuspendedsolids,anddisinfectionforpathogenreduction.Additionaltreatmentfornutrientremovalisalsoincludedintertiarytreatment.

5.5.7 Advanced treatment

Advancedtreatmentprocessesareusedwhenadditionalremovalofwaste-waterconstituentsisdesiredduetotoxicityofcertaincompounds,orforpotential water reuse applications. Examples include activated carbonadsorption for removalofvolatileorganic compounds, ion exchange forremovalofspecificions,etc.

5.6 RESIDUALS AND BIOSOLIDS MANAGEMENT

Eachofthetreatmentprocessesdescribedabovegeneratesacertainamountofwastesolids.Thewastegenerated is semisolid innatureand is termedsludge.Thewastegeneratedfrompreliminarytreatmentincludesgritandscreenings.Thesewasteresidualsarelowinorganiccontentandaredisposedofinlandfills.Thesludgegeneratedfromprimaryandsecondaryclarifiershasasignificantamountoforganicmatterandrequiresfurthertreatmentandprocessingpriortodisposal.Thetermbiosolidsisusedtodenotetreatedsludge. The cost of treatment of sludge and disposal of biosolids can beequivalentto40%to50%ofthetotalcostofwastewatertreatment.

The main objectives of sludge treatment are (a) to reduce the organiccontent, (b) toreduce the liquid fraction,and(c) toreduce thepathogencontent. If the sludge contains heavy metals or other toxic compounds,localorstateregulationsmayrequireadditionaltreatmentdependingonthefinaldisposalofthebiosolidsproduced.

Theliquidfractionisreducedbyanumberofprocesses.Theseincludegravitythickening,dissolvedairflotation,centrifugation,beltfilterpress,etc.Organiccontentandpathogenreductionisachievedbyprocessesthatinclude anaerobic digestion, aerobic digestion, air drying, heat drying,


thermophilicdigestion,composting,limestabilization,pasteurization,etc.Acombinationoftheseprocessesmaybeuseddependingonthequalityofbiosolidsdesired.

Overthelastfourdecades,mostoftheresearchhasfocusedontreatmentofwastewater,whiletreatmentofsludgehaslaggedbehind.Thetraditionalmethodofbiosolidsdisposalinlandfillsisstillusedextensively.Landappli-cationofbiosolidsispracticedinsomeareas.Inrecentyears,theconceptofbeneficialreuseofbiosolidsasasoilconditionerandfertilizeronagri-cultural landshasgained importance,both fromtheviewpointofgreenengineeringandnecessity.Asaresult,wehaveseenincreasedresearchonbiosolidsforthepurposeoffurtherreducingpathogensforsafereuseoftheproduct.DetaileddiscussiononbiosolidsisprovidedinChapter12.

5.7 FLOW DIAGRAMS OF TREATMENT OPTIONS

EXAMPLE5.1Drawaflowdiagram foraprocess to treatamunicipalwastewaterthathasahighconcentrationofsuspendedsolids,organicmatter,andpathogens.Also,illustrateasludgetreatmentoption.

SOLUTION

Thewastewatercanbetreatedwithaconventionalprocessconsistingofprimaryandsecondarytreatment.Thesludgecanbetreatedusinganaerobicdigestion.SeeFigure 5.1.

Screens Comminutor Gritchamber

Primaryclari�er

Biologicalreactor

Secondaryclari�er

Gravitythickener

Primarysludge

Secondarysludge

Grit toland�ll

Screens toland�ll

Sludge recycle

Disinfectant

Anaerobicdigester Centrifuge

Biosolidsfor land

application

E�uent recycledthru secondary

reactor

Preliminary and Primary Treatment Secondary Treatment

Sludge Treatment


E�uent Wastewater

in�uent

Figure 5.1 Flow diagram of a conventional wastewater treatment process with sludge digestion.


EXAMPLE5.2Drawaflowdiagramtotreatawastewaterthathasahighconcentra-tionofsuspendedsolids,organicmatter,pathogens,andahighcon-centrationofammonia–nitrogen.

SOLUTION

Thewastewatercanbetreatedusingprimarytreatmentfollowedbybiologicaltreatmenttoremoveorganicmatterandammonia–nitrogen.Ammonia is removed in a two-step biological process consisting ofnitrificationfollowedbydenitrification.ThenitrificationstepcanbecombinedwithBODremovalinanaerobicreactor.Thisisfollowedbydenitrificationinananoxicreactor.SeeFigure 5.2.

EXAMPLE5.3Drawaflowdiagramfortreatmentofawastewaterthathasahighcon-centrationofherbicides,aswellassuspendedsolidsandorganicmatter.

SOLUTION

Thewastewaterwillbetreatedusingprimary,secondary,andtertiaryadvanced treatment. Activated carbon adsorption is used to removetheherbicide.SeeFigure 5.3.

EXAMPLE5.4Drawaflowdiagramfor treatmentofawastewater thathasahighconcentration of suspended solids and organic matter. Effluent dis-chargeregulationsallowverylowconcentrationofsuspendedsolids.

SOLUTION

The wastewater treatment will include tertiary treatment togetherwith primary and secondary treatment. Tertiary treatment consistsof dual media filtration to remove residual suspended solids. SeeFigure 5.4.

Screens Gritchamber

Primaryclari�er

Aerobicreactor

Secondaryclari�er

Grit toland�ll

Screens toland�ll

Sludge recycle

Wastewater �owSludge �ow

Wastewaterin�uent

Combined BODRemoval & Nitri�cation

Anoxicreactor

Denitri�cationMethanol

Clari�erE�uent

Disinfectant

Sludge recycle

Sludge todigestion

Figure 5.2 Flow diagram for treatment of wastewater with high nitrogen concentration.


5.8 TYPES OF BIOLOGICAL TREATMENT PROCESSES

Therearetwomaintypesofwastewatertreatmentprocesses:

1.Suspended growth process—Themicroorganismsarekeptinsuspen-sioninabiologicalreactorbysuitablemixingdevices.Theprocesscanbeaerobicoranaerobic.Examplesofsuspendedgrowthprocessesincludeactivatedsludgeprocess,sequencingbatchreactor,pondsandlagoons,digesters,etc.

2.Attached growth process—Themicroorganismsresponsibleforbio-conversionattachthemselvesontoaninertmediuminsidethereactor,where they grow and form a layer called biofilm. The wastewaterflowingthroughthereactorcomesincontactwiththebiofilm,whereconversion and removal of organic matter takes place. The inertmediumisusuallyrock,gravel,slag,orsyntheticmedia.Theprocesscanbeoperatedaerobicallyoranaerobically.Examplesaretricklingfilters,biotowers,androtatingbiologicalcontactors(RBCs).

Screens Gritchamber

Primaryclari�er

Biologicalreactor

Secondaryclari�er

Grit toland�ll

Screens toland�ll

Sludge recycle


Wastewaterin�uent Activated

carbonadsorption

E�uent

Disinfectant

Sludge todigestion

Advancedtreatment

Figure 5.3 Flow diagram for an advanced wastewater treatment process.

Screens Gritchamber

Primaryclari�er

Biologicalreactor

Secondaryclari�er

Grit toland�ll

Screens toland�ll

Sludge recycle


Wastewaterin�uent Dual

media�ltration

E�uent

Disinfectant

Sludge todigestion

Tertiarytreatment

Figure 5.4 Flow diagram for a wastewater treatment system to achieve very low effluent solids concentration.


Effective design and successful operation of the processes depend on athoroughunderstandingofthetypesofmicroorganismsinvolved,growthrequirements,reactionkinetics,andenvironmentalfactorsthataffecttheirperformance.Selectionofaparticularprocessshouldbebasedonbenchscaleandpilot scale studieson thespecificwastewater, investigating theeffects of a variety of possible variables. Detailed discussion of each oftheseprocessesisprovidedinChapters8and9.

PROBLEMS

5.1 Whatarethecommonsourcesofwastewater?Namethem. 5.2 Whatarethemainobjectivesofwastewatertreatment? 5.2 Whatarepreliminarytreatmentandprimarytreatment?Whatdo

theyremove? 5.3 Whatisbiologicaltreatment?Whataretheadvantagesofbiological

treatment? 5.4 Whatischemicaltreatment?Ifyouweregivenanoption,wouldyou

prefer touse chemical treatmentorbiological treatment?Explainyourreasons.

5.5 Definethetermseffluent,sludge,andbiosolidsastheypertaintowastewatertreatment.

5.6 Definesuspended growthandattached growth processes.Giveanexampleofeach.

5.7 Whatarethemainobjectivesoftreatmentofsludge? 5.8 Drawaflowdiagramofaprocesstotreatawastewaterthathasa

highconcentrationofsuspendedsolids,BOD,pathogens,andphos-phorus.Alsoshowsludgetreatmentprocess.

5.9 Drawaflowdiagramforaprocesstotreatawastewaterthathasahighconcentrationofsyntheticorganiccompounds(SOCs)aswellassuspendedsolidsandorganicmatter.

REFERENCES

FederalRegister(2010)CodeofFederalRegulations,Title40.U.S.Government.Davis,M.(2011)Water and Wastewater Engineering: Design Principles and Practice.

McGraw-Hill,Inc.,NewYork.Hammer,M.J.,andHammerM.J.Jr.(2012)Water and Wastewater Technology.

Seventhedition.Pearson-PrenticeHall,Inc.,NewJersey.Metcalf and Eddy, Inc. (2003) Wastewater Engineering: Treatment and Reuse.

Fourthedition.McGraw-Hill,Inc.,NewYork.Neupane,D.,Riffat,R.,Murthy,S.,Peric,M.,andWilson,T.(2008)“Influenceof

SourceCharacteristics,ChemicalsandFlocculationonChemicallyEnhancedPrimaryTreatment.”Water Environment Research,vol.80,no.4,pp.331–338.


85

Chapter 6

Preliminary treatment

6.1 INTRODUCTION

Preliminarytreatmentinvolvestheremovaloflargersuspendedsolidsandinertmaterialsfromthewastewater.Physicaltreatmentprocessesareusedtoremovetheseparticlesanddebris thatmaycauseharmtopumpsandotherequipment,andforremovalofinertmatterpriortosecondarybio-logicaltreatment.Theunitoperationsusedincludesscreens,comminutors/grinders,andgritchambers.AtypicallayoutisillustratedinFigure 6.1.

6.2 SCREENS

Rawwastewatercontainsasignificantamountofsuspendedandfloatingmaterials.Theseincluderags,weeds,twigs,organicmatter,andavarietyofsolids.Thesolidscandamagepumpsandmechanicalequipmentandinter-ferewiththeflowinpipesandchannels.Screeningdevicesareplacedaheadofpumpstoremovethelargermaterialsfromthewastewaterstream.Theremoveddebris,calledscreenings,isusuallydisposedofinlandfillsorbyincineration.Differenttypesofscreensareavailabledependingonwaste-watercharacteristicsandsiterequirements.Thefollowingsectionsdescribethetypesofscreensthatareavailable,basedonthesizeoftheopenings.

6.2.1 Trash racks

These are screens that have large openings to exclude larger debris andgarbage.Theseconsistofrectangularorcircularsteelbarsarrangedinaparallelmanner,eitherverticallyoratan inclinetothehorizontalchan-nel.Sizeofopeningbetweenbarsrangesfrom50to150mm(2to6in).Mechanicalrakesareusedtoclearthesolidscollectedonthetrashracks.Rakemachinesareoperatedbyhydraulicjacks.Trashracksarefollowedbycoarsescreens(seeFigure6.2).


6.2.2 Coarse screens or bar screens

Thesearesimilartotrashracks,buthaveasmallersizeopening,rangingfrom25to75mm(1to3in).Coarsescreenscanbemanuallycleanedormechanicallycleaned.Manuallycleanedbarscreensmaybeusedatsmall-sizedwastewatertreatmentplants.Theyarealsousedinbypasschannelswhen other mechanically cleaned screens are being serviced, or in theeventof apower failure.Manually cleaned screens shouldbeplacedona slopeof 30° to45° from the vertical.This increases the cleaning sur-face,makescleaningeasier,andpreventsexcessiveheadlossbyclogging.Mechanicallycleanedscreenscanbeplacedat0°to30°fromthevertical.Lower maximum approach velocities are specified for manually cleaned

(a) Section

(b) Plan

Directionof flow

Inclinedscreen

Platform for screening

Perforated metalplatform forscreenings

Figure 6.2 Diagram of trash rack.

Trashrack

Gritchamber

Grit toland ll

Screens to land ll

Rawwastewater Bar

screenFine

screenComminutor

To secondarytreatment

Figure 6.1 Typical layout of a preliminary treatment process.

Preliminary treatment 87

screens(0.3–0.6m/s,or1.0–2.0ft/s),comparedwithmechanicallycleanedscreens(0.6–1.0m/s,or2.0–3.25ft/s)(MetcalfandEddy,2003).Therearefourmaintypesofmechanicallycleanedbarscreens:chaindriven,recip-rocatingrake,catenary,andcontinuousbelt.Figure 6.3presentstwotypesofautomatic,mechanicallycleanedbarscreens,manufacturedbyVulcanIndustries,Inc.,ofIowa.

6.2.2.1 Design of coarse screens

Thefollowingparametersareimportantdesignconsiderationsintheinstal-lationofcoarsescreens:

• Location• Approachvelocity• Clearopeningsbetweenbarsormeshsize• Headlossthroughthescreen• Disposalofscreenings

(a) (b)

Figure 6.3 (a) Mensch Crawler™ Bar Screen and (b) VMR™ Multi-Rake Bar Screen (Source: Courtesy of Vulcan Industries, Inc., Iowa).


Coarsescreensshouldbeinstalledaheadoffinescreensandgritcham-bers. For manually cleaned screens, the approach velocity should belimited to about 0.45 m/s (1.5 ft/s) at average flow. For mechanicallycleaned screens, an approach velocity of at least 0.4 m/s (1.25 ft/s) isrecommendedtominimizesolidsdepositioninthechannel.Atpeakflowrates,thevelocitythroughthescreenshouldnotexceed0.9m/s(3ft/s),to prevent the pass-through of solids (Metcalf and Eddy, 2003; EPA,1999).Velocity through thebar screencanbecontrolledby installingadownstreamheadcontroldevice,e.g.aParshallflume.Twoormoreunitsshouldbeinstalled,sothatoneunitmaybetakenoutofservicefor maintenance. The head loss through mechanically cleaned screensisusuallylimitedtoabout150mm(6in)byoperationalcontrols.Thehead loss ismeasuredas thedifference inwater levelbeforeandafterthescreen.

Theheadlossthroughascreenisafunctionoftheapproachflowvelocityandthevelocitythroughthebars.Bernoulli’sequationisusedtocalculatetheheadloss(Droste,1997),whichresultsinthefollowingequation:

HC

V v

gL

d

s= −

1

2

2 2

(6.1)

where:HL=Headlossthroughthescreen,m =h1–h2=upstreamdepthofflow–downstreamdepthofflowCd=Coefficientofdischarge,usually0.70–0.84foracleanscreen,

and0.6forcloggedscreenVS=Velocityofflowthroughtheopeningsofthebarscreen,m/sv=Approachvelocityinupstreamchannel,m/sg=accelerationduetogravity,9.81m/s2

The velocity of flow through the bar screen openings can be calculatedfromthenumberofbarsinthechannelwidthandthedepthofthewaterlevel.Theapproximatenumberofbarsis(Davis,2011)asfollows:

N =channelwidth –barspacing

barwidtbars

hh + barspace (6.2)

Numberofbarspaces=(Nbars+1) (6.3)

Areaofscreenopenings=(numberofbarspaces)×(barspacing) ×(waterdepth)m2 (6.4)

Therefore,Vs=(flowratem3/s)/(areaofscreenopeningsm2) (6.5)


EXAMPLE6.1Amechanicallycleanedbarscreenisusedinpreliminarytreatmentforthefollowingconditions:

Wastewaterflowrate=100,000m3/dApproachvelocity=0.6m/sOpenareaforflowthroughthescreen=1.6m2

Headlosscoefficientforcleanscreen=0.75Headlosscoefficientforcloggedscreen=0.60Inclinefromvertical=0°

a.Calculatethecleanwaterheadlossthroughthebarscreen. b.Calculate thehead loss after 40%of theflowarea is clogged

withsolids.

SOLUTION

Step1.CalculateVsforcleanscreenusingequation(6.5).

Flowrate,Q=(100,000m3/d)/(86,400s/d)=1.16m3/s

Vs=(1.16m3/s)/(1.6m2)=0.725m/s

Determinethecleanwaterheadlossusingequation(6.1).

HC

V v

gL

d

s= −

1

2

2 2

H L = −

×

1

0 75

0 725 0 6

2 9 81

2 2

.

. .

.=0.01m

Step2.CalculateVsforcloggedscreenusingequation(6.5).

Areaavailableforflow,A=1.6×(1–0.4)=0.96m2

Vs=(1.16m3/s)/(0.96m2)=1.21m/s

Calculateheadlossforcloggedscreenusingequation(6.1).

H L = −×

1

0 6

1 21 0 6

2 9 81

2 2

.

. .

.=0.09m

Note:Vsforcloggedscreenexceedsmaximumsuggestedvalueof0.9m/s.Thisindicatesthatthescreenshouldbecleaned.Screensareusu-allycleanedeitheratregulartimeintervalsorwhenaspecifiedmaxi-mumheadlossvalueisreached.


6.2.3 Fine screens

Thesescreenshaveopeningslessthan6mminsize.Finescreensareusedinpreliminarytreatmentaftercoarsescreens,inprimarytreatmentpriortosecondarytricklingfilters,andfortreatmentofcombinedseweroverflows.Differentfabricationtechniquesareusedtoprovidethesmallscreensizes.Theseincludethefollowing:

• Profilebarsarrangedinaparallelmannerwithopeningsfrom0.5mm(0.02in)

• Slottedperforatedplateswith0.8to2.4mm(0.03to0.09in)wideslots• Wedge-shapedbarsweldedtogetherintoflatpanelsections• Loopedwireconstructionwithopeningsof0.13mm(0.005in)• Wiremeshwithapproximately3.3mm(0.013in)openings• Wovenwireclothwithopeningsof2.5mm(1.0in)

A variety of fine screens are commercially available. Some of them aredescribedbelow:

1.Static wedgewire screen—Thesescreenshave0.2to1.2mmopeningsandaredesignedforflowratesof400to1200L/m2·minofscreenarea(MetcalfandEddy,2003).Headlossrangesfrom1.2to2m.Thescreenconsistsof small stainless steelwedge-shapedbars,with theflatpartofthewedgefacingtheflow.

2.Stair screen—This type of screen has two step-shaped sets of thinverticalplates, onefixedandonemovable.Thefixedandmovableplates alternate across the width of an open channel and togetherformasinglescreeningsurface.Themovableplatesrotateinaverticalmotion,liftingthecapturedsolidsontothenextfixedsteplanding,ultimatelytransportingthemtothetopofthescreen,fromwheretheyaredischargedtoacollectionhopper.Rangeofopeningsbetweenthescreenplatesis3to6mm(0.12to0.24in).Astairscreenmanufac-turedbyVulcanIndustries,Inc.,ofIowaisillustratedinFigure 6.4(a).

3.Drum screen—Thescreeningmediumismountedonadrumorcylin-derthatrotatesinaflowchannel.Dependingonthedirectionofflowintothedrum,thesolidsmaybecollectedontheinteriororexteriorsurface.Drumscreensareavailableinvarioussizesrangingfrom0.9to2m(3to6.6ft)indiameter,andfrom1.2to4m(4to13.3ft)inlength.ArotarydrumscreenisillustratedinFigure 6.4(b).

6.2.3.1 Design of fine screens

Aninstallationshouldhaveaminimumoftwoscreens,andeachshouldbecapableofhandlingpeakflowrates.Flushingwatershouldbeprovidedtoremovethebuildupofgreaseandothersolidsonthescreen.Theclearwater


(a)

(b)

Figure 6.4 (a) ESR™ Stair Screen and (b) Liqui-Fuge™ Rotary Drum Screen (Source: Courtesy of Vulcan Industries, Inc., Iowa).


headlossthroughafinescreenmaybeobtainedfromthemanufacturer’sratingtables.Itcanalsobecalculatedfromthefollowingequation(MetcalfandEddy,2003):

Hg

Q

C AL

d

=

1

2

2

(6.6)

where:HL=Headlossthroughthescreen,mCd=Coefficientofdischarge,usually0.6foracleanscreenQ=Wastewaterflowrate,m3/dA=Effectiveopenareaofsubmergedscreen,m2

g=accelerationduetogravity,9.81m/s2

ValuesofCdandAdependonscreendesignfactors,andmaybeobtainedfromthescreenmanufacturerordeterminedexperimentally.

6.2.4 Microscreens

Thesescreenshaveopeningsthatarelessthan50μm.Thistypeofscreenisusedintertiarytreatmenttoremovefinesolidsfromtreatedeffluents.Itinvolvestheuseofvariablelowspeedrotatingdrumscreensthatareoper-atedundergravityflowconditions.Thefabricfilterhasopeningsrangingfrom10to35μmandisfittedontheperipheryofthedrum.

6.3 SHREDDER/GRINDER

Coarsesolids,especiallylargerorganicsolids,arereducedtosmallersizesolidsbyusingshreddingprocesses.Thesecanbeusedinconjunctionwithmechanicallycleanedscreenstocutupthesolidsintosmallerparticlesofuniformsize,whicharethenreturnedtotheflowstreamforpassagetosec-ondarytreatmentunits.Therearethreemaintypesofshreddingdevices:

1.Comminutor—Theseareusedinsmallwastewatertreatmentplants,withflowrateslessthan0.2m3/s(5Mgal/d).Atypicalcomminutorhasastationaryhorizontalscreentointercepttheflowandarotatingcuttingarmtoshredthesolidstosizesrangingfrom6to20mm(0.25to0.77in).Abypasschannelwithamediumscreenisusuallyprovidedtomaintainflowwhenthecomminutoristakenoff-lineforservicing.Figure 6.5presentsadiagrammaticlayoutofacomminutor.Headlossthroughacomminutorcanrangefrom0.1to0.3m(4 to12in)andcanreach0.9m(3ft)inlargeunitsatmaximumflowrates(Metcalf


andEddy,2003).Comminutorscancreateastringofragsand/orplas-ticthatcancollectondownstreamequipmentandcauseoperationalproblems.Newerinstallationsusemaceratorsorgrinders.

2.Macerator—These are slow-speed grinders that chop or grind sol-ids to very small pieces.Themaceratorblade assembly is typicallybetween6to9mm(Davis,2011).Effectivechoppingactionreducesthepossibilityofproducingropesofragsandplasticsthatcancollectondownstreamequipment.

3.Grinder—High-speedgrindersareusedtopulverizesolidsinthewaste-water.TheyarealsocalledHammermills.Thesolidsarepulverizedastheypassthroughahigh-speedrotatingassembly.Washwaterisusedtokeeptheunitcleanandtotransportsolidsbacktothewastewaterstream.

6.4 GRIT CHAMBER

Gritisdefinedassand,gravel,orothermineralmaterialthathasanominaldiameterof0.15–0.20mmorlarger(Droste,1997).Gritmayalsoincludeash, wood chips, coffee grounds, egg shells, and other nonputrescibleorganicmatter.Somecomponentssuchascoffeegroundsareorganic,buttheyareessentiallynonbiodegradableoverthetimespanforgritcollectionanddisposal.Gritchambersaresedimentationtanksthatareplacedafterscreensandbeforeprimaryclarifiers.Thepurposeofagritchamberistoremovematerialsthatmayformheavydepositsinpipelines,protectpumps

In�uent E�uent

Comminutor

Motor and gear reducer

Valved drain for dewateringcomminutor channel

Figure 6.5 Diagrammatic layout of a comminutor.


andothermechanicalequipment fromabrasion, reduce the frequencyofdigestercleaningcausedbygritaccumulation,andseparateheavier inertsolidsfromlighterbiodegradableorganicsolidsthataresenttosecondarybiologicaltreatment.Theamountofgritcollecteddependsonthewastewa-tercharacteristicsandthetypeofgritchamberusedattheplant.Gritcanbecoatedbygreaseorotherorganicmatter.So,gritremovedfromthegritchambersisusuallywashedtoremoveorganicmatterandthentransportedtoasanitarylandfillfordisposal.

Ingeneral,gritchambersaredesignedtoremoveparticleswithaspecificgravityof2.65 (sand)andnominaldiameterof0.20mmor larger,atawastewater temperature of 15.5°C (60°F). The settling velocity of theseparticlesisabout2.3cm/s(4.5ft/min)basedoncurvesofwastewatergritsettling velocities developed by Camp (1942). Grit chambers are some-timesdesignedtoremove0.15mmsandparticleswithasettlingvelocityof1.3cm/s(2.6ft/min),basedonCamp’scurves.Subsequentresearchhasrevealedthatthespecificgravityofgritcanrangefrom1.1to2.7(Eutek,2008; Metcalf and Eddy, 2003). Wilson et al. (2007) suggested a sand equivalent size (SES),whereSES is the sizeofaclean sandparticle thatsettlesatthesamerateasagritparticle.

Therearemainlythreetypesofgritchambers:

1.Horizontal flow grit chamber—Thisisasquareorrectangularopenchannelwithasufficientdetentiontimetoallowsedimentationofgritparticles,andaconstantvelocitytoscourtheorganics.Thevelocityiscontrolledbychanneldimensions,aninfluentdistributiongate,andaneffluentweir.Itmaybecleanedmanuallyorbymechanicalsludgescrapers.Itisfoundinolderinstallations.

2.Aerated grit chamber—This isusedinnewer installations.Aspiralflowpatternisintroducedinthewastewaterasitflowsthroughthetank,bysupplyingairfromadiffuserlocatedononesideofthetank.The air provides sufficient roll velocity to keep the lighter organicparticlesinsuspension,whileheaviergritparticlessettleatthebot-tom.Thelighterorganicparticlesarecarriedoutofthetankwiththewastewater.Ahopperisprovidedalongonesideofthetankforgritcollection.Theadvantagesofthesystemincludeminimalheadlossandthefactthataerationhelpsreducesepticconditionsinthewaste-water.Disadvantagesincludehighpowerconsumption,laborinten-sive,andpossibleodorissues.Figure 6.6illustratestheflowpatterninanaeratedgritchamber.Aeratedgritchambersaregenerallydesignedtoremoveparticles0.21mmdiameterorlargerwithadetentiontimeof2to5minatpeakhourlyflowrates.Typicalwidthtodepthratiois1.5:1,andlengthtowidthratiois4:1.Airsupplyrangesfrom0.2to0.5m3/minpermof length (3–8 ft3/ft·min) (MetcalfandEddy,2003).DesignofanaeratedgritchamberisshowninExample6.2.


3.Vortex grit chamber—Thistypeofdevicegeneratesavortexflowpat-tern,whichtendstolift the lighterorganicparticlesupward,whilethegritsettlesinthehopperatthebottom.Settledgritisremovedbyagritpumporairliftpump.Ithasasmallfootprintwithminimalheadloss.Thedesignisproprietary,andcompactionofgritmaybeaproblem.Vortexgritchambersaretypicallydesignedtohandlepeakflowratesupto0.3m3/s(7Mgal/d)perunit.Figure 6.7illustratesavortexgritchamber.

EXAMPLE6.2Designanaeratedgritchamberforamunicipalwastewatertreatmentplant.Theaverageflowrateis20,000m3/d,withapeakingfactorof2.5.Useadepthof3m.Airissuppliedat0.35m3/minpermoflength.Assumegritcollectedis0.10m3/1000m3atpeakflow.Determinethetankdimensions,totalairsupplyrequired,andquantityofgrit.

Grit troughs Inspection bridge

Grit dischargeline

Grit pumpDiuser

Figure 6.6 Aerated grit chamber (Source: Adapted from Metcalf and Eddy, 2003).


SOLUTION

Step1.Determinetankdimensions.

Aeratedgritchamberisdesignedforpeakflowrates.Usetwocham-bersinparallel.

Peakflowrate=20,000×2.5=50,000m3/d Flowineachtankatpeakflow,Q=50,000/2=25,000m3/d Assumedetentiontimeatpeakflow,t=4min Volumeofeachtank=Q·t=(25,000m3/d)×(4min)/(1440min/d)

=70m3

Assumewidthtodepthratio=1:1 Depth=3m Therefore,width=3m Length=70/(3×3)=7.7m≈8m Tankdimensionsare8m×3m×3m.

Grit removal pipe

Drive unit

Drive torque tube

Outlet

Inlet

Impeller

Grit suction pipe

Grit movement pattern

Figure 6.7 Vortex grit chamber.


Step2.Determinetotalairrequired.

Airrequired=(0.35m3/minperm)×8m=2.80m3/minforeachtank Totalairrequiredfor2tanks=2.80×2=5.60m3/min

Step3.Calculatevolumeofgrit.

Volumeofgrit=(0.10m3/1000m3)×25,000m3/d=2.5m3/dineachtank

Totalgritvolume=2.5×2=5.0m3/d

PROBLEMS

6.1 Drawaflowdiagramofapreliminarytreatmentprocessthatcon-sists of trash racks, two bar screens, two rotary drum screens, amacerator with a bypass channel, and an aerated grit chamber.Whathappenstothewastesremovedfromeachunit?

6.2 Whataretheadvantagesofinstallingabarscreenatanincline? 6.3 Whyshouldyouprovideabypasschannelwithacomminutor? 6.4 Thewaterelevationsupstreamanddownstreamofabarscreenare

0.89mand0.85m.Iftheapproachvelocityis0.45m/s,whatistheflowvelocitythroughthescreen?Thedischargecoefficientof thescreenis0.7.

6.5 Amechanicallycleanedbarscreenisusedinpreliminarytreatmentforthefollowingconditions:

Inclinefromvertical=30°Wastewaterflowrate=150,000m3/dApproachvelocity=0.6m/sOpenareaforflowthroughthescreen=1.6m2

Headlosscoefficientforcleanscreen=0.74Headlosscoefficientforcloggedscreen=0.60

a. Calculatethecleanwaterheadlossthroughthebarscreen. b. Calculatetheheadlossafter50%oftheflowareaisclogged

withsolids. 6.6 Estimatetheheadlossforabarscreensetata30°inclinefromthe

vertical.Thewastewaterflowrateis90,000m3/d.Thebarsare20mmindiameter,with25mmclear spacing inbetweenbars.Thewaterdepthis1.2m,channelwidthis1.5m,approachvelocityis0.65m/s,andtheheadlosscoefficientis0.65.

6.7 Whatisgrit?Whataretheobjectivesofgritremovalfromthewaste-water?Whatisthedisposalmethodforcollectedgrit?

6.8 Find thenameof thewastewater treatmentplant that serves yourlocality.Whatunitoperationsareusedaspreliminarytreatmentattheplant?Drawaflowdiagramofthepreliminarytreatmentprocess.


6.9 Design a grit chamber for a wastewater treatment plant with anaverageflowrateof25,000m3/dandapeakflowrateof55,000m3/d.Thedetentiontimeatpeakflowis3.0min.Thewidthtodepthratiois2:1.Useadepthof2m.Theaerationrateis0.4m3/minpermoftanklength.Determinethetotalairrequiredanddimensionsofthegritchamber.

REFERENCES

Camp,T.R.(1942)“GritChamberDesign.”Sewage Works J.,vol.14,pp.368–381.Davis,M.(2011)Water and Wastewater Engineering: Design Principles and Practice.

McGraw-Hill,Inc.,NewYork.Droste,R.L.(1997)Theory and Practice of Water and Wastewater Treatment.John

Wiley&Sons,Inc.,Hoboken,NewJersey.EPA(1999)“CombinedSewerOverflowTechnologyFactSheet:Screens.”EPA832-

F-99-040, United States Environmental Protection Agency, Office of Water,Washington,D.C.

Eutek(2008)EutekSystems,Hillboro,Oregon.Metcalf and Eddy, Inc. (2003) Wastewater Engineering: Treatment and Reuse.

Fourthedition.McGraw-Hill,Inc.,NewYork.Peavy,H.S.,Rowe,D.R.,andTchobanoglous,G.(1985)Environmental Engineer-

ing,McGraw-Hill,Inc.,NewYork.Wilson, G., Tchobanoglous, G., and Griffiths, J. (2007)“The Nitty Gritty: Grit

SamplingandAnalysis.”Water Environment & Technology,July,pp.64–68.

99

Chapter 7

Primary treatment

7.1 INTRODUCTION

Theobjectiveofprimarytreatment is toremoveasignificant fractionofthe suspended solids andfloatingmaterial from thewastewaterby sedi-mentation.Thesuspendedsolidsremovedareorganicinnature,andthuscontribute to theBOD (biochemicaloxygendemand)of the sludge.Thefloatingmaterialcanincludeoil,grease,rags,etc.thatwerenotremovedinupstreamprocesses.Theseareremovedasscumfromthewatersurfaceinthetank.Theremovaloflargerorganicsolidshelpstoreducetheloadonthesecondarybiologicalreactors.Thesolidsremovedarefurthertreatedindigesters,orotherprocesses,andstabilizedbeforedisposal.

Primary treatment mainly involves sedimentation or settling by grav-ity.Thevarioustypesofsettlingthatareobservedinwaterandwastewa-tertreatmentoperationsaredescribedinthefollowingsections. Insomecases, sedimentation is enhancedby the additionof coagulatingorfloc-culationagents.Theprocessiscalledenhancedclarification,orchemically enhanced primary treatment.Thisisdiscussedinmoredetailattheendofthechapter.

7.2 TYPES OF SETTLING/SEDIMENTATION

Therearemainlyfourtypesofgravitationalsettlingobservedinwaterandwastewatertreatmentoperations:

1.Type I or discrete particle settling—Particleswhosesize,shapeandspecificgravitydonotchangewithtimearecalleddiscrete particles(Peavyetal.,1985).TypeIsedimentationreferstothesettlingofdis-creteparticlesinadilutesuspension,wheretheparticleconcentrationislowenoughthattheparticlessettleasindividualentities.Thereisno interferenceofvelocityfieldswithneighboringparticles.Thistypeofsettlingisusuallyobservedingritchambers.


2.Type II or flocculent settling—This refers to settlingobserved inasuspensionwithparticlesthatcoalesceorflocculateastheycomeincontactwithotherparticles.Thisresultsinincreasingthesize,shape,andmassof theparticles, thus increasing the settling rate.Type IIsedimentationisobservedinprimaryclarifiers,intheupperportionofsecondaryclarifiersinwastewatertreatment,andalsoinclarifiersfollowingcoagulation–flocculationinwatertreatmentoperations.

3.Type III or hindered settling—This is also called zone settling. Itreferstosettlingthatoccursinasuspensionofintermediateconcen-tration,whereinterparticleforcesaresufficienttohinderthesettlingof adjacent particles. The mass of particles settle as a unit, and asolid–liquid interfacedevelops at the topof themass (Metcalf andEddy,2003).Hinderedorzonesettlingisobservedinsecondaryclari-fiersfollowingbiologicaltreatment,suchasactivatedsludgereactors.

4.Type IV or compression settling—Thisoccursinhighlyconcentratedsuspensions,whereastructureisformedduetothehighconcentra-tion, and settling can takeplaceonlyby compressionof the struc-ture.Asmoreparticles are added to the structure from the liquid,theincreasingmasscausescompressionsettling.Thistypeofsettlingisobservedatthebottomofsecondaryclarifiersfollowingactivatedsludgereactorsandalsoinsolidsthickeners.

7.3 TYPE I SEDIMENTATION

7.3.1 Theory of discrete particle settling

ThesettlingofdiscreteparticlesinafluidcanbeanalyzedusingNewton’slawandStokes’slaw.Consideradiscreteparticlefallinginaviscousandquiescentbodyoffluid.Theforcesactingontheparticleare(1)Fgduetogravity in a downward direction, (2) Fb due to buoyancy in an upwarddirection,and(3)FDduetofrictionaldraginanupwarddirection(Droste,1997;MetcalfandEddy,2003).Theeffectivegravitationalforceisgivenby

FG=Fg–Fb=(ρp–ρw)gV (7.1)

where:ρp=densityofparticle,kg/m3

ρw=densityofwater,1000kg/m3at5°Cg=accelerationduetogravity,9.81m/s2

V =volumeofparticle,m3

Thedragforceisgivenby

FC A v

Dd p w p=

ρ 2

2 (7.2)

Primary treatment 101

where:Cd=dragcoefficientAp=cross-sectionalorprojectedareaofparticles in thedirectionof

flow,m2

vp=particlesettlingvelocity,m/s

ThedragforceFDactsinadirectionoppositetothedrivingforceFG,andincreasesasthesquareofthevelocity.Accelerationoccursatadecreasingrate,untilasteadyvelocityisreached,wherethedrivingforceequalsthedragforce:

ρ ρρ

p wd p w pgV

C A v−( ) =

2

2 (7.3)

Forsphericalparticleswithdiameter,dp,

V

Ad

pp= 2

3 (7.4)

Substituting vp for vt, the terminal settling velocity of the particle, andusingequation(7.4),equation(7.3)providesanexpressionfortheterminalsettlingvelocity:

vgd

Ct

p

d

p w

w

=−

4

3

1

2ρ ρρ

(7.5)

TheexpressionforCddependsontheflowregime(Peavyetal.,1985).FlowregimecanbedeterminedfromReynoldsnumber,Re,

Re=Φv d

µt w p

w

ρ (7.6)

where:µw=dynamicviscosityofwater,N·s/m2

Φ=shapefactordependingonsphericityofparticle.Forperfectspheres,Φ=1.

Forlaminarflow,

Re<1,Cd = 24

Re (7.7)


Fortransitionalflow,

1<Re<103,Cd = + +24 30 34

Re Re. (7.8)

Forturbulentflow,

Re>103,Cd=0.4 (7.9)

Determinationof vt involves the simultaneous solutionof equation (7.5)andanexpressionforCd.

7.3.1.1 Stokes equation

Forlaminarflowandsphericalparticles,equation(7.5)becomes

vg d

µt

p w p

w

=−( )ρ ρ 2

18 (7.10)

Equation(7.10)isknownasStokesequation,whichcanbeusedtocalcu-latetheterminalsettlingvelocityofadiscreteparticle,whentheconditionsoflaminarflowandparticlesphericityaresatisfied.

Example problems are provided below to illustrate the use of theaboveequations.

EXAMPLE7.1Calculatetheterminalsettlingvelocityofasphericalsandparticleset-tlingthroughwaterat25°C.Thediameteroftheparticlesis0.6mmandspecificgravityis2.65.At25°C,ρw=997kg/m3,andµw=0.89×10–3N·s/m2.

SOLUTION

Assumelaminarflowanduseequation(7.10)tocalculatevt.

ρp=2.65×1000kg/m3=2650kg/m3,wheredensityofwaterat4°C=1000kg/m3

vg d

µt

p w p

w

=−( )ρ ρ 2

18

=

−( ) ××

−. / / ( . )9 81 2650 997 0 6 10

18

2 3 3 2 2m s kg m m

(( . . / )0 89 10 3 2× − N s m=0.36m/s

(UnitsofNarekg·m/s2.)


NowwehavetocalculateRe,andcheckwhetherourassumptionoflaminarflowwascorrect.

Re=Φv d

µt w p

w

ρ

=

× × × ×

×

−

−

1 0 36 997 0 6 10

0 89 10

33

3

. .

. · /

m

s

kg

mm

N s m 22

=241.96→transitionalflow

Ourassumptionwasnotcorrect.Therefore,wehavetouseequation(7.5)tocalculatevt.Useatrialanderrorprocedure,sincebothReandvtareunknown.

Trial#1

AssumeRe=241.96(calculatedfrompreviousstep).

CalculateCRe Re

d = + +24 30 34. =0.63

Calculate

vgd

Ct

p

d

p w

w

=−

=×

4

3

4 9

1

2ρ ρρ

.. .

.

81 0 6 10

3 0 63

2650 997

997

23m

sm× ×

×−

−

=

1

2

0 14. /m s

Trial#2

Withvt=0.14m/s,calculateRe,Cd,andvt.

Re=94.1→transitionalflow

Cd=0.90

vt=0.12m/s


Trial#3

Withvt=0.12m/s,calculateRe,Cdandvt.

Re=80.66→transitionalflow

Cd=0.97

vt=0.116m/s=0.12m/s

Terminalsettlingvelocityoftheparticleis0.12m/s.

7.3.2 Design of ideal sedimentation tank

An ideal sedimentation tank is designed to achieve complete removalofparticles with a specified settling velocity vo, such that all particleswitha terminal settling velocity greater than vo will be completely removed.Particleswithaterminalsettlingvelocitylessthanvowillbefractionallyremoved.EarlierworkbyHazen(1904)andCamp(1945)haveprovidedthebasisforsedimentationtheoryanddesignofsedimentationtanks.

Let us consider an ideal horizontal-flow, rectangular sedimentationtankasshowninFigure 7.1.Thelength,width,andheightofthetankareL,W,andH,respectively.ThewastewaterflowrateisQ.Theflowpathsoftwoparticles,P1andP2,areillustrated,alongwiththehorizontalandverticalcomponentsofvelocity.ParticleP1hasasettlingvelocityofvoandiscompletelyremovedintimetd.td isthetimetakenbyP1totravelthelengthofthetankandbedepositedinthesludgezoneasthewastewaterflowsoutthroughtheoutletzone.Thedesigndetention timeofthetankisthustd.

Inle

t Zon

e

Out

let Z

one

L

HP1

P2

V0Vs

VhVh

Flow path of particles

Sludge Zone

Figure 7.1 An ideal rectangular sedimentation tank (Source: Adapted from Metcalf & Eddy, 2003).


Thefollowingassumptionsaremade:

1.Thereisnosettlingofparticlesintheinletandoutletzones. 2.Particlessettleinthesludgezoneandarenotresuspended. 3.Plugflowconditionsexist.

Thehorizontalcomponentofvelocityvhisequaltotheflow-throughveloc-ityandisrelatedtotheflowrate(Q)andcross-sectionalarea(Ax)inthefollowingmanner:

vQ

A

Q

W Hh

x

= = (7.11)

Thedetentiontimetdisrelatedtotheflowrateandtankvolume(V)as:

tV

Q

LW H

Qd = = (7.12)

Also

vL

th

d

= (7.13)

Thedesignsettlingvelocityisgivenby

vH

to

d

= (7.14)

Allparticleswithasettlingvelocityvs>vowillberemoved100%.Particleswithasettlingvelocityvs<vowillberemovedintheratioofv vs o.Thesettlingvelocityof theparticlevscanbecalculatedusingequation (7.5).Actualwastewaterflowshavealargegradationofparticlesizes.Todeter-minetheremovalefficiencyforagivendetentiontime,settlingcolumntestscanbeperformedtodeterminetherangeofsettlingvelocitiesoftheparti-clesinthesystem.Settlingvelocitycurvesareconstructedwithcorrespond-ingremovalsandintegratedtodeterminetheoverallremovalefficiency.

Theratevoatwhichtheparticlessettleinthetankisequaltotherateatwhichclarifiedwaterflowsoutfromthetank.Thisrateisadesignparam-eterandiscalledthesurface overflow rate.Itisdefinedastheflowrateperunitsurfacearea(As)andisgivenby

vQ

A

Q

LWo

s

= = (7.15)


Inactualpractice,designfactorshavetobeadjustedtoaccountforeffectsof inlet and outlet turbulence, short-circuiting, and velocity gradientscausedbysludgescrapers.

EXAMPLE7.2A wastewater contains sand particles of three major sizes: 0.002mm, 0.6 mm, and 60 mm. A settling basin is designed to achieve100% removal of 0.6 mm diameter particles. Assume water tem-peratureis25°C.Howmuchremovalcanbeachievedfortheotherparticlesizes?

SOLUTION

Thesettlingvelocityof0.6mmdiameterparticle,vt=0.12m/s(calcu-latedinExample7.1)

Calculatethesettlingvelocitiesoftheothertwoparticlesizes.Assumelaminarflow,sphericalparticlesanduseStokesequation(7.10).Calculatesettlingvelocityof0.002mmparticles,v1

vg d

µ

p w p

w1

2

18=

−( )ρ ρ

=−( ) × −. / / ( . )9 81 2650 997 0 002 10

1

2 3 3 2 2m s kg m m

88 0 89 10 3 2× × −( . · / )N s m

=4×10–6m/s<vt

CheckRe:

Re=Φv d

µw p

w

1ρ

= × × × ×

×

−

−1 0 36 997 0 002 10

0 89 10

3 3

3

. / / .

.

m s kg m m

NN s m· / 2

=0.9<1,laminarflowassumptioniscorrect.

Therewillbefractionalremoval,v

vt

104 10

0 12100 0 005= × × =

−

.% . %

Calculatesettlingvelocityof60mmparticles,v2,usingStokesequation.

v

m s kg m m2

2 3 3 2 29 81 2650 997 60 10

18=

−( ) × −. / / ( )

×× × −( . · / )0 89 10 3 2N s m=5628.07m/s


CheckRe:

Re

. / /

.= × × × ×

×

−

−1 0 36 997 60 10

0 89 10

3 3

3

m s kg m m

N ·· /s m 2

=24,196.85>103,turbulentflow

UseCd=0.4.

Calculatev2usingequation(7.5).

vgd

Cp

d

p w

w2

1

24

3

4 9 8

=−

= ×

ρ ρρ

. 11 60 10

3 0 4

2650 997

997

2 3m s m/

.

× ××

−

−

1

2

=1.80m/s>vt,therefore100%removalof60mmparticleswillbeachieved.

7.4 TYPE II SEDIMENTATION

TypeIIsedimentationinvolvesthesettlingofflocculentparticles.Asfloc-culationoccurs,theshapeandmassoftheparticlesincrease,resultinginanincreaseinsettlingvelocities.Asettlingcolumntestcanbeusedtodeter-minethesettlingcharacteristicsandremovalefficiencyofflocculentpar-ticles.Asettlingcolumnwithsamplingportssituatedatregularintervalsofdepthisused,asillustratedinFigure 7.2(a).Theheightofthecolumnshouldbeequaltotheproposedtankdepth.Thetestdurationshouldbeequaltotheproposeddetentiontime.Settlingshouldtakeplaceunderqui-escent conditions. A suspension with solids concentration similar to thewastewaterisintroducedatthetopofthecolumn.Atregulartimeinter-vals,samplesarewithdrawnfromalltheportsandanalyzedforsuspendedsolidsconcentration.

Thepercentremovalatithtimeintervalforjthportisgivenby

RC

Cij

ij

o

= −

×1 100% (7.16)

where:Co=initialconcentrationofsuspension,mg/LCij=concentrationatithtimeintervalforsamplefromjthport,mg/L


Thepercentremovalsareplottedaspointsagainsttimeanddepth.Thencurvesof equalpercent removalor isoremoval lines aredrawn, as illus-trated inFigure 7.2(b).Theoverflowrate foraparticularcurve isdeter-minedbynotingthevaluewherethecurveintersectsthexaxis(MetcalfandEddy,2003).Theoverflowrateorsettlingvelocityvoisgivenby

vH

to

c

= (7.17)

where:H=heightofsettlingcolumn,mtc=timecorrespondingtopointofintersectionofanisoremovalline

withxaxis,min

Thefractionofparticlesremovedisgivenby

Rh

H

R R

n

n

n n n=

+

=

+∑1

1

2

∆ (7.18)

where:R=suspendedsolidsremoval,%n=numberofisoremovallines

50% 60% 70% 80% 90%

Time

Dep

th

∆h1

∆h2

∆h3

∆h4

(b)(a)

Figure 7.2 (a) Settling column, (b) isoremoval lines for settling column analysis.


Rn=%removalofisoremovallinenumbernRn+1=%removalofisoremovallinenumbern+1Δhn=distancebetweentwoisoremovallines,mH=heightofsettlingcolumn,m

Theslopeatanypointonanyisoremovallineistheinstantaneousvelocityofthefractionofparticlesrepresentedbythatline(Peavyetal.,1985).ItcanbeseenfromFigure 7.2(b)thatthevelocityincreaseswithincreasingdepth,sincetheslopeoftheisoremovallinebecomessteeper.Thisisduetothecollisionandflocculationoftheparticles,whichresultsinincreasedmassandincreasedsettlingvelocities.Thesettlingcolumntestenablesustoobtainvelocityandremovaldataatvariousdepthsofsettling.

7.5 PRIMARY SEDIMENTATION

Primary sedimentation tanksor clarifiers aredesigned toachieve50% to70%removalofsuspendedsolidsand25%to40%removalofBOD.TheBODremovedisassociatedwiththeorganicfractionofthesuspendedsolids.Rectangularorcirculartanksmaybeusedasprimaryclarifiers.Thetypeofclarifierselecteddependsonthesiteconditions,sizeoftheplant,localregula-tions,andengineeringjudgment.Twoormoretanksshouldbeprovidedsothatclarificationremainsinoperationwhileonetankistakenoff-lineforser-viceormaintenance.Atlargeplantsthenumberoftanksisdictatedlargelyby size limitations.Typical design information forprimary sedimentationtanksfollowedbysecondarytreatmentisprovidedinTable 7.1.

Table 7.1 Design criteria for primary sedimentation tanks

Parameter Range Typical value

Detention time, h 1.5–2.5 2.0Overflow rate, m3/m2 · d At average flow 32–50 40 At peak hourly flow 78–120 100Weir loading rate, m3/m · d 125–500 260Rectangular tank Length, m 15–90 25–40 Widtha, m 3–24 5–10 Depth, m 3–5 4.5Circular tank Diameter, m 3–60 12–40 Depth, m 3–5 4.5a For widths greater than 6 m, multiple bays with individual sludge

removal equipment may be used.


AsoutlinedinTable 7.1,the importantdesignparametersforprimaryclarifiersare(1)detentiontime,(2)overflowrate,and(3)weirloadingrate.Historically,thetanksaredesignedforaverageflowrateconditions.Peakflowratescanbe2to3timestheaveragerates.Forsmallcommunitiesorsystemswithcombinedsewers,peakratescanbe10to15timesthedesignaveragerates.Iftheobjectiveistomaximizetheprimaryclarifierefficiencyandreducetheloadondownstreambiologicalprocesses,thenthehydraulicdesignshouldaddressthepeakflow(Davis,2011).Thismaybedonebysizingtheclarifierforpeakflowsand/orbyusingequalization tanks.

Flowequalizationisamethodofdampingthevariationsinflowrates,sothattheunitprocessesreceivenearlyconstantflowrates(MetcalfandEddy,2003).Thisisusuallydonetoreducepeakflowsandloadsandtoequalizecombinedstormsewerandsanitarysewerflows,especiallydur-ingwetweatherflows.Equalizationtankscanbelocatedbeforeprimaryclarifiers.OnepossiblearrangementisillustratedinFigure 7.3,wheretheequalizationtankiskeptoff-lineuntiltheflowexceedsaspecifiedvalue,atwhichpointtheflowispassedthroughtheequalizationtank.Equalizationtankscanalsobeplacedin-linebeforetheprimaryclarifier,wheretheyareusedtoeliminatediurnalflowvariationsandtominimizeshockloadingstothebiologicaltreatmentprocess.

7.5.1 Rectangular sedimentation tank

Rectangularsedimentationtankshavewaterflowingthroughinahorizon-talmanner.ArectangularsedimentationtankisillustratedinFigure 7.4.Ataminimum,twotanksareplacedlongitudinallyinparallelwithacom-monwall.The inlet zoneorstructureisdesignedtodistributethewaterovertheentirecross-section.Thesettling zoneisusuallydesignedbasedonoverflowratesanddetentiontimes.Intheory,thebasindepthorsidewaterdepthisnotadesignparameter.However,clarifierswithmechanicalsludgeremovalequipmentareusually3to5mdeep.Thistakesintoaccountthe

Screens Gritchamber

Overflowstructure

Grit tolandfill

Screens tolandfill

Wastewater flowSludge flow

Wastewaterinfluent Primary

treatmentEffluent

Equalizationtank

Flow meter

Secondarytreatment

Sludge to�ickener

and digester

Sludge to�ickener

and digester

Figure 7.3 Equalization tank in a wastewater treatment plant (Source: Adapted from Metcalf and Eddy, 2003).


minimum depth required for sludge removal equipment, control of flowthroughvelocities,andpreventionofscouringofsettledparticles.Topro-videplugflowandminimizeshortcircuiting,aminimumlengthtowidthratio(L:W)of4:1isrecommended.ApreferredL:Wis6:1.Intheoutlet zone,collectionchannelscalledlaundersareplacedparalleltothetanklength.Clarifiedwaterflows into the launders/weirsandexits the tank throughoverflowweirs.Thewaterlevelinthetankiscontrolledbyoverflowweirs,whichmaybeV-notchweirsorbroad-crestedweirs.Theweir loading rateistheeffluentflowrateovertheweirdividedbytheweirlength.Optimumweirloadingratedependsonthedesignofpriorandsubsequentprocesses.Itcanrangefrom125to500m3/m·d,withtypicalvaluesaround250m3/dpermeterofweirlength(MetcalfandEddy,2003;Peavyetal.,1985).Inthesludge zone,thebottomofthetankisslopedtowardasludgehopperforsolidscollection.Solidscollectionisaccomplishedbychainandflightcollectors,bridgecollectors,orcrosscollectors.Scumisusuallycollectedandremovedfromthewatersurfaceattheeffluentend.

Settling andsludge zone

Inletzone

Outletzone

(a) E�uent weir

Inle

t zon

e

Out

let z

one

Settling andsludge zone

E�uent weir

Sludge out

(b)

Figure 7.4 Rectangular sedimentation tank (a) plan, (b) elevation.


7.5.2 Circular sedimentation tank

A radial flow pattern occurs in circular sedimentation tanks. Circularclarifiersmaybecenter feedorperipheral feed.Acenter feedclarifier isillustrated inFigure 7.5. In the center feed tank,water enters a circularwellatthecenter,whichisdesignedtodistributethewaterequallyinalldirections. Ithasanenergydissipating inletwithin the feedwell. In theperipheralfeedtank,asuspendedcircularbaffleformsanannularspaceinwhichtheinfluentwastewaterisdischargedinatangentialdirection.Thewaterflowsspirallyaroundthetank,underthebaffle,andclarifiedwateriscollectedinacentrally locatedweirtrough(MetcalfandEddy,2003).Circular tanks can range from 3.6 to 10.5 m (12 to 35 ft) in diameterorlarger,dependingontheflowrateandsitespecifications.Figure 7.6(a)showsacircularclarifierand(b)showswaterflowingintotheeffluentlaun-deroverV-notchweirs.

EXAMPLE7.3Awastewatertreatmentplantusesrectangularsedimentationtanksforprimaryclarification.Theaveragedesignflowis14,000m3/d,withapeakingfactorof2.5.Twotanksareused.Thelength,width,anddepthare24m,7m,and4m,respectively.Singleeffluentweirsareprovidedattheoutletzone.Calculatethesurfaceoverflowrate,detentiontime,andweirloadingratesforthedesignflow.Whathappensatpeakflowconditions?Stateregulationsspecifyaminimumdetentiontimeof1h.

SOLUTION

Step1.Determineparametersforaveragedesignflowconditions,andcomparetovaluesprovidedinTable 7.1.

Flowineachtank,Q=14 000

2

, =7000m3/d.

Surfaceoverflowrate,vo=Q

LW

m d

m m=

×7000

24 7

3 /

=41.67m3/m2·d→

withinrange.

Walkway

Sludge scraper

SkimmerE�uentover�owweir

Flow Flow

In�uentSludge

Drive motor

In�uent well

Figure 7.5 Circular sedimentation tank (center feed).


(a)

(b)

Figure 7.6 (a) Circular clarifier, (b) water flowing over V-notch weirs into the effluent launder (photos by Rumana Riffat).


Detentiontime,td= LW H

Q

m m m

m d= × ×24 7 4

7000 3 / =0.096d=2.30h

→withinrange.

Weirlength=W=7m.

Weir loading rate =Q m d

mweirlength= /7000

7

3

= 1000 m3/m·d →

veryhigh.Asecondsetofweirmaybeaddedtoreducetheweirloadingrate

to500m3/m·d.

Step2.Determineparametersforpeakflowconditions,andcomparetovaluesprovidedinTable 7.1.

Peakflow=14,000×2.5=35,000m3/d.

Flowineachtank,Q=35 0002

, =17,500m3/d.

Surfaceoverflowrate,vo= Q

LW

m d

m m=

×, /17 500

24 7

3

=104.17m3/m2·d

→withinrange.

Detentiontime,td=LW H

Q

m m m

m d= × ×24 7 4

17 500 3, / =0.038d=0.92h→

slightlylessthanthespecified1hminimum.

Totalweirlengthfromaverageflowconditions=2W=14m.

Weirloadingrate= Q m d

mweirlength= , /17 500

14

3

=1250m3/m·d→

veryhigh.

Oneoption is toaddanothereffluentweir toreduce theweir load-ingrate.Anotheroptionis toaddanequalizationtanktostoretheadditional flow during peak flow periods. This would increase thedetentiontimeintheprimaryclarifier,aswellasreducetheweirload-ingrate.

EXAMPLE7.4Youhavebeenassignedtodesignprimaryclarifiersforawastewatertreatment plant. The average flow rate of the wastewater is 32,000m3/daywithaBOD5of220mg/Landsuspendedsolidsconcentrationof300mg/L.Thegoalistoremove30%BOD5and60%suspendedsolidsinprimarytreatment.Determinethefollowing:

a.Thediameteroftheprimaryclarifierforasurfaceoverflowrateof40m3/m2-day.

b.Thedetentiontimeintheprimaryclarifierandthemassofsolidsremovedinkg/day.

Assumeadepthof3.5m.


SOLUTION

Step1.Use2circularclarifiers.

Flowineachclarifier,Q=32 0002

, =16,000m3/d.

Surfaceareaofeachclarifier=As,withdiameterD.

Surfaceoverflowrate,vo= QA

Q

Ds

=π4

2

.

Therefore

As=16 00040

3, /

/

m d

m d =400m2.

As= π4

2D

Therefore

D=4 400 2×

m

π=22.56m=23m.

As= π4232=415.48m2

Step2.Detentiontime,td=A H

Q

m m

m ds = ×415 48 3 5

16 000

2

3

. .

, /=0.091d=2.18h.

Solidsin=300mg/L.

Solidsremoved=60%.

Massofsolidsremovedinprimary=flow×concentration

=32,000m3/d×300mg/L×0.60×103L/m3×10–6kg/mg

=5760kg/d

Note:Solidseffluent=300mg/L×0.40=120mg/Lgoingtosecondarytreatment.

Massofsolidstosecondary=32,000m3/d×120mg/L×103L/m3×10–6kg/mg

=3840kg/d

BOD5ineffluent=220mg/L×(1–0.30)=154mg/Lgoingtosecond-arytreatment.

MassofBOD5goingtosecondary=32,000m3/d×154mg/L×103L/m3×10–6kg/mg

=4928kg/d


7.6 CHEMICALLY ENHANCED PRIMARY TREATMENT

CEPT (chemically enhanced primary treatment) refers to a process thatuseschemicalsforcoagulation,flocculation,andprecipitationofparticu-late/dissolved solids in thewastewaterasaprimary step in clarification.AlthoughCEPTwasfirstusedaround1840inFrance,itsuseintheUnitedStatesstartedinthe1960s(Pericetal.,2008).Anumberofdifferentchemi-calsweredeveloped,tested,andused.Asinglechemicaloracombinationofchemicalscanbeused.Inrecentyears,CEPThasbeenusedatvariouswastewatertreatmentplantsforphosphateremoval,clarificationofwaste-water, reduction in sludgevolume,and increase in surfaceoverflowrate(SOR).Increasingtheefficiencyofprimarytreatmenthasdualbenefits:(a)Itreducestheloadfordownstreamprocesses,and(b)itenhancestherateofsecondarytreatment,becausesmaller,easilybiodegradableparticlesareavailableafterprimarytreatment(Odegaard,1998).Theselectionofchem-icalsforCEPTdependsontheprimaryobjectiveofusingthem.Thedoseofchemicalcoagulantandmethodofdosinghavetobeoptimizedforbetterclarification.Chemicalcoagulantssuchasferricchlorideareused,togetherwithpolymers,asflocculatingagents.Combinedflocculator–clarifierscanbeusedforthisprocess.

PerformanceofCEPTdependstoagreatextentoninfluentcharacter-isticsofwastewater.Influentcharacteristics includeTSS(totalsuspendedsolids), turbidity, BOD (biochemical oxygen demand), COD (chemicaloxygen demand), particle size distribution, septicity, etc. Characterizingincoming wastewater can provide a vast array of benefits, e.g. feedbackforchemicaldosing,analysisandpreventionofoperationalinefficiencies,establishingthetrendsofseasonalvariations,providingthebenchmarkonoperationalperformanceoftheplantitself,andprovidingparametersforcomparisonofqualityofwastewaterwiththatofotherplantsintheregionorcountry.Itisnecessarytostudysourcecharacteristicsofthewastewatertodesignanoptimizedsettlingenvironment.InfluentcharacteristicssuchasTSS, turbidity, and totalCODwere found tohave significant impactonCEPTinanexperimentalstudyconductedbyNeupaneatal. (2008).Rapidmixingtimesdidnotimpactperformance,butincreasedflocculationtimeimprovedperformance.Aminimumflocculationtimeof10minwasrequired foroptimizedCEPTperformance, asobservedbyParker et al.(2000)andNeupaneetal.(2008).

PROBLEMS

7.1 DefineTypeIandTypeIIsettling.Whatarethedifferencesbetweenthesetwotypesofsettling?Wherearetheyobserved?

7.2 Whatistheobjectiveofusinganequalizationtank?Whereisitused?


7.3 A1cmdiameterplasticspherefallsinaviscousliquidatatermi-nalvelocityof1cm/sec.Whatwouldbetheterminalvelocityofa10cmdiametersphere(ofthesameplasticmaterial)inthesamefluid?Clearly state anyassumptions that youmake toarrive atyouranswer.

7.4 Thesettlingbasinforatype-1suspensionistooperateatanover-flowrateof0.76m3/m2·h.Theflowratethroughtheplantis24,000m3/day. Determine the dimensions for a long rectangular basin,usingalengthtowidthratioof4:1.Depthshouldnotexceed4m.Usemorethanonetank.Determinethedetentiontimeinthetankandthehorizontalvelocity.

7.5 Arectangularsedimentationtankisdesignedwithadepthof3.5mandadetentiontimeof1h.Isthedesignsufficienttoachievecom-pleteremovalofparticleswithadiameterof0.01mm?Specificgrav-ityoftheparticlesis2.65.Theflowrateandtemperatureofwaterare10,000m3/dand20°C,respectively.At20°C,densityofwateris998kg/m3anddynamicviscosityofwateris10–3kg/m·s.Stateanyassumptionsyoumaketocalculateyourresults.

7.6 Anindustrialprocesswastewatercontainsamixtureofmetalfrag-mentsandsand.Themetalfragmentsrangeindiameterfrom0.5to10mmwithaspecificgravityof1.65.Thesandparticlesrangeindiameterfrom0.04to2.0mmwithaspecificgravityof2.65.Thewastewaterdischargerateis1400m3/dat20°C.Designasettlingtanktoremoveallthemetalandsandparticles.Ifthedepthofthetankis2m,calculatethedetentiontime.

7.7 Aprimaryclarifierremoves35%ofBOD5and55%ofsuspendedsolidsfromtheincomingwastewater.CalculatethemassofsolidsandmassofBOD5removedinkg/dforaplantprocessing4500m3/dofwastewaterwith275mg/LBOD5and400mg/LSS.

7.8 WhatisCEPT?WhataretheadvantagesofusingCEPT? 7.9 Whattypesofsettlingcanbeobservedinawastewatertreatment

plant?Drawaflowdiagramofaconventionaltreatmentplanttreat-ingmunicipalwastewater,andlabeltheunitprocessestogetherwiththetypeofsettlingobserved.

7.10 A wastewater treatment plant has four primary clarifiers, eachwithdiameterof15mand sidewaterdepthof4m.Theaver-age daily flow is 24,000 m3/d. The effluent weir is located ontheperipheryofeachtank.Calculatethesurfaceoverflowrate,detentiontime,andweirloadingrates.Ifonetankistakenoutofserviceformaintenance,whathappenstotheoverflowrateanddetentiontime?Isthedesignadequatefortheaverageflowwiththethreetanksinservice?

7.11 Awastewatertreatmentplanthasadesignaverageflowof15,000m3/d.Theengineerwishestouserectangularclarifiersforprimary


treatment.Design therectangular tanks foramaximumoverflowrateof42m3/m2·dandaminimumdetentiontimeof2h.Whichcriteriagovernthedesign?

7.12 AmoderateamountoftotalBODcanberemovedinaprimaryclari-fierthroughsettling.TherelationshipbetweenpercentremovaloftotalBODandthesurfaceoverflowrateinacircularclarifiercanbedescribedbythefollowingstraight-lineequation:

y=42–0.255x

whereyisthepercentremovaloftotalBODandxistheoverflowrateinm/d.Theaveragewastewaterflowrateis5000m3/d.Calculatethediameterof theclarifier thatwouldresult inremovalofone-thirdofthetotalBOD.Also,calculatethedetentiontimeintheprimaryclarifierwhenthesidewalldepthis3.6m.

REFERENCES

Camp,T.R.(1945)“SedimentationandtheDesignofSettlingTanks.”Transactions of the American Society of Civil Engineers,vol.111,pp.895–936.

Davis,M.(2011)Water and Wastewater Engineering: Design Principles and Practice.McGraw-Hill,Inc.,NewYork.

Droste,R.L.(1997)Theory and Practice of Water and Wastewater Treatment.JohnWiley&Sons,Inc.,NewJersey.

Hazen,A,(1904)“OnSedimentation.”Transactions of the American Society of Civil Engineers,vol.53,pp.46–48.


Neupane,D.,Riffat,R.,Murthy,S.,Peric,M.,andWilson,T.(2008)“InfluenceofSourceCharacteristics,ChemicalsandFlocculationonChemicallyEnhancedPrimaryTreatment.”Water Environment Research,vol.80,no.4,pp.331–338.

Odegaard, H. (1998) “Optimized Particle Separation in the Primary Step ofWastewaterTreatment.”Wat. Sci. Tech.vol.37,p.43.

Parker,D.,Esquer,M.,Hetherington,M.,Malik,A.,Robinson,D.,Wahlberg,E.,andWang,J.(2000)“AssessmentandOptimizationofaChemicallyEnhancedPrimaryTreatmentSystem.”Proc. 73rd Annual Conference and Exposition of Water Environment Federation,WEFTEC,Anaheim,California.

Peric,M.,Riffat,R.,Murthy,S.,Neupane,D.,andCassel,A.(2008)“DevelopmentofaLaboratoryClarifierTesttoPredictFull-ScalePrimaryClarifierPerformance.”International Journal of Environmental Research,vol.2,no.2,pp.103–110.


119

Chapter 8

Secondary treatmentSuspended growth processes

8.1 INTRODUCTION

Secondary treatment usually consists of biological treatment of primaryeffluentwastewater.Theobjectivesofsecondarytreatmentaretoreducethebiochemicaloxygendemand(BOD)andsuspendedsolidsoftheeffluenttoacceptablelevels.Insomecases,nutrientremovalmayalsobeanobjective.Dependingondischargelimits,thesecondaryeffluentmaybedischargedtosurfacewatersafterdisinfection,ormayproceedtotertiarytreatment.Twomajorcategoriesofbiologicaltreatmentprocessesare(1)suspendedgrowth and (2) attached growth processes. In this chapter, aerobic sus-pendedgrowthprocessesforBODremovalwillbedescribedindetail,withmajoremphasisontheactivatedsludgeprocess.Membranebiologicalreac-tors(MBRs)andland-basedsystems,suchaspondsandlagoons,arepre-sentedtowardthelatterpartofthischapter.Attachedgrowthprocessesarediscussed inChapter9.Biologicalprocessesusedfornitrogenandphos-phorusremovalarepresentedindetailinChapter13.

Inasuspendedgrowthprocess,themicroorganismsarekeptinsuspen-sioninabiologicalreactorbyusingasuitablemixingtechnique.Themicro-organismsusetheorganicmatterasfoodandconvertittonewbiologicalcells, energy, and waste matter. Municipal wastewater contains a widevarietyoforganics,consistingofproteins,fats,andcarbohydrates,amongothers.Asaresult,avarietyoforganismsoramixedcultureisrequiredforcompletetreatment.Eachtypeoforganisminthemixedcultureusesthefoodthatismostsuitabletoitsmetabolism(Peavyetal.,1985).Thelargerspecies,inturn,feedonthesmallerspecies.Forexample,therotifersandcrustaceansfeedontheprotozoa,theprotozoafeedonthebacteria,andsoon.Themicroorganismsusedinthebiologicaltreatmentprocessesareessentiallythesameasthosefoundinsurfacewaters,performingdeg-radationoforganicmatterduringnaturalpurificationprocesses.Naturalpurificationprocesses takeplaceover an extendedperiodof time, rang-ingfromdaystoweeks,dependingonthestrengthofthewastewaterandavailabilityofasuitablemicrobialpopulation,asdescribedinChapter4.


The role of the engineeristodesignbiologicaltreatmentprocessesusingthesamebasicprinciplesbutprovidingsuitableenvironmentalconditionsandprocessparametersthatenhancereactionrates,suchthatpurificationtakesplacewithinashortperiodoftime,e.g.inamatterofhours.Athor-ough understanding of microbial growth kinetics, substrate utilization,principlesofmassbalance,reactorkinetics,andoperationalparametersisnecessaryfordesignofbiologicaltreatmentprocesses.Thesearediscussedinmoredetailinthefollowingsections.

8.2 MICROBIAL GROWTH KINETICS

Therateofmicrobialgrowthandrateofsubstrateutilizationareamongthefundamentalkineticparametersofbiologicaltreatmentprocesses.Abatchexperimentcanbeconductedwithaspecificamountoffoodorsubstrate(S)inalaboratoryreactorinoculatedwithamixedcultureofmicroorgan-isms(X).TherateofbiomassgrowthdX/dt,andthecorrespondingrateofsubstrateutilizationovertimedS/dt,canberepresentedbythecurvesshowninFigure 8.1.Themicrobialgrowthcurvehasfourdistinctphases.ThesephaseshavebeendescribedpreviouslyinChapter3.

8.2.1 Biomass yield

From Figure 8.1 we can see that the rate of biomass growth increaseswithacorrespondingdecreaseintherateofsubstrateutilization.Ifallthe

Time, days

Conc

entr

atio

n, m

g/L

Lagphase

Exponentialphase

Stationaryphase

Deathphase

Food

Biomass

Figure 8.1 Relationship between microbial growth and substrate utilization.

Secondary treatment 121

substratewasconvertedtobiomass, thentherateofbiomassproductionwouldequaltherateofsubstrateutilization.Butpartofthefoodiscon-vertedtoenergyandwasteproducts,aswellasnewcells.Forthisreason,thiscanbeexpressedas:

dX

dt

dS

dt∝ − (8.1)

or

dX

dtY

dS

dt= −

(8.2)

or

rg=–Y(rsu) (8.3)

where:

dX

dt=rg=growthrateofbiomass,mg/L·d

dS

dt=rsu=rateofsubstrateutilization,mg/L·d

Y=biomassyieldor

Y=m gbiom assproduced

m gsubstrateutilized (8.4)

Thebiomassinareactorisusuallymeasuredintermsofconcentrationoftotalsuspendedsolids(TSS)orvolatilesuspendedsolids(VSS).Thesub-strateconcentrationcanbemeasuredintermsofBODorCOD.Therefore,

Y = m gVSSproduced

m gBO D rem oved or Y=

m gTSSproduced

m gCO D rem oved

TheyieldcoefficientYdependsonthemetabolicpathwayusedinthedeg-radationprocess.Aerobicprocesseshaveahigheryieldofbiomasscom-paredwithanaerobicprocesses.TypicalvaluesofYforaerobicprocessesrangefrom0.4to0.8kgVSS/kgBOD5,whiletheyrangefrom0.08to0.2kgVSS/kgBOD5foranaerobicprocesses.


8.2.2 Logarithmic growth phase

Inthelogarithmicgrowthphase,wecanusuallyassumefirstorderkineticstoobtainthefollowingrateexpression:

rdX

dtXg = = µ (8.5)

where:

dX

dt=growthrateofbiomass,mg/L·d

µ=specificgrowthrate,d–1

X=concentrationofbiomass,mg/L

8.2.3 Monod model

Anumberofmodelshavebeendevelopedtomodelthemicrobialgrowthinbiologicalreactors.OneoftheearliestmodelswastheMonod model,whichhasservedasthebasisfordevelopmentofnumerousmodelsthatareinusetoday.TheMonodmodelassumesthattherateofsubstrateutiliza-tion,andthereforetherateofbiomassproduction,islimitedbytherateofenzymaticreactionsinvolvingthelimitingsubstrate.TheMonodequationformicrobialgrowth(Monod,1949)isgivenby

µ µ= ⋅+m ax

S

K SS (8.6)

where:µmax=maximumspecificgrowthrateconstant,d–1

S=substrateconcentration,mg/LKS=halfsaturationcoefficient,mg/L

KSisthesubstrateconcentrationcorrespondingtogrowthrateµ=1/2µmax.Figure 8.2isagraphicalrepresentationoftheMonodequation,whichillus-tratesthatthegrowthrateofbiomassisahyperbolicfunctionofthesub-strateconcentration.

BasedontheMonodequationandFigure 8.2,athighsubstrateconcen-tration,thesystemisconsideredtobeenzymelimited(S>>Ks).Inthiscase,thegrowthrateisapproximatelyequaltothemaximumgrowthrate,andequation(8.6)becomes

µ≈µmax (8.7)


Another situation arises at low substrate concentrations, when the sub-strateislimiting(S<<Ks).Equation(8.6)canthenbewrittenas

µ µ= =m ax

S

S

KK′S (8.8)

whereµm ax

SK=K′.

Thegrowthrateofbiomassbecomesindependentoftheconcentrationofbiomasspresent.Thespecificgrowthratebecomesfirstorderwithrespecttosubstrateconcentration,asshowninequation(8.8)andrepresentedbytheinitialstraightlinesegmentofthecurveinFigure 8.2.

8.2.4 Biomass growth and substrate utilization

Combiningequations(8.5)and(8.6),wecanobtainanexpressionfortherateofbiomassproductionas

rdX

dt

SX

K Sg

max

S

= =+

µ (8.9)

Combiningequations(8.3)and(8.9),wecanwritethefollowingexpressionfortherateofsubstrateutilization,

rr

Y

SX

Y K Ssu

g

S

= − = −+

µm ax

( ) (8.10)

or

Limiting Food Concentration S, mg/L

Gro

wth

rate

μ, 1

/t

Ks

μmax

μmax

2

Figure 8.2 Graphical representation of the Monod model.


dS

dt

SX

Y K SS

= −+

µm ax

( ) (8.11)

or

rkSX

K Ssu

S

= −+

(8.12)

wherekYmax= µ

andisdefinedasthemaximumrateofsubstrateutilization

perunitmassofmicroorganisms.

8.2.5 Other rate expressions for rsu

Dependingonthesubstrateandspecificmicroorganismsinvolved,anum-ber of rate expressions have been used to describe substrate utilizationrates, inadditionto thesubstrate limitedrelationshippresented inequa-tion(8.11).Thesearebasedonexperimentalresultsobservedbyvariousresearchers.Someofthecommonlyusedrateexpressionsare

rsu=–k (8.13)

rsu=–kS (8.14)

rsu=–kSX (8.15)

wherek=substrateutilizationratecoefficient,time–1.

8.2.6 Endogenous metabolism

Inthedeathanddecayphaseofthemicrobialgrowthcurve,someendog-enousmetabolismtakesplace.Itisassumedthatthedecreaseinbiomasscaused by death and predation is proportional to the concentration ofmicroorganismspresent.Theendogenousdecayisthusassumedtobefirstorderandcanbewrittenas:

dX

dtend r k Xd d

= = − (8.16)

where:rd=rateofdecay,mg/L·dkd=endogenousdecaycoefficient,d–1


8.2.7 Net rate of growth

Combiningequations(8.9)and(8.16),wecanobtainanexpressionforthenetrateofgrowth:

rg(net)=rg+rd

dX

dtnet

dX

dt

dX

dtend

=

+

dX

dtnet

SX

K Sk X

Sd

=+

−m axµ (8.17)

Thenetbiomassyieldcanbeexpressedas

Ynet= −r

rg net

su

( ) (8.18)

Thenetbiomassyieldisusedasanestimateoftheamountofactivemicro-organismsinthesystem.

Table 8.1presentstypicalvaluesofthekineticcoefficientsfortheacti-vatedsludgeprocesstreatingdomesticwastewater.

8.2.8 Rate of oxygen uptake

Therateofoxygenuptakeisstoichiometricallyrelatedtotherateofutiliza-tionoforganicmatterandthebiomassgrowthrate.BasedontheformulaC5H7O2Nforbiomass,theoxygenequivalentofbiomass(measuredasVSS)isapproximately1.42gCODutilized/gVSSproduced(MetcalfandEddy,2003).Therefore,theoxygenuptakeratecanbeexpressedas:

ro=rsu=–1.42rg (8.19)

Table 8.1 Kinetic coefficients for activated sludge process treating municipal wastewater

Kinetic coefficient Range Typical value

µmax, mg COD/mg VSS · d 1–10 5Ks, mg/L COD 12–60 38kd, mg VSS/mg VSS · d 0.05–0.15 0.10Y, mg VSS/mg COD 0.25–0.60 0.40

Source: Adapted from Metcalf and Eddy (2003).


where:ro=rateofoxygenuptake,gO2/m3·drsu=rateofsubstrateutilization,gCOD/m3·d1.42=CODofcelltissue,gCOD/gVSSrg=rateofbiomassgrowth,gVSS/m3·d

8.2.9 Effect of temperature

Temperaturehasasignificanteffectonbiologicalreactions.Temperatureinfluencesthemetabolicactivitiesofthemicrobialpopulation,aswellasgas transfer rates and settling characteristics of the biomass. The van’tHoff–Arrheniusmodelcanbeusedtodescribetheeffectoftemperatureonreactionratecoefficientsasshownbelow:

kT=k20θ(T–20) (8.20)

where:kT=reactionratecoefficientattemperatureT°Ck20=reactionratecoefficientat20°Cθ=temperatureactivitycoefficientT=temperature,°C

Valuesofθrangefrom1.02to1.25dependingonthetypeofsubstrateandbiologicalprocess(MetcalfandEddy,2003).

8.3 ACTIVATED SLUDGE PROCESS (FOR BOD REMOVAL)

The most widely used suspended growth process is the activated sludgeprocess. It is used for biological treatment of municipal and industrialwastewaters. The process concept dates back to the work of Dr. AngusSmith in the early 1880s, who investigated the aeration of wastewatertanks toacceleratebiologicaloxidation. In1912and1913, experimentswere conducted by Clark and Adams with aerated wastewater to growmicroorganisms in bottles and tanks, at Lawrence Experiment Station(ClarkandAdams,1914).TheseresultswerethemotivationforadditionalresearchcarriedoutatManchesterSewageWorksinEnglandbyArdernandLockett(1914a,b).Theydevelopedtheprocessandnameditactivated sludge,becauseitinvolvedtheproductionofanactivatedmassofmicro-organismscapableofaerobicstabilizationoforganicmatterinwastewater(MetcalfandEddy,2003).

Thebasicactivatedsludgeprocessconsistsofthreecomponents,asillus-tratedinFigure 8.3:(1)abiologicalreactorwherethemicroorganismsare


keptinsuspensionandaerated,(2)asedimentationtankorclarifier,and(3)arecyclesystemforreturningsettledsolidsfromclarifiertothereactor.Wastewaterflowscontinuouslyintotheaerationtankorbiologicalreactor.Airisintroducedtomixthewastewaterwiththemicroorganisms,andtoprovidetheoxygennecessarytomaintainaerobicconditions.Themicroor-ganismsdegradetheorganicmatterinwastewater,andconvertthemtocellmassandwasteproducts.Themixturethengoestothesecondaryclarifier,whereclarificationofeffluentandthickeningofsettledsolidstakesplace.Theclarifiedeffluentisdischargedforfurthertreatmentordisposal.Thethickenedsolidsareremovedasunderflow.Aportionoftheunderflowiswasted(calledwaste activated sludge,WAS),whiletheremainder(20%to50%)isreturnedtotheaerationtankasreturn activated sludge(RAS).Thereturnsludgehelpstomaintainahighconcentrationofactive biomassintheaerationtank.

Alargenumberofvariationsoftheactivatedsludgeprocesshavebeendevelopedandarecurrentlyinuse.Descriptionsoftheseprocessesarepro-videdlaterinthischapter.Thebiologicalreactormaybeoperatedascom-pletelymixed (continuous-flowstirred tank reactor,CSTR)orplugflowreactor.Inrecenttimes,activatedsludgeprocessesareusedmorefrequentlyforBOD removal in conjunction with removal of nitrogen and/or phos-phorus. These are discussed in Chapter 13. A large body of knowledgeexistsbasedonpastandpresentresearchonthemicrobialcommunities,operationalparameters,processmodels,andremovalcapabilitiesofvari-ouspollutantsintheactivatedsludgeprocess(Jahanetal.,2011;Schmitet al.,2010;Plószetal.,2010;Jonesetal.,2009;Maetal.,2009;Riegeret al.,2010;amongothers).

8.3.1 Design and operational parameters

The following are definitions of basic design parameters for biologicaltreatmentreactors:

Aerationtank

Secondaryclari�er

Primary e uentSecondary

e uent

Sludgereturn

Sludgeunderow

Sludgewaste

Figure 8.3 Activated sludge process.


MLSS—mixed liquor suspended solidsconcentration inthebiologicalreactor.Itismeasuredasthevolatilesuspendedsolids(VSS)ortotalsus-pendedsolids(TSS)concentrationinthereactor,expressedasmg/Lorkg/m3.MLVSS(mixedliquorvolatilesuspendedsolids)representstheactivebiomassconcentrationinthereactor.Theconcentrationofactivebiomassplus the inert solids is calledMLSS.Usually the termMLSS isused forboth,withtheunitsofmeasurement(VSSorTSS)indicatingthedifference.

SRT—solids retention timeofthereactor.Itisalsocalledsludge ageormean cell residence time. Itistheamountoftimespentbyaunitmassofactivatedsludgeinthereactor.Itisdefinedastheratioofthemassofsolidsinthereactortothemassofsolidswastedperday.Itisgivenbytheequa-tionbelow:

θc = m assofsolidsin reactor

m assofsolidswwasted perday (8.21)

FortheactivatedsludgeprocessillustratedinFigure 8.3,θcisgivenby

θcw w e e

VX

Q X Q X=

+ (8.22)

where:θc=solidsretentiontime,dX=MLSSconcentrationinreactor,mg/LXw,Xe=biomass concentration in waste sludge and effluent, respec-

tively,mg/LQw=rateofsludgewastage,m3/dQe=effluentflowrate,m3/d

SRT is themost importantdesignandoperatingparameter,as itaffectsprocessperformance,aerationtankvolume,sludgeproduction,andoxy-genrequirements.ForBODremoval,anSRTof3dmaybeusedattem-peraturesrangingfrom18°Cto25°C.At10°C,SRTvaluesof5to6darerequired(MetcalfandEddy,2003).

F/M ratio—theratiooffoodtomicroorganismsinthereactor.Itiscal-culatedasthemassofBODremovedinthereactor,dividedbythemassofmicroorganismsinthereactor.Itisexpressedas

F

M

Q S S

VXo= −( )

(8.23)

where:


F/M=foodtomicroorganismratio,mgBOD/mgVSS·dSo=BOD5concentrationofsubstrateenteringthereactor,mg/LS=BOD5concentrationofsubstrateleavingthereactor,mg/L

TheF/Mratioisanimportantdesignvariablethatdictatesthephaseofoperationonthemicrobialgrowthcurve.AlowF/Mratioofabout0.05indicatesoperationinthedecayphase,whileahighF/Mratioaround1.0andaboveindicatesloggrowthphase.Conventionalactivatedsludgepro-cessesareoperatedatF/Mratiosfrom0.2to0.4.Thisindicatesoperationtowardtheendofthestationaryphaseandcorrespondstoalowsubstrateconcentration. This is desired when the aeration tank is operated as aCSTR,sinceconcentrationinthereactorwillbethesameasconcentrationintheeffluent.TheseareillustratedinFigure 8.4.

TheuseofSRTandF/Mratio indesignallows for trade-offbetweenreactorvolumeandMLSSconcentrationinthereactor.

Volumetric loading rate—themassofsubstrateorfoodappliedperunitvolumeofreactor.Itisgivenby

VL=Q S

Vo (8.24)

where:VL=volumetricloadingrate,kgBOD5/m3

So=substrateconcentrationenteringthereactor,kgBOD5/m3

Q=flowrateenteringthereactor,m3/dV=reactorvolume,m3

Time

Log

# of

Mic

roor

gani

sms

Lagphase

Exponentialphase

Stationaryphase

Deathphase

F/M ≈ 1.0 F/M ≈ 0.2to 0.4

F/M ≈ 0.05

Figure 8.4 F/M ratios corresponding to various microbial growth phases.


HRT—thehydraulic retention timeofthereactor.Itisthetimespentbyafluidparticleinthereactor,beforeitisdischarged.TheHRTisexpressedas

θ = V

Q (8.25)

where:θ=hydraulicretentiontime,dV=volumeofreactor,m3

Q=volumetricflowrate,m3/d

TheHRTinconventionalactivatedsludgereactorsrangesfrom3to8d.Itcanbereducedinhighrateprocesses.

EXAMPLE8.1Anactivatedsludgeprocessisusedtotreatawastewaterwithaflowrateof1800m3/dandBOD5concentrationof300mg/L.Theaerationtank is operated at anMLSSof 2500mg/L, andHRTof 7h.Thesludgeiswastedat34m3/dwithasolidsconcentrationof9000mg/l.The effluent BOD5 concentration is 25 mg/L. Calculate the volumeofaerationtank,SRT,volumetricloadingrate,andF/Mratiooftheprocess.

SOLUTION

Step1.Calculateaerationtankvolume.

HRT,θ=7h=0.29d

Usingequation(8.25),

θ = V

Q

or

0 291800 3

./

dV

m d=

or

V=525m3

Step2.CalculateSRT.


Assumesolidsintheeffluentisnegligible,andusingequation(8.21),

θc = m assofsolidsin reactor

m assofsolidswwasted perday= VX

Q Xw w

or

θc

m m g L

m

dm g L

= ×

×

525 2500

34 9000

3

3

/

/

or

θc=4.29d

Step3.CalculateF/Mratiousingequation(8.23).

F

M

Q S S

VXo= −( )

or

F

M

m

dm g L

m m g L=

−( )×

1800 300 25

525 2500

3

3

/

/

or

F

Md= 0 38 1. –

Step4.Calculatevolumetricloadingrateusingequation(8.24).

VL= Q SV

o

or

VL=1800 0 3

525

33

3

m

dkg m

m

× . /

or

VL=1.03kgBOD5/m3·d

8.3.2 Factors affecting microbial growth

Anin-depthknowledgeofthefactorsthataffectthegrowthofthemixedpopulationofmicroorganismsisimportantforefficientreactoroperation.


Aerobicheterotrophicbacteria arepredominant. Protozoaare alsopres-ent,whichconsumebacteriaandcolloidalparticles.TheimportantfactorsincludepH,temperature,alkalinity, typeofsubstrateandconcentration,presence of toxins, and dissolved oxygen concentration, among others.SomeofthesefactorshavebeendiscussedindetailinChapter3.

8.3.3 Stoichiometry of aerobic oxidation

Thefollowinggeneralequationsgiveasimplifieddescriptionoftheoxida-tionprocess(Davis,2011).AssumeCHONSrepresentsorganicmatterandC5H7O2Nrepresentsnewcells.

Thesynthesisreactionisgivenby

bacteria CHONS+O2+nutrients __________▶ C5H7O2N+CO2+NH3 +otherproducts (8.26)

Theendogenousrespirationisgivenby

bacteria C5H7O2N+5O2

__________▶ 5CO2+NH3+2H2O +energy (8.27)

8.4 MODELING SUSPENDED GROWTH PROCESSES

Inthissection,theprinciplesofmassbalancewillbeusedtogetherwiththekineticrelationshipsdescribedpreviouslytodevelopdesignequationsforsuspendedgrowthprocesses.Theexamplesaregivenforactivatedsludgereactors,buttheprinciplesareapplicabletoanysuspendedgrowthprocess.The mass balances for each specific constituent, e.g. substrate, biomass,willbeconductedacrossadefinedvolumeof thesystem.Thedevelopedmodelswillthenbeusedforpredictionofeffluentbiomassandsubstrateconcentrations,MLSSinthereactor,andoxygenrequirements.

8.4.1 CSTR without recycle

Considerthecompletelymixedsuspendedgrowthreactor(CSTR)showninFigure 8.5.Conductamass balance for biomass Xaroundthesystem.

Rateofaccum ulation

=Rateofinflow

–Rateofoutflow

+Rateof

netgrowth (8.28)


or

dX

dtV Q X Q X V

dX

dtneto

= − +

(8.29)

where:Q=influentandeffluentwastewaterflowrate,m3/dXo,X=biomassconcentrationsininfluentandeffluentrespectively,kg/m3

dX

dt=growthrateofbiomass,kg/m3·d

V=reactorvolume,m3

Thefollowingassumptionsaremadetosimplifyequation(8.29):

1.Thereactorisatsteadystatecondition.Therefore,accumulation=0. 2.Completemixingisachieved.Therefore,concentrationsinreactor=

concentrationsineffluent. 3.Concentrationofbiomassineffluentisnegligiblecomparedwithcon-

centrationofbiomassinreactor,i.e.Xo≈negligible.

Fromequation(8.17)weknowthatdX

dtnet

SX

K Sk X

Sd

=+

−µm ax .Equation(8.29)becomes

0= − ++

−

Q X VSX

K Sk X

Sd

µm ax

or

Q

V

S

K Sk

Sd=

+−µm ax (8.30)

Equation(8.30)canberewrittenusingHRT,θ=V

Qor

µ

θm axS

K Sk

Sd+

= +1 (8.31)

In�ow

Q, X0, S0

Out�ow

Q, X, S

V, X, S

Figure 8.5 Completely mixed reactor without recycle.


ForaCSTRwithoutrecycle,SRT=HRT.

Since,θ θc

VX

Q X

V

Q= = =

Conductamass balance for substrate Saroundthesystem.

Rateofaccum ulation

=Rateofinflow

–Rateofoutflow

+Rateofm ass

substrateutillization (8.32)

dS

dtV Q S Q S V

dS

dtsuo

= − −

(8.33)

where:

dS

dt=rateofsubstrateutilization,kg/m3·d

So=substrateconcentrationininfluent,kg/m3

S=substrateconcentrationinreactorandeffluent,kg/m3

Notethatthenegativesigninequations(8.32)and(8.33)indicatesdeple-tion of substrate. Using the above-mentioned assumptions and equation

(8.11)fordS

dtsu

,equation(8.33)becomes

0= −( ) −+

Q S SV

Y

SX

K So

S

µm ax

or

µm axS

K S

Q

V

Y

XS S

So+

= −( )or

µ

θm axS

K S

Y

XS S

So+

= −( ) (8.34)

Equatingequations(8.31)and(8.34),wecanwrite

1

θ θ+ = −( )k

Y

XS Sd o

Simplifyingweget,XY S S

ko

d

= −+( )

1 θ (8.35)


Equation(8.35)givesusanexpressionforXintermsofthesubstrate,HRT(orSRT)andkinetic coefficients fora suspendedgrowthCSTRwithoutrecycle.

To determine an expression for substrate S, substitute the value of Xfromequation(8.35)intoequation(8.34).

µ

θθ

m ax ( )( )

S

K S

Y S SY S S

kS

o

o

d

+= −

−+1

=1+ kdθ

θ

or

µθ

θm axs d

SK S k

=+( ) +( )1

Simplifyingweobtain,

SK k

ks d

d

=+( )−( ) −

1

1max

θθ µ

(8.36)

Equation(8.36)isthedesignequationforsubstrateSintermsofthekineticcoefficients and HRT (or SRT), for a suspended growth CSTR withoutrecycle.

8.4.2 Activated sludge reactor (CSTR with recycle)

Nowwewilldevelopthedesignequationsforanactivatedsludgereactor(operated as CSTR) with recycle, using the same concepts of mass bal-ancedescribedabove.Consider theactivatedsludgeprocesspresented inFigure 8.6.Theflowsaswellasbiomassandsubstrateconcentrationsfortheaerationtankandsecondaryclarifierareshown.

Conduct a mass balance for biomass X around the system boundaryrepresentedbythedashedline.

Rateofaccum ulation

=Rateofinflow

–Rateofoutflow

+Rateof

netgrowth (8.37)

or

dX

dtV Q X Q X Q X V

dX

dtneto e e w u

= − + +

( ) (8.38)


where:Q=influentwastewaterflowrate,m3/dQe=effluentwastewaterflowrate,m3/dQw=wastesludgeflowrate,m3/dXo,Xe=biomassconcentrationininfluentandeffluent,kg/m3

Xu=biomassconcentrationinunderflow,kg/m3

dX

dt=growthrateofbiomass,kg/m3·d

V=volume,m3

Equation(8.38)canbesimplifiedbymakingthefollowingassumptions:

1.Thereactorisatsteadystatecondition.Therefore,accumulation=0. 2.Completemixingisachieved.Therefore,concentrationsinreactor=

concentrationsineffluent. 3.Concentrations of biomass in influent and effluent are negligible

comparedwith theconcentrationsatotherpoints, i.e.XoandXe≈negligible.

4.Allreactionstakeplaceinthereactororaerationtank.Nofurtherconversionsofsubstrateorbiomassoccurintheclarifier.

5.ThevolumeVrepresentsthevolumeofthereactoronly,basedontheaboveassumption.Itdoesnotincludethevolumeoftheclarifier.

Usingtheseassumptionsandtheexpressionfornetgrowthratefromequa-tion(8.17),equation(8.38)canbewrittenas

0= −+

−

m axQ X VSX

K Sk Xw u

Sd

µ

Aeration tank Secondaryclarifier

V, X, S

Primary effluent

Q, X0, S0 Q + QR, X, S

Effluent

Qe, Xe, S

QR, Xu, SSludgereturn Sludge

underflow Qu, Xu, S

Sludgewaste Qw, Xu, S

System Boundary

Figure 8.6 Activated sludge process operated as CSTR.


or

µm axS

K Sk

Q X

VXSd

w u

+− = (8.39)

Now,θcw u

VX

Q X= ,usingthisinequation(8.39),weget

µ

θm axS

K Sk

S cd+

= +1 (8.40)

Conductamass balance for substrate Saroundthesystemboundaryrep-resentedbythedashedline.

Rateofaccum ulation

=Rateofinflow

–Rateofoutflow

+Rateofm ass

substrateutillization (8.41)

dS

dtV Q S Q S Q S V

dS

dtsuo w e

= − + −

( ) (8.42)

where:dS

dt=rateofsubstrateutilization,kg/m3·d

So=substrateconcentrationininfluent,kg/m3

S=substrateconcentrationinreactorandeffluent,kg/m3

Notethatthenegativesigninequations(8.41)and(8.42)indicatesdepletionofsubstrate.Fromcontinuity,Qe=(Q–Qw).Usingtheabove-mentioned

assumptionsandequation(8.11)fordS

dtsu

,equation(8.42)becomes,

0= −( ) −+

Q S SV

Y

SX

K So

S

µm ax

or

µm axS

K S

Q

V

Y

XS S

So+

= −( )

or

µ

θm axS

K S

Y

XS S

So+

= −( ) (8.43)


whereHRTθ=V

Q.

Combiningequations(8.40)and(8.43),weobtainthefollowing:

1

θ θcd ok

Y

XS S+ = −( ) (8.44)

Simplifyingequation(8.44)weobtainthefollowingexpressionforbiomassX,

XY S S

kc o

d c

= −+

θθ θ

( )

( )1 (8.45)

Note:Equation(8.45)reducestoequation(8.35)foraCSTRwithoutrecy-clewithθ=θc.

To determine an expression for substrate S, substitute the value of Xfromequation(8.45)intoequation(8.44)andsimplifytoobtain:

SK k

ks d c

c d

=+( )−( ) −

1

1

θθ µm ax

(8.46)

Note:Equations (8.45)and (8.46) reduce to equations (8.35) and (8.36)respectively,foraCSTRwithoutrecyclewithθ=θc.

8.4.2.1 Other useful relationships

Equation(8.40)canbewrittenas

m ax1

θµ

c Sd

S

K Sk=

+−

or

1

θc

µ= (8.47)

ThespecificsubstrateutilizationrateUisdefinedasfollows:

Ur

X

dS dt

X

Q S S

VX

S S

Xsu o o= − = = − = −/ ( ) ( )

θ (8.48)

Usingequation(8.11),equation(8.40)canberewrittenasfollows:


1

θc

sudY

r

Xk= − −

or

1

θcdYU k= − (8.49)

Aplot of 1/θc versus Uwill result in a straight line.The slope of thestraightlinewillbetheyieldcoefficientYandtheinterceptwillbekd.

Theefficiencyofsubstrateremovalisgivenby:

ES S

So

o

% %= − ×100 (8.50)

Ucanalsobeexpressedas:

U

F

ME

=

100 (8.51)

8.4.3 Activated sludge reactor (plug flow reactor with recycle)

AnactivatedsludgeprocessisillustratedinFigure 8.7,wheretheaerationtankisoperatedasaplugflowreactor(PFR).Plugflowmaybeachieved


VariableX, S

Primary effluent

Q, X0, S0 Q + QR,X, Se

Effluent

Qe, Xe, Se

QR, Xu, SeSludge return

Sludgeunderflow Qu, Xu, Se

Sludgewaste Qw, Xu, Se

System Boundary

Figure 8.7 Activated sludge process operated as PFR.


inlong,narrowaerationtanks.Inatrueplugflowmodel,alltheparticlesenteringthereactorspendthesameamountoftimeinthereactor.Someparticlesmayspendmoretimeinthereactorduetorecycle.Butwhiletheyareinthetank,allpassthroughinthesameamountoftime(MetcalfandEddy,2003). It isdifficult todevelopakineticmodelduetothevaryingconcentrationsofbiomassandsubstrateinthereactor.

LawrenceandMcCarty(1970)developedamodelfortheplugflowpro-cessbyusingtwosimplifyingassumptions:

1.Theconcentrationofmicroorganismsintheinfluenttotheaerationtankisapproximatelythesameasthatintheeffluentfromtheaera-tiontank.Thisholdstruewhenθc/θ≥5.Theresultingaveragecon-centrationofmicroorganismsinthereactorisdenotedbyXavg.

2.The rate of soluble substrate utilization as the wastewater passesthroughthereactorisgivenby

rSX

Y K Ssu

avg

S

= −+( )

maxµ (8.52)

Integratingequation(8.52)overthehydraulicretentiontimeinthereactorandsubstitutingboundaryconditionsandrecyclefactorprovidesthefol-lowingdesignequation:

1

1θµ

c

o

o s id

S S

S S K R S Sk= −

−( ) + +( ) −m ax( )

ln( / ) (8.53)

where:

R=recycleratio=Q

QR

Si=influentsubstrateconcentrationtoreactorafterdilutionwithrecy-cleflow,mg/L

or

SS RS

Ri

o= ++1

(8.54)

Othertermsarethesameasdefinedpreviously.OneofthemaindifferencesbetweenthedesignequationsforanactivatedsludgeCSTR(equation8.40)andactivatedsludgePFR(equation8.53)isthattheSRT(θc)isafunctionoftheinfluentsubstrateconcentrationSoforthePFR.


In practice, a true plug flow regime is almost impossible to maintainbecause of longitudinal dispersion caused by aeration and mixing. Theaeration tank may be divided into a series of reactors to approach plugflowkinetics.ThisalsoimprovesthetreatmentefficiencycomparedwithaCSTR.TheCSTR,however,canhandle shock loadsbetterdue to thehigherdilutionwithinfluentwastewater,ascomparedwithstagedreactorsinseries(MetcalfandEddy,2003).

8.4.4 Limitations of the models

In practice, the assumptions of ideal CSTR or PFR are extremely diffi-culttoachieve.Reallifereactorsfallsomewhereinbetween.Themodelsdescribed in theprevioussectionsprovideauseful startingpoint for thedesignandmodelingofactualprocesses.Thequantificationofsubstrateconcentrationisalsoimportant.ThesubstrateconcentrationSisthesolu-bleCODconcentrationthatisreadilybiodegradable,butitisnotthetotalBOD.SomefractionofthesuspendedsolidsthatremaininthesecondaryclarifiereffluentalsocontributestotheBODloadofthereceivingwaters.ThetotalBODconsistsofasolublefractionandaninsoluble/particulatefraction. It is importanttokeepthese inconsiderationwhendesigningatreatmentprocess.WecandeterminetheeffluentsubstrateconcentrationSinthefollowingmanner(Davis,2011):

S=TotalallowableBOD–BODineffluentsuspendedsolids (8.55)

Anotherassumptionwasthatnobiologicalreactionstakeplace intheclarifier.Basedon theconcentrationofbiomassand theamountof timespentintheclarifier,thisassumptionmaynotbeentirelycorrect.ThismayresultinerrorsincalculationofthevolumeVinthemodel.Itisimportanttounderstandtheselimitationswhenusingthemodelsfordesignoftreat-mentprocesses.

EXAMPLE8.2DevelopanexpressionfortherecycleflowQRforanactivatedsludgeprocess,usingtheconceptofmassbalances.

SOLUTION

Consider the activated sludge process illustrated in Figure 8.6. Wewilldoamassbalancearoundthesecondaryclarifierwiththesystemboundaryasshownbelow.Conductamassbalanceforbiomassaroundthesecondaryclarifier.Makealltheassumptionsthatwerestatedpre-viouslyinSection8.4.2.Sinceallbiologicalreactionstakeplaceintheaerationtank,thereisnogrowthintheclarifier.Accumulation=0.



V, X, S

Primary effluent

Q, X0, S0 Q + QR,X, S

Effluent

Qe, Xe, S

QR, Xu, SSludgereturn Sludge

underflow Qu, Xu, S

Sludgewaste Qw, Xu, S

System Boundary

Rate of accumulation = Rate of inflow – Rate of outflow

0=(QX+QRX)–QeXe–QuXu

or

QX+QRX=0+QRXu+QwXu

or

QR(Xu–X)=QX–QwXu

or

QQ X Q X

X XR

w u

u

= −−

(8.56)

EXAMPLE8.3Acompletelymixedhigh-rateactivatedsludgeplantistotreat15,000m3/dofindustrialwastewater.Theprimaryeffluentgoingtotheacti-vatedsludgereactorhasaBOD5of1100mg/Lthatmustbereducedto150mg/Lpriortodischargetoamunicipalsewer.Theflowdiagramof theplant is given inFigure 8.6.Pilotplant analysis gave the fol-lowingresults:meancellresidencetime=5d,MLSSconcentrationinreactor=6000mg/LVSS,Y=0.7kg/kg,kd=0.03d–1.Determinethefollowing:

a.Thehydraulicretentiontimeandvolumeoftheactivatedsludgereactor.

b.ThevolumetricloadingrateinkgBOD5/m3-dtothereactor. c.TheF/Mratiointhereactor.


d.Themassandvolumeofsolidswastedeachday,atanunderflowsolidsconcentrationXu=12,000mg/L.

e.Thesludgerecirculationratio. f.Thevolumeofsolidsthatmustbewastedeachday,ifthesolids

arewasteddirectlyfromtheactivatedsludgereactorinsteadoffromtheunderflow.

SOLUTION

Step1.Useequation(8.45)tocalculateHRT(θ).

XY S S

kc o

d c

= −+

θθ θ

( )

( )1

or

θ θθ

= −+

=( )( ) −(( )

( )

.c o

d c

Y S S

X k

d

1

5 0 7 1100 150))+ ×−

m g L

m g L d d

/

/ ( . )6000 1 0 03 51

or

θ=0.48d=11.56h

Volumeofaerationtank,V=Qθ=15000m3/d×0.48d=7200m3

Step2.Useequation(8.24)tocalculatevolumetricloadingrate.

So=1100mg/L=1.10kg/m3

VL=Q SV

m d kg m

mo = × =/ . /15000 1 10

7200

3 3

32.29kgBOD5/m3·d

Step3.Useequation(8.23)tocalculateF/Mratio.

F

M

Q S S

VX

m d m g Lo= − =

−( )( ) , / /15 000 1100 150

7200

3

mm m g L3 6000 /×

=0.33mgBOD5/mgVSS·d

Step4.Atsteadystate,theSRTisgivenby

θcw u

VX

Q X=

or

Q XVX m d kg m

dw u

c

= = ×/ /

θ7200 6

5

3 3

=8640kg/d


Masswastedeachday=8640kgVSS/d.

Volumewastedeachday,QQ X

X

kg d

kg mw

w u

u

= = 8640

12 3

/

/=720m3/d

Step5.Frommassbalancearoundsecondaryclarifierandusingequa-tion(8.56),

QQ X Q X

X X

m dkg

mR

w u

u

= −−

=×

−, / (15 000 633

88640

12 6 3

/ )

( ) /

kg d

kg m−

=13,560m3/d

Recirculationratio,RQ

Q

m d

m dR= = 13560

15000

3

3

/

/=0.90.

Step6.FromStep4,masswastedeachday=8640kg/d.Ifsolidsarewasteddirectlyfromtheaerationtank,thensolidscon-

centrationinwastesludge=MLSSconcentration.Therefore,volumewastedeachday,

QX

kg d

kg mw = =m asswasted each day /

/

8640

6 3=1440m3/d

EXAMPLE8.4ItisdesiredtodeterminethekineticcoefficientsYandkdforanacti-vated sludgeprocess treatingawastewater.Fivebench scaleCSTRswereoperatedatdifferentMLVSS concentrations and the followingdatawereobtained.Determinethekineticcoefficients.

Reactor #X

mg VSS/LU

mg BOD5/mg VSS · drg

mg VSS/L · d

1 1000 0.39 1942 1500 0.51 3993 3000 0.60 9604 5000 0.91 25305 6000 1.20 4080

SOLUTION

Step1.DeveloptheequationsfordeterminationofYandkd.Usingequations(8.47)and(8.49)wecanwrite,

µ YU kd= −

Usingequation(8.5),r

Xg = µ


Therefore,r

XYU kg

d= −

Step2.Prepareatableforagraphicalplotoftheaboveequation.

Reactor #X

mg VSS/Lrg /Xd–1

Umg BOD5/mg VSS · d

1 1000 0.194 0.39

2 1500 0.266 0.51

3 3000 0.320 0.60

4 5000 0.506 0.91

5 6000 0.680 1.20

Step3.Plotrg/XversusU.Drawthebestfitlineasshownbelow.

–0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.2 0.4 0.6 0.8 1 1.2 1.4

rg/X

, d–1

U, d–1

Fromthegraph,slope=Y=0.60mgVSS/mgBOD.Intercept=kd=0.04d–1

8.4.5 Aeration requirements

Airoroxygenissuppliedtotheaerationtankoftheactivatedsludgeprocesstoprovideoxygenrequiredbytheaerobicmicroorganismsfordegradationoforganicmatter.Theamountofoxygenaddedshouldbesufficientto(1)matchtheoxygenutilizationrate (OUR)of themicroorganisms,and(2)maintainasmallexcessinthetank,about0.5to2mg/Ldissolvedoxygen,toensureaerobicmetabolismatalltimes(Peavyetal.,1985).TheOURisafunctionofthecharacteristicsofthewastewaterandthetypeofreactor.Inaconventionalactivatedsludgeprocess,theOURisaround30mg/L·h.


Forextendedaerationprocess,theOURisabout10mg/L·h,whereasforhigh-rateprocessestheOURcanbeupto100mg/L·h.

TheoxygenrequirementmaybeestimatedfromthebiodegradableCOD(bCOD)ofthewastewaterandthebiomasswastedeachday.ThemassofoxygenrequiredforBODremovalmaybecalculatedfromthefollowingexpression(MetcalfandEddy,2003):

Mo=Q(So–S)–1.42Px (8.57)

where:Mo=MassofoxygenrequiredforBODremoval,kg/dQ=wastewaterflowrateintoaerationtank(withoutrecycleflow),

m3/dSo=influentbCOD,kg/m3

S=effluentbCOD,kg/m3

Px=biomasswasted,kgVSS/d1.42=CODofcelltissue,kgCOD/kgVSS

Asanapproximation,theoxygenrequirementforonlyBODremovalwillvaryfrom0.9to1.3kgO2/kgBODremovedforSRTsof5to20d,respec-tively(WEF,1998).Whennitrificationisincludedintheprocess,theoxy-genrequiredforoxidationofammoniaandnitritetonitrateisincludedasfollows(MetcalfandEddy,2003):

MO+N=Q(So–S)–1.42Px+4.33Q(NOx) (8.58)

where:NOx=TKNconcentration,kg/m3

MO+N=oxygenrequiredforBODandnitrogenremoval,kg/d

Othertermsarethesameasdefinedpreviously.The aeration devices have to provide adequate oxygen to satisfy the

demandforaverageandpeakflows.Apeakingfactorof2.0iscommonlyused.However,areviewofactualconditionsshouldbeperformed.Eachtypeofaerationdevicecomeswithacertainoxygentransferefficiencyandanoxygentransferrate inpurewateratstandardtemperatureandpres-sure(SOTR)specifiedbythemanufacturer.Theactualoxygentransferrate(AOTR)variesfromtheSOTRduetowastewatercharacteristics,pressurevariation,residualoxygenconcentration,etc.TheAOTRcanbecalculatedfromthefollowingexpression(MetcalfandEddy,2003):

AO TR SO TRC C

CFsT H L

s

T=−

( )( )−βθ α, ,

,20

20 (8.59)


where:AOTR=actualoxygentransferrateunderfieldconditions,kgO2/hor

kgO2/kWh(lbO2/horlbO2/hp·h)SOTR=standardoxygentransferrateincleanwaterat20°Candzero

dissolvedoxygen,kgO2/horkgO2/kWh(lbO2/horlbO2/hp·h)β=salinity–surfacetensioncorrectionfactor,typically0.95to

0.98Cs,T,H=oxygensaturationconcentrationincleanwateratwastewater

temperatureT,anddiffuserdepthH,mg/LCL=dissolvedoxygenconcentrationinwastewater,mg/LCs,20=dissolvedoxygensaturationconcentrationincleanwaterat

20°Cand1atmpressure,usually9.17mg/Lθ=correctionfactorfortemperature=1.024T=wastewatertemperature,°CF=foulingfactor,typically0.65to1.0fornofoulingα=oxygen transfer correction factor for wastewater, typically

0.3–0.4foractivatedsludgereactorswithBODremoval,and0.45–0.75forBODremovalandnitrificationsystems

8.4.5.1 Types of aerators

Differenttypesofaeratorsareusedinaerationtanks.Theselectionofaera-tionsystemdependsonthesitecharacteristicsandtypeofprocessused.Thefollowingaretwoofthecommonlyusedaerationsystems.Figures 8.8and8.9illustratethesesystems.

Figure 8.8 Aeration tanks with submerged fine bubble diffuser system (photo by Rumana Riffat).


Airdiffusers—usedtoinjectairintotheaerationtank.Thediffusersmaybemountedalongthesideofthetankortheymaybeplacedinamani-foldalongthebottomoftheaerationtank.Airdiffusersmayproducecoarse bubblesorfine bubbles.Coarsebubblesmaybeupto25mmindiameter,whilefinebubblesare2to2.5mmindiameter.Thereareadvantagesanddisadvantagesofbothtypesofdiffusers.Finebubblediffusershavegreaterenergyrequirementsandclogeasily,eventhoughtheyhavebetteroxygentransferduetolargersurfaceareapervolume.CoarsebubblediffusershaveloweroxygentransferratesbutrequirelessmaintenanceandhavelowerheadlossasshowninFigure8.8.

Mechanicalaerators—usuallyhaveimpellersthatproduceturbulenceattheair–waterinterface,whichenhancesthetransferofoxygenfromairtowater.High-speedimpellerscanaddlargequantitiesofairtorelativelysmallquantitiesofwater.Mixingoftheaeratedwaterwithreactor contents takesplace throughvelocity gradients.Brush-typeaeratorsareusedinoxidationditchestopromoteairentrainmentandalsomomentumtothewastewater.

EXAMPLE8.5Considerthecompletelymixedhigh-rateactivatedsludgeplantfromExample8.3.Finebubblemembranediffuserswithtotalfloorcover-agearetobeusedfortheaerationtank.TheSOTRspecifiedbythemanufactureris3.5kgO2/kWh,withαFof0.5.Theaveragewaste-water temperature is16°C.The residualDO in theaeration tank is

Figure 8.9 Typical air diffuser system.


4 mg/L,βis0.95,andsaturationoxygenconcentrationisat16°Candtankdepthelevationis9.81mg/L.Calculatetheoxygendemandandthepowerrequiredforaeration.

SOLUTION

Step1.CalculatetheoxygendemandforBODremoval.Usingequation(8.57)andthedatafromExample8.3,

Mo=Q(So–S)–1.42Px

=15,000m3/d(1.1–0.15)kg/m3–1.42×8640kg/d =1981.20kg/d

Useapeakingfactor=2.0.Oxygendemand=1982.Oxygendemand=1981.2×2.0=3962.40kg/d.

Step2.CalculatetheAOTR.

SOTR=3.5kgO2/kWh Cs,T,H=9.81mg/L Cs,20=9.17mg/L CL=4mg/L

Useequation(8.59)tocalculateAOTR.

AO TR SO TRC C

CFsT H L

s

T=−

( )( )−βθ α, ,

,20

20

or

AO TR = × −

−3 50 95 9 81 4

9 171 02416 20.

. .

..(( )( )0 5.

or

AOTR=0.92kgO2/kWh

Oxygentransferefficiency=AOTR/SOTR×100%=(0.95/3.5)×100%=27%whichistypicalforthistypeofaerationsystem.

Step3.Calculatepowerrequiredforaeration.Oxygendemand=3962.40kgO2/d=165.10kgO2/h.Powerrequired=oxygendemand/AOTR

=(165.10kgO2/h)/(0.92kgO2/kWh)

=179.46kW

≈180kW


8.5 TYPES OF SUSPENDED GROWTH PROCESSES

Awidevarietyofsuspendedgrowthprocessesareinoperationatwastewatertreatmentplantsworldwide.Each typehas its advantages and limitations.Wastewatercharacteristics,sitecharacteristics,effluentlimitations,regulatoryrequirements,andeconomicconsiderationsaresomeofthefactorsthatinflu-encethechoiceandselectionofaparticularprocess.Pilotstudiesshouldbeconductedbeforemakingafinalselection.SomeofthemorecommontypesofsuspendedgrowthprocessesusedforBODremovalaredescribedbelow.

8.5.1 Conventional activated sludge

Theconventionalactivatedsludgeprocessisthemostwidelyusedsuspendedgrowthprocessforthetreatmentofwastewater.ThebasicprocesshasbeendescribedindetailinSection8.3.Section8.4providesthedevelopmentofdesignmodels for completelymixedandplugflowactivated sludgepro-cesses.Typicaldesignvalues for completelymixed systemsare (Peavyetal.,1985)asfollows:HRT3–5h,F/M0.2–0.4,SRT4–15d,VL0.8–2.0kgBOD5/m3·d,withBODremovalefficiency85%–95%andrecycleratioof0.25–1.0.Forplugflowsystemstherecycleratiovariesfrom0.25to0.5,withanHRTof4–8handVL0.3–0.6kgBOD5/m3·d.TheSRTandF/Mratiosforthetwosystemsaresimilar.

Anumberofvariationsoftheconventionalprocesshavebeendevelopedandareinuse.Thefollowingsectionsprovidedescriptionsofafewofthem.

8.5.2 Step aeration or step feed process

Inthisprocess,alongandnarrowaerationtankisusedforplugflowcon-figuration.Theinfluentwastewaterenterstheaerationtankatseveralloca-tionsalongthelength.Thishelpstoreducetheoxygendemandattheheadinletpoint.Atthesametime,compressedair is injectedintothetankatseverallocationsalongthelength.Thishelpsprovideuniformaerobiccon-ditionsthroughoutthetank.This is illustrated inFigure 8.10(a).TypicaldesignvaluesareHRT3–5h,F/M0.2–0.4,SRT4–15d,VL0.6–1.0kgBOD5/m3·d,withBODremovalefficiency85%–95%.

8.5.3 Tapered aeration process

Plugflowconfigurationisused,withthewastewaterenteringtheaerationtankatoneend.Theairflowistaperedwiththehigherflowtowardtheinlet,graduallytaperingtolowairflowtowardtheoutlet.Airandmaxi-mumoxygenisprovidedattheinletwheretheorganicloadishighest.Asthewastewaterflowsthroughthetank,thesubstrateisdegradedandtheoxygendemandislowertowardtheoutlet.Thisresultsinefficientuseof


theairwhereitisneededmost.ThetaperedaerationprocessisillustratedinFigure 8.10(b).TypicaldesignvaluesareHRT4–8h,F/M0.2–0.4,SRT5–15d,VL0.3–0.6kgBOD5/m3·d,withBODremovalefficiency85%–95%.

8.5.4 Contact stabilization process

Thisprocessusestwoseparatetanksforthetreatmentofwastewaterandstabilizationofactivatedsludge.Theprocessconsistsofacontact tankwithashortHRTof30to60min,followedbyaclarifier.Thereadilybiodegrad-ablesolubleCODisoxidizedorstored,whileparticulateCODisadsorbedonthebiomassatthesametime.Thetreatedwastewaterisseparatedfromthebiomassintheclarifier.Thesettledbiomasswiththeadsorbedorganicmatteristhentransportedtoastabilization tank(HRT1–2h),wherethestoredandadsorbedorganicsaredegraded.Thebiomass is thenreturnedtothecontacttankasactivatedsludge.OverallBODremovalefficiencyis

(a)

(b)

Reactor Secondaryclari�er

Primary e�uent E�uent

Air

Sludge return

Sludgewaste

Reactor Secondaryclari�er

Primary e�uent E�uent

Air

Sludge return

Sludgewaste

Figure 8.10 Suspended growth processes used for BOD removal: (a) step feed, (b) tapered aeration, (c) contact stabilization, (d) staged activated sludge process.


80%–90%.SincetheMLSSisveryhighinthestabilizationtank,thisresultsinalowertankvolume.Theadvantageofthissystemisreductioninoveralltankvolume.ThecontactstabilizationprocessisillustratedinFigure 8.10(c).

8.5.5 Staged activated sludge process

Anumberof completelymixed reactorsareplaced in series followedbyafinalclarifier.Thereturnactivatedsludgecomesbacktothefirsttank.Threeormorereactorsinseriesapproximateaplugflowsystem.Thepro-cessiscapableofhandlinghighorganicloadswithhighBODremovaleffi-ciencies.ThisisillustratedinFigure 8.10(d).

(d)

(c)

Influent Effluent

Influent Effluent

Contactbasin

Secondaryclarifier

Secondaryclarifier

Aerationtank

Aerationtank

Aerationtank

Sludge return

Sludge waste

Sludge waste

Aeration tank

Air

Air Air Air

Figure 8.10 (Continued)


8.5.6 Extended aeration process

Theextendedaerationprocessisusedtotreatwastewaterfromsmallcommu-nitiesthatgeneratelowvolumesoffairlyuniformcharacteristics.Completelymixedactivated sludgeprocess configuration is used (Figure 8.3).TypicaldesignvaluesareHRT18–24h,F/M0.05–0.15,SRT20–30d,VL0.16–0.4kgBOD5/m3·d,withBODremovalefficiency75%–90%.Thereactorisoperatedintheendogenousdecayphase,asevidencedbytheF/Mratio.

8.5.7 Oxidation ditch

Thisprocessconsistsofanoval-shapedaerationchannel,wherethewaste-waterflowsinonedirection,followedbyasecondaryclarifier.Brush-typemechanicalaeratorsprovideaerationandmixing,andkeepthewaterflow-inginthedesireddirection.Theinfluententersthechannelandismixedwiththereturnactivatedsludge.Theflowinthechanneldilutestheincom-ingwastewaterbyafactorof20to30.Processkineticsapproachthatofacompletemixreactor,butwithplugflowalongthechannels.TheoxidationditchisillustratedinFigure 8.11.Thisprocessissuitableforuseinsmallruralcommunitieswherelargelandareaisavailable.TheoxidationditchcanbedesignedandoperatedtoachievebothBODandnitrogenremoval.

8.5.8 Sequencing batch reactor (SBR)

TheSBRisafill-and-drawtypeofsystemwhereaeration,biodegradation,andsettlingalltakeplaceinasinglereactor.Thereactorsequencesthroughanumberofsteps inonecycle.Thereactorcangothrough2to4cyclesperday.Atypicalcycleconsistsofthefollowingsteps:(1)Fill—wheresub-strateisadded;(2)React—mixingandaerationisprovided;(3)Settle—forclarificationofeffluent;(4)Decant—forwithdrawalofeffluent.Anidlestepmayalsobeincludedtoprovideflexibilityathighflows.Aerationisaccom-plishedbyjetaeratorsorcoarsebubblediffuserswithsubmergedmixers.

In�uent E�uent

Sludgewaste

Aeration rotor

Sludgeconcentrating

hopper

Figure 8.11 Oxidation ditch.


8.5.9 Membrane biological reactor (MBR)

The MBR process uses a biological reactor with suspended biomass forBODand/ornitrogenremoval,andmicro-orultrafiltrationmembranesforsolidsseparation.Thebioreactorsmaybeaerobicoranaerobic.Theefflu-entwaterqualityisveryhighandmakesthisprocessattractiveforwaterreuseapplications.MBRshavebeenusedfortreatmentofmunicipalandindustrialwastewater,aswellasforwaterreuse.Thistypeofreactorhasbeenfoundsuitableforremovalofavarietyofcontaminantsfrommunici-palwastewater,e.g.pharmaceuticalproductsandaromatichydrocarbons,amongothers(Kimuraetal.,2007;Francescoetal.,2011).Recentresearchhas focusedontheuseofnanomaterials,whichareappliedasacoatingonthemembranestoimprovethehydrophilicity,selectivity,conductivity,foulingresistance,andantiviralpropertiesofmembranes(Suetal.,2011;KimandvanderBruggen,2010;Luetal.,2009;Zodrowetal.,2007;BaeandTak,2005).

The MBR process has the following advantages (Metcalf and Eddy,2003):

• ItcanoperateathighMLSSconcentrations(15,000to25,000mg/L).• AsaresultofhighMLSS,itcanhandlehighvolumetricloadingrates

atshortHRTs.• LongerSRTresultsinlesssludgeproduction.• High-quality effluent in terms of low turbidity, TSS, BOD, and

pathogens.• Lessspaceisrequired,asnosecondaryclarifierisneeded.

Thedisadvantagesoftheprocessincludethefollowing:

• Highcapitalcosts• Membranefoulingproblems• Membranereplacementcosts• Higherenergycosts

Membranebioreactorsystemshavetwobasicconfigurations:(1)Aninte-gratedbioreactorhasamembranemodule immersed in the reactor, and(2) a recirculated MBR has the membrane module separately mountedoutside the reactor. Immersed membranes use hollow fiber or flat sheetmembranesmountedinmodules.Theyoperateatlowerpressuresandareused in activated sludge bioreactors. The separate systems use pressure-driven, in-pipe cartridge membranes. They are used more for industrialwastewaters(Davis,2011).Additionaldiscussiononmembranecharacter-istics,membranefouling,andfluxcalculationsisprovidedinChapter13(Section13.4.3).


8.6 STABILIZATION PONDS AND LAGOONS

Ponds and lagoons are land-based suspendedgrowth treatment systems.Usually there are no primary or secondary clarifiers. All treatment andsolids separation takes place in an earthen basin, where wastewater isretainedandnaturalpurificationprocessesresultinbiologicaltreatment.Mechanicalmixingisprovidedinalagoon,whereasthereisnomechanicalmixinginapond.Pondscanbe(a)aerobic—shallowpond,(b)anaerobic—deeppond,and(c)facultative.Lagoonscanbe(a)aerobic—withcompletemixing,and(b)facultative—withmixingoftheliquidportion.Themajor-ityofpondsandlagoonsarefacultative.AtreatmentsystemmayconsistofanaerobiclagoonfollowedbyfacultativepondstoachievesufficientBODremoval.Thesetypesofsystemsaresuitableforsmallcommunitiesandon-sitetreatmentofindustrialwastewaters.Lagoonsareusedextensivelyfortreatmentoflivestockwastewatersathogandpoultryfarms.

Theadvantagesoftheseland-basedtreatmentsystemsareasfollows:

• Lowcapitalcost• Lowoperatingcost• Largevolumeto inflowratioprovidesenoughdilutiontominimize

theeffectsofvariableorganicandhydraulicloadings

Thedisadvantagesincludethefollowing:

• Largelandareaisrequired.• Odorproblemsareaconcern.• High suspended solids concentration in the effluent. In the United

States,thedischargelimitsforsolidsintheeffluentare75mg/LasspecifiedbytheEnvironmentalProtectionAgency(EPA).

• At a cold temperature, biological activity is significantly reduced.Incoldclimates,itisoftennecessarytoprovidesufficientvolumetostoretheentirewinterflow.

8.6.1 Process microbiology

Inthissectionthemicrobiologicalprocessestakingplaceinafacultativepond will be discussed. This includes both aerobic and anaerobic pro-cesses.Thefacultativepondhasacomplexsystemofmicrobialprocessesthatresultindegradationoforganicmatter.Thefacultativepondhasanaerobic section in the top layers,anaerobic section in thebottom layers,andsomefacultativereactionstakingplaceinbetween.ThisisillustratedinFigure 8.12.

The wastewater enters the pond near the bottom. The biological andother solids settle at the bottom in a thin sludge blanket. Anaerobic


bacteriadegradetheorganicmatterandreleaseproductsofdecomposition.Theseproductsaremainlyorganicacidsandreducedcompoundsofcar-bon,nitrogen,sulfur,andphosphorus.Thereisafacultativeregioninthemiddlelayers,wherethebacteriacanswitchtheirmetabolismfromaero-bictoanaerobic,orviceversa,dependingontheloadingconditions.Theorganicacidsandreducedcompoundsarethenusedbytheaerobicbacteriaintheupper layersofthepond.Theaerobicdecompositionproductsareoxidizedcompoundsofcarbon,nitrogen,sulfurandphosphorus,e.g.CO2,

Algae

Oxidized productsCO2, NO3

–, PO43–, SO4

2–O2

Aerobicbacteria

CO2Sunlight

Ana

erob

ic zo

ne

Aero

bic z

one

Fluc

tuat

ing

facu

ltativ

e zon

e

Organic acids andreduced compoundsCH4, NH3, H2S, CO2

Sludge inlet

Pond bottom

Anaerobic bacteria

Figure 8.12 Microbiological processes in a facultative pond (Source: Adapted from Peavy et al., 1985).


NO3–,PO4

–3,SO4–2,etc.Algaeusetheseoxidizedcompoundsasfoodinpres-

enceofsunlightandreleaseO2asaby-product.Thereleasedoxygenhelpstoreplenishthedissolvedoxygenconcentrationofthepondandmaintainaerobicconditionsinthetoplayers.Thus,asymbioticrelationshipexistswithinthemicrobialcommunityinthepond.

Thedepthof theaerobic zonedependson thepenetrationof sunlightand wind action. Strong wind action and enhanced light penetration inclearwaterscanextendthedepthoftheaerobiczonedownward.Ontheotherhand,absenceofwindandcloudyskiescanresultintheanaerobiczone rising toward the surface.The facultative zone is the regionwheredissolvedoxygenconcentrationfluctuatesinthepond.Facultativemicroor-ganismsexistinthiszone,whicharecapableofadjustingtheirmetabolisminresponsetoloworhighdissolvedoxygenconcentrations.

8.6.2 Design of pond or lagoon system

Anumberofmodelsareavailableforthedesignofpondsandlagoons.Themostcommonlyusedmodelassumesacompletelymixedreactorwithoutsolidsrecycle.Therateofsubstrateutilizationisassumedtobefirstorder.Amassbalanceforthesolubleportionofsubstratecanbewritten,andthefollowingdesignequationcanbeobtained(MetcalfandEddy,2003;Peavyetal.,1985):

S

S ko

=+1

1 θ (8.60)

where:S=effluentsolubleBODconcentration,mg/LSo=influentsolubleBODconcentration,mg/Lk=firstorderratecoefficient,variesfrom0.5to1.5d–1

θ=HRT=V/Q,d

Whenapondorlagoonsystemisusedformunicipalwastewatertreatment,itiscommonpracticetodistributetheflowbetweentwotothreepondsinseries.Thisisdonetominimizeshort-circuitingthatcanoccurinonelargepond/lagoon. The first unit is usually designed as an aerated facultativelagoon,sinceitreceivesthewastewaterwiththehighestBODconcentra-tion.Thisisfollowedbytwoormorefacultativeponds.Thedesignequa-tionfornnumberofequallysizedpondsisgivenby

S

S k nn

on

=+

1

1( / )θ (8.61)


where:Sn=effluentsolubleBODconcentrationfromnthpond,mg/Lθ=totalHRTforthepondsystem,dn=numberofponds/lagoonsinseries

Othertermsarethesameasdescribedpreviously.Thevan’t-HoffArrheniusmodel (Equation (8.20) is used to correct k values for temperature.Arrheniuscoefficientcanrangefrom1.03to1.12.

8.6.3 Design practice

TheHRToffacultativepondscanvaryfrom7to30d,withaBODload-ingof2.2to5.6g/m2·d(20lb/acre·dto50lb/acre·d).Thelowerloadingisforcolderclimates,wherebiologicaldegradationisseverelyreducedinwinter.Sufficientvolumemayhavetobeprovidedtostoretheentirewinterflow.TheHRToffacultativelagoonscanvarybetween7and20d.Thewaterdepthrangesfrom1to2m,with1mofdikefreeboardabovethewaterlevel.Aminimumwaterlevelof0.6m(2ft)isrequiredtopreventthegrowthofrootedaquaticweeds(HammerandHammer,2012).

Asmentionedpreviously, it iscustomarytousetwoorthreeponds inseries,anddistributetheflowequallybetweentheponds.Athreecellsys-temis illustratedinFigure 8.13.Thefirstunit iscalledtheprimary cell,whichisoperatedasanaeratedlagoon.Thesecondcellmaybeoperatedasaprimaryorsecondary cell,dependingonthevolumeofflow.Thesemay

Effluent

Influent

Influen

Secondary ortertiary cell

Primary orsecondary cell

Primary cell

Figure 8.13 Three cell pond or lagoon system.


beoperatedinparallelorinseries.Thethirdcellprovidesadditionaltreat-mentandstoragevolume.Inthesizingofponds,thesecondarycellisnotincludedinBODloadingcalculations.However,thevolumeisincludedindeterminationofhydraulicretentiontimes.

Algae provides some dissolved oxygen replenishment to the faculta-tiveponds. For lagoons, aeration is providedusingmechanical aerators.AerationrequirementscanbecalculatedasdescribedinSection8.4.5.

Forpondsandlagoonstreatingmunicipalwastewater,thebottomandsides are sealed with bentonite clay to provide an impervious layer. Forindustrialwastewatertreatment,thesidesandbottomhavetobecoveredwithanimperviousliner.Inaddition,stateandlocalregulationsforhaz-ardouswastetreatmentwouldhavetobefollowed.

EXAMPLE8.6Apondandlagoonsystemistobedesignedformunicipalandindus-trialwastewatertreatmentforasmallcommunitywithapopulationof2500.Thewastewaterdesignflowis400L/capita·d(Lpcd)withaBODloadof70g/capita·d.Itisdesiredtouseathreecellsystemsimi-lartotheoneillustratedinFigure 8.13,withthefirsttwocellsusedasprimarylagoonsinparallel.TheallowableBODloadingis2.2g/m2·d.(1)Calculatetheareaofthepondsystem.(2)Calculatethewinterstor-ageavailableifthewaterdepthofthepondsis2m.Assumelossesduetoevaporationandseepageare0.5mm/d.

SOLUTION

Step1.Calculaterequiredareaofpondsystem.

Designflow=400Lpcd

Q=400Lpcd×2500people=1,000,000L/d=1,000m3/d

BODproduced=70g/capita‚d×2500people=175,000g/d=175kg/d

AllowableBODloading=2.2g/m2·d

Primarylagoonarearequired=175 0002 2 2

, /

. / ·

g d

g m d=79,545.45m2

≈80,000m2

Note:BODloadingiscalculatedforprimarycellsonly.Select twoprimary lagoonsof 40,000m2 area each.Adda third

pondofequalarea.Thehighwaterlevelis2mwith1moffreeboard.Sototaldepthis3m.


Primarylagoon

In�uent

Primarylagoon

Secondarypond

E�uent

Three-cell lagoon and pond system.

Step 2. Calculate winter storage available. The cross-section of thepondisshownbelowwiththelowandhighwaterlevels.

Assumelowwaterlevel=0.6m

Depthavailablebetweenlowandhighwaterlevel=2–0.6m=1.4m

Storagevolume=depth×totalarea=1.4m×3×40000m2=168,000m3

Evaporationandseepageloss=0.5mm/d=0.0005m/d

Wastewater in�uent Low water level

High water level

Free board

Pond cross-section showing high and low water levels.

Evaporationandseepagelossvolume=0.0005m/d×3×40,000m2

=60m3/d

Totalstoragetimeavailable= 168 000

1000 60

3

3

,

/

m

m d−( ) =179d


PROBLEMS

8.1 WhatistheMonodmodel?GraphicallyillustratetheMonodmodel.Showwiththehelpofequationswhathappensatverylowandveryhighsubstrateconcentrations.

8.2 DefineSRTandF/Mratio.Whyaretheseconsideredtobeimpor-tantdesignparameters?

8.3 Name five factors affecting the microbial growth in an activatedsludgereactor.

8.4 Whatarethebasiccomponentsofanactivatedsludgereactor?Whyisitcalledactivated sludge?

8.5 Drawadiagramofacompletelymixedsuspendedgrowthreactorwithoutrecycleandwritedownqualitativemassbalanceequationsforbiomassandsubstrate.Simplifythemassbalanceequationstoobtainexpressionsforbiomass(X)andsubstrate(S).Clearlystateanyassumptionsthatyoumake.

8.6 Developtheequation:1 1 1

U

K

k S ks=

+

wherekYmax= µ

,S=effluentsubstrateconcentration,andallother

terms are the same as defined previously. Describe how you candeterminethevaluesofthekineticcoefficients,Ksandk,byusingthisequationandexperimentaldata.

8.7 Anumberofbenchscalereactorswereoperatedinthelaboratoryascompletelymixedreactorswithrecycle, todeterminekineticcoef-ficients for awastewater.The reactorswereoperatedat the sameHRT(θ)andinitialsolublesubstrateconcentrationsbutatdifferentSRT(θc)values.Theinitialsolublesubstrateconcentrationwas500mgCOD/L.TheHRTwas6hforallthereactors.Theexperimentaldataareprovidedbelow.

Reactor So, mg/L S, mg/L X, mg VSS/L SRT, d

1 500 300 295 0.452 500 200 472 0.503 500 150 584 0.554 500 100 746 0.605 500 60 990 0.756 500 30 1516 1.05

a. CalculatethekineticcoefficientsYandkd.Useagraphicalpro-cedureanduseequations(8.48)and(8.49).

b. Calculatethekineticcoefficientsk,Ksandµmax.Useagraphicalprocedureandtheequationdevelopedinproblem8.6.


8.8 Acompletelymixedactivatedsludgeplantisdesignedtotreat10,000m3/dof an industrialwastewater.Thewastewaterhas aBOD5of1200mg/L.Pilotplantdataindicatesthatareactorvolumeof6090m3withanMLSSconcentrationof5000mg/Lshouldproduce83%BOD5removal.ThevalueforYisdeterminedtobe0.7kg/kgandthevalueofkdisfoundtobe0.03d–1.Underflowsolidsconcentra-tionis12,000mg/L.TheflowdiagramissimilartoFigure 8.6.

a. Determinethemeancellresidencetimeforthereactor. b. Calculatethemassofsolidswastedperday. c. Calculatethevolumeofsludgewastedeachday. 8.9 The city of Annandale has been directed to upgrade its primary

wastewater treatment plant to a secondary treatment plant withsludgerecyclethatcanmeetaneffluentstandardof11mg/lBOD5.Thefollowingdataareavailable:

Flow=0.15m3/s,MLSS=2,000mg/L. Kineticparameters:Ks=50mg/L,µmax=3.0d–1,kd=0.06d–1,Y=0.6 ExistingplanteffluentBOD5=84mg/L. a. CalculatetheSRT(θc)andHRT(θ)fortheaerationtank. b. Calculatetherequiredvolumeoftheaerationtank. c. Calculatethefoodtomicroorganismratiointheaerationtank. d. Calculatethevolumetric loadingrate inkgBOD5/m3-dforthe

aerationtank. e. Calculatethemassandvolumeofsolidswastedeachday,when

theunderflowsolidsconcentrationis12,000mg/L. 8.10 A completely mixed high rate-activated sludge plant with recycle

treats17,500m3/dayof industrialwastewater.The influent to theactivatedsludgereactorhasaBOD5of1000mg/L.ItisdesiredtoreducetheinfluentBOD5to120mg/L,priortodischargetoamunic-ipalsewer.Pilotplantanalysisgavethefollowingresults:meancellresidencetime=6d,MLSSconcentrationinreactor=5500mg/L,Y=0.6kg/kg,kd=0.03d–1.Determinethefollowing:

a. Thehydraulicretentiontimeandvolumeoftheactivatedsludgereactor.

b. ThevolumetricloadingrateinkgBOD5/m3-daytothereactor. c. TheF/Mratiointhereactor. d. Themassandvolumeofsolidswastedeachday,atanunderflow

solidsconcentration,Xu=10,000mg/L. e. Thesludgerecirculationratio. 8.11 Considerthecompletelymixedhigh-rateactivatedsludgeplantfrom

problem8.10.Finebubblemembranediffuserswithtotalfloorcov-eragearetobeusedfortheaerationtank.TheSOTRspecifiedbythemanufactureris3.2kgO2/kWh,withαFof0.45.Theaveragewastewater temperature is18°C.The residualDO in theaerationtankis4mg/L,βis0.90,andsaturationoxygenconcentrationisat


18°Candtankdepthelevationis9.54mg/L.Calculatetheoxygendemandandthepowerrequiredforaeration.

8.12 ThetownofOrlandParkusesstabilizationpondstotreatitswaste-water. The wastewater flow is 140,000 gpd with a BOD5 of 320mg/L.Totalsurfaceareaofthepondis14.8acres.Waterlossduringthewintermonthsis0.11in/day.

a. CalculatetheBODloadingonthepond. b. Calculate thedaysofwinterstorageavailable,whenoperating

waterdepthsrangefrom2ftto5ft.

REFERENCES

Ardern,E.andLockett,W.T. (1914a)“Experimentson theOxidationofSewageWithouttheAidofFilters.”Journal of Society of Chemical Industry,London,vol.33,p.523–539.

Ardern, E. and Lockett, W. T. (1914b) “Oxidation of Sewage Without the Aidof Filters.” Journal of Society of Chemical Industry, London, vol. 33,p. 1122–1124.

Bae, T., and Tak, T. (2005) “Preparation of TiO2 Self-Assembled PolymericNanocomposite Membranes and Examination of Their Fouling MitigationEffectsinaMembraneBioreactorSystem.”Journal of Membrane Science,vol.266,no.1–2,pp.1–5.

Clark,H.W.andAdams,G.O.(1914)“SewageTreatmentbyaerationandContactinTanksContainingLayersofSlate.”Engineering Record,vol.69,p.158–159.


Francesco,F.,DiFabio,S.,Bolzonella,D.,andCecchi,F.(2011)“FateofAromaticHydrocarbons in Italian Municipal Wastewater Systems: An Overview ofWastewaterTreatmentUsingConventionalActivated-SludgeProcesses(CASP)and Membrane Bioreactors (MBRs).” Water Research, vol. 45, no. 1, pp.93–104.

Hammer,M.J.,andHammerM.J.Jr.(2012)Water and Wastewater Technology.Seventhedition.Pearson-PrenticeHall,Inc.,UpperSaddleRiver,NewJersey.

Jahan,K.,Hoque,S.,Ahmed,T.,Türkdoğan,andPatarkine,V. (2011)“ActivatedSludgeandOtherAerobicSuspendedCultureProcesses.”Water Environment Research,vol.83,no.10,pp.1092–1149.

Jones,R.,Parker,W.,Zhu,H.,Houweling,D.,andMurthy,S.(2009)“PredictingtheDegradabilityofWasteActivatedSludge.”Water Environment Research,vol.81,no.8,pp.765–771.

Kim, J., andvanderBruggen,B. (2010)“TheUseofNanoparticles inPolymericandCeramicMembraneStructures:ReviewofManufacturingProceduresandPerformance Improvement for Water Treatment.” Environmental Pollution, vol.158,no.7,pp.2335–2349.

Kimura, K., Hara, H., and Watanabe, Y. (2007)“Elimination of Selected AcidicPharmaceuticalsfromMunicipalWastewaterbyanActivatedSludgeSystemandMembraneBioreactors.”Environmental Science and Technology,vol.41,no.10,pp.3708–3714.


Lawrence,A.W.,andMcCarty,P.L.(1970)“AUnifiedBasisforBiologicalTreatmentDesignandOperation.”Journal of Sanitary Engineering Division,vol.96,no.SA3,p.757.

Lu, Y., Yu, S., and Meng, L. (2009) “Preparation of Poly(Vinylidene Fluoride)UltrafiltrationMembraneModifiedbyNano-SizedAluminaandItsAntifoulingPerformance.”Harbin Gongye Daxue Xuebao/Journal of Harbin Institute of Technology, vol.41,no.10,pp.64–69.

Ma, F., Guo, J., Zhao, L., Chang, C., and Cui, D. (2009) “Application ofBioaugmentation to Improve theActivated Sludge System into the ContactOxidation System Treating Petrochemical Wastewater.” Bioresource Technology,vol.100,no.2,pp.597–602.


Monod, J. (1949) “The Growth of Bacterial Cultures.” Annual Review of Microbiology,vol.3,pp.371–394.


Plósz,B.,Leknes,H,Liltved,H.,andThomas,K.(2010)“DiurnalVariationsintheOccurrence and the Fate of Hormones andAntibiotics inActivated SludgeWastewaterTreatment inOslo,Norway.”Science of the Total Environment, vol.408,no.8,pp.1915–1924.

Rieger, L., Takács, I., Villez, K., Siegrist, H., Lessard, P., Vanrolleghem, P., andComeau, Y. (2010) “Data Reconciliation for Wastewater Treatment PlantSimulation Studies: Planning forHigh-QualityData andTypical Sources ofErrors.”Water Environment Research,vol.82,no.5,pp.426–433.

Schmit,C.G.,Jahan,K.,Schmit,K.,Aydinol,F.,Debik,E.,andPattarkine,V.(2010)“Activated Sludge and OtherAerobic Suspended Culture Processes.” Water Environment Research,vol.82,no.10,pp.1073–1123.

Su,Y.,Huang,C.,Pan,J.R.,Hsieh,W.,andChu,M.(2011)“FoulingMitigationbyTiO2CompositeMembraneinMembraneBioreactors.”Journal of Envi-ronmental Engineering (ASCE),doi:10.1061/(ASCE)EE.1943-7870.0000419.

WEF (1998) Design of Wastewater Treatment Plants. Fourth edition, Manual ofPracticeno.8,WaterEnvironmentFederation,Alexandria,Virginia.

Zodrow, K., Brunet, L., Mahendra, S., Li, D., Zhang,A., Li, Q., andAlvarez, P.J. (2007) “Polysulfone Ultrafiltration Membranes Impregnated with SilverNanoparticles Show Improved Biofouling Resistance and Virus Removal.”Polymers for Advanced Technologies, vol.18,no.7,pp.562–568.

165

Chapter 9

Secondary treatmentAttached growth and combined processes

9.1 INTRODUCTION

Thetwomajorcategoriesofbiologicaltreatmentare(1)suspendedgrowthand(2)attachedgrowthprocesses.Thefocusofthischapterisaerobicbio-logicaltreatmentusingattachedgrowthprocessesforbiochemicaloxygendemand(BOD)removal.Acombinationofsuspendedandattachedgrowthprocessesmaybeusedforbiologicaltreatmentofwastewater.Someofthesehybridprocessesarediscussedattheendofthechapter.SuspendedgrowthprocesseshavebeendiscussedinChapter8.BiologicalprocessesusedfornitrogenandphosphorusremovalwillbedescribedindetailinChapter13.

Inattachedgrowthsystemsthemicroorganismsareattachedtoaninertmedium,formingabiofilm.Asthewastewatercomesincontactwithandflowsoverthebiofilm,theorganicmatterisremovedbythemicroorgan-ismsanddegradedtoproduceanacceptableeffluent.Asecondaryclarifierisused,butsludgerecirculationtothebiologicalreactorisnotnecessary.The settled sludge consisting of sloughed biofilm is usually recirculatedback to the wet well or primary clarifier. Attached growth systems arecharacterizedbyahighdegreeofliquidrecirculation(100%to300%)tothebiologicalreactor.Themediumisusuallyaninertmaterialwithahighporosityandsurfacearea,e.g.rock,gravel,syntheticmedia.Thesystemcanbeoperatedasanaerobicoranaerobicprocess.Themediacanbewhollyorpartiallysubmergedinthewastewater.Thecommontypesofattachedgrowthprocessesincludetricklingfilters,biotowers,androtatingbiologi-calcontactors(RBCs).Thesewillbedescribedinthefollowingsections.

Themainadvantagesof the attachedgrowthprocesses are as follows(MetcalfandEddy,2003):

• Simplicityofoperation• Lowenergyrequirement• Lowmaintenancerequired• Abilitytohandleshockloads• Lowersludgeproduction


• Noproblemsofsludgebulkinginsecondaryclarifiers• Bettersludgethickeningproperties

Disadvantagesincludethefollowing:

• Lowefficiencyatcoldtemperatures• Possibilityofmasstransferanddiffusionlimitations• Problemswithbiofilmmaintenanceduetoexcesssloughing• HigherBODandsolidsconcentrationintheeffluent• Possibilityofodorproblems

9.2 SYSTEM MICROBIOLOGY AND BIOFILMS

Themicroorganisms inthebiofilmaresimilartothosefoundinactivatedsludgereactors.Theyaremostlyheterotrophic,withfacultativebacteriabeingpredominant.Fungiandprotozoaarepresent.Ifsunlightisavailable,algaegrowthisfoundnearthesurface.Largerorganismssuchassludgeworms,insectlarvae,rotifers,etc.mayalsobepresent.Whenthecarboncontentofthewastewaterislow,nitrifyingbacteriamaybepresentinlargenumbers.

Inanattachedculturereactor,microorganismsattachthemselvesontothe inertmediaandgrowintodensefilms.Thesearecalledbiofilms.Asthe wastewater passes over the biofilm, suspended organic particles areadsorbedon thebiofilmsurface.Theadsorbedparticlesaredegraded tosolubleproducts,whicharethenfurtherdegradedtosimplerproductsandgasesbythebacteria.Dissolvedorganicspass intothebiofilmaccordingtomasstransferprinciples,duetothepresenceofconcentrationgradients.Dissolvedoxygenfromthewastewaterdiffusesintothebiofilmfortheaer-obicbacteria.Wasteproductsandgasesdiffuseoutwardfromthebiofilmand are carried out of the reactor with the wastewater. This process isillustratedinFigure 9.1.

With the passage of time, the thickness of the biofilm increases. Thebiofilm grows in a direction outward from the media. As the thicknessincreases,theouter0.1to0.2mmofbiofilmremainsaerobic(Peavyetal.,1985).Theinnerlayersofthebiofilmbecomeanaerobic,asoxygencannotpassintotheinnerlayersduetodiffusionlimitations.Thetotalthicknessmayrangefrom100µmto10mm.Withincreasingthicknessthebiofilmattachmentbecomesweak,andshearingactionofthewastewaterdislodgesitfromthemediaandtransportsittothesecondaryclarifier.Thisprocessisknownassloughingofbiofilm.Regrowthofbiofilmoccursquicklyinplacesclearedbysloughing.Sloughingisafunctionofthehydraulicandorganicloadingonthereactor.Thehydraulic loadingaccountsforshearvelocities,andtheorganicloadingcontrolstherateofmetabolisminthebiofilmlayer.


TherateofBODremovaldependsonthefollowing(Peavyetal.,1985):

• Wastewaterflowrate• Organicloadingrate• Temperature• RatesofdiffusionofBODandoxygenintothebiofilm,withoxygen

diffusionrateusuallyalimitingfactor

9.3 IMPORTANT MEDIA CHARACTERISTICS

Selection of the media is an important aspect of the design of attachedgrowthprocesses.Inadditiontothesizeandunitweight,thefollowingareimportantcharacteristicsofthemediaorpackingmaterialusedinattachedculturesystems:

1.Chemical and biological inertness—Themediamaterialshouldnotundergoanychemicalorbiologicalreactionswiththeconstituentsofthewastewater.

2.Porosity—Porosityisdefinedastheratioofthevolumeofvoidstothetotalvolumeofaparticleormaterial.Itisgivenby

Air

O2

O2

CO2

Filtermedium Biofilm

Wastewaterfilm

Airspace

Anaerobic Aerobic

Organic matter

Wastewater flow

Figure 9.1 Mass transfer of organic matter and gases in a biofilm.


PorosityV

Vv

T

= (9.1)

where:Vv=volumeofvoidsVT=totalvolume

Porosityisoftenexpressedasapercent.Stonemediacanhaveporosi-tiesrangingfrom40%to50%,whilesyntheticmediacanhaveporos-ities up to 95%. Higher porosity is desired, as that provides morepassageforwastewaterandgases.Figure 9.2illustratestwotypesofmediaandanunderdrainpanelfortricklingfilters.

3.Specific surface area—Thisisdefinedastheamountofsurfaceareaofthemediathatisavailableforgrowthofbiofilm,perunitvolumeofmedia.Stonemediacanhavespecificsurfaceareasrangingfrom45to70m2/m3.Forsyntheticmedia,thisvaluecanrangefrom100to200 m2/m3.

9.4 LOADING RATES

TheorganicloadingorBODloadingiscalculatedusingonlytheBODloadcomingwiththeprimaryeffluent.TheBODloadingintherecycleflowisnotincluded.

BO D loading=Prim aryeffluentBO D

volum eoffilterm edia= Q S

V0 (9.2)

where:Q=wastewaterflowrate,m3/d(mgal/d)So=BODconcentrationofprimaryeffluent,kg/m3(lb/mgal)V=volumeofmedia,m3(1000sofft3)BODloading=kgBOD/m3·d(lbBOD/1000ft3·d)

Theorganic loading for stonemedia tricklingfilterscanvary from0.08to1.8kgBOD/m3·dforlow-tohigh-ratefilters,respectively.Theorganicloading forplasticmediafilters ranges from0.31 toabove1.0kgBOD/m3·d(Davis,2011).

Thehydraulicloadingistheamountofwastewaterappliedtothefiltersurfaceincludingprimaryeffluentandrecycleflows.

H ydraulicloading= +Q Q

Ar

s

(9.3)


(a)

(b)

(c)

Figure 9.2 Trickling filter media: (a) plastic media (Bio-Pac SF30™), (b) PVC sheet media (Dura-Pac XF31™), and (c) random media underdrain (ND 330™) (Photos courtesy of Jaeger Environmental of Virginia, and Jaeger Products, Inc. of Texas).


where:Q=wastewaterflowrate,m3/d(mgal/d)Qr=recirculationflow,m3/d(mgal/d)As=surfaceareaoffilter,m2(acres)Hydraulicloading=m3/m2·d(mgal/acre·d)

Thehydraulic loading for stonemedia tricklingfilters can range from4to40m3/m2·dforlow-tohigh-ratefilters,respectively.Forplasticmediafilters,thehydraulicloadingcanvaryfrom60to180m3/m2·d.

9.5 STONE MEDIA TRICKLING FILTER

Thetricklingfilterisoneoftheearliesttypesofattachedgrowthprocessesthatwereused. Ithasbeenused for secondary treatment since theearly1900s (MetcalfandEddy,2003).The term trickling filter ismisleading,asmostof thephysicalprocesses involved infiltrationareabsent in thisprocess. Instead, sorptionandsubsequentdegradationoforganicmatterareusedforsubstrateremoval(Peavyetal.,1985).AtypicaltricklingfilterisillustratedinFigure 9.3.

A trickling filter consists of a shallow tank filled with crushed stone,rocks,orslagasmedia.Thetankdepthrangesfrom0.9to2.5mforstonemediatricklingfilters.Theseprovidedurable,chemicallyinertsurfacesforgrowthofbiofilm.Media size ranges from50 to100mm(2–4 in)withporositiesof40%to50%.Thewastewater isappliedonthemediabyarotarydistributorarmfromthetopofthetank.Asthewaterflowsthroughthetank,organicmatterisremovedasitcomesincontactwiththebiofilm.Anunderdrainsystemtransportsthetreatedwastewaterandsloughedbio-filmtothesecondaryclarifier.

Feed pipe E�uent channel

Center column Underdrains

Distributor arm

Filter medium

Outlet ori�ce

Figure 9.3 Section through a trickling filter.


Recirculation is an important aspect of trickling filters. Recirculationratiosrangingfrom0.5to3areused.Liquidrecirculationisusedtoprovidethedesiredwettingratetokeepthemicroorganismsaliveandraisethedis-solvedoxygenoftheinfluent.Ithelpstodilutethestrengthofshockloads.Typical diurnal variation ofwastewater flow is illustrated in Figure 9.4.Recirculationisusedtodampenthevariationinloadingsovera24hperiod(Davis,2011).

Whenaportionoftheeffluentfromthetricklingfilterisrecycledbacktothefilter,whiletheremaindergoestothesecondaryclarifier,itiscalleddirect recirculation.ThisisillustratedinFigure 9.5(a).Therecirculationofsettledsludgefromthesecondaryclarifiertothewetwellortotheprimaryclarifieris termed indirect recirculation.Figure 9.5(b) illustrates indirectrecircula-tionofsettledsludgeandliquideffluentfromthesecondaryclarifier.

Tricklingfiltersmaybeusedassinglestageorwithtwostagesinseries.An intermediateclarifiercanbeusedbetweenthe twofilters.Usingtwofiltersinseriesaidsinimprovementofefficiency.

9.5.1 Design equations for stone media

Thefirstempiricaldesignequationsweredevelopedforstonemediatrick-lingfiltersbytheNationalResearchCouncil,basedonperformancedataatmilitaryinstallationstreatingdomesticwastewaterduringWorldWarII(Mohlman,1948a,b).TheseareknownastheNationalResearchCouncil,orNRC,equations.TheseequationswereusedtopredicttheefficiencyoftricklingfiltersbasedonBODload,volumeoffiltermedia,andrecircula-tionratio.

Forasingle-stagerockfilter,orforthefirststageofatwo-stagefilter,theefficiencyisgivenby

EW

VF

1

1

100

1 0 4432

=+ .

(9.4)

A.M.

Was

tew

ater

Flo

w

Noon P.M.

Daily average

Figure 9.4 Variation of wastewater flow over a typical 24 h period.


where:E1=BODremovalefficiencyforfirststagefilterat20°Cincluding

recirculation,%W1=BODloadingtofilter,kg/dV=volumeoffiltermedia,m3

F=recirculationfactor

Therecirculationfactoriscalculatedas

FR

R= +

+(1

1 0 1 2. ) (9.5)

whereR=recirculationratio=Q

Qr = recycleflow rate

wastewaterflow rate

(a)

(b)

Primary clarifier

Secondary clarifier

Sludge tolandfill

Sludge recirculationWastewater flow Sludge flow

Wastewaterinfluent

Trickling filter

Effluent

Effluent recycle

Primaryclarifier

Secondaryclarifier

Sludge tolandfill

Sludge recirculationWastewater flowSludge flow

Wastewaterinfluent Trickling

filter

Effluent

Effluent recycle

Figure 9.5 (a) Trickling filter with direct recirculation of effluent, (b) trickling filter with indirect recirculation.


Foratwo-stagetricklingfilter,theBODremovalefficiencyofthesecondstageisgivenby

E

E

W

VF

2

1

2

100

10 4432

1

=+

−.

(9.6)

where:E2=BODremovalefficiencyforsecondstagefilterat20°Cincluding

recirculation,%W2=BODloadingtosecond-stagefilter,kg/dE1=fractionofBODremovalinfirststagefilter

An intermediateclarifier isassumedtobesituatedbetweenthefirstandsecondstagefilters.TheeffectofwastewatertemperatureonBODremovalefficiencyiscalculatedusingaformofthevan’tHoff–Arrheniusequation:

ET=E20(1.035)T–20 (9.7)

where:T=wastewatertemperature,°CET=BODremovalefficiencyattemperatureT°CE20=BODremovalefficiencyat20°C

WhenusingtheNRCequationsitshouldbenotedthatthemilitaryinstalla-tions,whichwerethebasisfortheNRCstudy,hadhigherinfluentBODcon-centrationsthandomesticwastewatertoday.Theclarifierswereshallowerandcarriedhigherhydraulicloadingthancurrentpracticetoday(Davis,2011).

EXAMPLE9.1A wastewater treatment plant uses a single stage rock-media trick-lingfilterforsecondarytreatment,asillustratedinFigure 9.5(a).Thewastewaterflowrateis2000m3/dwithaBOD5concentrationof400mg/L.Primaryclarificationremoves30%oftheBOD5.Thefilteris12mindiameterand1.5mindepth.Directrecirculationpumpoperatesat2.78m3/mintothefilter.Wastewatertemperatureis20°C.Calculatethehydraulicloadingrate,organicloadingrate,effluentBOD5concen-tration,andoverallplantefficiency.

SOLUTION

Step1.Calculatefilterareaandvolume.

Tricklingfilterarea,As=π π4 4

122 2

D( ) = ( )=113.09m2

Tricklingfiltervolume,V=As×h=113.09m2×1.5m=169.65m3


Step2.Calculatehydraulicloadingrateonfilter.

BOD5intotricklingfilter,So=400(1–0.30)=280mg/l=0.28kg/m3

Recirculationflow,Qr=2.78m3/min=2.78×60×24=4003m3/d

Recirculationratio,R=Q

Qr = 4003

2000=2

Hydraulicloadingrate=Q Q

A

m d

mr

s

+ = +2000 4003

113 09

3

2

/

.

=53.08m3/m2·d

Step3.Calculateorganicloadingrate.

Organic/BODloadingrate=Q S

V0 =

2000 0 28

169 65

33

3

m

dkg m

m

× . /

.

=3.30kg/m3·d

Step4.Calculatethefilterefficiency.

Recirculationfactor, FR

R= +

+(1

1 0 1 2. )=

1 2

1 0 1 22

+

+ ×( ).=2.08

W1/V=3.3kg/m3·d

EW

VF

1

1

100

1 0 4432

100

1 0 44323 30

2 08

=+

=+. .

.

.

=64.17%

PlanteffluentBODconcentration=280mg/L(1–0.6417)

=100.32mg/L

Overallplantefficiency=400 100 32

400100

− ×.% =74.92%

Comment:ThehydraulicandBODloadingratesarehigh.Thesecanbereducedbyaddingmorefiltersinparalleltothefirstone.


9.6 BIOTOWER

Biotowers are deep bed trickling filters with plastic or synthetic media.Depths up to 12 m can be utilized, since lightweight media are used.Varioustypesandshapesofmediaareusedforpacking.Smallplasticcyl-inderswithperforatedwallsmaybeusedasillustratedinFigure 9.2.Thespecificsurfacearearangesfrom100to130m2/m3withaporosityofabout94%.Randompackingallowsthewastewatertobedistributedthroughoutthemedia,allowingenoughcontacttimebetweenthesubstrateandbio-film.Modularmediaconsistingofcorrugatedandflatpolyvinylchloride(PVC)sheetsweldedtogetherinalternatingpatternsarealsoused.TheseareillustratedinFigure 9.2.

9.6.1 Design equations for plastic media

Thedesignmodelsforplasticmediatricklingfiltersorbiotowerswerebasedon the early work of Velz (1948), Howland (1958), and Schulze (1960).Eckenfelder(1961)appliedtheSchulzeequationtoplasticmediabiotowersas

S

See

o

kD

qn=−

(9.8)

where:Se=solubleBODconcentrationofsettledfiltereffluent,mg/LSo=solubleBODconcentrationoffilterinfluent,mg/LD=depthofmedia,mq=hydraulicapplicationrateexcludingrecirculation,m3/m2·mink=wastewaterandfiltermediatreatabilitycoefficient,min-1

n=coefficientrelatedtopackingmedia,takenas0.5formodularplasticmedia

Thevalueofkrangesfrom0.01to0.1min–1.kat20°Cisaround0.06formunicipalwastewateronmodularplasticmedia(Germain,1966).kispri-marilyaffectedbytemperature,andtemperaturecorrectionsareperformedusingthevan’tHoff–Arrheniusequation:

kT=k20(1.035)T–20 (9.9)

where:kT=treatabilityconstantattemperatureT°Ck20=treatabilityconstantat20°CT=wastewatertemperature,°C


Equation(9.8)canbemodifiedtakingintoaccounttheeffectofrecircula-tion,asshownbelow:

S

S

e

R Re

e

a

kD

q

kD

q

n

n

=

+( ) −

−

−1

(9.10)

where:Sa=BODconcentrationofthemixtureofprimaryeffluentand

recycledwastewater,mg/LSe=effluentBODconcentration,mg/LR=recirculationratioq=hydraulicloadingratewithrecirculation,m3/m2·min

Allothertermsareasdefinedpreviously.FrommassbalanceSaiscalcu-latedas

SS RS

Ra

o e= ++1

(9.11)

Othermodelshavebeenproposedtakingintoaccountthespecificsur-faceareaofthemedia.ThemodifiedVelzequationincorporatesthespecificsurfaceareaasshownbelow(HammerandHammer,2012):

S

See

o

k A D

q

sn

=− 20

(9.12)

whereAs=specificsurfaceareaofmedia,m2/m3.Allothertermsareasdefinedpreviously.Equation(9.12)canbemodi-

fiedtoincorporatetheeffectofrecirculationandobtainanequationsimi-lartoequation(9.10).

EXAMPLE9.2Anindustryhasdecidedtotreatitsprocesswastewaterinabiotowerwithaplasticmodularmedium(n=0.6).Theflowrateofthewastewa-teris1000m3/dwithaBOD5of500mg/landanaveragetemperatureof18°C.The treatability constantk is0.04min–1 for the systemat20°C.Depthofthemediumis5.5m.ThedesiredeffluentBOD5is15mg/L.Calculatethefollowing:


a.Theareaofbiotowerrequired,withoutanyrecycle. b.Theorganicloadingrateforthebiotowerwithoutrecycle. c.Theareaofbiotowerrequiredwhendirectrecirculationratiois

3:2. d.Theorganicloadingrateforthebiotowerwithrecycle. e.Whichoftheabovedesignsseemsbettertoyouandwhy?

SOLUTION

Step1.Adjustkfortemperature.Usethevan’tHoff–Arrheniusequation(9.9):

k18=k20(1.035)18–20

=0.04(1.035)–2

=0.037min–1

Step2.Calculatesurfaceareaforbiotowerwithoutrecycle.Usingequation(9.8),calculateq:

S

See

o

kD

qn=−

or

15

500

0037 5 506m g L

m g Le q/

/

. ..

=− ×

or

loge(0.03)= − ×0 037 5 506

. ..q

or

q0.6=0.058

or

q=0.0087m3/m2·min=12.528m3/m2·d

Q=1000m3/d

Surfacearearequired=Q

q

m d

m

md

= 1000

12 528

3

3

2

/

. ·

=79.82m2≈80m2


Step3.Calculateorganicloadingrateusingequation(9.2).

BODloading = Q S

V0

=×

×

1000 0 5

80 5 5

33

2

m

dkg m

m m

. /

.=1.14kgBOD/m3·d

Step4.CalculateSausingequation(9.11),withR=3/2=1.5.

SS RS

Ra

o e= ++1

or

S

m g

Lm g L

a =+ ×

+

. /

.

500 1 5 15

1 1 5=209mg/L

Step5.Calculateqforbiotowerwithrecycle.Useequation(9.10)tocalculateq:

S

S

e

R Re

e

a

kD

q

kD

q

n

n

=

+( ) −

−

−1

or

15

2091 1 5 1 5

0037 5 506

/

/. .

. ..

m g L

m g L

e q

=

+( ) −

− ×

. ..

e q− ×0 037 5 5

06

or

0 179 0 108

02035 0 203506 0 6

. .

. .. .

= +− −e eq q

or

0 179

1 108

0203506.

.

..

=−e q

or

log ..

.eq

0 16160 2035

06( ) = −


or

q0.6=0.1116

or

q=0.026m3/m2·min=37.27m3/m2·d

Step6.Calculatesurfaceareaofbiotowerwithrecycle.

Surfacearearequired=Q R

q

m d

m

md

( ) ( .) /

. ·

1 1000 1 1 5

37 27

3

3

2

+ = +=67m2

Step7.Calculateorganicloadingrateforbiotowerwithrecycle,usingequation(9.2).

BODloading = Q S

V0

=×

×

1000 0 5

67 5 5

33

2

m

dkg m

m m

. /

.=1.36kgBOD/m3·d

Note:Thebiotowerwithrecycleseemstobethebetterdesign.Asmallersurfaceareaisrequired,andatthesametimeitcanhandlehigherBODloadingdue to recirculation.Construction costwouldbe lower, butadditionalpumpingcostforrecirculationwillhavetobeconsidered.

9.7 ROTATING BIOLOGICAL CONTACTOR

Therotatingbiologicalcontactor,orRBC,wasfirstinstalledinGermanyin1960andlaterintroducedintheUnitedStates(MetcalfandEddy,2003).RBCisatypeofattachedgrowthprocesswherethemediumisinmotionaswellasthewastewater.Aseriesofcloselyspacedcirculardisksofpoly-styreneorpolyvinylchloridearemountedonahorizontalshaft,whichisrotatedinatankthroughwhichthewastewaterisflowing(Figure 9.6).Themediaispartiallysubmergedinthewastewater.Itcomesincontactwithairandwastewaterinanalternatingfashion,thusmaintainingaerobiccondi-tions,astheshaftwiththedisksrotatesinthetank.Rotationalspeedvariesfrom1to2rpm.Itmustbesufficienttoprovidethehydraulicshearneces-saryforsloughingofbiofilmandtomaintainenoughturbulencetokeepthesolidsinsuspensioninthewastewater(Peavyetal.,1985).Themediadiskshaveadiameterrangingfrom2to4m,andthicknessofabout10mm.Spacingbetweenthedisksisabout30to40mm.Eachshaftwiththemedium,alongwithitstankandrotatingdevice,iscalledamodule.Several


modulesarearranged in seriesorparallel toobtain thedesired removalefficiencies.Figure 9.7illustratesaflowdiagramofanRBCprocess.

TheRBCmodulesprovidealargeamountofsurfaceareaforbiomassgrowth.Onemoduleof3.7mdiameterand7.6mlengthcontainsapproxi-mately10,000m2ofsurfacearea.Thelargeamountofbiomassisabletoproduceacceptableeffluentswithinashortcontacttime.Recirculationofeffluent through the reactor is not necessary. Advantages of the processinclude low power requirements, simple operation, and good ability forsludgetosettle.Disadvantagesincludehighcapitalcostandsusceptibilitytocoldtemperatures.Covershavetobeprovidedtomaintainthebiomassinwinter.RBCscanbeoperatedasaerobicoranaerobicprocesses.

NitrificationcanbeachievedinanRBCsystembyoperatinganumberofmodulesinseries.BODremovaltakesplaceinthefirststages.Thenwhen

(a) Plan view

(b) Side view

Primarye�uent

To secondaryclari�er

Drivemotor

RBC shaft

Radialpassages

Concentricpassages

Supplementalair header

Central steel shaft

Figure 9.6 Plan and side view of an RBC module.


thecarboncontentislow,nitrificationtakesplace.Typicallyfivemodulesinseriesarerequiredforcompletenitrification.

ManufacturersofRBCsystemsoftenspecifyasolubleBODloadingratefortheirequipment,sincethesolubleBODisusedmorerapidlyinthefirststageofanRBCsystem.AsolubleBODloadingintherangeof12to20gsBOD/m2·d(2.5to4.1lbsBOD/1000ft3·d)iscommonlyspecifiedforthefirststage(MetcalfandEddy,2003).ThetotalBODloadingcanrangefrom24to40gsBOD/m2·d,assuminga50%solubleBODfraction.Toaccommodatehigherloadingratesduetohigh-strengthwastewaters,mul-tiplemodulesareusedinparallelforthefirststage.AnumberofempiricaldesignapproacheshavebeenusedforRBCsystemsbasedonpilotplantandfull-scaleplantdata.AreviewofthesemodelsisprovidedinWEF(2000).

9.8 HYBRID PROCESSES

Activatedsludgesystemsthatincorporatesomeformofmediainthesus-pendedgrowthreactoraretermedhybrid processes(Davis,2011).Theseincludemovingbedbiofilmreactor(MBBR),integratedfixed-filmactivatedsludge(IFAS),andfluidizedbedbioreactor(FBBR),amongothers.Hybridactivatedsludge/MBBRprocesseshavebeeninvestigatedfortreatmentofmunicipalwastewaterincoldclimates(DiTrapanietal.,2011).

9.8.1 Moving bed biofilm reactor (MBBR)

The MBBR process was developed in the late 1980s in Norway. Smallcylinder-shaped polyethylene media elements are placed in the aerationtanktosupportbiofilmgrowth.Thetankmaybemixedwithaerationormechanicalmixers.Aperforatedplateorscreenisplacedattheoutlettopreventthelossofmediawiththeeffluent.Figure 9.8illustratesatypicalMBBR.Thebiofilmcarrierelementsareabout10mmindiameterand7

In�uent E�uent

Sludge waste

Primaryclarier

Secondaryclarier

Rotating biological contractors

Figure 9.7 Flow diagram of an RBC process (adapted from Peavy et al., 1985).


mminheight,withadensityofabout0.96g/cm3.Thespecificsurfacearearanges from 300 to 500 m2/m3. The biofilm in this process is relativelythinnerandmoreevenlydistributedoverthecarriersurface,ascomparedwithotherfixed-filmprocesses.Toobtainthistypeofbiofilm,thedegreeofturbulenceinthereactorisimportant(Ødegaard,2006).Themixingorturbulencetransportsthesubstratetothebiofilmandalsomaintainsalowthicknessofthebiofilmbyshearingforces.

Advantages of the MBBR process include (1) continuous operation ofthereactorwithoutthethreatofclogging,(2)nobackwashrequirement,(3)sludgerecirculationisnotnecessary,(4)lowheadloss,and(5)ahighspecificbiofilmsurface(Ødegaardetal.,1994;RustenandNeu,1999;andAndreottolaetal.,2003).

Whenatreatmentplantneedsto increase itscapacitydueto increasedBODloading,thiscanbeachievedbyaddingmorebiofilmcarrierelementstothereactortoincreasethebiofilmsurfacearea(Aspegrenetal.,1998;andRustenetal.,1995).Forthesamereason,existingactivatedsludgeprocesscanbeupgraded toanMBBRprocess tohandle increased loadswithoutexpansionofexistingreactorvolume.Costofthesyntheticmediahastobeconsideredwithrespecttoothercosts.TheMBBRprocessmaybeusedforaerobic,anoxic,oranaerobicprocessesforcarbonandnitrogenremoval.

9.8.2 Integrated fixed-film activated sludge (IFAS)

In the IFASprocesses, afixedpackingmaterial is placed in an activatedsludgereactor.Thepackingcanbeintheformofframes,foampads,etc.suspendedintheaerationtank.AnumberofproprietaryprocessesincludeBioMatrix®process,Bio-2-Sludge®process,Ringlace®,andBioWeb®.These

(a) (b)

Air

Mixer

In�uent E�uent In�uent E�uent

Screen Screen

Suspendedpacking

Suspendedpacking

Figure 9.8 Moving bed biofilm reactor (MBBR) (a) mixing with external air and (b) mechanical mixing (Source: Adapted from Ødegaard, 2006).


processesdifferfromtheMBBRinthattheyuseareturnsludgeflow.Thepurposeofthefixed-filmmediumistoincreasethebiomassconcentrationinthereactor.Thisisadvantageousinincreasingthecapacityoftheactivatedsludgeprocesswithoutincreasingtankvolume.

9.8.3 Fluidized bed bioreactor (FBBR)

In an FBBR, wastewater enters at the bottom of the aeration tank andflowsupwardthroughabedofsandoractivatedcarbon.Activatedcarbonprovidesbothadsorptionpropertiesandmediasurfaceforbiofilmgrowth.Thespecificsurfaceareaisabout1000m2/m3ofreactorvolume,withbeddepthsof3to4m.Effluentrecirculationisperformedtoprovidethefluidvelocitywithin thenecessarydetention times.AdiagramofanFBBR isprovidedinFigure 9.9.Asthebiofilmincreasesinthickness,themediumaccumulatesatthetopofthebedfromwhereitisremovedandagitatedtoremove excess solids at regular intervals. In aerobicFBBRs, recirculatedeffluentispassedthroughanoxygentanktosaturatewithdissolvedoxy-gen.Airisnotaddeddirectlytothereactor.Thesystemhasanumberofadvantages,including(1)longsolidsretentiontime(SRT)forthemicroor-ganismsnecessarytodegradetoxiccompounds,(2)abilitytohandleshockloads,(3)productionofhigh-qualityeffluentthatislowintotalsuspendedsolids (TSS)andchemicaloxygendemand(COD)concentration,and(4)systemoperationissimpleandreliable(MetcalfandEddy,2003).

Formunicipalwastewater,FBBRshavebeenused forpost-denitrifica-tion.TheFBBRprocess is suitable for removal of hazardous substancesfromgroundwater.

In�uent

E�uent

Screen

Air

O2

Oxygenation

Recycle

Distributionplenum

Sand or activatedcarbon packing

Figure 9.9 Fluidized bed bioreactor (FBBR).


9.9 COMBINED PROCESSES

Acombinationoftricklingfilterandactivatedsludgecanbeusedfortreat-mentofwastewater.Combinedprocesseshaveresultedaspartofaplantupgradewhereatricklingfilteroractivatedsludgereactorisaddedtoanexistingsystem,or theyhavealsobeen incorporated intonewtreatmentplantdesigns (Parker et al.,1994).Combinedprocesseshave theadvan-tagesofeachoftheindividualprocesses,includingthefollowing:

(a)

(b)

Primarye�uent E�uent

Aeration basin

Returnactivated

sludgeSludge

Tric

klin

g�l

ter

Under�owRecycle

Secondaryclari�er with�occulating

center feed well

Sludgereaeration


Aeration basin

Returnactivated

sludgeSludge

Tric

klin

g�l

ter

Under�owRecycle

Secondaryclari�er

Figure 9.10 (a) Trickling filter/activated sludge (TF/AS) process, (b) trickling filter/solids contact (TF/SC) process, and (c) series trickling filter/activated sludge pro-cess (Source: Adapted from Metcalf and Eddy, 2003).


• VolumetricefficiencyandlowenergyrequirementofattachedgrowthprocessforpartialBODremoval

• Stabilityandresistancetoshockloadsoftheattachedgrowthprocess• Highqualityofeffluentwithactivatedsludgetreatment• Improvedsludgesettlingcharacteristics

Figure 9.10presents flowdiagrams of a number of combinedprocesses.Figure 9.10(a),(b),and(c)illustratesatricklingfilter/activatedsludge(TF/AS)process,atricklingfilter/solidscontact(TF/SC)process,andaseriestricklingfilter/activatedsludgeprocess,respectively.

PROBLEMS

9.1 Whatisthemajordifferencebetweensuspendedgrowthandattachedgrowthprocesses?

9.2 Explain with the help of flow diagram the meaning of the termsdirect recirculationandindirect recirculation.

9.3 Awastewatertreatmentplantusesasinglestagerock-mediatricklingfilterforsecondarytreatment,asillustratedinFigure 9.5(a).Thewaste-waterflowrateis2000m3/dwithaBOD5concentrationof400mg/L.Primaryclarification removes30%of theBOD5.Thefilter is12mindiameterand1.5mindepth.Directrecirculationpumpoperatesat2.78m3/mintothefilter. It isobserved,asshowninthesolution

(c)


Aeration basin

Returnactivated

sludgeSludge

Tric

klin

g�l

ter

Under�owRecycle

Secondaryclari�er

Intermediateclari�er

Sludge

Figure 9.10 (Continued)


toExample9.1, that theuseofa singlefilterproducesaplant effi-ciencyofabout75%,withveryhighhydraulicandBODloadingrates.Toreducetheloadingratesandincreaseplantefficiency,theengineerdecidestoaddanidenticalfilterinparalleltothefirstone.Thedirectrecirculation ratio is maintained at 2.0. Wastewater temperature is20°C.Calculatethenewhydraulicloadingrate,organicloadingrate,effluentBOD5concentration,andoverallplantefficiency.Isthenewdesignbetterthantheoldone?

9.4 Consider the single stage trickling filter plant from the solutionin Example 9.1. The use of a single stage filter produced a plantefficiencyofabout75%.Toincreaseplantefficiency, theengineerdecides toaddan identical secondstagefilter inseries to thefirstone.Thewastewaterflowrateis2000m3/dwithaBOD5concentra-tionof400mg/L.Primaryclarificationremoves30%oftheBOD5.Thefiltersareeach12mindiameterand1.5mindepth.Thedirectrecirculationratioforeachstageis2.0.Wastewatertemperatureis20°C. Calculate the second stage hydraulic loading rate, organicloading rates, effluentBOD5concentration,andoverallplant effi-ciency.Commenton the advantages/disadvantages of using singlestageversustwostagetricklingfilters.

9.5 Reworkproblem9.3forawastewatertemperatureof15°C. 9.6 Reworkproblem9.4forwastewatertemperaturesof15°Cand22°C. 9.7 Aplasticmediabiotowerhasverticalflowpackingwithn=0.5and

k20of0.045min–1.Thetoweriscylindricalwithadiameterof10m.Depthofpackingmediumis6m.Primaryeffluentflowrateis2500m3/d with a soluble BOD5 of 250 mg/L. The average wastewatertemperature is16°C.Direct recirculation ispracticedat1:1 ratio.Calculatethefollowing:

a. Organicloadingrateofthebiotower b. Hydraulicloadingrateofthebiotower c. Recirculationratio d. EffluentsolubleBOD5concentration 9.8 Anindustryhasdecidedtotreatitsprocesswastewaterinabiotower

with a plastic modular medium (n = 0.55). The flow rate of thewastewateris1400m3/dwithaBOD5of630mg/l.Averagesum-merandwintertemperaturesare22°Cand15°C,respectively.Thetreatabilityconstantis0.04min–1forthesystemat20°C.Depthofthemediumis4m.CalculatetheareaofthebiotowerrequiredtoproduceaneffluentwithaBOD5of20mg/l,witharecycleratioof2:1.

9.9 Rework problem 9.8 using a winter recirculation ratio of 3:1.Summerrecirculationratioremainsthesameat2:1.Commentontheprosandconsofusingahigherrecirculationratio.

9.10 Whatisahybridprocess?Giveanexample.


REFERENCES

Andrettola, G., Foladori, P., Gatti, G., Nardelli, P., Pettena, M., and Ragazzi, M.(2003)“UpgradingofaSmallOverloadActivatedSludgePlantUsingaMBBRSystem.”Journal of Environmental Science and Health,vol.A38,no.10,pp.2317–2338.

Aspegren,H.,Nyberg,U.,Andersson,B.,Gotthardsson, S., and Jansen, J. (1998)“PostDenitrificationinaMovingBedBiofilmReactorProcess.”Water Science and Technology,vol.38,no.1,pp.31–38.


DiTrapani,D.,Christensso,M.,andØdegaard,H.(2011)“HybridActivatedSludge/BiofilmProcessfortheTreatmentofMunicipalWastewaterinaColdClimateRegion: A Case Study.” Water Science and Technology, vol. 63, no. 6, pp.1121–1129.

Eckenfelder,W.W. Jr. (1961)“Trickling Filter Design and Performance.” Journal Sanitary Engineering Division,vol.87(SA6),p.87.

Germain,J.E.(1966)“EconomicalTreatmentofDomesticWastebyPlastic-MediaTricklingFilters.”Journal Water Pollution Control Federation,vol.38,no.2,p.192.

Hammer,M.J.,andHammer,M.J.Jr.(2012)Water and Wastewater Technology.Seventhedition.Pearson-PrenticeHall,Inc.,UpperSaddleRiver,NewJersey.

Howland,W.E.(1958)“FlowoverPorousMediainaTricklingFilter.”Proceedings of the 12th Industrial Waste Conference,PurdueUniversity,Lafayette,Indiana,p.435.


Mohlman,F.W. (1948a)“SewageTreatmentatMilitary Installations–SummaryandConclusions.”Sewage Works Journal,vol.20,p.52–95.

Mohlman,F.W. (1948b)“High-RateFilterPerformance.”Sewage Works Journal,vol.20,p.618–622.

Ødegaard, H. (2006) “Innovations in Wastewater Treatment: The Moving BedBiofilmProcess.”Water Science and Technology,vol.53,no.9,pp.17–33.

Ødegaard, H., Ruaten, B., andWestrum,T. (1994)“A New Moving Bed BiofilmReactor:Applications and Results.” Water Science Technology, vol. 29, no.10–11,pp.157–165.

Parker,D.S.,Krugel,S.,andMcConnell,H.(1994)“CriticalProcessDesignIssuesintheSelectionoftheTF/SCProcessforaLargeSecondaryTreatmentPlant.”Water, Science and Technology,vol.29,p.209.


Rusten,B.,andNeu,K.E.(1999)“DowntoSize.”Water Environment & Technology,vol.11,no.1,pp.27–34.

Rusten, B., Hem, L. J. and Ødegaard, H. (1995) “Nitrification of MunicipalWastewaterinMoving-BedBiofilmReactors.”Water Environment Research,vol.67,no.1,pp.75–86.

Schulze, K. L. (1960) “Load and Efficiency in Trickling Filters.” Journal Water Pollution Control Federation, vol.33,no.3,pp.245–260.


Velz,C.J.(1948)“ABasicLawforthePerformanceofBiologicalFitters.”Sewage Works Journal,vol.20,no.4.

WEF (2000) Aerobic Fixed Growth Reactors: A Special Publication. WaterEnvironmentFederation,Alexandria,Virginia.

189

Chapter 10

Secondary Clarification

10.1 INTRODUCTION

Thetermsecondary clarificationdenotesclarificationofeffluentfromsec-ondarybiologicalreactorsinsettlingtanks.Secondaryclarifiersareplacedafterthebiologicalreactors,andconstitutesecondarytreatmenttogetherwiththebiologicalunit.Theassumptionisthatallbiochemicalreactionstakeplaceinthebioreactor,andthefunctionoftheclarifierisseparationofsolidsfromtheliquidfractionandthickeningofsettledsolidsinmostcases.

Thedesignofsecondaryclarifiersfollowingsuspendedgrowthprocessesis slightly different from clarifiers following attached growth processes.Thecharacteristicsofthebiologicalsolidsinthesetwotypesofprocessesare significantly different, so the design and operation of the secondaryclarifiersforthesesystemsarealsodifferent(Peavyetal.,1985).

AccordingtotheUrbanWastewaterTreatmentDirective(91/271/EEC)oftheEuropeanUnion(EU),thelimitsforsecondarytreatmentBOD5(bio-chemicaloxygendemand)andtotalsuspendedsolids(TSS)are25mg/Land35mg/L, respectively.TheU.S.EnvironmentalProtectionAgency (EPA)specifiesaneffluentBOD5lessthanorequalto30mg/Landaneffluentsuspendedsolidsconcentrationlessthanorequalto30mg/Lforsecondarytreatment.Multipleunitscapableofindependentoperationarerequiredforallplantswheredesignaverageflowsexceed380m3/d(GLUMRB,2004).Thehydraulicloadingisusuallybasedonpeakflowrates.

10.2 SECONDARY CLARIFIER FOR ATTACHED GROWTH PROCESS

Theprimaryobjectiveof secondary clarifiers followingattachedgrowthprocesses is to achieve clarification of treated wastewater. Sludge thick-ening is not considered in the design. The goal is to settle the sloughedbiofilmorhumus,whichexhibitsType IIorType I settling.Asaresult,this type of secondary clarifier is designed similar to primary clarifiers.


Sidewaterdepthsrangefrom2to5m,withcorrespondingmaximumover-flowratesof18to65m3/m2·d(Davis,2011).GLUMRB(2004)specifiesapeakhourlyoverflowrateof2.0m/h.

High-ratetricklingfiltersandbiotowersareusuallydesignedwithahighdegree of liquid recirculation,which can range from100% to300%. Ifindirectrecirculationisused,thentheclarifiersizeisincreasedsignificantly.Direct recirculationmaybeanoption tohandlehighrecirculationrates,togetherwiththeuseofmodularsyntheticmedia.Thereisnosludgerecyclefromtheclarifiertothebioreactor.Sludgemaybepumpedtotheprimaryclarifiertobesettledwiththerawwastewatersolids,thenundergofurtherprocessingpriortodisposal.Figure 10.1illustratesasecondaryclarifierforabiotowersystemwith(a)directrecirculationand(b)indirectrecirculation.

EXAMPLE10.1Design secondary clarifiers for a wastewater treatment plant usingbiotowers for treatmentof amunicipalwastewater.Thewastewaterflowrateis1500m3/dwithaBOD5of180mg/Landsuspendedsolidsof200mg/L.Thebiotowerusesindirectrecirculationandoperatesatarecycleratioof2:1.Adesignsidewaterdepthof3misselectedwithamaximumoverflowrateof1.6m3/m2·h.

SOLUTION

Design2circularsecondaryclarifiersfortheplant.

Totalwastewaterflowwithrecycle,QT=1500m3/d×(2+1)=4500m3/d

Useapeakingfactorof2.0.

Totaldesignflow,Q=4500m3/d×2.0=9000m3/d

Flowineachclarifier=90002

=4500m3/d=187.50m3/h

Surfacearea,As=187 501 6

3

3 2

. /

. / ·

m h

m m h=117.19m2

Diameterofeachclarifier,D= 4 117 19 2× . m

π=12.21m=roundoffto12.5m

Note:Weshouldnotroundoffto12m,asthatwillreducethesurfaceareaand increase theoverflow ratebeyond themaximumvalue. Soroundofftothenext0.5mincrement.Therefore,twocircularclari-fierswillbeusedwith12.5mdiameter.

Secondary Clarification 191

10.3 SECONDARY CLARIFIER FOR SUSPENDED GROWTH PROCESS

Secondaryclarifiersfollowingsuspendedgrowthprocessesaredesignedtoachieve twomajor functions: (1)clarificationofeffluentand(2) thicken-ingofbiologicalsolids.Theeffluentfromtheactivatedsludgereactororothersuspendedgrowthprocesshastobeclarifiedtoreducethesuspendedsolidsconcentrationtomeetdischargelimits.Atthesametime,thesludgehastobethickenedpriortorecyclingbacktotheactivatedsludgereactor(Figure 10.2)andbeforefurthertreatment.Thesecondaryclarifierhastoachievebothofthesecriteria.

Clarificationisduetosettlingofthelighterflocculentparticles;thicken-ingisduetomassfluxofsolids inthehinderedsettlingzone,wherethesolidsconcentrationismuchhigher.Itisnotpossibletoselectanoverflowrate to represent the settling velocity of such a complex composition of

(a)

(b)

Primaryclarifier

Secondaryclarifier

Sludge tolandfill


Wastewaterinfluent Bio

towerEffluent

Effluent recycle

Primaryclarifier

Secondaryclarifier

Sludge tolandfill


Wastewaterinfluent Bio

towerEffluent

Effluent recycle

Figure 10.1 Secondary clarification for a biotower system with (a) direct recirculation and (b) indirect recirculation.


biosolids.Thesurfaceareaforeachofthesefunctionshastobedeterminedseparately,andthenthesurfaceareathatsatisfiesbothcriteriaisselected.The surface area required for clarification is determined from the sameprinciplesasthoseusedforaprimaryclarifier.Thesurfacearearequiredforthickeningcanbedeterminedbyoneofthefollowingmethods:(1)sol-ids flux analysisor(2)state point analysis.Bothmethodsarebasedonthesameprinciples.Thesolidsfluxmethodisdiscussedinmoredetailinthefollowingsections.Thestatepointanalysisisusedmoreforoptimizationofexistingsystems.ReadersarereferredtoMetcalfandEddy(2003)foradditionaldetailsofthatmethod.

Inasecondaryclarifier,TypeI,TypeII,TypeIII,andTypeIVsettlingmaybeobservedatdifferentdepths,dependingonthesolidsconcentration.Thesecondaryclarifierisdesignedtoincreasetheincomingsolidsconcen-trationXitoamuchhigherunderflowsolidsconcentrationXu.Asaresult,thesettlingcharacteristicschange,resultinginzoneswithdifferenttypesofsettling.Thiscanbedemonstratedwithabatchsettlingcolumntest.Thesettlingcolumntestcanbeusedtodeterminethehinderedsettlingvelocitycorrespondingtotheinitialsolidsconcentration(MetcalfandEddy,2003).

10.3.1 Settling column test

AclearPlexiglascylinderisusedforthesettlingcolumntest.Theheightofthecylindershouldbeequaltotheheightoftheclarifier.ThecolumnisfilledwithasuspensionwithasolidsconcentrationX1,whichisallowedtosettleinanundisturbedmanner.Afterashorttimeatt2,fourdistinctzoneswilldevelopinthecolumn,asillustratedinFigure 10.3.Zone1istheclarifiedeffluentzonewithalowconcentrationofparticles,wheredis-crete(TypeI)settlingandsomeflocculent(TypeII)settlingoccurs.Inzone2,theinitialconcentrationremainswheretheparticlessettleatauniformvelocity.Hindered(TypeIII)settlingisobservedinthiszone.Duetothe

Primaryclari�er

Secondaryclari�er

Sludge toland�ll Sludge recycle

Wastewater owSludge ow

Wastewaterinuent Aeration

tank

E�uent

Sludge waste

Figure 10.2 Secondary clarifier in activated sludge process with recycle.


highconcentrationofparticles, thewater tends tomoveupthroughtheintersticesofthecontactingparticles.Thecontactingparticlestendtoset-tleasablanket,maintainingthesamerelativepositionwithrespecttoeachother,resultinginhinderedsettling.Zone3representsatransitionzone.The concentration increases from the interface of zones 2 and 3 to theinterfaceofzones3and4,creatingaconcentrationgradient.Compression(TypeIV)settlingisobservedinzone4.Assolidssettletothebottomofthecylinder,particles immediatelyabovefallontopof them, formingazonewhere the solidsaremechanically supported frombelow.Solids inthecompressionzonehaveanextremelylowvelocitythatresultsmainlyfromconsolidation.

Aftersometimet3, theheightof theclarifiedzone1andcompressionzone4increases,whilezone2decreases.Withtime,asmoreandmorepar-ticlessettle,zone2disappears(t4).Eventually,zone3decreasesuntilonlyzones1and4remain(t5).Theinterfacebetweenzones1and4willtraveldownward at a very slow rate as the solids consolidate from their ownweightandreleasewatertotheclarifiedzone.Onlytheinterfacesinvolvingzone1withtheotherzoneswillbevisibleuponobservation.Theinterfacesbetweentheotherzoneswillnotbereadilyvisibleastheconcentrationdif-ferencesarenotsignificant.

From the settlingcolumn testwithX1 initial solids concentration, theheight of the interface ismeasured at regular time intervals andplottedversustime,toobtainacurvesimilartothatshowninFigure 10.4.Theinitialportionofthecurveisastraight line.TheslopeofthisportionisthehinderedsettlingvelocitycorrespondingtotheinitialconcentrationX1.

Inte

rface

hei

ght

Initial concentration, X1

1

2

3

4t0 t1 t2 t3 t4

4 4 4

1

11

2

3 3

Figure 10.3 Settling column test from time t0 to t4: Zone 1–clarified zone; zone 2–uniform settling zone at solids concentration X1; Zone 3–hindered settling with concentration gradient; and Zone 4–compression settling.


Thestraightlineportionrepresentshinderedsettling,whilethehorizontalflatendportionofthecurverepresentscompressionsettling.AnumberofsettlingcolumntestscanberunatdifferentinitialsolidsconcentrationstogeneratethesettlingcurvesillustratedinFigure 10.4.Thecorrespondinghinderedsettlingvelocitiescanbecalculatedfromslopesofthestraightlineportions.Astheinitialconcentrationincreases,thecurvesbecomeflatterwithlowersettlingvelocities.Thisisduetothepresenceofzone2foraveryshortperiodoftime,withzones3and4becomingpredominantathighersolids concentrations. Compression settling becomes more important athighersolidsconcentrations.

10.3.2 Solids flux analysis

Themajorprocessparameterforsolidsthickeningisthesolidsloadingrate.Itistherateofsolidsfedperunitcross-sectionalareaofclarifier(kg/m2·d).Thesolidsloadingratehastobedeterminedbasedonsludgesettlingprop-ertiesandclarifierreturnsludgeflowrate.Oneofthemethodsforsecond-aryclarifieranalysisisthesolids fluxmethod.

Figure 10.5presentsasecondaryclarifieratsteadystateconditions.Qerepresentstheeffluentoroverflowfromtheclarifier,Qtheplantflowrate,Qrtherecycleflow,andQutheunderflowrate.Thesolidsconcentrationsaregivenbythefollowing:X,themixedliquorsuspendedsolids(MLSS)concentration;Xe,theeffluentsolids;andXu,theunderflowsolidsconcen-tration.Theinterfacebetweenzones1and2isstationary,aswaterintheclarifiedzonerisestowardtheoverflowatarateequaltothehinderedset-tlingvelocityofthesolidswithconcentrationX(Peavyetal.,1985).Thissatisfiestheclarificationfunction.

Time

Inte

rface

Hei

ght

X1

X2

X3

X4

X4 > X3 > X2 > X1

V1

V2

V3

V4

Figure 10.4 Settling curves corresponding to various initial solids concentrations.


Thethickeningfunctiondependsonthelimitingsolidsfluxthatcanbetransported to the bottomof the clarifier. The solids flux is affectedbythesludgecharacteristics,andsettlingcolumntestshavetobeconductedtodeterminetherelationshipbetweensettlingvelocityandsolidsconcen-tration.Datafromthesettlingcolumntestisthenusedtodeterminethearearequiredforthickeningusingthesolidsfluxmethod.ThesolidsfluxmethodwasfirstproposedbyCoeandClevenger(1916)andlatermodifiedbyanumberofresearchers,e.g.Yoshiokaetal. (1957),DickandEwing(1967),andDickandYoung(1972),asreportedbyPeavyetal.(1985).

10.3.2.1 Theory

Solidsfluxisdefinedasthemassofsolidspassingperunittimethroughaunitareaperpendicular to thedirectionofflow. It iscalculatedas theproductofsolidsconcentration(kg/m3)andthevelocity(m/h),resultinginunitsofkg/m2·h.

Inasecondaryclarifieratsteadystate,aconstantfluxofsolidsmovesin a downward direction. The downward velocity of the solids has twocomponents:(1)transportvelocityduetothewithdrawaloftheunderflowsludgeataconstantrateQu,and(2)gravity(hindered)settlingofthesolids.

Clari�ed e�uentzone

Hindered settlingzone

Transitionzone

Compressionzone

Over�owQe, Xe

Clari�erin�uent(Q + Qr)

Solidsinterface

Under�owQu, Xu

Figure 10.5 Secondary clarifier operating at steady state.


Thetransportvelocityvuduetounderflowisgivenby

vu=Q

Au

s

(10.1)

where:vu=underflowvelocity,m/hAs=surfaceareaofclarifier,m2

Qu=underflowflowrate,m3/h

Atanypointintheclarifier,theresultingunderflowsolidsfluxGuis

Gu=vuXi=Q

AXu

si (10.2)

where:Xi=solidsconcentrationatthepointinquestion,kg/m3

Gu=solidsfluxduetounderflow,kg/m2·h

Atthesamepointintheclarifier,themassfluxofsolidsduetogravityset-tlingisgivenby

Gg=vgXi (10.3)

where:Gg=solidsfluxduetogravity,kg/m2·hvg=settlingvelocityofsolidsatconcentrationXi,m/h

Thetotalmassfluxisthesumoftheunderflowfluxandthegravityflux,andisgivenby

GT=Gu+Gg (10.4)

GT=vuXi+vgXi (10.5)

Figure 10.6illustratesthenatureofthetotal,underflow,andgravityfluxcurves.Thegravityfluxdependsonthesolidsconcentrationandthecor-respondingsettlingcharacteristics.Atlowsolidsconcentration,thesettlingvelocityisessentiallyindependentofconcentration.Ifthevelocityremainsthesameas thesolidsconcentration increases, thenthegravityfluxalsoincreases.Atveryhighsolidsconcentration,asthesolidsapproachthecom-pressionzone,thegravity(hindered)settlingvelocitybecomesnegligible,


andthegravityfluxapproacheszero.So,thefluxduetogravitymustpassthroughamaximumvalueastheconcentrationisincreased,asshowninFigure 10.6.

Thesolidsfluxduetounderflowisa linear functionofsolidsconcen-tration.The slopeof theunderflowfluxcurve is equal to theunderflowvelocityvu.Theunderflowvelocityisusedasaprocesscontrolparameter.Thetotalfluxcurveisdrawnasthesumofthegravityandunderflowfluxcurves.Increasingordecreasingtheunderflowflowratecanshiftthetotalfluxcurveupwardordownward.Thelowestpointonthetotalfluxcurvecorresponds toa limiting solidsflux for theclarifier.The limiting solids fluxGLcorrespondstothemaximumsolidsloadingthatcanbeappliedtotheclarifier.Thisgovernsthethickeningparameterandisusedtocalculatethearearequiredforthickening.

10.3.2.2 Determination of area required for thickening

Thefirststeptodeterminethearearequiredforthickeningistoobtaindatafor the gravity flux curve. Settling column tests described previously inSection10.3.1arerunwithdifferentinitialsolidsconcentrations.GraphssimilartoFigure 10.4aregeneratedwiththedata.Theslopesoftheinitialstraightlineportionsarecalculated.Thesearethegravity(hindered)set-tlingvelocities(vg)correspondingtothedifferentinitialsolidsconcentra-tions(Xi).Equation(10.3)isusedtocalculatethegravityfluxvalues(Gg)fordifferentXivalues.ThegravityfluxcurveisthenplottedsimilartotheoneinFigure 10.6.Asmentionedpreviously,theunderflowvelocityisusedas aprocess controlparameter.Avalueof vu is selected, anda straightlineindrawnthroughtheorigintorepresenttheunderflowfluxGu.Thetotalfluxcurve(GT)isthendrawnasthesumofthetwofluxcurves.Ahorizontallineisdrawntangenttothelowestpointonthetotalfluxcurve.

Solids Concentration, Xi

XL Xu

GL

Solid

s Flu

x, G

i

Total flux curve, GT

Underflow flux curve, Gu

Gravity flux curve, Gg

Figure 10.6 Total, underflow and gravity flux curves for solids flux analysis.


Itsintersectionwiththey-axisgivesthelimiting solids fluxGLthatcanbehandledbytheclarifier.ThecorrespondingunderflowsolidsconcentrationXu,isobtainedastheabscissaofthepointwherethehorizontallineinter-sectstheunderflowfluxcurve.Ifthequantityofsolidscomingtotheclari-fierisgreaterthanthelimitingsolidsfluxvalue,thensolidswillbuildupintheclarifierandmayoverflowfromthetopifadequatestoragecapacityisnotavailable.

Thesurfacearearequiredforthickeningiscalculatedas

A T = Totalflow in to clarifierx solidsconceentration

Lim itingsolidsflux (10.6)

AQ Q X

GT

r

L

=+( )

(10.7)

where:AT=surfacearearequiredforthickening,m2

Qr=recycleflow,m3/hQ=plantflowrate,m3/hX=MLSSconcentration,kg/m3

GL=limitingsolidsflux,kg/m2·h

Thedepthofthethickeningportionoftheclarifiermustbesufficientto(1)maintainanadequatesludgeblanketdepthsothatthickenedsolidsarenotrecycled,and(2)temporarilystoreexcesssolidsthatmaycomeintotheclarifier(MetcalfandEddy,2003).

AslightmodificationtotheabovemethodwasproposedbyYoshiokaetal.(1957)todeterminethelimitingsolidsflux.ThismethodisillustratedinFigure 10.7.Thegravityfluxcurveisplottedfirst.AvalueofunderflowsolidsconcentrationXuisselected.ThenalineisdrawnfromXuonthex-axisandtangenttothegravityfluxcurve.Thetangent isextendedtothey-axis.Theintersectionpointofthetangentwiththey-axisprovidesthelimitingfluxvalueGL.Theabsolutevalueoftheslopeofthetangentistheunderflowvelocityvu.Theordinatevaluecorrespondingtothepointof tangency is the gravity solids flux, while the intercept GL–Gg is theunderflowflux (Peavyetal.,1985).Thismethod isuseful todeterminetheeffectofvariousunderflowsolidsconcentrationsonthelimitingsolidsflux. As illustrated in Figure 10.8, increasing the value of Xu results indecreasingthemaximumsolidsloadingthatcanbeappliedtotheclari-fier.Conversely,ahighersolidsloadingcanbeappliedtoaclarifierwithalowerdesiredunderflowsolidsconcentration.Thesemethodsareillus-tratedintheexamplesthatfollow.


10.3.3 Secondary clarifier design

Secondary clarifiers following suspended growth processes have to bedesignedtoachievetwofunctions:(1)clarificationand(2)thickening.Thesurfacearearequiredtoachieveclarificationofeffluentisdeterminedusingthesameprinciplesasthoseusedforprimaryclarifiers.Thesurfacearearequiredforthickeningisdeterminedusingoneofthemethodsmentioned

Solids Concentration

Solid

s Flu

x

XuXL

Gg

GL

Gravity �ux curve

Under�ow transport

Slope, Vu

Figure 10.7 Alternative graphical method for determination of limiting solids flux (Adapted from Yoshioka et al., 1957).

Solid

s Flu

x

Xu2Xu1Solids Concentration

Xu3

Gravity �ux curve

GL1

GL2

GL3

Figure 10.8 Effect of underflow solids concentration on limiting solids flux.


previously.Thisrequiresdatafromsettlingcolumntestsrunwithappropri-atesludgesamples.Foranewplant,theactivatedsludgesystemthatwouldproducethesludgemayalsobeinthedesignphase.Asaresult,itwouldbeverydifficulttoobtainrepresentativesludgesamples.Thedesignprocedureoutlinedsofarismoreapplicablefortheevaluationandoptimizationofanexistingsystemratherthanforthedesignofanewsystem.Foranewsys-tem,whereanalyticaldataarenotavailable,designparametersfromlitera-turemaybeused.Designvaluesfrompriorinstallationsthathaveworkedsuccessfully are presented in Table 10.1. However, careful considerationofwastewatercharacteristicsandtypeofreactorshouldbemadepriortoselectionofdesignparametersfromliterature.

Thephysicalunitsusedassecondaryclarifiersaresimilartothoseusedasprimaryclarifiers.Circularor rectangular tanksmaybeused.Sludgeremovalmechanismsaresomewhatdifferent,duetothenatureofthebio-logicalsolids.Thesludgeshouldberemovedasrapidlyaspossibletoensurereturnof activemicroorganisms to theactivated sludge reactor.Effluentoverflowratesbasedonpeakflowconditionsarecommonlyusedtopreventlossofsolidswiththeeffluentifdesigncriteriaareexceeded.Analternativemethodistousetheaveragedryweatherflowratewithacorrespondingsurfaceloadingrate,andalsocheckforpeakflowandloadingconditions.Eitherconditionmaygovernthedesign(MetcalfandEddy,2003).

EXAMPLE10.2Youhavetodesignasecondaryclarifierforanactivatedsludgepro-cess. The MLSS in the activated sludge reactor is 2500 mg/L. It isdesiredtothickenthesolidsto10,000mg/Linthesecondaryclarifier.Theplantflowrateis6500m3/d.Thesludgerecirculationrateis45%.Batchsettlingcolumntestswereconductedatdifferent initial solidsconcentrations,andcorrespondingsettlingvelocitieswerecalculated.Theresultsaregivenbelow.

Table 10.1 Typical design data for secondary clarifiers for activated sludge systems

Type of system

Overflow rate, m3/m2 · d Solids loading, kg/m2 · hDepth

mAverage Peak Average Peak

Clarifier following air-activated sludge (excluding extended aeration)

16–32 40–64 4–6 8 3.5–6

Clarifier following oxygen activated sludge

16–32 40–64 5–7 9 3.5–6

Clarifier following extended aeration

8–16 24–32 1–5 7 3.5–6

Source: Adapted from Peavy et al. (1985) and Metcalf and Eddy (2003).


Solids concentrationmg/L

Settling velocitym/h

1000 52000 3.23000 24000 1.15000 0.56000 0.288000 0.11

10,000 0.07512,000 0.06

SOLUTION

Step1.Calculatethesolidsfluxforthegivendata.Useequation(10.3),andnotemg/L=g/m3

Gg(kg/m2·h)=vg Xi=settlingvel(m/h)xsolidsconcn.(g/m3)/1000g/kg

Solids concentrationg/m3

Solids fluxkg/m2 · h

1000 52000 6.43000 64000 4.45000 2.56000 1.688000 0.88

10,000 0.7512,000 0.72

Step 2. Use the alternative graphical method for solids flux analy-sis.DrawthegravityfluxcurveusingthedatafromStep1.Sincethedesiredunderflowconcentrationis10,000mg/Lorg/m3,usethisvalueonthex-axisasthestartingpointanddrawatangenttothegravityfluxcurve.Thetangentintersectsthey-axisat4.0kg/m2·h,whichisthelimiting solids fluxvalueorGLfortheclarifier.Thisisthemaxi-mumsolidsloadingthatcanbeappliedtotheclarifier,andgovernsthethickeningfunction.Thisisillustratedinthefigurebelow.


0

1

2

3

4

5

6

7

0 2000 4000 6000 8000 10000 12000 14000

Solid

s Flu

x, k

g/m

2 .h

Solids Concentration, g/m3

GL

Solids flux method for calculation of GL.

Step3.Determinethearearequiredforthickening.

Limitingsolidsflux=4kg/m2·h

PlantflowrateQ=6500m3/d

RecycleflowrateQr=0.45Q

Totalflowtoclarifier=Q+Qr=1.45×6500m3/d=9425m3/d

Solidsloadingtoclarifier=MLSS×(Q+Qr)

=2.5kg/m3×9425m3/d× d

h24

=981.77kg/h

Surfaceareaofclarifierrequiredforthickening=981 77

42

. /kg hkg

mh

AsT=245.44m2

Step4.Determinethearearequiredforclarification.ThesettlingvelocityofparticlescorrespondingtotheMLSSconcen-trationcanbeusedastheoverflowrateforclarification.TheMLSSconcentrationis2500mg/L.Drawsettlingvelocityversusthesolidsconcentration for the given data from settling column tests. This isillustratedinthefollowingfigure.


0

1

2

3

4

5

6

Settl

ing

Velo

city

, m/h

Solids Concentration, mg/L0 2000 4000 6000 8000 10000 12000 14000

Settling velocity curve.

ThesettlingvelocitycorrespondingtoMLSSof2500mg/Lisdeter-minedas2.6m/hor2.6m3/m2·hfromthefigure.Thisistheoverflowrateforclarificationofeffluent.

Assume the rate of sludgewastingQw is negligible.Then effluentflowrateQe=Q

Q=6500m3/d× d

h24=270.83m3/h

Surfacearearequiredforclarification= 270 832 6

3. /

. /

m h

m h=104.16m2

AsC=104.16m2

AsT>AsC;therefore,thickeningfunctiongovernsthedesign.Designsurfaceareaofsecondaryclarifier=245.44m2.

Step5.Calculatedimensionsforclarifier.

Selectdepth=4.5m

Selectacircularclarifier.

Diameter=4 245 44 2× . m

π=17.68m=18m

Therefore,designsurfacearea=π418 2552( ) =

m2


EXAMPLE10.3Consider the activated sludge system described in Example 10.2.CalculatetheunderflowrateQuandtheunderflowvelocityvu,assum-ingthatsludgewastagerateQwisnegligible.Alsoestimatethemaxi-mumMLSSthatcanbemaintainedinthereactor.

SOLUTION

Step1.CalculateunderflowrateQu.

Qe, XeQ + Qr, Xm

Qu, Xu

Clarifier

Qu=Qw+Qr≈Qr(sinceQwisnegligible)

Underflowrate,Qu=Qr=0.45Q=0.45×6500m3/d=2925m3/d

Step2.Calculateunderflowvelocityvu.Underflow velocity is the slope of the limiting solids flux line inExample10.2.

Slopeofline=(4kg/m2·h)/10kg/m3=0.4m/h

vu=0.4m/h

Step3.EstimatemaximumMLSSforreactor(Xm)Considerthediagramofthesecondaryclarifier.

Fromequationofcontinuity,

Inflow=Outflow

Q+Qr=Qe+Qu=Qe+Qr

Therefore,Q=Qe.

Writeamassbalanceforsolidsaroundthesecondaryclarifier.

(Massrateofsolids)in=(Massrateofsolids)out

(Q+Qr)Xm=QuXu+QeXe

SinceXe<<Xu,thenXecanbeconsiderednegligible.Also,Qu=Qr


(Q+Qr)Xm=QrXu

(1.45Q)Xm=(0.45Q)(10,000mg/L)

Xm=3103.45mg/L

EXAMPLE10.4Consider the activated sludge system described in Example 10.2. ItisdesiredtooperatethesystematahigherMLSSof3000mg/Landincreasetheunderflowsolidsconcentrationto12,000mg/L.Canthisbedonewiththeselecteddesignsurfaceareaof255m2?

SOLUTION

Steps1and2.CompleteStep1andStep2similartoExample10.2togeneratethefigurebelow.

0

1

2

3

4

5

6

7

14000120001000080006000400020000

Solid

s Flu

x, k

g/m

2 .h

Solids Concentration, g/m3

GL

Atangentdrawnthroughdesiredunderflowsolidsconcentrationof12,000g/m3givesaGLof2.6kg/m2·h.This is themaximumsolidsloadingthatcanbeappliedtotheclarifier.

Step3.Checkarearequiredforthickening.

Totalflowtoclarifier=Q+Qr=1.45×6500m3/d=9425m3/d Solidsloadingtoclarifier=MLSS×(Q+Qr)

=3kg/m3×9425m3/d×d

h24

=1178.13kg/h

Surfaceareaofclarifierrequiredforthickening=1178 13

2 62

. /

. /

kg hkg

mh

AsT=453.13m2


Step4.Checkarearequiredforclarification.Settling velocity corresponding to MLSS of 3000 mg/L is 2.0 m/h(giveninsettlingcolumntestdata).Thisisequivalenttotheoverflowrateforclarification.

Assume the rate of sludgewastingQw is negligible.Then effluentflowrateQe=Q.

Q=6500m3/d× d

h24=270.83m3/h

Surfacearearequiredforclarification=270 832 0

3. /

. /

m h

m h=135.42m2

AsC=135.42m2

AsT>AsC;therefore,thickeningfunctiongovernsthedesign.

Designsurfaceareaofsecondaryclarifier=453.13m2

ThesurfacearearequiredisalmostdoublethatofExample10.2.Thesurfaceareawillhave tobe increased to453.13m2 from255m2 inordertoincreasetheMLSSandunderflowsolidsconcentrationtothenewvalues.

PROBLEMS

10.1 Whatisthemaindesignobjectiveofsecondaryclarifiersforattachedgrowthprocesses?Whyisitdifferentfromthedesignofsecondaryclarifiersfollowingsuspendedgrowthprocesses?

10.2 Designsecondaryclarifiersforatricklingfilterplanttreatingwaste-water fromamunicipality.Theaverageflowrate is500m3/d,witharecirculationratioof1.5to1.Themaximumoverflowrateis2.1m3/m2·h.

10.3 What are themajordesign considerations for secondary clarifiersfollowingactivatedsludgeprocesses?

10.4 Whatisasettlingcolumntest?Howcanyouuseittodeterminethesettlingvelocity?

10.5 Howcanyoudesignasettlingcolumntesttolocatethevariousset-tlingzoneswithinthecolumn?Illustrateyourdesign.

10.6 Writeanexpressionforthetotalsolidsfluxinasecondaryclarifier.Defineeachofthecomponentfluxes.

10.7 Illustratewiththehelpofagraphwhathappenstothelimitingsol-idsfluxandunderflowvelocitywhentheunderflowsolidsconcen-trationisincreasedordecreasedfromitsdesignvalue.

10.8 Anactivated sludgeplantwith recycle is evaluating its secondaryclarificationsystem.Settlingcolumntestsareconductedandana-lyzedforsettlingvelocitiesatdifferentinitialsolidsconcentrations.


Thedataareprovidedbelow.Theplantflowrateis10,000m3/d,witharecirculationratioof0.5.TheMLSSinthereactoris4000 mg/L.Thedesiredunderflowconcentrationis17,000mg/L.

Solids concentrationmg/L

Settling velocitym/h

1500 6.53000 5.14500 46000 2.957500 29000 1.4

12,000 0.515,000 0.2717,000 0.2

a. Calculatetherequiredsurfaceareaifoneclarifierisused.b. Calculateunderflowrate,underflowvelocity,andoverflowrate

forsingleclarifierdesign.Clearlystateyourassumptions.c. Calculatetherequiredsurfaceareaiftwoclarifiersareused.

10.9 Designsecondaryclarifiersfortheactivatedsludgeplantofproblem10.8,witharecycleratioof0.6.Calculatetherequiredsurfaceareaiftwoclarifiersareused.Alsocalculateunderflowrate,underflowvelocity,andoverflowrates.Clearlystateyourassumptions.

10.10 A wastewater treatment plant consists of a primary clarifier andanactivatedsludgereactor(withrecycle)followedbyasecondaryclarifier.Theaverageinflowofwastewatertotheprimaryclarifieris14,500m3/daywithaBOD5of250mg/Landsuspendedsolidsconcentrationof300mg/L.Therecirculationratiointhesecondarysystem is0.75,with anunderflow solids concentrationof12,000mg/L.Calculatethearearequiredforsecondaryclarificationwhentheslopeof the limitingsolidsflux line is0.5m/hfromaplotofsolidsflux (kg/m2-h) versus solids concentration (kg/m3).The set-tlingvelocityofthesolidsattheMLSSconcentrationof6500mg/Lis0.7 m/h.

REFERENCES

Coe,H.S.,andClevenger,G.H.(1916)“DeterminingThickenerUnitAreas.”Trans AIME,vol.55,no.3,p.356.



Dick,R. I., andEwing,B.B. (1967)“EvaluationofActivated SludgeThickeningTheories.”Journal of Sanitary Engineering Division, ASCE,vol.96,p.423.

Dick,R.I.,andYoung,K.W.(1972)“AnalysisofThickeningPerformanceofFinalSettling Tanks.” Proceedings of 27th Industrial Waste Conference, PurdueUniversity,Indiana,p.33.

GLUMRB(2004)Recommended Standards for Wastewater Facilities.GreatLakes–Upper Mississippi River Board of State and Provincial Public Health andEnvironmentalManagers,HealthEducationServices,Albany,NewYork,pp.70–73.

MetcalfandEddy,Inc.(2003)Wastewater Engineering: Treatment and Reuse.Fourthedition.McGraw-Hill,Inc.,NewYork.


Yoshioka,N.,Hotta,Y.,Tanaka,S.,Naito,S.,andTsugami,S.(1957)“ContinuousThickening of Homogeneous Flocculated Slurries.” Chemical Engineering, Japan,vol.21,p.66–74.

209

Chapter 11

Anaerobic wastewater treatment

11.1 INTRODUCTION

Thebiologicaltreatmentofwastewaterandsludgeinabsenceofoxygenistermedasanaerobic treatment.LouisPasteurwasthefirstscientisttodis-coveranaerobiclifeduringhisresearchonfermentationprocessesin1861(Madiganetal.,2010).Heobservedthattheclostridiumbacteria,whichcausedbutyricfermentation,werestrictlyanaerobic.Exposuretooxygenwastoxictothebacteria.Pasteurintroducedthetermsaerobicandanaer-obic to designate biological life in the presence and absence of oxygen,respectively.Pasteurobservedthattherewasadifferenceinyieldbetweenaerobicandanaerobicprocesses.Anaerobicfermentationresultedinlowermicrobialmassinyeastproductionthanaerobicconditions.

Historically,anaerobic treatmenthasbeenusedmore for treatmentofsludgeorbiosolidsratherthanforwastewater.Theseptic tankwasoneofthefirstformsofanaerobictreatmentusedforsewagesludgeorbiosolids.The development and use of the first septic tank dates back to 1896 atExeter,England,asreportedbyFuller(1912).Wastewaterclarificationanddigestiontookplaceinthesametank.Thiswaswidelyusedforwastetreat-mentinEuropeandtheUnitedStates.In1904,WilliamTravisdevelopedatwo-storyseptictankinGermany,wheresuspendedmaterialwasseparatedfromthewastewaterbysettlinginthefirststage.Thesecondstagewasahydrolyzingchamberthroughwhichthesupernatantwasallowedtoflow.TheTravis hydrolytic tankwasmodifiedbyKarlImhoffin1907toprovidea treatment system,which laterbecameknownas the Imhoff tank.TheImhofftankdidnotallowthewastewatertoflowthroughthehydrolyzingtank.Instead,thesludgewaskeptinthehydrolyzingtankforalongperiodoftimetoallowfordigestionandstabilization.TheImhofftankreducedthecostofsludgedisposalandrapidlybecamepopularinbothEuropeandtheUnitedStates(Imhoff,1915;McCarty,1981).

The importance of temperature on anaerobic treatment was observedandinvestigatedbyanumberofresearchersasearlyasthe1920s.Rudolfs(1927) observed that the total amount of gas produced from a gram of


organicmatterunderanaerobicconditionswasnotdependentontempera-ture,buttherateofgasproductionwastemperaturedependent.Eventuallythe mesophilic (35°C) and thermophilic (55°C) temperature ranges wereidentifiedforanaerobictreatment(Heukelekian,1933).Extensivestudieswereconductedbyresearcherstogainabetterunderstandingofthemicro-biologyofanaerobictreatment,aswellasthebiochemicalandenvironmen-talfactorsthataffecttheprocess(BabbittandSchlenz,1929;Heukelekian,1958;FairandMoore,1932;Sawyeretal.,1954;McCartyetal.,1963;Dagueetal.,1966).In1964,PerryL.McCartypublishedaseriesofpapersonanaerobicwastetreatmentthatprovidedacomprehensivesummaryofthefundamentalsofanaerobictreatment(McCarty,1964a,b,c).Overtime,a large number of suspended and attached growth processes have beendevelopedforanaerobictreatmentofwastewater.

Anaerobic treatment of wastewater involves the stabilization oforganic matter, with a concurrent reduction in odors, pathogens, andthemassofsolidorganicmatterthatrequiresfurtherprocessing.Thisisaccomplishedbybiologicalconversionoforganicstomethaneandcar-bon dioxide in an oxygen-free or anaerobic environment (Parkin andOwen,1986).

Themainadvantagesofanaerobictreatmentprocessesoveraerobicpro-cessesare(McCarty,1964a;MetcalfandEddy,2003)asfollows:

• Ahighdegreeofwastestabilizationispossibleathighorganicloads.• Thereislowproductionofwastebiologicalsludge.• Lessenergyisrequired.• Therearelownutrientrequirements.• Methanegasproducedisausefulsourceoffuel.• Smallerreactorvolumeisrequired.• Nooxygenisrequired,sotreatmentratesarenotlimitedbyoxygen

transferrates.• Rapid reactivation of biomass is possible with substrate addition,

afterlongperiodsofstarvation.

Theanaerobicprocesshassomedisadvantages,asfollows:

• Relativelyhightemperature(35°C)isrequiredforoptimaloperation.• Longerstart-uptimeisrequiredtodevelopnecessaryamountofbio-

mass,duetoslowgrowthrateofmethane-formingbacteria.• Itmaybenecessarytoaddalkalinityorotherspecificions,depending

onthecharacteristicsofthewastewater.• Maybemoresusceptibletotoxicsubstances.• Odorproductionmaybeaproblem.• Biologicalnitrogenandphosphorusremovalmaynotbepossible.

Anaerobic wastewater treatment 211

This chapter will provide an overview of the process microbiology, fol-lowedbydiscussionoffactorsthataffecttheprocess,andprocesskinetics.Anaerobic suspended and attached growth processes used for wastewa-tertreatmentwillbediscussedindetailinthelatterpartofthischapter.Anaerobicprocessesusedfortreatmentofsludgeandbiosolidswillbedis-cussedinChapter12.

11.2 PROCESS CHEMISTRY AND MICROBIOLOGY

Anaerobicwastetreatmentisacomplexbiologicalprocessinvolvingvari-ous types of anaerobic and facultative bacteria.A four-stepprocess canbeusedtodescribetheoverall treatment.Althoughthebacteriaarerep-resentedbyseparategroups,itisnotpossibletoseparatethemetabolismofeachgroup.Theyareinterdependent.TheanaerobicbiotransformationprocessisillustratedinFigure 11.1.

Complex Organic Compounds(Carbohydrates, proteins, lipids)

Simple Organic Compounds(Sugars, amino acids, peptides)

Long Chain Fatty Acids(Propionate, butyrate, etc.)

H2O, CO2 Acetate

CH4, CO2

Hydrolysis

Acidogenesis

Acetogenesis

Acetogenesis

MethanogenesisMethanogenesis

1

1

2

3

4 5

Figure 11.1 Metabolic steps involved in anaerobic biotransformation (Source: Adapted from McCarty and Smith, 1986). The numbers represent the different micro-bial groups.


Five groups of bacteria are thought to be involved, each deriving itsenergyfromalimitednumberofbiochemicalreactions(Novaes,1986):

1.Fermentativebacteria:Thisgroupisresponsibleforthefirsttwostagesofanaerobicconversion,hydrolysisandacidogenesis.Anaerobicspe-ciesbelongingtothefamilyofStreptococcus andEnterobacterandto thegeneraofClostridium eubacteriumaremainly found in thisgroup.

2.Hydrogen-producing acetogenic bacteria: These catabolize sugars,alcohols, and organic acids to acetate and carbon dioxide. TheseincludetheSyntrophobacter woliniiandSyntrophomonus wolfei.

3.Hydrogen-consumingacetogenicorhomoacetogenicbacteria:Thesebacteriausehydrogenandcarbondioxidetoproduceacetate.TheyincludetheClostridium aceticumandButyribacterium methylotro-phicum,amongothers.

4.Carbondioxide–reducingmethanogens:Theseutilizehydrogenandcarbondioxidetoproducemethane.

5.Aceticlasticmethanogens:Thesecleaveacetatetoformmethaneandcarbondioxide.

Thefourstepsofanaerobicbiotransformation,discussedbelow,areasfollows:

1.Hydrolysisandliquefaction 2.Fermentationoracidogenesis 3.Hydrogenandaceticacidformation,oracetogenesis 4.Methaneformationormethanogenesis

Step 1: Hydrolysis and liquefaction. The first step involves hydrolysisandliquefaction.Insolubleorganicsmustfirstbesolubilizedbeforetheyareconsumed.Inaddition,largesolubleorganicmoleculesmustbediminishedinsizetofacilitatetransportacrossthecellmembrane.Thereactionsarehydrolyticandcatalyzedbyenzymessuchasamylase,proteinase, lipase,andnuclease.Nowastestabilizationtakesplaceduringthisstep,butrathertheorganicmatter isconverted intoa formthatcanbe takenupby themicroorganisms.Anaerobicdigestionmaybelimitedinthehydrolysisandliquefactionstep,ifthewastecontainslargeportionsofrefractoryornon-biodegradableorganicmaterialthatisnothydrolyzedbymicroorganisms.Particulateorganicmatter(lipids,polysaccharides,protein)isconvertedtosolublecompoundsandafterwardhydrolyzedtosimplemonomers (fattyacids,monosaccharides,aminoacids)inthisstep.

Step 2: Fermentation or acidogenesis.The simplemonomers resultingfromhydrolysisareusedascarbonandenergysourcesbytheacid-produc-ingbacteria.Theoxidizedendproductsofthisstepareprimarilyvolatilefattyacids(VFAs),suchasacetic,propionic,butyric,valeric,andcaproic


acid,togetherwithproductionofammonia(NH3),carbondioxide(CO2),hydrogensulfide(H2S),andotherby-products.

Step 3: Hydrogen and acetic acid formation, or acetogenesis.Inthethirdstep,VFAsandalcoholsproducedintheacidogenesissteparedegradedprimarilytoaceticacidtogetherwithproductionofCO2andH2.Inthisconversion,partialpressureofH2isanimportantfactor.Freeenergychangeassociatedwiththeconversionofpropionateandbutyratetoacetateandhydrogenrequireshydro-genconcentrationtobelowinthesystem(H2<10–4atm)ortheconversionwillnottakeplace(McCartyandSmith,1986).Hydrogenisproducedbythefermentativeandhydrogen-producingacetogenicbacteria.Acetateisalsopro-ducedbythesegroupsinadditiontothehomoacetogenicbacteria.

Step 4: Methane formation or methanogenesis. Waste stabilizationoccurs in the fourth and final stage when acetic acid or acetate is con-vertedtomethanebythemethanogenicbacteria.Approximately72%ofmethaneformedcomesfromacetatecleavagebyaceticlasticmethanogens(McCarty,1964c).Theproposedreactionis

CH3COOH ___▶ CH4+CO2 (11.1)

Theremaining28%resultsfromreductionofcarbondioxide(13%frompropionicacidand15%fromotherintermediates),usinghydrogenasanenergysourcebycarbondioxide–reducingmethanogens,formingmethanegasintheprocess:

CO2+H2___▶ CH4+H2O (11.2)

Thefollowingreactionsdescribetheoverallanaerobicbiotransformationofaceticacid,propionicacid,butyricacid,ethanol,andacetone:

CH3COOH ___▶ CH4+CO2 (11.3)

4CH3CH2COOH+2H2O ___▶ 7CH4+5CO2 (11.4)

2CH3CH2CH2COOH+2H2O ___▶ 5CH4+3CO2 (11.5)

2CH3CH2OH ___▶ 3CH4+CO2 (11.6)

CH3COCH3___▶ 2CH4+CO2 (11.7)

11.2.1 Syntrophic relationships

TheratelimitingstepintheentireanaerobicprocessistheconversionofhydrogentomethanebyCO2-reducingmethanogens.Thehydrogenpartial


pressuremustbemaintainedatanextremelylowleveltoenablefavorablethermodynamicconditionsfortheconversionofvolatileacidsandalcoholstoacetate.Understandardconditionsof1atmofhydrogenpartialpres-sure, the freeenergychange ispositive for thisconversionand thuspre-cludesit.Thefreeenergychangeforconversionofpropionateandbutyratetoacetateandhydrogendoesnotbecomenegativeuntilthehydrogenpar-tialpressuredecreasesbelow10–4atm(Speece,1983;McCartyandSmith,1986).Itisthereforeobligatoryforthehydrogen-utilizingmethanogenstoutilizehydrogenrapidlyandmaintaintheseextremelylowhydrogenpartialpressuresinthesystem.Otherwise,highervolatileacids,suchaspropionicandbutyricacids,willaccumulateandwastestabilizationwillnotoccur.

11.3 METHANOGENIC BACTERIA

Methanogens are often considered to be the key class of microorgan-isms in anaerobic treatment. Methanogens are classified as Archaea orArchaebacteriaandcanbedistinguishedbythecomparativecatalogingof16SrRNAsequences(Batchetal.,1979)aswellasbiochemicalproperties,morphology,and immunologicalanalyses (MacarioandConway,1988).Theyareobligateanaerobeswithrelativelyslowreproductionrates,sincelessenergyisreleasedinthereactionsinvolvedintheanaerobicstabiliza-tionoforganicmatter.Thisslowgrowthratelimitstherateatwhichtheprocess can adjust to changing substrate loads, temperatures, and otherenvironmentalconditions.

Avarietyofmethanogensareobservedaccordingtothefollowing:

• Morphology:longorshortrods,smallorlargecocci,numerouslacentand spirillum shapes. The cell walls of methanogens are based onthreemajorcomponents:pseudomurien,protein,andheteropolysac-charide(ArcherandHarris,1986).

• Gramstaining:allaregram-negative.• Growthtemperature:somearethermophilic(55°Cto65°C)andsome

aremesophilic(30°Cto35°C)organisms.• Generation time: range is from 1.8 to 3.5 hrs (Dubach and

Bachofen,1985).

Methanogenscanonlyuseasmallnumberofsimplecompoundsthatcon-tain one or two carbons (Wose et al., 1978; Wose, 1987). The primaryreactions of methane formation with their associated Gibbs free energyvalues are shown in Table 11.1. The methanogenic bacteria are depen-dentonotherorganismsfortheirsubstrates.Henceacomplexfoodwebofanaerobesisrequiredtoconvertmostoftheorganicsubstratestolowmolecularweightorganicacids,CO2andhydrogen.Themethanogensuse


thelattertwooftheseproductsandeventuallyconvertacetatetomethane.Ithasbeenestimatedthatapproximately70%ofthemethaneformedinnatureisviaacetatecleavagetomethaneandcarbondioxide.Theoptimumdegradationperformancedependsonanumberofbiochemicalandphysi-cal interactionsbetweenmethanogensandnonmethanogens (ArcherandHarris,1986).

Themajorityofthespeciesusehydrogenandcarbondioxideforbothcarbonand energy sources.Other substrates include formate,methanol,carbon monoxide, methylamines, and acetate. Three types of methano-genic bacteria have been identified that utilize acetate: Methanosarcina sp.,Methanothrix soehngenii,andMethanococcus mazei.Formateisusedby several genera, including Methanobacterium, Methanogenium, andMethanospirillum(Novaes,1986;Daniels,1984).

Thereisavariationingrowthrateamongdifferentspeciesofmethanogens.GujerandZehnder(1983)evaluatedgrowthkineticsforMethanosarcinaand Methanothrix on acetate. Methanosarcina had a sharply increasinggrowthcurvewithamaximumspecificgrowthrate(µmax)of0.3d–1andhalf saturation coefficient (Ks)of200mg/L.Methanothrix hadaflattergrowth curve with a maximum specific growth rate of 0.1 d–1 and halfsaturation coefficient (Ks)of30mg/L.Thismeant that at low substrateconcentrations,theMethanothrixoutcompetetheMethanosarcina.Butathighsubstrateconcentrations,theMethanosarcinapredominate.

Even though the methanogens are the most important and sensitivemicrobial species in anaerobic treatment, a balance must be maintainedbetweentheacid-formingandhydrogen-formingbacteriaandthemethane

Table 11.1 Gibbs free energy values for selected methanogenic reactions

Reactants Products Go (kJ/mol CH4) Organisms

4 H2 + HCO3– + H+ CH4 +3 H2O –135 Most methanogens

4 HCO3– + H+ + H2 CH4 +3 HCO3

– –145 Most hydrogenotrophic methanogens

4CO + 5H2O CH4 +3 HCO3– +3 H+ –196 Methanobacterium and

Methanosarcina2CH3CH2OH +HCO3

– 2 CH3COO– + H+ + CH4 + H2O

–116 Some hydrogenotrophic methanogens

CH3COO– + H2O CH4 + HCO3– –31 Methanosarcina and

Methanothrix4CH3OH 3 CH4 +HCO3

– +H2O +H+

–105 Methanosarcina and other Methylotrophic methanogens

CH3OH + H2 CH4 + H2O –113 Methanoshaera stadtmanii and Methylotrophic methanogens

Source: Adapted from Wose (1987) and Thauer et al. (1977).


formersinordertoachievecompleteconversionoforganiccompoundstomethaneandcarbondioxide.Theproperenvironmentalconditionshavetobemaintainedforgrowthandmetabolism.

11.4 SULFATE-REDUCING BACTERIA

Onegroupofbacteriaoften found inassociationwith themethanogens isthesulfate-reducing bacteria.Theseproducehydrogen,acetate,andsulfides,whichareusedbythemethanogens.Insulfate-richenvironments,thesulfate-reducingbacteria have a thermodynamic advantageover themethanogens(Thaueretal.,1977).Thesulfatereducershavelowerhalfsaturationcoef-ficient(Ks)valuesforH2andacetate,ascomparedtothevaluesformetha-nogens. The productionof sulfidemight inhibitmethanogenesis, since thesulfate-reducingbacteriautilizehydrogenandaceticacidasenergysourcesandoutcompetemethanogensforthesesubstrates.Also,solublehydrogensul-fideinexcessof200mg/L(ParkinandOwen,1986)istoxictomethanogens.

11.5 ENVIRONMENTAL REQUIREMENTS AND TOXICITY

Optimumenvironmentalconditionsareveryimportantinthedesignandoperationofanaerobictreatmentprocesses.Theseconditionsareusuallydictatedbytherequirementsofthemethanogens,whosegrowthratelimitstheprocessofwaste stabilization.The followingare important environ-mentalfactorsaffectingtheprocess:

1.Temperature—Temperature is an important factor influencing theanaerobicbacteria.Thereisalimitedrangeoftemperaturesforopti-mumgrowth.Methanebacteriaareactiveintwotemperaturezones,themesophilicandthethermophilicranges,andespeciallyinthepartofmesophilicrangebetween30°Cand35°C.Theratesofdegradationaresloweratlowertemperatures.Thetreatmentprocesshastobeoperatedatlongerdetentiontimes,orthemicrobialpopulationshouldincreasetoobtainthesamedegreeofstabilizationatlowertemperatures.

A rapid change of temperature is also detrimental to anaerobictreatment.Changingthetemperaturebyafewdegreescancauseanimbalancebetweenthemajorbacterialpopulations,whichcanleadtoprocessfailure(GradyandLim,1980).

2.pH—pHisanimportantparameteraffectingtheenzymaticactivity,since a specific and narrow pH range is suitable for the activationofeachenzyme.AnaerobictreatmentprocessesoperatebestatapHsystemofnearneutrality.ThepHhasaneffectonbothacid-forming


andmethane-formingbacteria.TheoptimumpHforanaerobictreat-mentisintherangeof6.5to7.6(McCarty,1964a).IfthepHdropsbelow6.3orincreasesbeyond7.8,therateofmethanogenicactivityreducessignificantly.AsharppHdropbelow6.3indicatesthattherateoforganicacidsproductionisfasterthantherateofmethanefor-mation.Forreactorstreatingawastewaterwithahighconcentrationofprotein,thebufferingeffectofammoniareleasedfromaminoacidfermentationcanpreventthepHfromdroppingbelowtheoptimumrange.Ontheotherhand,asharppHincreaseabove7.8canbeduetoashift inNH4

+toNH3,thetoxic,un-ionizedformofammonia(Gomecetal.,2002).Bufferscanalsobeaddedintheformofbicar-bonatesorhydroxidestomaintainpH.

3.Nutrients—Nitrogen and phosphorus are the two major nutrientsrequiredformicrobialgrowthandreproduction.Inaddition,sulfur,iron,cobalt,nickel,calcium,andsometracemetalsarenecessaryforgrowthofmethanogens.Sulfideisrequiredbymethanogens,eventhoughitmayadverselyaffectmethaneproductionbyprecipitatingessentialtracemet-als.Itistoxicatconcentrationsabove100to150mg/Lofun-ionizedhydrogensulfide(Speece,1983).Molybdenum,selenium,andtungstenhavealsobeenreportedastracemetalsusedbymethanogens.

4.Toxic materials—Themethanogensarecommonlyconsideredtobethemostsensitivetotoxicityamongallthemicroorganismsinvolvedin anaerobic conversion of organic matter to methane. However,acclimation to toxicity and reversibility of toxicity are frequentlyobserved.Whetherasubstanceistoxictoabiologicalsystemdependsonthenatureof thesubstance, itsconcentration,andthepotentialforacclimation.Changes in theconcentrationof thesubstancecanchange the classificationof the substance from toxic tobiodegrad-able.Table 11.2presents a summaryof concentrationsofdifferentcationsatwhichtheyarereportedtobestimulatoryorinhibitorytotheanaerobicprocess(McCarty,1964c).

Controloftoxicantsisvitaltothesuccessfuloperationofananaer-obicprocess.Toxicitymaybecontrolledby(1)dilutiontoreducecon-centration below the toxic threshold, (2) removal of toxic materialfromthefeed,(3)removalbychemicalprecipitation,(4)neutraliza-tion,or(5)acclimation.

11.6 METHANE GAS PRODUCTION

11.6.1 Stoichiometry

Asignificantfractionofthechemicaloxygendemand(COD)removedinananaerobicprocessisconvertedtomethane.Sothemethanegasproduction


canbeestimatedfromtheamountofCODthatisbiodegraded.TheCODequivalenceofmethanecanbedeterminedfromstoichiometry.TheCODofmethaneistheamountofoxygenneededtocompletelyoxidizemethanetocarbondioxideandwaterasfollows:

CH4+2O2___▶ CO2+2H2O (11.8)

Fromtheaboveequation,(2×32)or64goxygenarerequiredtooxidizeonemoleofmethane.Thevolumeoccupiedbyonemoleofgasatstandardtemperatureandpressure(STP)conditionsof0°Cand1atmis22.4L.SothemethaneequivalentofCODconvertedunderanaerobicconditionsis

22 4

640 35 0 34. /

/. .

L m ol

g CO D m ol

L CH

g CO Dor= 55

34m CH

kg CO D (11.9)

Equation(11.9)providesanestimateofthemaximumamountofmethaneproducedperunitofCODatSTPconditions(MetcalfandEddy,2003).

Theamountofmethanegasproducedatothertemperatureandpressureconditionscanbedeterminedbyusingthe idealgas law.This isdemon-stratedinExample11.1.

EXAMPLE11.1Awastewatertreatmentplanttreats2000m3/dofhighstrengthwaste-water in an anaerobic reactor operated at 35°C. The biodegradablesolubleCODconcentrationofthewastewateris3500mg/L.Calculatethe amount of methane gas that will be produced with 90% CODremoval, and net biomass yield of 0.04 g volatile suspended solids(VSS)/gCODused.AssumeCODequivalent ofVSS equals 1.42kgCOD/kg VSS. If the total gas contains 65% methane, calculate thetotalgasproducedfromthewastewater.

Table 11.2 Stimulatory and inhibitory concentrations of some compounds on anaerobic treatment

SubstanceStimulatory

(mg/L)Moderately inhibitory

(mg/L)Strongly inhibitory

(mg/L)

CalciumMagnesiumPotassiumSodiumAmmonia-nitrogen

100–200 75–150200–400100–200

50–1000

2500–45001000–15002500–45003500–55001500–3000

80003000

12,0008000

>3000

Source: Adapted from McCarty (1964c).


SOLUTION

Step1.ConductasteadystatemassbalancefortheCODintheanaer-obicreactor.

Accum ulation = Influent

CO D–Effluent

CO D–CO D connverted

to new cells–CO D converted

to m ethanee

(11.10)or

0=CODin–CODout–CODVSS–CODmethane (11.11)

CODin=2000m3/d×3.5kg/m3=7000kg/d

CODout=(1–0.9)7000kg/d=700kg/d

CODVSS=0.9×7000kgCOD/d×0.04kgVSS/kgCOD ×1.42kgCOD/kgVSS

=357.84kg/d

Usingthesevaluesinequation(11.11),weobtain

0=7000kg/d–700kg/d–357.84kg/d–CODmethane

or

CODmethane=5942.16kg/d

Step2.Determinevolume(V)occupiedby1moleofmethanegasat35°C.Fromtheidealgaslawwehave,

VnRT

P= (11.12)

where:n=numberofmolesR=idealgasconstant=0.082057atm·L/mole·KT=temperature,KP =pressure,atm

Here,T=273+35=308K.

n=1mol,andP=1atm

Therefore, Vm ol atm

L

m olK K

atm=

× ×=

. · ·.

1 0 082057 308

125 227 L


Step3.CalculatethemethaneequivalentofCODconverted.ThemethaneequivalentofCODconvertedunderanaerobicconditionsis

25 27

640 395 04. /

/.

L m ol

g CO D m ol

L CH

g CO Dor= ..395

34m CH

kg CO D

Step4.Calculatemethanegasproduced.

CH4produced=5942.16kgCOD/d×0.395m3CH4/kgCOD =2347.15m3/d

Step5.Calculatetotalgasproduced.Totalgascontains65%CH4.

Totalgasproduced= 2347 150 65

3. /

.

m d =3611m3/d.

11.6.2 Biochemical methane potential assay

Thebiochemicalmethanepotential(BMP)assaymeasurestheconcentra-tionoforganicpollutantsinawastewaterthatcanbeanaerobicallycon-vertedtomethane,thusindicatingwastestabilization.TheBMPmeasuresanaerobic biodegradability and can be used to identify aerobic nonbio-degradable components that are amenable to anaerobic biodegradation(Speece,2008).Itcanbeusedtoevaluateprocessefficiency.

TheBMPtestwasdevelopedbyMcCartyandhisresearchgroupasanindicatoroftheanaerobicpollutionpotentialofawaste(Owenetal.,1979).JustastheBODtestisusedtodeterminetheaerobicpollutionpotentialofawaste, theBMPtest isusedasacorrelative indicator in theanaerobicprocess.IthasnotbeenincorporatedintotheStandard Methods(AWWAetal.,2005),butitiswidelyusedinpractice.

IntheBMPtest,asampleofwastewaterisplacedinaserumbottlewithananaerobicinoculum.Careshouldbetakenthattheanaerobicinoculumorbiomassisacclimatedtothewastewaterbeingtested.Asmallamountof nutrients is added to the bottle.Theheadspace is purgedwith70:30nitrogen:carbondioxidegastoensureanaerobicconditionsandforpHcon-trol.Theserumbottleiscappedandincubatedat35°Cforaperiodrangingfrom30to60d.Acontrolwithonlythe inoculumisalsoplaced intheincubator.Gasproductionandcompositionismonitoredatregularinter-vals.Gasvolumeproducedismonitoredbyinsertingahypodermicneedleconnectedtoacalibratedfluidreservoirthroughthebottlecap.SimilartotheBODtest,anumberofdifferentsamplevolumesofthewastewaterare


used in the serumbottles.Averagegasproduction shouldbe similar.At35°C,395mlofCH4productionisequivalentto1gofCODused(Speece,2008).ThisstoichiometricrelationshipcanbeusedtocalculatetheCODreductionintheliquidphase.

11.6.3 Anaerobic toxicity assay

Theanaerobictoxicityassay,orATA,isusedtomeasurethepotentialtox-icityofawastewatersampleorcompoundtotheanaerobicbiomass.TheprocedureissimilartotheBMPtest,withtheexceptionthatexcesssub-stratesuchasacetateisaddedinitiallytotheserumbottlestoavoidsub-stratelimitation.Iftoxicityispresentinthesample,itwillbedemonstratedbyareducedinitialrateofgasproductioninproportiontothevolumeofwastewateradded,as comparedwith thecontrol.The test is runwitharangeofdilutionsofthewastewatersample.TheATAwasalsodevelopedbyMcCartyandhisresearchgroup(Owenetal.,1979).

Intheanaerobicbiomassconsortium,theaceticlasticmethanogensarethemostsensitivetotoxicity.Forthisreason, it isusuallyrecommendedtoaddacetateasthesubstrate,atabout1000mg/LCOD(Droste,1997).Morecomplexsubstratessuchasglucose,ethanol,orotherscanbeaddedtoevaluatetoxicitytoothermicroorganismsintheconsortia.

11.7 ANAEROBIC GROWTH KINETICS

Monodmodelisthemostwidelyusedamongthemodelsdevelopedfortheanalysisofanaerobicgrowthkinetics.Thismodelassumesthattherateofsubstrateutilization,andthereforetherateofbiomassproduction,islim-itedbytherateofenzymereactionsinvolvingthesubstrate.ThishasbeendescribedindetailinChapter8.ThegrowthkineticsdescribedinSection8.2arealsoapplicableforanaerobictreatmentreactors.

Table 11.3showsthekineticparametersforacetateutilizationatvarioustemperaturesusingbatch, semicontinuous,andcontinuous systems fromvariousstudies.Becauseofthedifferenttemperaturesanddifferentsystemsused,thevaluesofthekineticparametersfromthesestudiesshowawiderangeofvariation.

Anaerobic process is stable when sufficient methanogenic populationexistsinthereactorandsufficienttimeisavailableforVFAminimizationandformethanogenstoutilizeH2.TheratelimitingstepistheconversionofVFAsbymethanogenicorganismsandnotthefermentationofsolublesubstrates by acidogens. Therefore, most interest in anaerobic processdesignisgiventomethanogenicgrowthkinetics.


11.8 ANAEROBIC SUSPENDED GROWTH PROCESSES

Historically,anaerobictreatmenthasbeenusedforstabilizationofsludgeandbiosolids.Over the last 50 years, a lot of research anddevelopmenthasresultedintheapplicationofanaerobicprocessesforwastewatertreat-ment.Bothsuspendedandattachedgrowthprocessesareinuse,especiallyfortreatmentofhigh-strengthwastewaters.Conventionalanaerobictreat-mentusingcompletelymixedreactorsisusedfordigestionofsludgeandisdescribed indetail inChapter12.Someof themorecommonsuspendedgrowthprocessesusedforwastewatertreatmentaredescribedinthissection.

11.8.1 Anaerobic contact process

Theanaerobiccontactprocessissimilartotheactivatedsludgeprocessinmanyaspects.Thesystemconsistsofacompletelymixedanaerobicreac-torwithgascollection,followedbyaclarifierforsolids–liquidseparation.Partof the settledsludge is recycled to the reactor to increase the solidsretentiontime(SRT).TheSRTisusuallygreaterthanthehydraulicreten-tiontime(HRT).ByseparatingtheHRTandtheSRT,thereactorvolumecanbereduced.ForanaerobicprocessestheminimumSRTat35°Cis4d,witharecommendeddesignSRTof10to30d.Fortheanaerobiccontactprocess,theHRTrangesfrom0.5to5dwithorganicloadingratesof1to8kgCOD/m3·d(MetcalfandEddy,2003).TheprocessflowdiagramisillustratedinFigure 11.2(a).

Theanaerobic sludge contains a large amountof entrainedgases thatareproducedduringanaerobicdegradation.Thesegasescandecreasetheabilityofthesludgetosettle.Variousmethodsareusedtoremovethegasbubbles from the sludge. These include vacuum degasification, inclined-plateseparators,andchemicalcoagulation,amongothers.

Table 11.3 Kinetic coefficients for acetate utilization

Temp. °Cμmax d–1

YkgbiomasskgCOD

Kdd–1

KsmgCOD/L Reference

37 0.11 0.023 ND 28 Zehnder et al., 198035 0.34 0.04 0.015 165 Lawrence and McCarty, 196935 0.44 0.05 ND 250 Smith and Mah, 198030 0.24 0.054 0.037 356 Lawrence and McCarty, 196925 0.24 0.05 0.011 930 Lawrence and McCarty, 1969

Note: µmax = maximum specific growth rate, Y = yield coefficient, Kd = decay coefficient, Ks = half saturation coefficient.


(a)

(b)

E�uentClari�er

Screened in�uent

Gas

Sludgeblanket

E�uentFlocculatorClari�er

Gas

In�uent

Sludge

Sludge recycle

Figure 11.2 (a) Anaerobic contact process, (b) upflow anaerobic sludge blanket (UASB) process, (c) SEM image of granule formed in a UASB reactor (Source: Courtesy of Somchai Dararat and Kannitha Krongthamchat).


11.8.2 Upflow anaerobic sludge blanket process

Theupflowanaerobicsludgeblanket(UASB)processwasdevelopedinTheNetherlandsbyLettingaandco-workers(Lettingaetal.,1980).Thiswasoneofthemostimportantdevelopmentsofanaerobictechnologyfortreat-mentofhigh-strengthwastewaters.Morethan500installationsarelocatedallovertheworldandtreatawiderangeofindustrialwastewaters(MetcalfandEddy,2003).

TheUASBprocessisillustratedinFigure 11.2(b).Thewastewaterentersthereactoratthebottomandisdistributedupwardthroughasludgeblan-ket.Organicmatterisdegradedinthesludgeblanket,afterwhichtheliq-uideffluentisdischargedatthetop.Gasproductionandevolutionprovidesufficientmixinginthesludgeblanket.Aquiescentzoneabovethesludgeblanketisprovidedforsolidssettling.Theliquideffluentispassedthrougha settling tank to collect solids thathave escaped from the reactor.Thecollectedsolidsarerecycledbacktothereactor.Criticaldesignelementsincludetheinfluentdistributionsystem,gas–solidsseparator,andeffluentwithdrawalsystem.

ThemaincharacteristicoftheUASBprocessistheformationofadensegranularsludge.Thesolidsconcentrationcanrangefrom50to100g/Lat the reactorbottom, to5 to40g/Lat the topof the sludgeblanket.Severalmonthsmayberequiredtoformthegranules,andseedisoftensupplied from other installations to accelerate the process. It was sug-gested that the UASB system promoted a selection between the sludgeingredients,suchthatlighterparticleswerewashedoutandheavierpar-ticleswereretained.Growthwasconcentratedontheseparticles,whichresultedintheformationofgranulesupto5mmindiameter(HulshoffPoletal.,1983).AtypicalgranuleisillustratedinFigure 11.2(c).Mostoftheorganismsgrowonthesurfaceandintheintersticesofthegran-ules,while thecoremaycontain inert extracellularmaterial.A symbi-oticrelationshipexistsbetweenthemicrobialconsortiaassociatedwithgranular sludge particles that is advantageous in enhancing biologicalactivity.Veryhigh specificactivitieshavebeenobserved, ranging from2.2to2.3kgCOD/kgVSS·d.McCartyandSmith(1986)reportedthatreactorswithgranularsludgeproducedlowerhydrogenpartialpressuresandmorerapidhydrogenutilizationthanreactorswithdispersedsludge,resulting in increased efficiency. Granule development is influenced bywastewatercharacteristics,reactorgeometry,upflowvelocity,HRT,andorganicloadingrates.TheseareallimportantdesignconsiderationsfortheUASBprocess.

Volumetricloadingratescanvaryfrom0.5to40kg/m3·d(0.03–2.5lb/ft3·d) foraUASBprocess (Droste,1997).TheHRTcanvary from6 to14 h.Upflowvelocitiesrangefrom0.8to3.0m/h,dependingonthetypeofwastewaterandreactorheight.


11.8.2.1 Design equations

Theareaofthereactorisgivenby

AQ

v= (11.13)

where:A=areaofreactor,m2

Q=influentflowrate,m3/dv=designupflowsuperficialvelocity,m/d

Therequiredreactorvolumedependsontheorganicloadingrateandeffec-tivetreatmentvolume.Theeffectivetreatmentvolumeisthevolumeoccu-pied by the sludge blanket and active biomass. An additional volume isprovidedbetweenthesludgeblanketandgascollectionunit,wheresolidsseparationoccurs.Thenominaloreffectiveliquidvolumeofthereactorisgivenby(MetcalfandEddy,2003)

VQ S

Ln

o

org

= (11.14)

where:Vn=effectiveornominalliquidvolumeofreactor,m3

So=influentCOD,kgCOD/m3

Lorg=acceptableorganicloadingrate,kgCOD/m3·d

Thetotalliquidvolumeofreactorexclusiveofthegasstorageareaisgivenby

VV

EL

n= (11.15)

where:VL=totalliquidvolumeofreactor,m3

E=effectivenessfactor,representingthevolumefractionoccupiedbysludgeblanket,canvaryfrom0.8to0.9.

Thereactorheight(HL)basedonliquidvolumeis

HV

AL

L= (11.16)


So,thetotalheightofthereactoris

HT=HL+HG (11.17)

where:HT=totalreactorheight,mHG=reactorheightcorresponding togascollectionandstoragevol-

ume,usuallyabout2.5to3m.

TheseconceptsareillustratedinExample11.2.

11.8.3 Expanded granular sludge bed

The expanded granular sludge bed (EGSB) process is a variation of theUASBprocess. Itconsistsof twoormoreUASBreactorssituatedontopof each other. The EGSB system has been reported to successfully treatwastewaterswithhigh lipid content,which cause foamingand scum,aswellashandleorganicloadingratesthreetosixtimesgreaterthanthatofaconventionalUASBsystemwithsimilarefficiency(Vallingaetal.,1986).

EXAMPLE11.2DesignaUASBreactorfortreatmentofadairywastewaterat35°C.The wastewater flow rate is 1500 m3/d with a soluble COD con-centrationof3000mg/L.Also,calculate theeffluentsolubleCODconcentrationandthereactorefficiency.The followingparametersaregiven:

SRT=60dSludgeblanketoccupies80%ofliquidvolumeHeightforgascollection=2.5mUpflowvelocity=1.5m/hDesignorganicloadingrate=16kgsCOD/m3·dY=0.08kgVSS/kgCODkd=0.04d–1

µmax=0.35d–1

Ks=160mgsCOD/L

SOLUTION

Step1.DeterminetheUASBreactorcross-sectionalareaanddiameterbasedonupflowvelocityusingequation(11.13).

v=1.5m/h×24h/d=36m/d

AQ

v

m d

m dm= = =1500

3641 67

32/

/.


AD

m= =π 22

441 67.

or

D=7.28m≅7.3m

Step2.Calculatetheliquidvolumeofthereactorusingequation(11.14).

VQ S

L

m

dkg m

kg

md

no

org

= =×

=1500 3

16281 25

33

3

/

·. m 3

Step3.Calculatetotalliquidvolumeofreactorusingequation(11.15).

VV

E

mmL

n= = =.

..

281 25

0 8351 56

33

Step4.Calculateliquidheightusingequation(11.16).

HV

A

m

mmL

L= = =351 56

41 678 44

3

2

.

..

Calculatetotalheightofreactorusingequation(11.17).

HT=HL+HG=8.44m+2.5m=10.94m≅11m

Therefore,UASBreactorheight=11m,anddiameter=7.3m.

Step5.CalculateeffluentsCODconcentrationusingthekineticcoef-ficientsandequation(8.46)fromChapter8.

SK k

k

s d c

c d

=+( )−( ) −

1

1

θθ µm ax

or

S

kg

md d

d=

+ ×

−

−0 16 1 0 04 60

60 0 35

31. ( . )

. 00 04 11. d−( ) −=

0.0309kg/m3=30.90mg/L

Step6.CalculatethesCODremovalefficiency.

Em g L

m g L= − × = ≅( .) /

/% . % %

3000 30 9

3000100 98 97 99


11.8.4 Anaerobic sequencing batch reactor

Theanaerobicsequencingbatchreactor(ASBR)wasdevelopedbyDagueand co-researchers in the late 1980s at Iowa State University in Ames,Iowa.It isasuspendedgrowthprocesswherebiologicalconversionsandsolids–liquidseparationalltakeplaceinthesamereactor.Gasiscollectedonacontinuousbasis.Oneoftheadvantagesoftheprocessistheforma-tionofadense,granularsludgethathasahighactivityandsettleswell.TheASBRsequencesthroughfourstepsasillustratedinFigure 11.3(SungandDague,1992):

Gas

Supernatant

Settledbiomass

(a) Feed (b) React

(c) Settle (d) Decant

Figure 11.3 Operational steps of an anaerobic sequencing batch reactor (Source: Adapted from Riffat, 1994).


1.Feed—Aspecificvolumeofsubstrateisfedtothereactorataspecificstrength.Reactorcontentsareusuallymixedduringfeeding.

2.React—The reactor contents are mixed intermittently to bring thesubstrateintoclosecontactwiththebiomass.Thisisthemostimpor-tantstepintheconversionoforganicmattertobiogas.

3.Settle—Mixing is turned off and the biomass is allowed to settle,leavingalayerofclearliquidatthetop.

4.Decant—Aspecificvolumeofclearsupernatantisdecantedfromthetop.Thevolumedecantedisusuallyequaltothevolumefedinthefirststep.

Thesefourstepsconstituteacycleorsequence.Thetimeforonesequenceiscalledthecycle length. TheASBRisaveryflexiblesystem.Thenumberofsequencesperdaymaybevaried,togetherwiththetimerequiredforthevarioussteps.Thefeedinganddecantingtimesareshort,whilethetimeforthereactstepisthelongest.IdeallythereactstepshouldcontinueuntiltheF/Mratioisquitelow,sincealowF/Mratioisassociatedwithimprovedflocculationandsettling.TheASBRiscapableofachievinga lowerF/MratioattheendofthereactcyclethanasimilarlyloadedCSTR,whichwasdemonstratedbySungandDague(1992)andisillustratedinFigure 11.4.

Thetimeforsettlingdependsonthesettlingcharacteristicsofthebio-mass.HRTcanvaryfrom6to24h,whiletheSRTcanrangefrom50to200d.TheASBRhasbeendemonstratedforsuccessfultreatmentofvari-oustypesofhigh-strengthwastewaters.

6 hr 6 hr 6 hr 6 hr

Time of Day

Food

Con

cent

ratio

n, m

g/L

Minimum F/M ratio

Maximum F/M ratio

AverageF/M ratio

Figure 11.4 Typical variation of F/M ratio during ASBR operation (Source: Adapted from Sung and Dague, 1992).


AnumberofvariablesinfluenceefficientoperationofanASBR.Theseincludeorganicloadingrate(OLR),HRT,SRT,andMLSSamongothers.TheratioofOLRtoMLSSdefinestheF/Mratio,whichis important inachievingefficient solids separation.TheASBRpromotesgranulationbyimposingaselectionpressureduringthedecantcycle.Thedecantprocesstendstowashoutpoorlysettlingflocs,sothattheheavier,morerapidlyset-tlingaggregatesremaininthereactor.Reactorgeometry,HRT,andOLRinfluencethesizeandcharacteristicsofthegranules.Settlingvelocitiesof0.98 to 1.2 m/min were obtained for the granular sludge formed in theASBR(SungandDague,1992).

EXAMPLE11.3AlaboratoryscaleASBRisoperatedat35°Ctotreatasyntheticwaste-water.Thefollowingoperationalparametersaregiven:

Totalliquidvolume=10LLengthofcycle=6hFeedphase=15minReactphase=300minSettlephase=30minDecantphase=15minVolumefed/wastedpercycle=2.5L

a.CalculatetheHRTforthegivenconditions. b. Ifthecyclelengthisincreasedto8h,whatwillbethenewHRTof

thesystem? c. Ifthecyclelengthremainsthesame,whatcanyoudotoincrease

theHRT?

SOLUTION

Step1.Numberofcyclesperday=24h/cyclelength=24h/6h=4.Theflowperday,Q=2.5L×4=10L/d

H RT

V

Q

L

L dd= = =

/

10

101

Step2.Numberofcyclesperday=24h/cyclelength=24h/8h=3.

Q=2.5L×3=7.5L

H RTV

Q

L

L dd= = =

. /.

10

7 51 33

Step3.Ifthecyclelengthremainsthesame,theHRTcanbeincreasedbyreducingthevolumefed/wastedpercycle.


11.8.5 Anaerobic migrating blanket reactor

The anaerobic migrating blanket reactor (AMBR) consists of a numberof compartments separated by over and under baffles, as illustrated inFigure 11.5. Mixing is provided in each compartment as the wastewaterflowsthrough.Thesludgeblanketineachcompartmentrisesandfallswithgasproductionandflow,andalsomovesthroughthereactorataslowrate.Aftersometimeofoperation,theinfluentfeedportischangedtotheeffluentport,andviceversa.Thishelpstomaintainauniformsludgeblanketacrossthereactor.Usuallytheflowisreversedwhenalargequantityofsolidsaccu-mulatesinthelastcompartment.TheAMBRwasdemonstratedtoachievehighCODremovalefficienciesat lowtemperaturesof15°Cand20°C inbenchscaletestswithnonfatdrymilksubstrate(Angenentetal.,2000).

11.9 ANAEROBIC ATTACHED GROWTH PROCESSES

Similartotheaerobicprocess,amediaisusedinthisprocessthatthebac-teriaareallowedtoattachtoandgrowon.Anaerobicconditionsaremain-tainedinthereactorforconversionoforganicmattertomethaneandothergases.Examplesincludeanaerobicfilterorfixed-filmreactorandanaerobicrotatingbiologicalcontactor(RBC),amongothers.

11.9.1 Anaerobic filter

Ananaerobicfilterisacolumnorreactorpackedwithhighlyporousmaterial/medium.Thewastewaterusuallypassesthroughthereactorwithverticalflow,eitherupflowordownflow(Figure 11.6).Themicroorganismsinthereactorattachtotheporousinertmediumorbecomeentrapped.Theeffluentgasflowsupwardthroughthesupportmediaandthegasproducediscollectedatthetop.Anaerobicfiltersarealsoknownasfixed-film reactorsorpacked bed reactors.

Gas

E�uentE�uent

(�ow reversed)

In�uent(�ow reversed)In�uent

Figure 11.5 Anaerobic migrating blanket reactor (AMBR).


Thefirstanaerobicfilters,constructedbyYoungandMcCarty (1969),wereusedtotreatwastesof intermediatestrengthrangingfrom6000to15,000 mg/L of COD, synthetic protein, carbohydrate and volatile acidwastesat25°C.Thefiltersconsistedofupflowreactorsfilledwithsmallstones.Thefirstfull-scaleanaerobicfilterwasdescribedbyTaylorandBurm(1972).Thefilterswereoperatedinseriestotreatwheatstarchwastes.Thesystemaccomplishedupto70%CODreduction.Afterashutdownperiodof26days,thefilterwasabletorecovertomaximumefficiencywithin24h.

Anaerobicfiltersarecapableoftreatingawidevarietyofwastewatersatahighloadingratewithahighrateofmethaneproduction.Ananaerobicfiltercan switch from treatmentof onewastewater to anotherwithout adverseeffects,andcanoperateattemperaturesaslowas10°C(vandenBerg,1981).

(a)

(b)

Gas

E�uent

Down�owfeed

Sludgerecycle

Gas

E�uent

Up�owfeed

Sludgerecycle

Figure 11.6 (a) Anaerobic upflow filter and (b) anaerobic downflow filter.


Theanaerobicfiltercaneffectively treatorganicwastes in thepresenceofsometoxicsubstancesthatarebelowathresholdlevel(ParkinandSpeece,1982).Effluentrecyclingcanaidtoreducethetoxicconcentrationandmain-tainauniformpHthroughthefilter.AveryhighSRTinexcessof100dcanbeachieved.Theeffectsoftemperatureanddetentiontimecanbeminimized.

Thedisadvantagesoftheprocessincludetheinabilitytohandlewaste-waterswithhighsuspendedsolidsconcentration.Thecostofpackingorfiltermaterialishigh.Cloggingofthemediacancauseproblems.Seedingisnecessaryforstart-up,whichmaytakefromafewweekstoafewmonthstodevelopsufficientbiomassforcompletemethanogenesis.

11.9.2 Anaerobic expanded bed reactor

Theanaerobicexpandedbedreactor(AEBR)isavariationoftheupflowanaerobicfilter.Thepackingmaterialisusuallysilicasandwithadiameterof0.2to0.5mm.Theupflowvelocityisdesignedtoachieveabout20%expansionofmedia(MetcalfandEddy,2003).TheAEBRprocesshasbeenusedmostlyfortreatmentofdomesticwastewaters.

11.10 HYBRID PROCESSES

Hybrid processes are a combination of suspended and attached growthprocesses. One example of this is the anaerobic fluidized bed reactordescribedbelow.

11.10.1 Anaerobic fluidized bed reactor

Theanaerobicfluidizedbedreactor(AFBR)consistsofareactorfilledwithapackingmediumsuchassand,andoperatedathighupflowvelocitiestokeepthemediainsuspension.Upflowvelocitiesof20m/hmaybeusedtoprovide100%expansionofthepackedbed(MetcalfandEddy,2003).Theeffluentisrecycledtomaintainahighupflowvelocity.Reactordepthrangesfrom4to6m.Theflowdiagramoftheprocessissimilartoanupflowfilterwitheffluentrecycle.TheAFBRissuitablefortreatmentofwastewaterswithmainlysolubleCODandverylowsolidsconcentration.Itcanhandleorganicloadingratesof10to20kgCOD/m3·dorhigher,withgreaterthan90%removal,dependingonthewastewatercharacteristics.Reactorbiomasscon-centrationsof15to20g/Lcanbeestablished(MalinaandPohland,1992).

Varioustypesofpackingmaterialscanbeused.Theseincludesand,dia-tomaceousearth,resins,andactivatedcarbon.Activatedcarbonisgener-allymoreexpensive,butitismoreefficientfortreatmentofindustrialandhazardouswastewaters.Granular activated carbon (GAC) can achieve ahighbiomassconcentrationduetoitsporousstructureandcanreducetox-icityandshockloadsbyadsorption.


11.10.2 Anaerobic membrane bioreactorTheanaerobicmembranebioreactorsystemconsistsofananaerobicbiore-actorcoupledwithamembraneseparationunit.Theeffluentfromthebio-reactorpassesthroughthemembraneunit,wheresolids–liquidseparationtakesplace.Theliquideffluentorpermeateisdischarged,whilethesolids(concentrate)are recycledback to the reactor.ThemembranebioreactorprocessisillustratedinFigure 11.7.

Themajoradvantagesoftheanaerobicmembranebioreactorprocessare(1)high-qualityeffluentduetoefficientsolidscapture;(2)higherbiomassconcentration in the reactor, which results in higher COD loadings andsmallerreactorsize;and(3)higherSRTsareachievedinthereactorduetosolidsrecycle.Disadvantagesoftheprocessincludethehighcostofmem-branesandthepotentialformembranefouling.Alotofrecentresearchhasfocusedonfabricationofmembranesandapplicationofcoatingsthatcanreducefoulingproblems.Moredetaileddiscussiononmembranebioreac-torsisprovidedinChapter13.

PROBLEMS

11.1 Listthreeadvantagesandthreedisadvantagesoftheanaerobictreat-mentprocess.

11.2 Brieflydescribethefourstepsofanaerobicbiotransformation.Whatgroupsofbacteriaareinvolvedineachstep?

Influent

Anaerobicbioreactor

Gas Effluent(permeate)

Membraneseparation unit

Return solids(concentrate)

Figure 11.7 Anaerobic membrane bioreactor process.


11.3 List the factors thatare important foranaerobic treatment.WhattemperatureandpHrangesarebestfortheprocess?

11.4 DesignaUASBreactortotreatwastewaterat30°Cfromafoodpro-cessingplant.Thewastewaterflowrateis500m3/dwithasolubleCODconcentrationof6000mg/L.Thedesignparameters are asfollows:

Reactoreffectivenessfactor(E)=0.85Upflowvelocity=1.2m/hOrganicloadingrate=12kgsCOD/m3·dY=0.08kgVSS/kgCODkd=0.03d–1

μmax=0.25d–1

Ks=360mg/LHeightforgascollection=3mUsingthegiveninformation,determinethefollowing:

a. Thereactorareaanddiameter b. Thereactorliquidvolume c. Liquiddepthandtotalheightofthereactor d. TheaverageSRT(assuming97%degradationofsCOD)

11.5 Determine themethanegasproduction rate (m3/d) for the reactorfromproblem11.4.AssumeCODequivalentofVSSequals1.42kgCOD/kgVSS.

11.6 An anaerobic reactor operates at 35°C with an SRT of 30 d.Suddenly,themethanegasproductionratedecreasessignificantly.Explainthepossiblereason(s)tobeinvestigatedforthereductionofmethane.

11.7 Ananaerobicsequencingbatchreactor(ASBR)operatesatanHRTof1.5dat35°C.Calculatethevolumetobewastedpercycleforthefollowingoperationalparameters:

Totalliquidvolume=15LFeedphase=30minReactphase=240minSettlephase=60minDecantphase=30min

11.8 Briefly differentiate between anaerobic suspended and attachedgrowthprocesses.Givetwoexamplesofeachoftheprocesses.

11.9 Whataretheadvantagesanddisadvantagesofanaerobicfluidizedbedreactor(AFBR)andanaerobicmembranebioreactor?


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Dague,R.R.,McKinney,R.E.,andPfeffer,J.T.(1966)“AnaerobicActivatedSludge.”Journal Water Pollution Control Federation,vol.38,no.2,pp.220–226.

Daniels,L.(1984)“BiologicalMethanogenesis:PhysiologicalandPracticalAspects.”Trends in Biotechnology,vol.2,no.4,pp.91–98.

Droste,R.L.(1997)Theory and Practice of Water and Wastewater Treatment.JohnWiley&Sons,Inc.NewJersey.

Dubach, A. C., and Bachofen, R. (1985) “Methanogens: A Short TaxonomicOverview.”Experientia,vol.41,p.441.

Fair,G.M.,andMoore,E.W.(1932)“HeatandEnergyRelationintheDigestionofSewageSolids, III.EffectofTemperature if Incubateupon theCourseofDigestion.”Sewage Works Journal,vol.4,p.589.

Fuller,G.W.(1912)Sewage Disposal.McGraw-Hill,Inc.,NewYork.Gomec, C. Y., Kim, M., Ahn, Y., and Speece, R. E. (2002)“The Role of pH in

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Gujer,W.,andZehnder,A.J.B.(1983)“ConversionProcessinAnaerobicDigestion.”Water Science and Technology,vol.15,pp.127–167.

Heukelekian, H. (1933) “Digestion of Solids between the Thermophilic andNonthermophilicRange.”Sewage Works Journal,vol.5,no.5,pp.757–762.

Heukelekian,H.(1958)“BasicPrinciplesofSludgeDigestion.”Biological Treatment of Sewage and Industrial Wastes,vol.II.ReinholdPublishingCorp.,NewYork.

HulshoffPol,L.W.,deZeeuw,W.J.,Velzeboer,C.T.M.,andLettinga,G.(1983)“GranulationinUASB-Reactors.”Presented at the 66th Annual Conference of Water Environmental Federation,Anaheim,California.

Imhoff,K.(1915)“EightYearsofImhoffTankDesignandOperation.”Engineering News(NewYork),vol.75,p.14–19,52-55.

Lawrence,A.W.,andMcCarty,P.L.(1969)“KineticsofMethaneFermentationinAnaerobicTreatment.”Journal of Water Pollution Control Federation,vol.41,pp.R1–R17.


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Macario,A.J.L.,andConwaydeMacario,E.(1988)“QuantitativeImmunologicalAnalysis of the Methanogenic Flora of Digesters Reveals a ConsiderableDiversity.”Applied Environmental Microbiology,vol.54,p.79.

Madigan, M., Martinko, J., Stahl, D., and Clark, D. (2010) Brock Biology of Microorganisms.Thirteenthedition.PrenticeHall,NewYork.

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McCarty,P.L., Jeris, J. S., andMurdoch,W. (1963)“IndividualVolatileAcids inAnaerobicTreatment.” Journal Water Pollution Control Federation, vol. 35,p.1501.

McCarty, P. L. (1964a) “Anaerobic Waste Treatment Fundamentals—Part One:ChemistryandMicrobiology.”Public Works,vol.95,no.9,pp.107–112.

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McCarty, P. L. (1964c)“Anaerobic Waste Treatment Fundamentals—Part Three:ToxicMaterialsandTheirControl.”Public Works,vol.95,no.11,pp.91–94.

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239

Chapter 12

Solids processing and disposal

12.1 INTRODUCTION

Solids thataregenerated fromprimary, secondary,andadvancedwaste-watertreatmentprocessesarecalledsludge.Sludgeisusuallyintheformofliquidorsemisolidliquid,whichtypicallycontainsfrom0.25%to12%solidsbyweight.Itisclassifiedinthefollowingcategories:primarysludge,secondary sludge, and sludge produced in advanced treatment process.Primarysludgeconsistsofsettleablesolidscarriedintherawwastewater;secondarysludgeconsistsofbiologicalsolidsaswellasadditionalsettleablesolids.Sludgeproducedintheadvancedwastewatermayincludeviruses,heavymetals,phosphorous,ornitrogen.

In general, municipal sludge consists of primary and waste-activatedsludgeandmustbetreatedtosomeextentbeforedisposal.Itcontainsvari-ousorganicsandinorganics,e.g.biomassproducedbythebiologicalcon-versionoforganics,oil andgrease,nutrients (nitrogenandphosphorus),heavymetals, syntheticorganic compounds, andpathogens.Disposalofsludgerepresentsupto50%oftheoperatingcostsofawastewatertreat-mentplant(Appelsetal.,2008).

Treated wastewater sludge, commonly referred to as biosolids, is thematerial produced as the ultimate by-product of the processes used totreatmunicipalwastewaterinwastewatertreatmentfacilities.Biosolidsarenutrient-richorganicmaterial.Theycanbeusedforsoilenrichmentandcansupplementcommercial fertilizers.Biosolidsmustmeetstrict regula-tionsandqualitystandardsbeforebeingapplied to land.ApproximatelyeightmilliontoninemilliontonsofbiosolidsareproducedeachyearbymunicipalwastewatertreatmentfacilitiesintheUnitedStates(Hongetal.,2006).In2003,about60%ofthebiosolidswasreused.Beneficialreuseofbiosolidsisexpectedtoincreaseinthenearfuture.

Thefirststepinsludgehandlingisusuallythickening.Thepurposeofthickeningistoreducethevolumeofsludgebeforefurthertreatment.Themainthickeningmethodsusedaregravitythickening,floatationthickening,


centrifugation, gravity-belt thickening, and rotary-drum thickening(MetcalfandEddy,2003).

Thesecondstepissludgestabilization.Thepuposeofsludgestabilizationis toreduceorganicmattercontentof thesludge,reducepathogens,andeliminateoffensiveodors.Themainsludgestabilizationprocessesarealka-linestabilization,anaerobicdigestion,aerobicdigestion,andcomposting.

Afterstabilization,treatedsludgeisusuallydewateredtoreducethevol-umefurther.Mostwidelyuseddewateringprocessesarecentrifuge,belt-filterpress,andsludgedryingbeds(MetcalfandEddy,2003).

Finaldisposalmethodsforbiosolidsare(1)landfilling;(2)landapplication,whichisadisposalmethodwithbeneficialuse;and(3)incineration,whichistotalconversionoforganicsolids tooxidizedendproductsofcarbondiox-ide,water,andash.Incinerationisusuallyappliedtodewateredanduntreatedsludge.Figure 12.1illustratesthevarioussludgetreatmentanddisposaloptions.

Landapplication is themajormunicipal sludgeandbiosolidsdisposalmethod.Agricultural landapplication is abeneficialuseofbiosolids. Inordertoproducebiosolidswithaqualitysuitableformeetingtherequire-ments for agricultural land application, both stabilization of sludge andpathogenreductionareofimportance.

Inthischapter,thevariousprocessesusedforsludgethickening,stabi-lization,anddisposalwillbedescribedindetail.Methodsusedforsludgetreatment, such as anaerobic digestion processes, will be emphasized.Energy generation from anaerobic digestion in the formofmethane gaswillbediscussed.

12.2 CHARACTERISTICS OF MUNICIPAL SLUDGE

Consideringconventionalwastewatertreatment,municipalsludgeisgener-allycomprisedofprimarysludgefromprimarysedimentationtanksandsecondarysludgefromthesecondarysedimentationtanksfollowingbio-logical treatment of wastewater. Primary sludge is composed of organicandinorganicparticlescomingfromrawwastewater.Itisinfluencedbythewastewatersourceandprimarysedimentationtankoperation.Thesecond-arysludge,whichisalsocalledwaste-activatedsludge,includestheexcessmicroorganismcellsfromthebiologicaltreatmentprocess.Typicalproper-tiesofprimaryandsecondarysludgearegiveninTable 12.1.

12.3 SLUDGE QUANTIFICATION

Themassandvolumeofsludgeareimportantquantitiesthatareusedindesign.Thequantityofsludgeproduceddependsonthecharacteristicsofthewastewater,thespecificprocessesusedfortreatment,andtheirefficiencies.

Solids processing and disposal 241

Primaryclarifierstypicallyremove40%to60%ofthetotalinfluentsol-ids.Themassofprimarysludgecanbecalculatedasfollows:

Mp=QX(Ep/100) (12.1)

where:

(a)

(b)

(c)

(d)

�ickener Dryer

Primary andsecondary

sludgeBelt �lter

press Incineration

Ash todisposal

E�uent recycled toplant headwork

�ickener Dryingbeds

Primary andsecondary

sludge Anaerobicdigestion

To disposal orland application

E�uent recycled toplant headwork

�ickener Belt �lterpress

Primary and secondary

sludge Chemical

conditioningLime

treatment

To landapplication

E�uent recycled toplant

Filtrate to plantin�uent

�ickener Anaerobicdigestion

Wasteactivated

sludge Chemicalconditioning Centrifuge

Biosolids

To disposal,processing or

land application

E�uent recycled toplant

Filtrate to plantin�uent

Primary sludge

�ickening

Figure 12.1 Flow diagrams for sludge treatment and disposal (Source: Adapted from Metcalf and Eddy, 2003).


Mp=massofprimarysludge,kg/dQ=wastewaterflowrate,m3/dX=totalsuspendedsolidsininfluent,kg/m3

Ep=solidsremovalefficiencyofprimaryclarifier,%

Secondaryclarifiersfollowingsuspendedgrowthprocessesareusedforthickeningofsludgeandclarificationofeffluent.Theamountofsludgegen-erateddependsontheamountofnewcellsthatareproduced.Thisdependsonthefood-to-microorganism(F/M)ratioofthereactor,aswellasorganicloadingratesandotherfactors.Themassofsecondarysludgecanbecalcu-latedasfollows(Peavyetal.,1985):

Ms=Q(So–S)Y′ (12.2)

where:Ms=massofsecondarysludge,kg/dQ=wastewaterflowrate,m3/dSo=influentBOD5concentrationtosecondaryreactor,kg/m3

S=effluentBOD5concentrationfromsecondaryreactor,kg/m3

Y′=biomassconversionfactor =fractionofBOD5convertedtobiomass,kg/kg

ThevalueofY′dependsprimarilyontheF/Mratioofthebiologicalreac-tor.Y′canbedeterminedfromFigure 12.2.

Thetotalsludgeproducedisgivenby:

MT=Mp+Ms (12.3)

Table 12.1 Typical properties of primary and secondary activated sludge

Parameter Primary sludge Secondary activated sludge

pH 5.5–8.0 6.6–8.0Alkalinity (mg/L as CaCO3) 600–1500 550–1200Total solids % (TS) 4–9 0.6–1.2Volatile solids (% of TS) 65–80 60–85Protein (% of TS) 18–30 30–40Fats and grease (% of TS)

Ether soluble 5–30 —Cellulose (% of TS) 8–16 —Nitrogen (N, % of TS) 1.4–4.2 2.5–5.0Phosphorus (P2O5, % of TS) 0.6–2.9 3–10Organic acids (mg/L as HAc) 250–1800 1000–Energy content, kJ/kg TS 24,000–28,000 18,000–23,000

Source: Adapted from U.S. EPA (1979) and Metcalf and Eddy (2003).


where MT = Mass of total sludge produced, kg/d. Mp and Ms are asdefinedpreviously.

Primarysludgeisgranularinnatureandconcentrated.Secondarysludgefromactivatedsludgeprocesseshasalowsolidscontent,andislightandflocculentincharacter.Sometimesprimaryandsecondarysludgearemixedtogetherpriortothickeningtofacilitatefurthertreatment.Or,theymaybethickenedseparatelyandthensenttodigesters.

Solids content is usually determined on a mass/volume basis, andexpressedaspercent.For example, a5%sludge contains95%waterbyweight.Thespecificgravityofsludgeisusuallyaround1.02to1.05.Whenthesludgecontainslessthan10%solids,thespecificgravityofsludgecanbeassumedtobeequaltothatofwater,or1.00,withoutintroducingsig-nificanterror(Peavyetal.,1985).Eachpercentsolidsthencorrespondstoasolidsconcentrationof10,000mg/L.

Thevolumeofsludgecanbecalculatedusingthefollowingequation:

VM

SG Ps w s

=ρ

(12.4)

where:V=volumeofsludge,m3/dM=massofsludge,kg/dSGs=specificgravityofsludgePs=percentsolidsexpressedasadecimalρw=densityofwater=1000kg/m3

Fraction of BOD Converted to Excess Solids0 0.1 0.2 0.3 0.4 0.5

0.05

0.07

0.10

0.15

0.2

0.3

0.4

0.5

F/M

, kg

BOD

per

Day

per

kg

MLS

S Conventionaland

step aerationprocesses

Extendedaeration

andbiologicalfiltration

Figure 12.2 Typical variation of excess sludge production with F/M ratio. Actual quanti-ties will vary from plant to plant (Source: Adapted from Peavy et al., 1985).


For a given solids content, the following relationship can be used forapproximatecalculations(MetcalfandEddy,2003):

V

V

P

P1

2

2

1

= (12.5)

where:V1,V2=volumesofsludgeP1,P2=percentofsolidsinV1andV2,respectively

ThecalculationofsludgevolumesisillustratedinExample12.1.

EXAMPLE12.1A conventional wastewater treatment plant treats 15,000 m3 /d ofmunicipalwastewaterwithaBOD5(biochemicaloxygendemand)of220mg/landsuspendedsolidsof200mg/l.Thetreatmentconsistsofprimaryfollowedbysecondarytreatment.TheeffluentBOD5fromthefinalclarifieris20mg/L.Thefollowingdataareprovided:

Primaryclarifier:removalefficiency:SS=55%,BOD5=30%watercontent=95%,specificgravity=1.04

Aerationtank:F/M=0.33Secondary clarifier: 2% solids in waste-activated sludge, specific

gravity=1.02

a.Calculatethemassandvolumeofprimarysludge. b.Calculatethemassandvolumeofsecondarysludge. c.Calculatethetotalmassofprimaryandsecondarysludge.

SOLUTION

Step1.Calculatethemassofprimarysludge.

SS=200mg/L=0.2kg/m3

Calculatemassofprimarysludgeusingequation(12.1).

Mp=QX(Ep/100)=15,000m3/d×0.2kg/m3×0.55=1650kg/d

Step2.Calculatevolumeofprimarysludge.

Solidscontent=100–watercontent=100–95%=5%


Useequation(12.4)tocalculatesludgevolume.

VM

SG Pp

s w s

=ρ

=× ×

/

. .

1650

1 04 1000 0 053

kg dkg

m

=31.73m3/d

Step3.Calculatemassofsecondarysludge.Primaryclarifierremoves30%BOD5.Therefore,BOD5goingtoaerationtank=220mg/L(1–0.30).Or,So=154mg/L=0.154kg/m3.Given,S=20mg/L=0.02kg/m3.For F/M of 0.33, biomass conversion factor Y′ = 0.38 from

Figure 12.2.Useequation(12.2)tocalculatemassofsecondarysludge.

Ms=Q(So–S)Y′

=15,000m3/d(0.154–0.02)kg/m3×0.38

=763.80kg/d

Step4.Calculatevolumeofsecondarysludgeusingequation(12.4).

VM

SG Ps

s w s

=ρ

=× ×

. /

. .

763 80

1 02 1000 0 023

kg dkg

m

=37.44m3/d

Step5.Calculate totalmassofprimaryand secondary sludgeusingequation(12.3).

MT=Mp+Ms

=1650+763.8kg/d

=2413.80kg/d


12.4 SLUDGE THICKENING

Theobjectiveofsludgethickening is toreducethevolumeofsludgeandincreasethesolidscontent.Thesludgegeneratedfromprimary,secondary,andtertiarytreatmentprocessescanhaveawiderangeofsolidsconcentra-tionandcharacteristics.Reducing thewater content isadvantageous forsubsequenttreatmentprocesses.Volumereductionreducespipesize,pump-ingcost,andtanksizesforfurthertreatment.

Allwastewatertreatmentplantsusesomemethodofsludgethickening.Insmallplantstreatinglessthan4000m3/d(lessthan1Mgal/d),thicken-ingisaccomplishedintheprimaryclarifierand/orsludgedigestionunits(MetcalfandEddy,2003). In largerplantsseparate thickeningprocessesareused.Examplesofthesearegravitythickener,dissolvedairflotation,centrifugation,gravity-belt thickener,androtary-drumthickener,amongothers.Thethickenedsludgeispumpedtoasubsequentsludgestabilizationprocess,whiletheliquideffluentisusuallyrecycledtoprimarytreatment.Thethickenershavetobedesignedtomeetpeakdemandsandpreventsep-ticityandodorproblemsduringthethickeningprocess.Anumberofthemajorsludgethickeningprocessesaredescribedinthefollowingsections.

12.4.1 Gravity thickener

Gravitythickeningisusedforprimarysludgeoracombinationofprimaryandwaste-activatedsludge.Thedesignofagravitythickenerissimilartoasecond-aryclarifier.Thethickeningfunctionisthemajordesignparameter,andtanksdeeperthansecondaryclarifiersareused.Thesurfacearearequiredforthick-eningmaybedeterminedusingthesolids flux analysisorthestate point analy-sismethods.AtypicalcirculargravitythickenerisillustratedinFigure 12.3.Dilute sludge is fed toacenter feedwell,where it isallowed tosettle.Thesludgescrapermechanismcanbeintheformofverticalpicketsordeeptrusses.Thescraperstirsthesludgegently,whichhelpstoreleasewatertrappedinthesludgeandpromotescompaction.Thethickenedsludgeispumpedtodigestersordewateringprocesses,andstoragespacehastobeprovidedforthesludge.Theliquideffluentisrecycledtotheheadworksoftheplant.

Asludgeblanketismaintainedatthebottomofthethickenertohelpinconcentratingthesludge.Blanketdepthscanrangefrom0.5to2.5m(2to8ft),withshallowerdepthsinwarmermonths.Anoperatingvariableisthesludge volume ratio,whichisthevolumeofsludgeblanketinthethickenerdividedbythevolumeofthickenedsludgeremoveddaily.Thesludgevol-umeratiocanrangefrom0.5to20d.Thesolidsloadingraterangesfrom100to150kg/m2·d,withmaximumhydraulicoverflowratesof15.5to21m3/m2·dforprimarysludge.Foracombinedprimaryandwaste-activatedsludgethickener,thesolidsloadingraterangesfrom25to80kg/m2·d,withmaximumhydraulicoverflowratesof6to12m3/m2·d(MetcalfandEddy,


2003).Highhydraulicloadingcanresultinexcesssolidscarryoverinefflu-ent, while low hydraulic loadings can cause septic conditions and sludgefloatation.

EXAMPLE12.2Consider the wastewater treatment process described in Example12.1. The primary sludge is thickened in a gravity thickener. Thethickenerhasadiameterof4.5mwithasidewaterdepthof5m.Asludgeblanketof1.2mismaintainedatthebottom.Primarysludgeisappliedat31.73m3/dwith5%solidstothethickener.Anadditional270m3/doftreatedwastewaterisappliedtothethickenertoincrease

(a) Plan

(b) Elevation

Walkway

Rake arm

Drive unit

Counter-�ow in�uent �ow

Blade

Direction of rotation

In�uentpipe

E�uentpipe

WalkwayDrive motor

Verticalpicket

Figure 12.3 Diagram of a typical gravity thickener: (a) plan, and (b) elevation.


theoverflowrateandimproveodorcontrolandthickening.Thethick-enedsludgeiswithdrawnat17m3/dwith7%solidscontent.Calculatethefollowing:

a.Hydraulicoverflowrate b.Solidsloadingrate c.Sludgevolumeratio d.Percentsolidscapturedinthickener

SOLUTION

Step1.Calculatethesurfaceareaofthickener.

As= π44 5 2( .) = 15.90m2

Step2.Calculatehydraulicoverflowrate.

Qin=(31.73+270)m3/d=301.73m3/d

Qthickened=17m3/d

Qeffluent=Qin–Qthickened=301.73–17=284.73m3/d

Overflowrate= QA

m d

meffluent

s

= . /

.

284 73

15 90

3

2=17.90m3/m2·d

Step3.Calculatesolidsloadingrate.FromExample12.1,massofprimarysludgesolids=1650kg/d.

Solidsloading=1650

15 90 2

/

.

kg d

m=103.77kg/m2·d

Step4.Calculatesludgevolumeratio.

Volumeofsludgeblanketinthickener=1.2m×15.90m2=19.08m3

Sludgevolumeratio= volum eofsludgeblanket

rateofthickened slludgewithdrawal= .

/

19 08

17

3

3

m

m d =1.12d

Step5.Calculatethesolidscapture.

Massofsolidscomingin=1650kg/d

Massofsolidsinthickenedsludge=17m3/d×0.07×1000kg/m3=1190kg/d


Solidscapture= 11901650

100/

/%

kg d

kg d× =72.12%

12.4.2 Dissolved air flotation

Dissolvedairflotation(DAF)isusedforthickeningwastesludgefromsus-pendedgrowthprocesses, suchaswaste-activated sludge.Theprocess isespeciallysuitableforthickeningthelight,flocculentsludgethatisgener-atedfromtheactivatedsludgeprocess.Itcanalsobeusedforthickeningofcombinedprimaryandwaste-activatedsludge.

Inthisprocess,waterorsecondaryeffluentisaeratedunderapressureofabout400kPa.Thesupersaturatedliquidisreleasedatthebottomofthetankthroughwhichsludgeispassedatatmosphericpressure.Fineairbubblesarereleasedintothetank.Theairbubblesattachthemselvestothesludgeparticles,floatingthemuptothetanksurface.Thefloatingsludgeisremovedfromthetopwithaskimmer,whiletheliquidisremovedandrecycledtotheplant.Polymercanbeaddedforsludgeconditioning.ThedissolvedairflotationsystemisillustratedinFigure 12.4.

Ba�e

E�uent

Recycle�ow

Auxiliary recycleconnectionPressurizing

pump

Vent

Air compressor

Pressuretank

Pressurecontrol

valve

In�uent feed

Mix tank

Sludge discharge

Float collector

Floatation tank

Figure 12.4 Typical dissolved air flotation system.


ImportantfactorsthataffectthedesignofDAFsystemsincludeair-to-solidsratio,hydraulicloading,polymeraddition,andsolidsloadingrate,amongothers (WEF,1998).Forwaste-activated sludgewithoutpolymeraddition, solids loading rates ranging from2 to5kg/m2·h canproducethickenedsludgewith3%to5%solids.Withpolymeraddition,theloadingratecanbeincreasedby50%to100%.Operationaldifficultiescanarisewhenthesolidsloadingrateexceeds10kg/m2·h.

EXAMPLE12.3ConsiderthewastewatertreatmentprocessdescribedinExample12.1.Thesecondarywaste-activatedsludge is thickened inadissolvedairflotationprocess.IftheDAFprocessthickensthesolidsto3.5%,cal-culatethevolumeofthickenedsludge.Assumethattheprocesscap-tures95%ofthesolids.

SOLUTION

MassintoDAF=Mi=763.80kg/d

VolumeintoDAF=Qi=37.44m3/d

Sludge in�ow

�ickened sludge tostabilization

Liquid recycledto plant

DAF

QT, MT

Qe, MeQi, Mi

DAFprocesscaptures95%solids.

Therefore,MT=0.95×Mi=0.95×763.80kg/d=725.61kg/d

Assume,specificgravityofthickenedsludge=specificgravityofwater=1.0.Useequation(12.4)tocalculatethevolumeofthickenedsludge:

QM

SG PT

T

s w s

=ρ

=× ×

. /

. .

725 61

1 0 1000 0 0353

kg dkg

m

=20.73m3/d


12.4.3 Centrifugation

Theprocessofcentrifugationisusedforboththickeninganddewateringofsludge.Thesolid-bowlcentrifugeisusedmainlyforthickeningofwaste-activatedsludge.Thebasicprinciple involvesthethickeningofsludgebytheuseofcentrifugalforces.Thickenedsolidsconcentrationof4%to6%canbeachieved.Thickeningcanusuallybeachievedwithoutpolymeraddi-tion.Maintenanceandpowercostsarehighfortheprocess.

Thesolid-bowlcentrifugeconsistsofalongcylindertaperedatoneend,whichismountedonahorizontalplaneandrotatesataparticularspeed.Sludgeflowsintothecylinderandthesolidsconcentrateontheperiphery.Aninternalhelicalscrollrotatesatadifferentspeedandmovesthecon-centratedsolidstowardthetaperedend,fromwheretheyaredischarged.Theliquidcentrateiscollectedattheotherendandrecycledtotheplant.Figure 12.5illustratesthesolid-bowlcentrifugeprocess.

12.5 SLUDGE STABILIZATION

Thickenedsludgemaybestabilizedbyvariousmeansatwastewatertreat-mentplants.Thecommonlyusedmethodsforsludgestabilizationare(1)alkalinestabilization,usuallywithlime,(2)anaerobicdigestion,(3)aerobicdigestion,and(4)composting.Thesearedescribedindetailinthefollow-ingsections.Notallplantspracticesludgestabilizationafterthickening.Someplantsdewaterthickenedsludgeandthenuselimestabilizationpriorto disposal. Other plants use anaerobic digestion to stabilize thickenedsludge.Thisisfollowedbydewateringandfinaldisposal.Theselectionoftreatmentmethodsdependsonregulatoryrequirementsforfinaldisposalofbiosolids.

Figure 12.5 Diagram of a solid-bowl centrifuge.


The objectives of sludge stabilization are the following (Metcalf andEddy,2003):

• Reducepathogens• Eliminateoffensiveodors• Inhibit,reduce,oreliminatethepotentialforputrefaction

12.5.1 Alkaline stabilization

Quicklimeorhydratedlimeisaddedtothesludgeforstabilization.LimeisaddedtoraisethepHto12orhigher.Thealkalineenvironmentinhib-itspathogenicmicroorganismsandsignificantlyreducesorhaltsbacterialdecompositionoforganicmatterinthesludge.Thispreventsodorproduc-tionandvectorattraction.HealthhazardsarenotaproblemaslongasthepHismaintainedatthislevel.Limecanbeusedforpretreatment orpost-treatmentofsludge.

12.5.1.1 Chemical reactions

Avarietyofchemicalreactionscanoccurdependingonthecharacteristicsandconstituentsofthesludge.Someofthesearegivenbelow(MetcalfandEddy,2003;WEF,1998):

Withcalcium: Ca2++2HCO3

–+CaO→2CaCO3+H2O (12.6)

Withphosphorus: 2PO4

3–+6H++3CaO→Ca3(PO4)2+3H2O (12.7)

WithCO2: CO2+CaO→CaCO3 (12.8)

Withfats: Fat+Ca(OH)2→glycerol+fattyacids+CaCO3 (12.9)

Withacids: RCOOH+CaO→RCOOCaOH (12.10)

Other reactions also take place with proteins, carbohydrates, and poly-mers.Asthereactionsprogress,thepHcandecreaseduetoproductionofacidsetc.,soexcesslimeisadded.Ammoniaisproducedfromaminoacids,inadditiontovolatileoff-gases,whichrequirecollectionandtreatmentforodorcontrol.


Whenquicklime (CaO) isused, its reactionwithwater is exothermic,producingabout64kJ/g·mol(2.75×104BTU/lb·mol).Reactionofquick-limewithCO2illustratedinequation(12.8)isalsoexothermic,releasingapproximately180kJ/g·mol(7.8×104BTU/lb·mol)(U.S.EPA,1983).

12.5.1.2 Lime pretreatment

Pretreatmentinvolvestheapplicationoflimetoliquidsludgebeforedewa-tering.Thisrequiresmorelimeperunitweightofsludge.Limepretreatmentisusedfordirectapplicationofsludgeonlandorforconditioningandsta-bilizationpriortodewatering.ThedesignobjectiveistomaintainthepHabove12forabout2htoensurepathogendestructionandtoprovidesuf-ficientalkalinitytomaintainthepHabove11forseveraldays.Excesslimeisusedtoensurethelatter.

12.5.1.3 Lime posttreatment

Inposttreatment,hydratedlimeorquicklimeisappliedtodewateredsludge.Theadvantagesofposttreatmentarethatdrylimecanbeused,andthereare no special requirements for dewatering. Scaling problems are elimi-nated.Adequatemixingisimportanttoavoidtheformationofpocketsofputresciblematerial.Thestabilizedbiosolidshaveagranulartexture,canbestoredforlongperiods,andareeasilyspreadonlandbyaconventionalmanurespreader.

12.5.2 Anaerobic digestion

Anaerobicdigestion is the traditionalmethodforstabilizationofmunic-ipal sludge, which results in volatile solids reduction, biogas productionas an energy source, pathogen reduction, and reduced odor production.Anaerobicdigestionprocessesaregenerallyoperatedatmesophilicorther-mophilic temperatures.Overtheyears,manyprocessmodificationshavebeen developed. In addition to using single-stage digesters, two-phaseddigestionprocesses(stagingthedigestionprocessbyaddingapretreatmentstepforacidproduction)ortemperature-phaseddigestionprocesses(usingmesophilicandthermophilicdigestion)arealsoused.Thermal,mechani-cal,andchemicalpretreatmentoptionscanbeusedaswell,beforeameso-philicanaerobicdigestionprocess.

Themainadvantagesofanaerobicdigestionoveraerobicprocessesarereducingtheenergyneedbyeliminatingthenecessityofaeration,lownutri-entrequirements,energyproductionintheformofmethanegas,andloweramount of bacterial synthesis (Gomec et al., 2002). Energy in the formof methane can be recovered from the biological conversion of organic


substrates. Sufficient digester gas can be produced to meet the energyrequirementsof digester heating andoperationof otherplantprocesses.Another advantage is that anaerobic processes can handle higher volu-metricorganicloadscomparedwithaerobicprocessesresultinginsmallerreactorvolumes.Forthesereasons,anaerobicdigestionistheprimarypre-ferredmethodfortreatmentofmunicipalsludgeandhigh-strengthorganicwastes.

Anaerobicdigestionhassomedisadvantagesaswell.Someofthesedis-advantagesarelongerstart-uptimerequiredtodevelopnecessaryamountofbiomassduetoslowgrowthrateofmethane-formingbacteria,possiblenecessityofalkalinityand/or specific ionaddition,andsensitivity to theadverseeffectoflowertemperaturesonreactionrates.

The following are important factors that should be considered in thedesignofanaerobicdigesters(MetcalfandEddy,2003):

• pH• Temperature• Alkalinity• Presenceoftoxiccompounds• Bioavailabilityofnutrients• Solidsretentiontime• Hydraulicretentiontime• Volumetricloadingofvolatilesolids

Process description. Anaerobicdigestion comprises fourmajor steps: (1)hydrolysis, (2) acidogenesis, (3) acetogenesis, and (4)methanogenesis, asdescribedpreviouslyinChapter11.Inconventionalsingle-stageanaerobicdigestionofmunicipalsludge,all foursteps takeplace inthesamereac-tor.However,metaboliccharacteristicsandgrowthratesofacidproducingandmethaneproducingbacteria aredifferent.Methanogens convert theendproducts(mainlyH2andacetate)frompreviousstepstomethaneandCO2,thereforemaintainingalowpartialpressureofH2andshiftingtheequilibriumof fermentationreactions toward formationofmoreH2andacetate.WhenthisbalanceisdisturbedandmethanogensdonotutilizetheH2formedbyacidogensfastenough,accumulationofVFAs(volatilefattyacids)andadropinpHareobservedduetoslowfermentationofpropio-nateandbutyrate,resultingindigesterfailure(MetcalfandEddy,2003).

Inordertomaintainafavorableenvironmentforthismixedcultureofmicroorganisms,VFAproductionandutilizationratesshouldbebalanced.Withshortretentiontimes,VFAproductionmayexceedVFAutilization.Therate-limitingstepistheconversionofVFAsbymethanogenicorgan-ismsandnotthefermentationofsolublesubstratesbyacidogens.Digesterupsetcanoccurduetodisturbanceoftheproperbalancebetweenacidandmethaneformers(GhoshandPohland,1974).


pH isan importantparameteraffecting theenzymaticactivity sinceaspecificandnarrowpHrangeissuitablefortheactivationofeachenzyme.pHrangeinwhichthemethanogensworkefficientlyisfrom6.7to7.4.AsharppHdropbelow6.3indicatesthattherateoforganicacidsproductionisfasterthantherateofmethaneformation.Ontheotherhand,asharppHincreaseabove7.8canbeduetoashiftinNH4

+toNH3,whichisthetoxic,un-ionizedformofammonia(Gomecetal.,2002).

BufferingeffectofammoniareleasedfromaminoacidfermentationcanpreventthepHfallinanaerobicdigesters.Primarysludgefromdomesticwastewaterconsistsofhighamountsofproteinanddetergent.Alkalinity-generating cations like ammonium ions from protein degradation andsodiumfromsoapdegradationincreasethealkalinityandpH.

ThemicrobiologyofanaerobictreatmentprocessisdiscussedindetailinChapter11.Factorsaffectinggrowthandtoxicityarealsoprovided.Thediscussioninthefollowingsectionswillfocusonthedesignandoperationofanumberofanaerobicdigestersforstabilizationofsludge.Theseinclude(1) single-stage mesophilic digestion, (2) two-stage mesophilic digestion,(3) thermophilic anaerobic digestion, (4) temperature-phased anaerobicdigestion (TPAD), (5) acid-gas phased digestion, (6) Enhanced EnzymicHydrolysis™,and(7)Cambi™process.

12.5.2.1 Single-stage mesophilic digestion

Single-stagemesophilicdigesterscanbestandard-rateorhigh-ratedigest-ers.Standard-ratedigestersareusedmainlybysmallplantsprocessinglessthan4000m3/d,whilehigh-ratedigestersareusedby largerwastewatertreatmentplants (Peavy et al., 1985).Digesters canhavefixed coversorfloatingcoverstoadjustforvariablevolumesofsludgeandgasproduction.Single-stageconventionalfloatingcoverdigestersperformthreefunctions:(1)volatilesolidsdestruction,(2)gravitythickeningofdigestedsludge,and(3) storage of digested sludge (Hammer and Hammer, 2012). Optimumoperatingtemperatureis35°C,witharangeof30°Cto38°C.

Figure 12.6 illustratesa single-stage standard-ratemesophilicanaerobicdigester (MAD). The sludge is fed continuously or at regular intervals tothedigester.Thetemperatureismaintainedat35°Cbypassingthesludgethrough a separate sludge heater. The sludge is mixed to some extent bypumpingactiontoandfromthesludgeheater,inthezoneofactivedigestion.Ascumlayerformsontop,withthesupernatantliquidseparatingoutfromthesolids.Thesupernatantiswithdrawnandrecycledtotheplant.Thetotalsolidsarereducedby45%to50%.Thedigestedsludgeiswithdrawnfromthebottomandtransportedtodewateringprocesses.Solidsconcentrationofdigestedsludgerangesfrom4%to6%.Theproducedgasiscollected,whichconsistsofabout60%to70%methane,25%to35%carbondioxide,andtraceamountsofothergases.Thegascanbeusedforheatingpurposes.


High-ratedigestersarecompletelymixed,andthereisnoseparationofsolids fromthe liquid.Mixingmaybeconductedbygasrecirculationordraft tubemixers.Theentirecontentsof thedigesterare transported todewateringprocesses.

12.5.2.1.1 Design of digester

Anaerobic digesters can be designed based on the principles outlined inChapter8forsuspendedgrowthprocesses.Anumberofempiricalmethodshavealsobeenused.Theseincludemethodsbasedonvolumetricloadingrate of solids, solids retention time, volatile solids destruction, observedvolumereduction,andloadingfactorsbasedonpopulation(MetcalfandEddy,2003).

Thesolidsloadingratecanrangefrom1.6to4.8kgvolatilesuspendedsolids(VSS)/m3·dforcompletelymixedhigh-rateanaerobicdigesterswithasolidsretentiontime(SRT)of15to20d(U.S.EPA,1979).Forconven-tionaldigesters,thesolidsloadingratecanrangefrom0.32to1.0kgVSS/m3·dwithSRTvaluesof30to90d.Volatilesolidsdestructionof55%to65%canbeachievedatSRTvaluesof15to30d(WEF,1998).Inprac-tice, the design SRT ranges from 10 to 20 d. McCarty (1964) observedthataminimumSRTof4dwasrequiredat35°Ctopreventwashoutofthemethanogens.HesuggestedadesignSRTof10d.Gradyetal.(1999)

Digested sludgestorage

Activedigestion

SupernatantSupernatant out

Sludgeheater

Raw sludge in

Gasstorage

Gas out

Digested sludge out

Scum

Floatingcover

Figure 12.6 Diagram of single-stage standard-rate anaerobic digester.


proposedalowerSRTlimitof10dtoensureanadequatefactorofsafetyagainstwashout.TheyobservedthatincrementalchangesinvolatilesolidsdestructionwererelativelysmallforSRTvaluesabove15dat35°C.

Whenpopulationequivalentloadisusedtodesigndigesters,typicalval-uesusedare0.17m3(6ft3)tankvolumepercapitafordigestionofprimaryandwaste-activatedsludge,and0.11m3(4ft3)percapitafordigestionoftricklingfiltersludge(HammerandHammer,2012).

Whenthecharacteristicsofrawanddigestedsludgeareknown,thevol-umerequiredforasingle-stagestandard-ratedigestercanbecalculatedfromthefollowingequation(Peavyetal.,1985;HammerandHammer,2012):

VV V

t V tS = + +1 21 2 2

2 (12.11)

where:VS=volumeofstandard-ratedigester,m3

V1=rawsludgeloadingrate,m3/dV2=digestedsludgeaccumulationrate,m3/dt1=digestionperiod,dt2=digestedsludgestorageperiod,d

Forasingle-stagehigh-ratedigester,thevolumecanbecalculatedbasedonsolidsloadingrates,ordetentiontimes,oranyoftheotherempiricalmeth-odsmentionedabove.High-ratedigestersaredesignedascompletelymixedreactorswithoutsolidsrecycle.Foradesigndigestionperiod,thevolumecanbecalculatedfromthefollowing:

VH=V1t1 (12.12)

where:VH=volumeofhigh-ratedigester,m3

t1=digestionperiodorSRT,d

V1isasdefinedpreviously.ThedesignofmesophilicdigestersisillustratedinExample12.4.

12.5.2.1.2 Gas production and use

Gasproducedfromanaerobicdigestionusuallycontainsabout65%to70%methane; 25% to 30% carbon dioxide; and trace amounts of nitrogen,hydrogen,hydrogensulfide,watervapor,andothergases.Thevolumeofmethanegasproducedcanbeestimatedfromthefeedconcentrationsandbiomassproduced.Anumberofmathematicalrelationshipsareavailablein


literature.Totalgasproductioncanbeestimatedfromtheamountofvola-tilesolidsreduction.Typicalvaluesrangefrom0.75to1.12m3/kgvolatilesolids(VS)destroyed(12to18ft3/lbVSdestroyed).Afirstapproximationofgasproductioncanalsobemadefromthepopulation.Forprimaryplantstreatingdomesticwastewater,thegasproductionisabout15to22m3/1000persons·d(0.6to0.8ft3/person·d),whileforsecondaryplantsthevalueisabout28m3/1000persons·d(1.0ft3/person·d)(MetcalfandEddy,2003).

Naturalgashasaheatingvalueof37,300kJ/m3 (1000BTU/ft3).Puremethanegasatstandardtemperatureandpressure(20°Cand1atm)hasaheatingvalueof35,800kJ/m3(960BTU/ft3).Digestergashasabout65%methane,whichhasaheatingvalueofapproximately22,400kJ/m3(600BTU/ft3).Digestergascanbeusedfuelforboilersandinternalcombustionengines.Theelectricitygeneratedisthenusedforpumpingwastewater,andheatingdigesters,amongotherpurposes.Itcanalsobeusedincogenera-tionofelectricityandsteam.

12.5.2.1.3 Digester heating

Heathastobeprovidedtoadigestertoachievethefollowing:(1)raisethetemperatureoffeedsludgetotemperatureofdigestiontank;(2)compen-sateforheatlossesthroughthefloor,wallsandcoverofthedigester;and(3)compensateforlossesinthepipingtotheheatexchanger.Thesludgeisheatedbytransportingthesludgetoanexternalheatexchangerandpump-ingitbacktothedigester.

Tocalculatetheenergyrequiredtoheattheincomingfeedsludge,itisassumedthatthespecificheatofincomingfeedsludgeisequaltothatofwater (4.186kJ/kg·K).Theheatrequiredtoraisethetemperatureof theincomingsludgecanbecalculatedusingthefollowingequation(MetcalfandEddy,2003;Davis,2011):

qr=MDCp(TD–TI) (12.13)

where:qr=heatrequired,kJ/dMD=massofsludgefedtodigester,kg/dCp=specificheatofwater=4.186kJ/kg·KTD=digestiontemperature,KTI=temperatureofincomingfeedsludge,K

Theheatlossesfromthewalls,floor,andcoverofthedigestercanbecal-culatedfromthefollowingequation:

ql=UAΔT (12.14)


where:ql=heatloss,J/sU=heattransfercoefficient,J/m2·s·KorW/m2·KA=cross-sectionalareaofheatloss,m2

ΔT=temperaturechangeacrossthesurface,K

Typicalheattransfercoefficientscanbefoundinvarioussources(U.S.EPA,1979;MetcalfandEddy,2003).SometypicalvaluesareprovidedinTable 12.2.

12.5.2.2 Two-stage mesophilic digestion

Inatwo-stagemesophilicdigestionprocess,thefirsttankisdesignedasahigh-ratedigesterwithmixing,whilethesecondtankisusedfordewater-ingandstorageofdigestedsludge.Usually,thesecondtankisnotheatedormixed.Mostofthegasisgeneratedinthefirststage.Lessthan10%ofthetotalgasisgeneratedinthesecondstage.Bothtanksareequippedwithgascollectionsystems.Atwo-stagemesophilicdigestersystemisillustratedinFigure 12.7.

Table 12.2 Heat transfer coefficients for anaerobic digesters

Part of digester U, W/m2 · K

Fixed concrete cover100 mm thick and covered with built-up roofing, no insulation 4.0–5.0100 mm thick and covered, with 25 mm insulation 1.2–1.6225 mm thick, no insulation 3.0–3.6

Fixed steel cover 6 mm thick 4.0–5.4Floating cover

35 mm wood deck, built-up roofing, no insulation 1.8–2.025 mm insulating board installed under roofing 0.9–1.0

Concrete floor300 mm thick in contact with dry soil 1.7300 mm thick in contact with moist soil 2.85

Concrete walls above ground300 mm thick with insulation 0.6–0.8300 mm thick without insulation 4.7–5.1

Concrete walls below groundSurrounded by dry soil 0.57–0.68Surrounded by moist soil 1.1–1.4

Source: Adapted from U.S. EPA (1979) and Metcalf and Eddy (2003).


EXAMPLE12.4A conventional wastewater treatment plant treats 30,000 m3/d ofmunicipalwastewaterwithaBOD5of240mg/landsuspendedsolidsof200mg/l.TheeffluentBOD5fromthefinalclarifieris15mg/l.Theflowdiagramoftheplantisgivenbelow.Thefollowingdataareprovided:

Primaryclarifier:removalefficiency:SS=50%,BOD5=35%Watercontent=94%,specificgravity=1.06Aerationtank:F/M=0.33,biomassconversionfactor=0.40Finalclarifier:1.5%solidsinwaste-activatedsludge,specificgrav-

ity=1.02Flotationthickener:96.5%waterinthickenedsludgeAnaerobicdigestion:sludgeis74%organic,55%reductioninVSS

duringdigestion,solidscontentofdigestedsludge=6.5%

a.Calculatethemassandvolumeofprimarysludge. b.Calculatethemassandvolumeofsecondarysludge. c.Calculatethetotalmassandvolumeofsludgeenteringtheblend-

ingtank. d.Calculatethepercentageofsolidsintheblendedsludge. e.Calculatethevolumeofasingle-stagestandard-ratedigesterfor

adigestionperiodof25daysand sludge storageperiodof60days.

f.Calculatethetotalvolumeofatwo-stagedigester,ifthediges-tionperiodinthehigh-ratefirststageis10days.Thedewateringtime is5daysandsludge storage is for60days in the secondstage.

Sludgeinlet

Sludgeheater

Slud

ge

inle

t

Slud

ge

outle

t

Supernatantoutlets

SludgeoutletsDigested

sludge

Supernatant layer

Gas storagescum layer

Floating coverFixed cover

Digested gas outlet

Gas storage

Mixer

First stage(completely mixed)

Second stage(strati�ed)

Figure 12.7 Diagram of two-stage mesophilic digester (Source: Adapted from Peavy et al., 1985).


Supernatantreturn

Returnsludge Filtrate return

Waste activated sludge Spreading on farmland

Primaryclari�er

Aerationtank

Secondaryclari�er

Blendingtank

Floatationthickener

Anaerobicdigestion

Mechanical dewatering

Flow diagram of wastewater treatment plant.

SOLUTION

Step1.Calculatethemassandvolumeofprimarysludge.

Massofsolidsininfluent=30,000m3/d×0.20kg/m3=6,000kg/d

With50%removal,massofprimarysludgesolidsMp=0.50×6,000 kg/d=3,000kg/d


Vp=Volumeofprimarysludge=3000

1000 1 06 1 0 943

/

. ( . )

kg dkg

m× × −

=47.17m3/d

Step2.Calculatemassandvolumeofsecondarysludge.

BOD5goingtoaerationtank=(1–0.35)×240mg/L=156mg/L=0.156kg/m3

BOD5consumedinaerationtank=156–15mg/L=141mg/L=0.141kg/m3

Useequation(12.2)tocalculatemassofsecondarysludgesolids.

Ms=Massofsecondarysludgesolids=0.4×0.141kg/m3×30,000m3/d

Or,Ms=1692kg/d



Vs=Volumeofsecondarysludge=1692

1000 1 02 0 0153

/

. .

kg dkg

m× ×

=110.59m3/d

Step3.Calculatevolumeofthickenedsludgeinflotationthickener.

Massofsolidsgoingtothickener=1692kg/d

Assume100%captureofsolids.

Therefore,massofsolidsinthickenedsludge=1692kg/d

Volumeofthickenedsludge=1692

1000 1 02 1 0 9653

/

. ( . )

kg dkg

m× × −

=52.44m3/d

Step4.Calculatethemassandvolumeofsludgeenteringtheblend-ingtank.

Massofsludgesolidsenteringtheblendingtank=primarysolids+thickenedsecondarysolids.

MB=3000+1692kg/d=4872kg/d

Volumeofsludgeenteringtheblendingtank:

VB=47.17+52.44=99.61m3/d

Step5.Calculate%solids(Ps)inblendedsludge.

Assumespecificgravityofblendedsludge=1.0.

VM

SG PB

B

s w s

=ρ ( / )100

or

PM

SG Vs

B

s w B

=ρ

or

Pkg d

kg

mm

s =× ×

×

4872

1 0 10001

10099 61

3

/

. . 33 /d

=4.9%


Step6.Single-stagedigesterdesign.

Rawsludgeloadingrate,V1=99.61m3/d

Massofsolidstodigester=4872kg/d

Organicfraction=4872kg/d×0.74=3605.28kg/d

Inorganicfraction=4872kg/d×(1–0.74)=1266.72kg/d

DigestionreducesVSSororganicfractionby55%.

Therefore,organicfractionremaining=3605.28kg/d×(1–0.55)=1622.38kg/d

Totalmassremaining=1266.72+1622.38kg/d=2889.10kg/d Digestedsludgeaccumulationrate,V2= 2889 10

1000 1 0 0 0653

. /

. .

kg dkg

m× ×

=44.45m3/d

Useequation(12.11)tocalculatesingle-stagedigestervolume.

VV V

t V tS = + +1 21 2 2

2

or

V

m dd

m

ddS = + × + ×( . . ) /

.99 61 44 45

225 44 45 60

3 3

=4467.75m3

Step6.Two-stagedigesterdesign.Useequation(12.12)tocalculatevolumeoffirststage.

V1ststage=V1t1=99.61m3/d×10d=996.10m3

Useequation(12.11)tocalculatevolumeofsecondstage.

V

m dd

mnd stage2

3 399 61 44 45

25 44 45

( . . ) /.= + × +

ddd×60 =3027.15m3

Therefore,totalvolume=996.10+3027.15m3=4023.25m3

Note:thetotalvolumerequiredforthetwo-stagedigesterislessthanthatrequiredforthesingle-stagedigester.Totaldigestiontimeisalsolessforthetwo-stagedigester.


EXAMPLE12.5A single-stage mesophilic digester is used for sludge stabilizationatthewastewatertreatmentplantmentionedinExample12.4.Themassofsludgefedtothedigesteris4872kg/d.Thetemperatureofthe feed sludge is12°C.Calculate the totalheat thatmustbepro-vided to maintain the digester temperature at 35°C, based on thedatagivenbelow:

Digester:Diameter=27mTotaldepth=8m,withdepthbelowground=5mDigesterhasafixedconcretecoverwithinsulation,concretefloor

and walls in contact with dry soil, and concrete walls abovegroundwithinsulation.

Temperatureofsoilsurroundingdigester=8°CAmbientairtemperatureinwinter=5°C

35° CDigester

Air temp5°C

Soil temp8°C

3 m

5 m

27 m diameter

SOLUTION

Step1.Calculateheatrequiredtoheatthefeedsludgeusingequa-tion(12.13).

qr=MDCp(TD–TI)

TD=273+35=308K

TI=273+12=285K

Cp=4.186kJ/kg·K

Therefore,qr=4872kg/d×4.186kJ/kg·K(308K–285K)=469,066.42kJ/d

Step2.Calculatethesurfaceareaofthefloor,walls,andcover.

Floorarea=π/4×(27)2=572.55m2


Areaoffixedcover=572.55m2

Wallareaaboveground=π×27m×3m=254.47m2

Wallareabelowground=π×27m×5m=424.12m2

Step3.Calculatetheheatlossesfromthefloor,cover,andwallsusingequation(12.14)andheattransfercoefficientsfromTable 12.2.

ql=UAΔT

HeatlossfromconcretefloorwithU=1.7W/m2·K.

qfloor=(1.7W/m2·K)(572.55m2)(308K–(273+8)K)=26,280.05WorJ/s=26,280.05J/s×86400s/d×10–3kJ/J=2.27×106kJ/d

HeatlossfromconcretecoverwithU=1.4W/m2·K.

qcover=(1.4W/m2·K)(572.55m2)(308K–(273+5)K)=24,047.10WorJ/s=24,047.10J/s×86400s/d×10–3kJ/J=2.08×106kJ/d

HeatlossfromconcretewallbelowgroundwithU=0.6W/m2·K.

qbg=(0.6W/m2·K)(424.12m2)(308K–(273+8)K)=6870.74WorJ/s=6870.74J/s×86400s/d×10–3kJ/J=0.59×106kJ/d

HeatlossfromconcretewallabovegroundwithU=0.7W/m2·K.

qag=(0.7W/m2·K)(254.47m2)(308K–(273+5)K)=5343.87WorJ/s=5343.87J/s×86400s/d×10–3kJ/J=0.46×106kJ/d

Totalheatloss=(2.27+2.08+0.59+0.46)×106kJ/d=5.4×106kJ/d

Step4.Calculatetotalheatrequiredforsludgeanddigester. qTotal=qr+qloss=0.47×106+5.4×106kJ/d=5.87×106kJ/d

EXAMPLE12.6Assumethat1m3ofgasisproducedperkgVSdestroyedinthemeso-philicdigestergiveninExample12.5.Theheatingvalueofthegasis22,400kJ/m3,withamethanecontentof65%.Thegaswillbeusedtofuelaboiler,whichwillthenbeusedtoheatthedigester.Theeffi-ciencyof theboiler is75%.Consider the treatmentplantdata fromExamples12.4and12.5.Will thepowergenerated from thegasbesufficienttoheatthedigester?


SOLUTION

FromExample12.4:

VSSdestroyed=3605.28kg/d×0.55=1982.90kg/dTotalgasproduced=1982.90kg/d×1m3/kg=1982.90m3/dHeatingvalueofgas=22,400kJ/m3×1982.90m3/d=44.42×106kJ/dBoilerefficiency=75%Heatgeneratedbygas=44.42×106kJ/d×0.75=33.31×106kJ/d

FromExample12.5:Totalheatrequiredfordigester=5.87×106kJ/d<<heatgeneratedbygasinboiler.The heat generated from the gas will be more than sufficient tomaintainthedigesterheatingrequirements.Excessgascanbeusedforotherpurposesattheplant,e.g.provideenergyforpumpingorotherpurposes.

12.5.2.3 Thermophilic anaerobic digestion

Thermophilic digestion takes place at temperature ranges from 50°C to60°C, with an optimum at 55°C. Advantages of thermophilic diges-tionwhen comparedwithmesophilic digestion are higher reaction ratesofdestructionoforganicmatter resulting in shorter retention timesandtherefore smaller reactor volumes, increased methane production dueto increased solids destruction, improved dewatering characteristics ofdigestedsludge,andhigherdestructionofpathogenicorganisms.Ontheotherhand,therearedisadvantagesassociatedwiththermophilicdigestionsuchashigherenergyrequirementsforheating,poorsupernatantquality,poor process stability, and increased odor problems (Metcalf and Eddy,2003). Thermophilic digestion is seldom used as a single-stage digester.Itistypicallyusedasafirststageinastagedprocess.Increasedpathogendestructionmakesitdesirable,especiallywhenthedigestedbiosolidsaretobeusedforspecificlandapplications.

Oneofthemajordrawbacksofthermophilicdigestionishighersen-sitivity of this process to environmental changes, e.g. temperature,accumulationofintermediateproductssuchasH2andacetate,andpro-pionateresultinginineffectiveconversionofVFAstomethane(vanLieretal.,1993).

Despite the higher substrate utilization and specific growth rates ofthermophilicmicroorganismswhencomparedwithmesophilicmicroor-ganisms,theyieldofthermophilicbacteriaperunitamountofsubstrateislower.Theloweryieldofthermophilicmicroorganismsmaybeduetotheirhigherenergyrequirementformaintenanceorthespecificmolecu-larpropertiesofenzymereactionsatthermophilictemperatures(Zeikus,1979).


12.5.2.4 Temperature-phased anaerobic digestion (TPAD)

Temperature-phasedanaerobicdigestion(TPAD)isatwo-stagedigestionsystem consistingof a thermophilic stage as thefirst step followedby amesophilicstageasthesecondstep(Hanetal.,1997).Bycombiningthethermophilicandmesophilicdigestionprocessesintoone,TPADofferstheadvantagesofbothwhileeliminatingtheproblemsassociatedwiththesesystemswhenoperatedindependently(HarikishanandSung,2003).TheTPADprocessisillustratedinFigure 12.8(a).

Thethermophilicstepprovidesincreasedrateofdegradationofcomplexorganicsandimprovespathogenreduction.Ontheotherhand,themeso-philicstageisusedasthepolishingstagehelpingtodiminishthedrawbacks

(a)

(b)

(c)

(d)

Rawsludge

42°C 42°C 42°C 55°C 55°C 55°C 35°C

Gas

Treatedsludge

E�uent

SRT = 1–2d SRT > 10d

Raw sludge 55°C

Acid phase Gas phase

35°Cor

55°C

35°Cor

55°C

SRT = 1–3d SRT > 10d

Raw sludge 35°C

Acid phase Gas phase

35°C

SRT = 3–5d SRT = 7–15d

Raw sludge 55°C

Figure 12.8 (a) TPAD process, (b) acid-gas phased digestion with mesophilic acid phase, (c) acid-gas phased digestion with thermophilic acid phase (d) Enhanced Enzymic Hydrolysis (EEHTM) process.


ofthethermophilicstagesuchaspoorprocessstabilityandpooreffluentquality(SungandSantha,2003).

12.5.2.5 Acid-gas phased digestion

Theacid-gas-phaseddigestionsystemprovides the separationof the twomain stages of anaerobic biodegradation—hydrolysis/acidogenesis/aceto-genesisandmethanogenesissteps—increasingtheprocessstability.Ingen-eral,thermophilictemperaturesareusedfortheacid-formingstep,havingtheadvantageofhigherdestructionratesoforganicsolidsandincreaseddestructionofpathogens.HydraulicretentiontimehasaconsiderableeffectonthepopulationlevelsofmethanogensandcompositionoffermentativeproductslikeVFAs(Fukushietal.,2003).Inadditiontothermophilictem-peratures, short retention timesareadopted for thepretreatment step inorder to inhibit methanogenic population and increase acid production.Thesecondstepisthemethane-formingstep,whereneutralpHconditionsandalongerSRTareprovidedforthegrowthofmethane-formingbacteriaandformaximizinggasproduction.

Oneoptionfortwo-phasesystemsisemployingthermophilicanaerobicdigestionasthepretreatmentstepfollowedbymesophilicanaerobicdiges-tion.Anotheroptionistouseamesophilicacid-formingstepfollowedbyamesophilicorthermophilicmethane-formingstep.Theseoptionsareillus-tratedinFigure 12.8(b)and(c).

Enhanced pathogen destruction in two-phase anaerobic digestion isthoughttobearesultofthecombinedeffectofpHandacidconcentration.Anumberofstudieshavebeenconductedtoobservetheseparateandcom-binedeffectsofpHandorganicacidconcentrationonpathogendestruc-tion(Fukushietal.,2003andSalsalietal.,2006).

12.5.2.6 Enhanced enzymic hydrolysisTM

In January 2002, legislation was enacted in the United Kingdom (UK)thatrequiredpathogenreduction inmunicipalwastewatersludgefor thefirsttime.Thisnewrequirementledmanyutilitiestosearchformethodstooptimize theirexistinganaerobicdigestionsystems(Cumiskey,2005),particularlymesophilicdigesters,which included themajorityofoperat-ingsystems intheUKatthattime.OnesuchprocesswastheEnhancedEnzymicHydrolysisTM(EEHTM)processdevelopedbyUnitedUtilities(inUK)inpartnershipwithMonsalLimited.

The EEH process uses acid-phase digestion for hydrolysis of complexorganiccompoundsandVFAproduction,followedbybatchthermophilicanaerobicdigestionforpasteurization,andcontinuousmesophilicanaero-bicdigestionformethaneproductionandstabilization.Thecombinationof


42°Cand55°Ctemperaturesprovideimprovedhydrolyticactivitiestogetherwithpasteurizationtoachieverequiredpathogenreduction(Werkeretal.,2007).Theenzymehydrolysisstepbreaksdowncellwalllipoproteinstruc-tures,enhancingthedigestionprocess.TheEEHprocessschematicispre-sentedinFigure 12.8(d).

TheEEHprocessutilizesanovelplug-flowdigesteroperationthatpro-vides the ideal condition for maximum production of digestive enzymesthatare responsible forpathogendestructionandVFAproduction.Thisenablesapathogendestructionrateof99.9999%(LeandHarrison,2006).Inthisprocess,thesludgeisprefermentedat42°Cfollowedbypasteuriza-tionat55°C,fromwherethesludgeistransferredtoamesophilicdigester.AccordingtoWerkeretal.(2007),theEEHprocesshasachieved6logE. coliremoval,eliminationofSalmonella,andhasenhancedvolatilesolidsdestructionby10%attheBlackburnWastewaterTreatmentPlantintheUK.

12.5.2.7 CambiTM process

TheCambiTMprocesswasdevelopedinNorwayinthe1990s.Itisapat-entedsludgepretreatmentprocess.Theprocesshasbeeninstalledinwaste-water treatment plants in Norway, Denmark, Japan, Ireland, Scotland,and England (Greater Vancouver Regional District, 2005). The processinvolvestheoxidationofsludgeunderelevatedtemperatureandpressure.Undertheseconditions,pathogensaredestroyedandcellhydrolysisoccurs,releasingenergy-richcompounds.Followinghydrolysis,sludgeisfedtoananaerobicdigester,whereitreadilybreaksdown,resultinginhighvolatilesolidsdestruction (approximately65%)and increasedbiogasproductioncomparedwithconventionalanaerobicdigestion.

In the Cambi process, primary and secondary sludge is dewatered toapproximately17%solidsbeforeenteringapulpingvessel.Inthepulpingvessel,themixedsludgeisheatedtoapproximately80°C,andthentrans-ferredtothethermalhydrolysisdigestervessel,whereitisheatedto160°Catapressureofapproximately5.5barfor15to30minutes.Afterdiges-tion,thesludgeisreleasedtoaflash tank,whichisatatmosphericpres-sure.Thepressuredropbetweenthedigesterandtheflashtankcausescelllysisandadecreaseintemperatureto100°C.Aseriesofheatrecoveryandheattransfersystemsisrequiredtooptimizetheenergyuseoftheprocess.Sludgeintheflashtankisdilutedwithtreatedeffluenttoensurethatthesolidsconcentrationinthedigesterisnotexcessive.Figure 12.9providesaflowdiagramoftheCambiprocess.Afterthermalhydrolysistheviscosityofsludgeissignificantlyreduced,thusallowingthedigestertobeoperatedatsolidsconcentrationsofabout9%.ThedigestersizescanbesignificantlyreducedintheCambiprocess.


12.5.3 Aerobic digestion

Aerobicdigestionisthebiologicalconversionoforganicmatterinthepres-enceofair,usuallyinanopen-topreactor.Aerobicdigestionistheoxidativemicrobialstabilizationofsludge.Itisbasedontheprinciplethatwheninad-equate external substrates are available,microorganismswillmetabolizetheirowncellularmass,resultinginanoverallreductionofvolatilesolids.

Aerobicdigestionissimilartotheactivatedsludgeprocess.Microorganismsstarttoconsumetheirownprotoplasmasanenergysourceasthesupplyofavailablesubstrateisdepleted.Whencelltissuesbecometheenergysource,microorganismsaresaidtobeinanendogenousstage.Celltissueisoxi-dizedtocarbondioxide,water,andammonia,whicharesubsequentlyoxi-dizedtonitrate.Between20%and25%ofcelltissueisnonbiodegradable,whichremainsafterthedigestionprocessasthefinalproduct.

Advantagesofaerobicdigestionareasfollows(MetcalfandEddy,2003):

• In a well-operated aerobic digester, the volatile solids reduction isapproximatelyequaltothatobtainedinananaerobicdigester.

• BODconcentrationislowerinthesupernatantliquor.• Itproducesanodorless,biologicallystableendproduct.• Operationisrelativelyeasy.• Capitalcostislower.• Thisprocessissuitablefordigestingnutrient-richsludge.

Figure 12.9 Flow diagram of CambiTM process.


Disadvantagesofaerobicdigestionareasfollows:

• Thecostofpowerishigher,relativetothesupplyofrequiredoxygen.• Digestedsolidshaveinferiormechanicaldewateringcharacteristics.• Theprocess is significantlyaffectedby temperature, location, tank

geometry,concentrationoffeed,andtypeofmixing/aeration.

Somevariationsandcombinationsofaerobicandanaerobicdigestionpro-cessesarepresentedinthefollowingsection.

12.5.3.1 Autothermal thermophilic aerobic digestion

Autothermalthermophilicaerobicdigestion(ATAD)isasolidstreatmentprocess where heat is released by the aerobic microbial degradation oforganicmatter(Layden,2007).InATAD,theheatreleasedbythediges-tionprocessisthemajorheatsourceusedtoachievethedesiredoperatingtemperature.For theATADoperation, feedsludge is typically thickenedto4%–6%totalsolids(TS)andVSdestructionprovidesheatproductionthatresultsinautothermalconditions.Thermophilictemperaturesbetween55°Cand70°Ccanbeachievedwithoutexternalheatinputbyusingtheheat released from themicrobial oxidationprocess.About20,000kJofheatisproducedperkgVSdestroyed(MetcalfandEddy,2003).However,sometimesanoutsideheatsourceisrequiredwhenthesolidscontentoftherawsludgeisnothighenoughtoachievethedesiredtemperature.

ThemainadvantagesoftheATADprocessareasfollows:

• Shorterretentiontimes(5to6d)canbeusedtoachieve30%to50%VSdestruction.

• Pathogendestructionisgreater.• Simplicityofoperation.


• Odorproduction• Lackofnitrification• Poordewateringcapabilitiesofdigestedsludge

12.5.3.2 Dual digestion

Thedualdigestion(DD)processinvolvestheuseofaerobicthermophilicdigestion followed by anaerobic digestion. Typically, an ATAD processwitharelativelyshortretentiontimeisusedasapretreatmentsteptomeso-philicanaerobicdigestion.This is termedasdual digestion (Wardetal.,1998; Zabranska et al., 2003). In the ATAD step, solids are pretreated


bysolubilizationandpartialacidification,resultinginenhanceddigestiontogetherwith improvedpathogendestruction(Nosratietal.,2007).ThedualdigestionprocessisillustratedinFigure 12.10.

Thermophilicaerobicdigestionstepprovideshydrolyzedandhomoge-nizedsolids,whichimprovevolatilesolidsdestructioninthedownstreamanaerobic digester. ATAD is operated under an oxygen-limiting condi-tion,which, inconjunctionwith shorthydraulic retention time (HRT),resultsintheformationofVFAsthroughthefermentativemetabolismofthermophilic bacteria (Borowski andSzopa, 2007).TheATAD reactorprovides consistent feedwithhighVFAconcentration to the anaerobicstage,whichperformsasthemethane-formingstep.Inaddition,ammo-nificationintheATADreactorproducesapHbufferedfeedtotheanaero-bicstage.Inadditiontoenhancingefficiencyoftheanaerobicdigestionstep,ATADalsoprovidesbetterpathogenremoval.Intheshortretentiontime, thermophilic phase, high levels of VFAs and ammonia producedresultinareductionofpathogenicbacteria.

Adetailedlaboratory-scaleevaluationofthedualdigestionprocesswasconductedbyAynuretal.(2010,2009a,2009b,2008).AdualdigestionsystemconsistingofATADfollowedbyMAD(mesophilicanaerobicdiges-tion)wasoperatedandcomparedwithotherenhanceddigestionprocesses.TheeffectsofpretreatmentHRTs,oxygenflowrates,andorganicloadingrateswereevaluated.TheATADprocessproducedheatof14,300J/gVSremovedfromhydrolyticandacetogenicreactionswithoutcompromisingoverallmethaneyields,whentheHRTwas2.5daysorlowerandthetotalO2usedwas0.20LO2/gVSfedorlower.ATADfollowedbyTPADwasalsoevaluatedbytheresearchers.

TheTacomaCentralTreatmentPlantinthestateofWashingtonusesaDDsetupwithacombinationofATADfollowedbyTPAD.ATADisusedasthefirstaerobicstep,atwhichpreconditioningandClassApasteurization

55°Cdigester

�ickenedsludge

Gas

E�uent E�uent recycledto secondarytreatment

Gas

Treatedsludge

Air or O2

Aerobicdigestion

Anaerobicdigestion

35°Cdigester

Figure 12.10 Schematic of dual digestion process.


isachieved.ATADisfollowedbyTPAD,whereVSdestructionandsludgestabilizationtakesplace.TheTPADstepconsistsofthreeanaerobicphasesinatemperature-phasedmodefromthermophilictohighmesophilictolowmesophilic. Implementationof theTPADsystemafterATADresulted ineliminationofodor(EschbornandThompson,2007).

12.5.4 Composting

Thecompostingprocessinvolvesbiologicaldegradationoftheorganicmat-terinsludgetoproduceastableendproduct.Theprocesscanbeaerobicoranaerobic.Inmostcases,aerobiccompostingisused,asitenhancesdecom-positionoforganicmatterandresultsinthehighertemperaturenecessaryforpathogendestruction. Itcanbeusedforstabilizationofprimaryandwaste-activatedsludge,aswellasdigestedanddewateredsludge.Theendproduct is a humuslikematerial that canbeused as a fertilizer and soilconditioner.

Compostingiscarriedoutinthefollowingsteps(MetcalfandEddy,2003):

1.Preprocessing—sludgeismixedwithanorganicamendmentand/ora bulking agent. Commonly used amendments are sawdust, straw,andrecycledcompost,whichareusedtoreducemoisturecontentandincreaseairvoids.Abulkingagentsuchaswoodchipsisusedtopro-videstructuralsupportandincreaseporosity.

2. High-rate decomposition—thecompostpileisaeratedbymechanicalturningorairaddition.Thetemperaturefirstincreasesfromambienttoabout40°C(mesophilic).Asmicrobialdegradationproceeds,thetemperaturefurtherincreasestothermophilicrange(50°Cto70°C),where maximum degradation, stabilization, and pathogen destruc-tionoccurs.Thistakesplacefor21to28d.

3.Recovery of bulking agent. 4.Curing—thecoolingperiod.Asthetemperaturegoesdown,waterof

evaporationisreleasedtogetherwithcompletionofhumicacidforma-tionandpHstabilization.Thecuringperiodcanlastfor30dorlonger.

5.Postprocessing—nonbiodegradable materials are screened andremoved.Grindingisusedforsizereductionofthefinishedproduct.

Commonly used methods of composting include the aerated static pileandwindrowsystems.AwindrowsystemisillustratedinFigure 12.11(a).In-vesselcompostingsystemsarealsoavailablefrommanufacturers,wherethecompostingtakesplaceinanenclosedvesselorreactor.Theseareusedforsmall-scaleapplicationsandhavebetterodorcontrol,fasterthrough-put,andsmallarearequirement.Anin-vesselcompostingsystemisillus-tratedinFigure 12.11(b).


12.6 CONDITIONING OF BIOSOLIDS

Chemicalssuchaspolymersareusedtoimprovethedewateringcharacter-isticsofbiosolids.Conditioning isusedaheadofmechanicaldewateringsystems,suchasbeltfilterpresses,centrifuges,etc.Chemicalssuchasferric

(a)

(b)Composted materialAir plenum

Composting mix

Material to becompostedMixer

Air and gasesto odor control

system

2 to 4.5 m

1 to

2 m

Figure 12.11 Composting systems: (a) windrow system, (b) in-vessel composter.


chloride,lime,alum,andpolymersareaddedtothebiosolids.Thesecausecoagulationofthesolidsandthereleaseofabsorbedwater.Themoisturecontentcanbereducedfrommorethan90%toarangeof65%to85%.Besides chemical conditioning, other methods such as heat treatment orfreeze–thawarealsousedtoalimitedextentatsomeplants.

12.7 BIOSOLIDS DEWATERING

Theprocessofdewateringisusedtoreducethevolumeoftreatedsludgeorbiosolidsbyreducingthewatercontent.Dewateringisaphysicalunitoperation.Dewateredsludgeiseasiertohandleandtransportforfinaldis-posal.Volumereductionreducestransportationcosts.Dewateringisusu-allyrequiredpriortocomposting,incineration,andlandfilling.Anumberofdewateringmethodsareusedatvarioustreatmentplants.Theseincludecentrifugation,belt-filterpress,recessed-platefilterpress,dryingbeds,andlagoons.Heatdryingisalsousedinsomeinstallations.Someofthesemeth-odsaredescribedindetailinthefollowingsections.

12.7.1 Centrifugation

Centrifugationisapopularmethodfordewateringbiosolids.Centrifugationisusedforthickeningaswellasdewateringstabilizedsludges.Thesolid-bowl centrifugeusedforsludgethickeninghasbeendescribedinSection12.4.3. The high-solids centrifuge used for dewatering biosolids is pre-sentedinthissection.

12.7.1.1 High-solids centrifuge

Thehigh-solidscentrifugeisamodificationofthesolid-bowlcentrifuge.It has a longer bowl length, lower differential bowl speed to increaseresidencetime,andamodifiedscroll toprovideapressingactionattheend.Polymerapplicationisrequired.Solidscontentrangingfrom10%to35%maybeachieved.Thecentrate isusuallyhigh insuspendedsolids,whichcanposeproblemswhenitisrecycledbacktotheplant.Chemicalconditioningorincreasedresidencetimeinthecentrifugecanbeusedtoincreasesolidscaptureandreducesolidsloadinthecentrate.Thepowercostsarehigh.

Thereareanumberofadvantagesofthecentrifugationprocess:

• Lowinitiallost• Smallerfootprintcomparedwithotherdewateringprocesses• Highsolidsconcentrationindewateredcake• Lowodorgenerationastheunitisenclosed


12.7.2 Belt-filter press

Thebelt-filterpressisoneofthemostwidelyusedpiecesofdewateringequip-mentintheUnitedStates.Itisacontinuous-feedprocessthatcanbeusedformosttypesofmunicipalwastewatersludgesandbiosolids.Dewateredcakesolidscontentrangesfrom12%to32%,dependingonthefeedsolidscontentandsludgecharacteristics.Theprocessuseschemicalconditioning,gravitydrainage,andmechanicalpressuretodewaterbiosolids.

Abelt-filterpressisillustratedinFigure 12.12.Inthefirststage,polymeris added to condition the solids. In the second stage, conditioned sludgeisappliedtotheupperbeltofthegravitydrainagezone.Waterdrainsbygravitythroughtheporousbeltandiscollected.Abouthalfofthewaterisremovedbygravity.Inthethirdstage,thesludgedropsontothelowerbelt,whereitissqueezedbetweenopposingporousbelts.Thisisfollowedbyahigh-pressuresectionwherethesludgeissubjectedtoshearingforcesasthebeltspassthroughaseriesofrollers.Attheend,scraperbladesremovethedewateredsludgecakefromthebelts.

Sludgeloadingratesrangefrom90to680kg/m·hbasedoncharacter-istics andconcentrationof feed (Metcalf andEddy,2003).Thebelt sizevariesfrom0.5to3.5minwidth.Beltsizesof2marecommonlyusedformunicipalsludgedewateringoperations.Beltspeedscanvaryfrom1.0to2.5m/min(Davis,2011).

12.7.3 Drying bedsTheuseofdryingbedsisapopularmethodfordewateringdigestedbiosol-idsandunthickenedprimaryandwaste-activatedsludge.Theadvantages

Cakedischarge

Filtrate and wash-water collection

Belttensionroller

Wedge zone

High pressure zone

Belt wash

Polymer Static box

Gravity drainage zone

Sludge feed

Figure 12.12 Belt-filter press (adapted from Hammer and Hammer, 2012).


ofthemethodarelowcost,lowmaintenance,andhighsolidscontentinthe dried cake. The disadvantages include large land requirement, odorproblems, rodentproblems, impactofweather,and labor-intensivedriedproduct removal.There are a number of different types of dryingbeds,includingconventionalsandbeds,pavedbeds,artificialmediabeds,solardryingbeds,andvacuum-assistedbeds.

12.8 DISPOSAL OF BIOSOLIDS

Afterthickening,stabilization,anddewateringcomesthedisposalofthetreated sludgeandbiosolids.Disposal canbeby incinerationorby landapplicationandlandfilling.Theselectionofdisposalmethoddependsonregulatoryrequirementsandthedegreeoftreatmentreceivedbythesludge.Someofthecommonmethodsaredescribedinthefollowingsections.

12.8.1 Incineration

Incinerationisthecompletecombustionoforganicmatterinthesludgetoendproductssuchascarbondioxide,water,andash.Dewatered,untreatedsludgeisusedasfeedtotheincinerator.Theproductashhastobedisposedofinanappropriatemannerdependingonwhetheritcontainshazardousmaterials.Thegeneratedgasesarepassedthroughscrubbersandotherairpollutioncontroldevicestoremoveairpollutantsofconcernbeforereleas-ing them to the atmosphere. Examples of combustion processes includemultiplehearthincineration,fluidizedbedincineration,andcoincinerationwithmunicipalsolidwaste.

Theadvantagesofincinerationareasfollows(MetcalfandEddy,2003):

• Maximumvolumereductionisachieved.• Pathogensandtoxiccompoundsaredestroyed.• Thereispotentialforenergyrecovery.


• Capitalandoperatingcostsarehigh.• Hazardouswastemaybeproducedasaby-product.• Emissionofairpollutantsisamajorconcern.

12.8.2 Land disposal methods

Biosolidsmaybedisposedon land inanumberofways, including landapplication, landfilling,andbeneficial reuse.Landapplicationcanbeon(1)agriculturalland,(2)forestland,(3)disturbedland,or(4)adedicated


landdisposalsite.Recyclingbiosolidsthroughlandapplicationhasseveraladvantages(U.S.EPA,2000):

• Biosolidsprovideessentialnutrients,suchasnitrogenandphospho-rus,toplants.Theyalsocontainothermicronutrients,e.g.nickel,zinc,andcopper.Theycanserveasanalternativetochemicalfertilizers.

• Thenutrientsinbiosolidsarepresentinorganicform.Assuch,theyarereleasedslowlytotheplantsandarelesssusceptibletorunoff.

• Biosolidsimprovesoiltextureandwater-holdingcapacity.Theycanenhancerootgrowthandincreasedroughttoleranceofvegetation.

• Biosolidscanbeusedtostabilizeandrevegetatelandsimpairedbymining, dredging, and construction activities, as well as by firesandlandslides.

• Biosolids are used in silviculture to increase forest productivity byacceleratingtreegrowth,especiallyonmarginallyproductivesoil.

Theselectionofdisposalmethodandsiteisdictatedbylocalandstatereg-ulationsandthedegreeoftreatmentreceivedbythesludge.TheregulatoryrequirementsfordisposalofbiosolidsintheUnitedStatesarepresentedinthefollowingsection.

12.9 BIOSOLIDS DISPOSAL REGULATIONS IN THE UNITED STATES

OnFebruary19,1993,theU.S.EnvironmentalProtectionAgencyundertheauthorityoftheCleanWaterActpromulgatedrisk-basedregulationsfor theuseanddisposalof sewagesludge (U.S.EPA,1993).Theregula-tionswereforsludgefrommunicipalwastewatertreatmentplantsthatwasappliedon land,soldorgivenawayforuse inhomegardens,and incin-erated. The regulations pertaining to land application are known as 40CFRPart503,aspublishedintheFederalRegisterastheCodeofFederalRegulations(CFR),andwillbediscussedinthissection.Theregulationsareapplicabletoalltreatmentplantsthatuselandapplicationforfinaldis-posalofbiosolids.Theregulationsareselfimplementing,i.e.permitsarenotrequiredbytheplants.However,failuretoconformtotheregulationsisaviolationofthelaw.Frequencyofmonitoringandreportingrequire-mentsareprovidedindetail.

The40CFRPart503regulationsspecifyanumberofmethodsforsludgestabilization,whichincludedigestion,composting,andlimestabilization,amongothers.Maximumconcentrationsofmetalsthatcannotbeexceededaregivenasceiling concentrations.Inaddition,cumulative pollutant load-ing rates for eightmetals are established,whichmaynotbe exceeded atland application sites. A third set of metals criteria, known as pollutant


concentrations, areprovided. If these concentrationsarenot exceeded inthebiosolids,thenthecumulativepollutantloadingratesdonothavetobemonitored.

ThePart503ruledefinestwomaintypesofbiosolidsaccordingtothelevelofpathogenreduction:ClassAandClassB.Class Abiosolidscanbeappliedtolandwithoutanyrestrictions.ClassAbiosolidshavepathogensbelowdetectionlimits,moststringentmetallimits,andvectorattractionstandards.Thetermexceptional quality biosolidsisalsousedforClassAbiosolids.Class Bbiosolidshavelessertreatmentrequirements.ClassBbio-solidscanbeappliedonlandbutaresubjecttorestrictionswithregardtopublicaccesstothesite,livestockgrazing,andcropharvestingschedules.

According to the 40 CFR Part 503 standards for Class A biosolids,fecalcoliformindicatorlevelsoflessthan1000MPN/gramTSshouldbeachievedorSalmonella sp.bacterialevelsshouldbebelowdetectionlimits(3MPN/4gTS)aftertreatment.Entericvirusesandviablehelminthovashouldeachbelessthan1per4gTS.

ThepathogenrequirementforClassBbiosolidsiseitherafecalcoliformconcentrationbelow2×106MPN/gTSorthatthesludgeistreatedinaprocesstosignificantlyreducepathogens(PSRP).Asapointofreference,theconcentrationoffecalcoliformsinundigestedsludgeisapproximately108MPN/gTS,andSalmonella sp.concentrationisapproximately2×103MPN/gTS(U.S.EPA,1994).

12.9.1 Class A biosolids

AccordingtoU.S.EPA(1993),therearesixpathogenreductionalternativesbywhichsludgetreatmentprocessescanbeconsideredtoproduceClassAbiosolids.ClassAbiosolidscanbeappliedon land immediatelywithoutanyrestrictions.Thepathogenreductionalternativesarethefollowing:

Alternative 1: Thermally treated sewage sludge—Thisalternativemaybe used when pathogen reduction process uses specific time–tem-perature regimes to reduce pathogens. Required time–temperatureregimes must be met as well as the following requirement: eitherfecalcoliformdensitiesshouldbebelow1000MPN/gTS(dryweightbasis) or Salmonella sp. bacteria should be below detection limits(3 MPN/4gTS).

Alternative 2: Sewage sludge treated in a high pH–high temperature process—High temperature–highpHprocess involves elevating thepH to greater than 12 and maintaining the pH for more than 72hours,orkeepingthetemperatureabove52°Cforatleast12hoursatpHgreaterthan12,orairdryingtoover50%solidsafterthe72hperiodofelevatedpHisnecessary.


Alternative 3: Sewage sludge treated in other processes—Thisalterna-tiveappliestosewagesludgetreatedbyprocessesthatdonotmeettheprocessconditionsrequiredbyAlternatives1and2.Theprocessmustbedemonstratedtoreducethedensityofentericvirusesandhelminthovainthesewagesludgetolessthan1PFU/4gTS,inbothcases.

Alternative 4: Sewage sludge treated in unknown processes—Forthisalternative,demonstrationoftheprocessisnotnecessary.Instead,thedensityofentericvirusesandhelminthovainthebiosolidsmustbelessthan1PFU/4gTS.Inaddition,asforallClassAbiosolids,thesewagesludgemustmeetthefecalcoliformorSalmonella sp.limits.

Alternative 5: Use of PFRP—Biosolids are considered to be Class Aif they have been treated in one of the processs to further reducepathogens(PFRPs)aslistedbyU.S.EPA(1993).Thesearecompost-ing,heatdrying,heattreatment,thermophilicaerobicdigestion,betarayirradiation,gammarayirradiation,andpasteurization.Inaddi-tion,productsmustmeet theClassAfecalcoliformorSalmonella sp.requirements.

Alternative 6: Use of process equivalent to PFRP—Oneofthealterna-tivestoachieveClassAbiosolidsistouseaprocessequivalenttoaPFRP,asdeterminedbythepermittingauthority.Inaddition,prod-uctsofallequivalentprocessesmustmeettheClassAfecalcoliformorSalmonella sp.requirements.

12.9.1.1 Processes to further reduce pathogens

ThePFRPsdefinedbyU.S.EPAarelistedinAppendixBof40CFRPart503(FederalRegister,2010).Theseareoutlinedbelow.

Composting—maintaintemperatureat55°Corhigherfor3dforstaticaeratedpileorin-vesselcompostingmethod.Withwindrowcompost-ing,maintaintemperatureat55°Corhigherfor15dorlonger.

Heatdrying—at80°Cbydirectorindirectcontactwithhotgases.Heattreatment—at180°Corhigherfor30min.Thermophilicaerobicdigestion—at55°Cto60°CwithanSRTof10d.Betarayirradiation—at1.0megaradatroomtemperature.Gammarayirradiation—at1.0megaradatroomtemperature.Pasteurization—at70°Corhigherfor30minorlonger.

12.9.2 Class B biosolids

TherearethreepathogenreductionalternativesbywhichsludgetreatmentprocessescanbeconsideredtoproduceClassBbiosolids(U.S.EPA,1993).Inadditiontothepathogenreductionalternative,thebiosolidsmustalsomeetoneofthevectorattractionreductionrequirements.Siterestrictions


areplacedforlandapplicationofClassBbiosolids,withrespecttocropharvesting,animalgrazing,andpublicaccesstothesitewherethebiosolidsareapplied.ThedetailsoftheseareavailableintheFederalRegister(2010).

ThepathogenreductionalternativesforproductionofClassBbiosolidsarethefollowing:

Alternative 1: Monitoring of indicator organisms—Seven samples ofbiosolidsshouldbecollectedeachtimeformonitoring.Thegeometricmeanofthedensityoffecalcoliformsinthosesamplesshouldbelessthan2×106MPN/gTS.

Alternative 2: Use of PSRP—ThesludgehastobetreatedbyaPSRP.Theseincludeaerobicdigestion,anaerobicdigestion,airdrying,com-posting,andlimestabilization.

Alternative 3: Use of processes equivalent to PSRP—ClassBbiosolidscanbeproducedusingaprocessthatisequivalenttoPSRP,asdeter-minedbythepermittingauthority.

12.9.2.1 Processes to significantly reduce pathogens

ThePSRPsdefinedbyU.S.EPAarelistedinAppendixBof40CFRPart503(FederalRegister,2010).Thesearepresentedbelow:

Aerobicdigestion—at20°CwithSRTof40d,orat15°CwithSRTof60d.

Anaerobicdigestion—at35°Cto55°CwithSRTof15d,orat20°CwithSRTof60d.

Airdrying—sludgeshouldbedriedinsandbeds,orpavedorunpavedbedsforatleast3months,whenambienttemperatureisabove0°C.

Composting—at40°Corhigherfor5d.For4hduringthe5dperiod,thetemperatureinthecompostpileshouldexceed55°C.

Limestabilization—sufficientlimeshouldbeaddedtoraisethepHofthesludgeto12after2hofcontact.

PROBLEMS

12.1 Define the termbiosolids.List the fourstepsof sludgeprocessinganddisposal.Giveexamplesofeachstep.

12.2 Differentiatebetweenprimaryandsecondarysludge.12.3 Amunicipalwastewatertreatmentplantprocessesanaverageflow

of15,000m3/dwith200mg/LofBOD5and320mg/Lofsuspendedsolids.Thepeakflowis1.5timestheaverageflow.DeterminethemassofBOD5andsolids(kg/day)thatareremovedassludgefrom


theprimaryclarifierforaverageandpeakflowconditions.AssumeareasonableefficiencyfortheprimaryclarifierandthesamepeakflowfactorforBOD5andsuspendedsolids.

12.4 A wastewater treatment plant consists of primary treatment fol-lowedbyanactivated-sludgesecondarysystem.Theplantprocesses7000m3/dofwastewaterwith180mg/LofBOD5and150mg/Lofsuspendedsolids.Theprimarysludgecontains500kgofdrysolidsperdaywith4.5%solidscontent.Theplantproduces760kgoftotalsludge(primaryandsecondary)perday.Assume30%removaleffi-ciencyoftheprimaryclarifierandF/Mratioof0.2fortheaerationtank.Determinethefollowing:

a. Primary sludge volume and removal efficiency of primaryclarifier

b. InfluentandeffluentBOD5of the secondaryactivated-sludgesystem

c. Volumeofsecondarysludgewith1%solidscontent12.5 Why it is necessary to thicken theprimary and secondary sludge

beforefurtherprocessing?Giveexamplesofdifferentsludge-thick-eningprocesses.Describethemostcommonmethodsavailableforvolumereductionofsludge.

12.6 Inproblem12.4,theprimaryandsecondarysludgesaremixedina gravity thickener and sent for further treatment.The thickenedsludgecontains4%solids.Calculatethepercentvolumereductioninthegravitythickener.

12.7 What are the objectives of sludge stabilization? List the commonmethodsusedtostabilizethesludgebeforedisposal.

12.8 Briefly describe the advantages and disadvantages of anaerobicdigestion.What factorsshouldbeconsidered for thedesignofananaerobicdigester?

12.9 Thethickenedsludgefromproblem12.6istobedigestedinastan-dard-ratemesophilicanaerobicdigester(MAD).Thesludgehas68%organiccontent,andapproximately60%oftheorganicfractionisdigestedaftera30-dperiod.Thedigestedsludgehasasolidscontentof6%andisstoredforaperiodof90d.Calculatethevolumeofthestandard-ratedigester.

12.10 Rework problem 12.9 for a two-stage digester system employing amixed,heatedfirststagewithadigestionperiodof10dandsecondstagewithathickeningperiodof3d.Determinethevolumeofthefirst-stageandsecond-stagedigesters,andcomparethetotalvolumewiththatofthesingle-stagedigesterinproblem12.9.

12.11 What are the advantages and disadvantages of aerobic digestion?Giveexamplesofdifferenttypesofaerobicdigestionprocesses.

12.12 Brieflyexplainthecompostingprocess.


12.13 Listtheprocessesbywhichwatercontentofthesludgeisreduced.Whichoneisthemostwidelyusedfordewatering?Brieflydescribetheprocess.

12.14 Nameanddescribethemostcommonmethodsofsludgedisposal.WhatisthebasicdifferencebetweenClassAandClassBbiosolids?

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Chapter 13

Advanced treatment processes

13.1 INTRODUCTION

When the effluent from secondary treatment does not meet regulatoryrequirementsfordischarge,additionaltreatmentmaybeneededtoreducethe levels of specific contaminants. This is usually termed as advanced treatmentortertiary treatment.Advancedtreatmentprocessesareusedforremovalofnutrientssuchasnitrogenandphosphorus,removalofresidualtotalsuspendedsolids,removalofspecificheavymetalsorinorganics,andremovalof emerging contaminantsof concern, amongothers.Advancedtreatmentprocessesmaybeincorporatedwithintheprimaryorsecondarytreatmentunits,e.g.forbiologicalnutrientremoval,ortheymaybeaddedseparatelyfollowingsecondarytreatment,e.g.forwastewaterreclamation.These includechemicalprecipitation,carbonadsorption,granularmediafiltration,andmembranefiltration,amongothers.Anumberofthesepro-cesseswillbediscussedindetailinthischapterwithregardtothetypesofcontaminantsthatareremovedbytheprocesses.Thefocusofthischapterwillbetheremovalofnitrogen,phosphorus,suspendedanddissolvedsol-ids,andotherinorganicsfromwastewater.

13.2 NITROGEN REMOVAL

Excess nutrients, such as nitrogen and phosphorus, can cause eutrophi-cation problems in water bodies as described previously in Chapter 3(Section3.5).Effluentdischargelimitsonnutrientscannecessitatetheuseofadvancedprocesses for removalofnitrogenandphosphorus fromthetreatment plant effluent. Wastewater treatment plants that use biologi-calprocessesfornutrientremovalareknownasBNR(biologicalnutrientremoval) plants. The most commonly used method of nitrogen removalisbiological nitrification–denitrification.Thiscanbeaccomplishedbyanumber of different suspended and attached growth processes. Some ofthesearedescribedinthefollowingsections.Emergingtechnologiesthat


usebiological deammonificationarealsopresented.Physicochemicalpro-cessescanbeusedfornitrogenremoval.Oneexampleofthisisairstrip-ping,whichisalsodiscussed.

13.2.1 Biological nitrogen removal

Nitrogen compounds are formed in domestic wastewater from the bio-degradationofproteinsandureadischarged inbodywaste.Theorganicnitrogencompoundsarefurtherconvertedtotheaqueousammoniumion(NH4

+) or gaseous free ammonia (NH3). These two species together arecalledammonia-nitrogen(NH4-N),andremaininequilibriumaccordingtothefollowingrelationship:

NH4+⇌NH3+H+ (13.1)

ThepHandtemperatureaffecttherelativeconcentrationsofthetwospe-ciesinwater,asillustratedinFigure 13.1.

The removal of ammonia-nitrogen or ammonia from water is carriedoutbybiological (a)nitrification–denitrificationprocess, (b)nitritation–denitritationprocess,and (c)deammonificationprocess.Descriptionsofeachprocessareprovidedinthefollowingsections.

13.2.1.1 Nitrification–denitrification

Nitrification involves theconversionofammonia tonitrates,whiledeni-trificationinvolvestheconversionofthenitratestonitrogengaswhichisreleased to the atmosphere. The overall nitrification–denitrification pro-cessisillustratedinFigure 13.2.Theconditionsandprocessrequirementsfornitrificationanddenitrificationareverydifferentfromoneanotherasdescribedbelow.

13.2.1.1.1 Nitrif ication stoichiometry

Nitrification is a two-stepprocesswhere ammonia is oxidized tonitrite(NO2

–)inthefirststep,andthenitriteisfurtheroxidizedtonitrate(NO3–)in

thesecondstep,asdescribedpreviouslyinChapter3(Section3.2).Aerobicautotrophicbacteriacarryoutthesereactionsasshownbelow(MetcalfandEddy,2003),

nitrosomonas 2NH4

++3O2____________▶ 2NO2

–+4H++2H2O+energy (13.2)

nitrobacter 2NO2

–+O2 ____________▶ 2NO3–+energy (13.3)

Advanced treatment processes 289

0

10

20

30

40

50

60

70

80

90

100

5 6 7 8pH

10 11 12100

90

80

70

60

50

40

30

20

10

0

9

NH

3, %

NH

4+ , %

0°C

40°C

20°C

Figure 13.1 Relative distribution of ammonia and ammonium ion in water according to pH and temperature (Source: Adapted from EPA, 1977).

1 mol Ammonia(NH3/NH4

+)

1 mol Nitrite(NO2

–)

1 mol Nitrate(NO3

–)

1 mol Nitrite(NO2

–)

½ mol Nitrogen Gas(N2)

75% O2

25% O2

40% Carbon

60% Carbon

Aerobic

Anoxic

Figure 13.2 Schematic of nitrification–denitrification process (Source: Adapted from Murthy, 2011).


Thetotaloxidationreactionis

NH4+

+2O2____▶ NO3

–+2H++H2O+energy (13.4)

Fromequation(13.4),theoxygenrequiredfortotaloxidationofammoniais4.57gO2/gNoxidized,with3.43gO2/gNusedfornitriteproductionand1.14gO2/gNusedfornitrateproduction.Whencellsynthesisiscon-sidered,theamountofoxygenislessthan4.57gO2/gN,asaportionoftheammoniaisassimilatedintocelltissue.

Neglectingcelltissue,theamountofalkalinityrequiredtocarryouttheoxidationreactiongiveninequation(13.4)is

NH4++2HCO3

–+2O2→NO3–+2CO2+3H2O (13.5)

Fromequation(13.5),7.14gofalkalinityasCaCO3isrequiredforeachgramofammonia-nitrogen(asN)converted.

Thebiomasssynthesisreactionisgivenby

NH4–+4CO2+HCO3

–+H2O→C5H7O2N+5O2 (13.6)

whereC5H7O2Nrepresentssynthesizedbacterialcells.

13.2.1.1.2 Nitrif ication kinetics

Theratelimitingstepinnitrificationistheconversionofammoniatonitrite,asrepresentedbyequation(13.2).Thisistrueforsystemsoperatedbelow28°C.So,designofnitrificationsystemsoperatedbelow28°Cisbasedonsaturationkinetics of ammoniaoxidation as shownbelow (Metcalf andEddy, 2003), with the assumption that excess dissolved oxygen (DO) isavailable.

µ µN

m axN

NdN

N

K Nk=

+

− (13.7)

where:µN=specificgrowthrateofnitrifyingbacteria,d–1

µmaxN=maximumspecificgrowthrateofnitrifyingbacteria,d–1

N=ammonia-nitrogenconcentration,g/m3

KN=halfsaturationcoefficientforammonia-nitrogen,g/m3

kdN=endogenousdecaycoefficientfornitrifyingbacteria,d–1

Equation(13.7)isaformoftheMonodmodelandcanbeappliedtocom-pletely mixed activated sludge systems. Temperature, pH, and dissolved


oxygen concentration are important parameters in nitrification kinetics.Temperatureeffectscanbemodeledbythevan’tHoff–Arrheniusequationas shown in Chapter 8, equation (8.20). The maximum specific growthratefornitrifiersvariesfrom0.25to0.77d–1,dependingonsite-specificconditions(Randalletal.,1992).Thegrowthratefornitrifyingorganismsismuchlowerthanthecorrespondingvaluesforheterotrophicorganisms,requiringmuchlongersolidsretentiontime(SRT)forthenitrificationpro-cess.TypicaldesignSRTvaluesrangefrom10to20dat10°C,and4to7dat20°C.Above28°C,bothammoniaandnitriteoxidationkineticshavetobeconsideredindesign.

Dissolvedoxygenconcentrationsof3to4mg/Linthewatercanincreasenitrificationrates.Toincorporatetheeffectsofdissolvedoxygen,equation(13.7)canbemodifiedasshownbelow,

µ µN

m axN

N odN

N

K N

DO

K DOk=

+

+

− (13.8)

where:DO=dissolvedoxygenconcentration,g/m3

Ko=halfsaturationcoefficientforDO,g/m3

Allothertermsareasdefinedpreviously.Thesekineticmodelsarebestusedtodescribenitrificationinsystemsatlowtomoderateorganicloadings,butwillusuallyoverpredictratesinsystemswithhighorganicloadings.

13.2.1.1.3 Denitrif ication stoichiometry

The final step in biological nitrogen removal is denitrification, whichinvolvesthereductionofnitratetonitricoxide(NO),nitrousoxide(N2O),andnitrogengas,andiscarriedoutbyavarietyofheterotrophicandauto-trophicbacteria.Inwastewaterprocesses,mostarefacultativeanaerobesofthePseudomonasspecies.Themetabolicpathwayofdenitrificationcanberepresentedbythefollowingequation:

NO3–→NO2

–→NO→N2O→N2 (13.9)

Denitrificationcanbeconsideredasatwo-stepprocess,sincenitritesmayappearasanintermediate.Thistwo-stepprocessistermeddissimilation.Thefirststeprepresentsreductionofnitratetonitrite,andthesecondstepareductionofnitritetonitrogengas(McCartyetal.,1969).Nitricoxidegas(NO)isonlyanintermediateproduct,butnitrousoxidegas(N2O)couldbethefinalproductofafewdenitrifiers.Inmostcases,nitrogengas(N2)istheendproductofdenitrification.


Denitrificationtakesplaceinthepresenceofnitrateandabsenceofoxy-gen.Thedissolvedoxygenlevelmustbeatornearzero,andacarbonsupplymustbeavailable for thebacteria.Thenitrateactsasanelectronaccep-torfororganicor inorganicelectrondonors.Sincea lowcarboncontentisrequiredforthepreviousnitrificationstep,additionalcarbonhastobeaddedforthedenitrificationstep.Thiscanbeaddedintheformofprimaryeffluentwastewater,whichhasbiochemicaloxygendemand(BOD),orbyaddinganexternalcarbonsourcesuchasmethanol,ethanol,acetate,orglycerol.Anexternalcarbonsource isusedwhen it isdesiredtoachieveverylowlevelsofnitrogenintheeffluentduetoregulatoryrequirements.

Whenwastewateristheelectrondonorfordenitrification,thereactioncanbeasfollows(MetcalfandEddy,2003):

C10H19O3N+10NO3–→5N2+10CO2+3H2O+NH3+10OH (13.10)

whereC10H19O3Nisageneralizedformulaforthewastewater.Thefollowingreactiontakesplacewithmethanolastheelectrondonor:

5CH3OH+6NO3–→3N2+5CO2+7H2O+6OH– (13.11)

When ethanol is used as the external carbon source, the reaction is thefollowing:

5CH3CH2OH+12NO3–→6N2+10CO2+9H2O+12OH– (13.12)

Thefollowingreactiontakesplacewithacetateastheelectrondonor:

5CH3COOH+8NO3–→4N2+10CO2+6H2O+8OH– (13.13)

In equations (13.10)–(13.13), one equivalent of alkalinity (OH–) is pro-ducedperequivalentofNO3–Nreduced.Thisamountsto3.57galkalin-ityasCaCO3pergnitrogenreduced.Thisindicatesthatabouthalfofthealkalinityconsumedinnitrificationcanberecoveredindenitrification.Theoxygenequivalentsofnitrateandnitritecanbecalculatedas2.86gO2/gNO3–Nand1.71gO2/gNO2–N,respectively.

Thedenitrificationpotentialofwastewaterisprimarilydeterminedasthestoichiometricratiobetweentheorganiccompoundusedandthenitrate,whichisusuallyexpressedasthechemicaloxygendemand(COD)/NortheBOD/Nratio.An importantdesignparameter fordenitrificationprocessis theamountofBODorbiodegradablesolubleCOD(bsCOD)requiredaselectrondonorfornitrogenremovalfromwastewater.Thiscanbeesti-matedfromthefollowingrelationship.ReadersarereferredtoMetcalfandEddy(2003)foracompletederivationofthefollowing:


gbsCOD/gNO3–N=2 86

1 1 42

.

.− Yn

(13.14)

where:Yn=netbiomassyield,gvolatilesuspendedsolids(VSS)/gbsCOD1.42=oxygenequivalentofbiomass,gO2/gVSS2.86=oxygenequivalentofnitrate,gO2/gNO3–N

The net biomass yield can be calculated from the following equation(MetcalfandEddy,2003):

YY

kn

d c

=+1 θ

(13.15)

where:Y=biomassyieldfordenitrifiers,gVSS/gbsCODkd=decaycoefficientfordenitrifiers,d–1

θc=anoxicSRT,d

13.2.1.1.4 Denitrif ication kinetics

The Monod model similar to equation (13.7) can be developed for thedenitrifyingbacteria.Thespecificdenitrificationrate(SDNR)ortherateofsubstrateutilizationcanbecalculatedfromtheMonodmodelandtheconceptspresentedinChapter8,togetherwithatermtoaccountforthelowerutilizationrateintheanoxiczone.

rS X

Y K Ssu

m ax

S

= −+( )

µ η (13.16)

where:rsu=rateofsubstrateutilization,mg/L·dη=fractionofdenitrifyingbacteriainthebiomass

Allothertermsareasdefinedpreviously.Thevalueofηcanrangefrom0.2to0.8forpreanoxicdenitrification(StenselandHorne,2000).Forpostan-oxicprocessesηis1.0,wherethebiomassismainlydenitrifyingbacteria.

13.2.1.1.5 External carbon sources for denitrif ication

Whenawastewatertreatmentplantrequiressignificantnitrogenremoval,theorganicmatternaturallypresentinthewastewatermaybeinsufficient


toachievetherequiredlevelofdenitrification.Thisrequirestheadditionofanexternalcarbonsource.Someexternalcarbonsourcesincludemetha-nol,ethanol,aceticacid,glucose,glycerol,etc.Inthelasttwodecades,asignificantamountofresearchhasbeenconductedoninvestigatingdiffer-entcarbonsources,theirapplications,kineticparameters,andtemperatureeffects.Theimpetusforthishasbeenconcernsofdetrimentalenvironmen-taleffectsofeffluentnitrogendischargestowaterbodies.

Oneexampleofthis is the loweringofeffluent limitsto3to4mg/LoftotalNforwastewatertreatmentplantsdischargingintotheChesapeakeBaywatershed in theeasternUnitedStates.TheChesapeakeBay is the largestestuaryintheUnitedStates,encompassingsixstates—Delaware,Maryland,NewYork,Pennsylvania,Virginia,andWestVirginia—andtheDistrictofColumbia.Bothpointandnonpointsourceshavecontributedexcessnutrientstothebay,resultinginseveredeteriorationandimpairedwaters.Asaresult,stricter effluent limitshavebeen imposedon thepoint sources,whicharemainlythewastewatertreatmentplants.Mostofthetreatmentplantsinthatareauseanexternalcarbonsourcefordenitrification.Butsizingandopera-tionoftreatmentunitsbasedonpreviousexistingkineticparametershavefailedtoproducethedesiredresults,especiallyatlowtemperatures.Thathasprovidedthemomentumfornewresearchinmethodologyanddetermina-tionofkineticparametersfordenitrificationwithexternalcarbonsources.Understandingthekineticsandstoichiometryofthedenitrifyingorganismsisofprimeimportanceindesigningandoptimizingnitrogenremovalprocesses.

Researchondenitrificationhasbeenconductedforseveraldecadesnow.Various researchers have measured kinetic parameters of denitrificationforavarietyofprocessconfigurations.Inearlierstudies,usingmethanol(MeOH)astheexternalcarbon,Stenseletal.(1973)reportedmaximumspecific growth rates (µmaxDEN) of 1.86 and 0.52 d–1 at 20°C and 10°C,respectively.Decaycoefficientsat these two temperatureswere0.04and0.05d–1,respectively.RecentresearchershaveobservedlowergrowthratesformethanolutilizersinextensivestudiesconductedatalargenumberoftreatmentplantsintheeasternUnitedStates.Doldetal.(2008)observedmaximumspecificgrowthrateof1.3d–1at20°CwithanArrheniuscoef-ficient (θ) of 1.13, based on a decay rate (kdDEN) of 0.04 d–1. Nichols etal. (2007) obtained maximum specific growth rate of 1.25 d–1 at 20°Cwith anArrhenius coefficient of 1.13.The carbon tonitrogen ratiowasapproximately 4.73 mg MeOH COD/mg NO3–N. The variation of µmaxwithtemperatureisillustratedinFigure 13.3,withthedashedlinerepre-sentingthevan’tHoff–Arrheniusmodel.Maximumspecificgrowthratesof1.0and0.5d–1at19°Cand13°C,withanArrheniuscoefficientof1.12,wereobservedbyMokhayerietal. (2006).The lowgrowthrate(similartothatofnitrifiers)indicatedthatsystemsshouldbedesignedbasedonalongenoughanoxicSRTtoensurestablegrowthandavoidwashout.This


isexacerbatedbythestrongtemperaturedependencyforplantsoperatingatlowtemperatures.

Threeexternalcarbonsources—methanol,ethanol,andacetate—wereevaluatedfordenitrificationbyMokhayerietal.(2009,2008,2007).At13°C,theSDNRsforbiomassgrownonmethanol,ethanol,andacetatewere 10.1 mg NO3–N/g VSS/h, 29.6 mg NO3–N/g VSS/h and 31.0 mgNO3–N/g VSS/h, respectively, suggesting that acetate and ethanol wereequallyeffectiveexternalcarbonsourcesfollowedbymuchlowerSDNRusingmethanol.Theyieldcoefficientswereobservedtobe0.45g/g,0.53g/g,and0.66g/gformethanol,ethanol,andacetate,respectively.Ethanolcouldbeusedwithmethanolbiomasswithsimilarratesasthatofmetha-nol.Additionally,methanolwasrapidlyacclimatedtoethanolgrownbio-mass,suggestingthatthetwosubstratescouldbe interchangedtogrowrespectivepopulationswithaminimumlagperiod.MethanolisusedatalargenumberoftreatmentplantsintheUnitedStatesbecauseofitslowcost, but the rates reduce significantly with methanol at cold tempera-tures.Thismaybeovercomebyusinganalternativesubstratetomethanolinwinter.

Studies conducted with glycerol as an external carbon source byHinojosaetal.(2008)determinedthefollowingkineticcoefficients:maxi-mumspecificgrowthrateof3.4d–1at21°C,stoichiometricC:Nratioof4.2mgCOD/mgNOx–N,growthyield(YDEN)of0.32mgbiomassCOD/mgNOx–N,SDNRof1.4mgNOx–N/gVSS/h,andhalfsaturationcoefficient(KSDEN)5–8mgCOD/L.

5 10 15 20 25 300.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Temperature (°C)

μ MA

X (d

–1)

Figure 13.3 Variation of maximum specific growth rate of denitrifiers with temperature (Source: Data from Nichols et al., 2007; and Hinojosa, 2008).


EXAMPLE13.1Amunicipalwastewatertreatmentplant isplanningtoupgradetoanitrogenremovalplant. Ithas successfully incorporatednitrificationwithBODremovalintheexistingactivatedsludgeprocess.Theplantwantstoaddaseparatedenitrificationsystemconsistingofananoxictank followedbya clarifier.Designa suspendedgrowthdenitrifica-tionsystemfortheplantusingmethanolasacarbonsource.Calculatethe tank volume and methanol dose required to achieve an effluentNO3–Nconcentrationof3mg/L.Thefollowingdataareprovided.

Wastewatereffluentfromnitrificationsystem:

Flowrate=3000m3/d,Temperature=20°C

NO3–N=30mg/L,TSS=20mg/L

Denitrificationkineticcoefficientswithmethanolat20°C:

µmax=1.3d–1,kd=0.04d–1,Ks=4mgbsCOD/L,Y=0.35kgVSS/kgbsCOD

CODequivalentofmethanol=1.5kgCOD/kgmethanolDenitrificationtank:mixedliquorsuspendedsolids=2,500mg/L,

SRT=6d,hydraulicretentiontime(HRT)=2hClarifier:overflowrate=24m3/m2·d

SOLUTION

Step1.DeterminethetankvolumebasedonHRT.

V=Q×HRT=3000m3/d×2h×1

24 /h d=250m3

Usetwotanks,withvolumeofeachtank=250/2=125m3.

Step2.Determinemethanolrequiredfornitratereduction.

NO3–Nreduced=(30–3)mg/L=27mg/L=0.027kg/m3

Calculatenetbiomassyieldusingequation(13.15).

YY

kn

d c

=+1 θ

or

Ykg kg

d dn =

+ ×−0 35

1 0 04 61

. /

( . )=0.282kg/kg


CalculateC:Nratiousingequation(13.14).

kgbsCOD/kgNO3–N=2 86

1 1 42

2 86

1 1 42 0 282

.

.

.

(. . )−=

− ×Yn

=4.767kg/kg

Methanolrequiredfornitratereduction=4.767kg/kg×0.027kg/m3

=0.129kg/m3asCOD

Dailymethanoldose=3000m3/d×0.129kg/m3=387kg/dasCOD

CODequivalentofmethanol=1.5kgCOD/kgmethanol

or

Dailymethanoldose=387

1 5

kg CO D d

kg CO D kg m ethanol

/

. /=258kg/d

Step3.Calculateareaofclarifierbasedonoverflowrate.

AQ

v

m d

m d= = =3000

24

3 /

/125m2

Usetwoclarifiers.

Areaofeachclarifier=125/2=62.5m2

Diameterofeachclarifier=8.92m≅9.0m

13.2.1.1.6 Nitrif ication–denitrif ication processes

Biologicalnitrogenremovalprocessesrequireanaerobiczonefornitrifica-tionandananoxiczonefordenitrification.Theprocessesmaybebroadlyclassifiedintothreetypes:(1)preanoxic,whereananoxiczoneisfollowedby an aerobic zone (Figure 13.4); and (2) postanoxic, where an aerobiczone is followed by an anoxic zone (Figure 13.4); and (3) a third type,wherenitrificationanddenitrificationoccurinthesamereactor/tank,e.g.SBR(sequencingbatchreactor).Suspendedgrowthnitrogenremovalpro-cessescanbefurtherclassifiedas(1)singlesludgesystem,whereoneclari-fierisusedforsolidsseparation,thoughinternalrecirculationmaybeusedbetweentanks(Figure 13.4);and(2)twosludgesystem,wherethenitrifica-tiontankisfollowedbyaclarifierandthedenitrificationtankisfollowedbyanotherclarifier.Thisisapostanoxicprocess,usuallywhereanexternalcarbonsourceisadded.ThisisillustratedinFigure 13.5(a).ThefirsttankmaybeusedforcombinedBODremovalandnitrification.

Examples of preanoxic processes are the MLE (modified Ludzack–Ettinger) process, step feed process, and others. Postanoxic processes


includetheoxidationditchandothers.TheBardenphoprocessisacombi-nationofpreanoxicandpostanoxicprocesses.Membranebioreactorscanbeusedfornitrogenremoval.

Attachedgrowthprocessesarealsousedfornitrogenremoval.TricklingfiltersandRBCs(rotatingbiologicalcontactors)canbeusedforbothBODremoval and nitrification. Coarse media deep-bed anaerobic filters havebeenusedfordenitrificationforalongtime.Anexternalcarbonsuchasmethanol is usually added to the filter. The moving bed biofilm reactor(MBBR) process can be used for both nitrification and denitrification.Someoftheseprocessesaredescribedindetailinthefollowingsections.

Modified Ludzack-Ettinger (MLE) process.Thisisawidelyusedpro-cess that consists of the preanoxic system illustrated in Figure 13.4(a).Wastewaterflowsintotheanoxiczoneandprovidesthecarbonnecessaryfordenitrification.TheinternalrecyclewasdesignedbyBarnard(1973)toincreasenitratefeedtotheanoxiczone,asamodificationoftheoriginaldesign.Withsufficient influentBODandanoxiccontact time(2to4h),averageeffluentNO3–Nconcentrationsof4 to7mg/Lcanbeachieved.

(a)

(b)

E�uent

Secondaryclari�er

Inuent Aerobic/Nitri�cation

Sludge

Return activated sludge

Anoxic

E�uent

Secondaryclari�er

Inuent Aerobic

Sludge


Anoxic

Internal recycle

Figure 13.4 (a) Preanoxic modified Lutzack-Ettinger (MLE) process and (b) postanoxic process.


Efficiencycanbeincreasedbydividingtheanoxiczoneintothreetofourstagesinseries.

Step-feed process.Thisissimilartotheactivatedsludgestep-feedpro-cess,exceptthatthetankisdividedintoanoxicandaerobiczonesasshowninFigure 13.5(b).Theportionofflowgoingtothelastanoxic/aerobiczoneiscritical, since thenitrateproduced in the lastaerobiczonewillnotbereducedandwillremaininthefinaleffluent.EffluentNO3–Nconcentra-tionslessthan8mg/Lcanbeachieved(MetcalfandEddy,2003).

BardenphoTM (four-stage) process.TheBardenphoTMprocessusesbothpreanoxic and postanoxic stages (Figure 13.6a). The mixed liquor fromthefirstaerobiczoneisrecycledtothepreanoxiczonetoprovidenitratefordenitrification.TheprocesswasdevelopedandappliedatfullscaleinSouth Africa in the 1970s. Since then it has been used worldwide. It iscapable of achieving both nitrogen and phosphorus removal. The nameoftheprocessisderivedfromthefirstthreelettersoftheinventor’sname,Barnard;denitrification;andphosphorus(Barnard,1974).

Oxidation ditch.This isapostanoxicprocessconsistingofanaerobiczonefollowedbyananoxiczoneinanoxidationditch.Wastewaterenters

(a)

(b)

E�uent

Secondaryclari�er

Inuent

Sludge

Primary clari�er

Sludge

Nitri�cationclari�er

Sludge

Nitri�cation tank

Air


Methanol


Air

E�uent

Secondaryclari�er

Inuent

Aerobic

Sludge


Anoxic Aerobic Anoxic Aerobic Anoxic Aerobic Anoxic

Figure 13.5 (a) Two sludge or two-stage nitrification–denitrification system and (b) step feed process.


intheaerobiczoneclosetotheaerator.Asitflowstowardtheanoxiczone,dissolvedoxygenisusedupbythemicroorganismsduringdegradationofBODandnitrification.Eventuallywhenthedissolvedoxygenisdepleted,an anoxic zone is formed and denitrification occurs due to endogenousrespiration by the bacteria using nitrate. The process is illustrated inFigure 13.6(b).LargetankvolumesandlongSRTshavetobemaintained.

MBBR (moving bed biofilm reactor).TheMBBRprocesscanbeusedforBODremovalaswellasnitrificationanddenitrification.ThisattachedgrowthprocesshasbeendescribedpreviouslyinChapter9(Section9.6).Figure 13.7(a,b) illustrates theMBBRwithtwotypesofmixingoptions.Figure 13.7(c) illustrates biofilm growth on an MBBR media. Full-scaleplantapplicationsinNorwayhavedemonstratedhighratesofnitrificationanddenitrification(Ødegaard,2006).

Denitrificationrateof1.8gN/m2·dwithmethanolat15°CwasobservedbyRustenetal.(1995)foranMBBRsystem.Aspegrenetal.(1998)obtainedmaximum denitrification of 2.0 g N/m2·d at 16°C using methanol for

(a)

(b)

Secondaryclari�er

Sludge In�uent


Aerobic

Anoxic

Aerator

E�uent

E�uent

Secondaryclari�er

In�uent

Sludge


Aerobic Anoxic AerobicAnoxic

Mixed-liquor return

Figure 13.6 (a) Four-stage BardenphoTM process and (b) oxidation ditch.


postdenitrificationprocessinanMBBRsystem.PilotplantMBBRstudiesatBluePlainsAdvancedWastewaterTreatmentPlantinWashington,D.C.(Peric et al., 2010), produced specific denitrification rates ranging from1.3to2.0gNOx–N/m2–day,andstoichiometricC:Nratiosof4.6–5.8mgCOD/mgNOx–N,foratemperaturerangeof13–18°C.ValuesofeffectiveKsNOx–Nwereobservedbetween0.6and2.6mgN/L(Shresthaetal.,2009).

13.2.1.2 Nitritation–denitritation

Intheconventionalnitrification–denitrificationprocess,ammoniaiscon-verted to nitrites and then to nitrates, followed by reduction of nitrates

Influent Effluent

Air

Influent Effluent

(b)(a)

(c)

Figure 13.7 (a) Aerobic MBBR, (b) anoxic and aerobic MBBR (Source: Adapted from Ødegaard, 2006), and (c) biofilm on MBBR media (Source: Photo courtesy of Arbina Shrestha).


again to nitrites and finally to nitrogen gas, as illustrated in equations(13.2),(13.3),and(13.9).Anumberofresearchershaveinvestigatednitro-genremovalbypartialnitrificationornitritation (oxidationofammoniato nitrite) followed by partial denitrification or denitritation (reductionof nitrite to nitrogen gas). This results in significant savings in oxygendemand,lowercarbonrequirementfordenitrification,andareductioninthe amountof excess sludgeproduced (Ruiz et al., 2003;Ciudad et al.,2005).TheoverallprocessisillustratedinFigure 13.8.Nitriteaccumula-tionisobtainedbyoptimizingdissolvedoxygen,pH,andtemperature.AnexampleofthisistheSHARONTMprocessdescribedbelow.

13.2.1.2.1 SHARONTM process

TheSHARONTM(singlereactorsystemforhighammoniumremovalovernitrite)processwasdevelopedinTheNetherlands,initiallytoreducetheloadofwastewaterstreamsandsidestreamswithhighammoniumconcen-tration.Thereactorisdesignedtoselectforammoniumoxidizersbywash-ingoutnitriteoxidizers,usingashortretentiontimeofapproximately1d,andatemperatureabove30°C(vanDongenetal.,2001).Longeraerobicandshorteranoxicphasesareused,withmethanoladditionintheanoxicphase.Comparedwith the conventionalnitrification–denitrificationpro-cess, theoxygendemand is reducedby25%and equals3.43gO2/gN,andthecarbonrequirementisreducedby40%andequals2.4gCOD/gN(Mulderetal.,2001;Hellingaetal.,1998).Thereareanumberoffull-scaleinstallationsoftheSHARONTMprocessinTheNetherlands.

13.2.1.3 Deammonification

The term deammonification is used to describe an ammonium removalprocess that does not depend on the supply of organic matter (Hippen


+)

1 mol Nitrite( NO2

–) 1 mol Nitrite

( NO2–)


75% O2

60% Carbon

Amm

onia

oxid

izers

(e.g.

Nitros

omon

as)

Figure 13.8 Schematic of nitritation–denitritation process (adapted from Murthy, 2011).


et al.,1997).Itusesaerobicandanaerobicammoniumoxidizerstoconvertammoniumdirectlytonitrogengasunderoxygen-limitedconditions.Theammoniumreactswithnitriteactingasanelectronacceptor toproducenitrogengas.Theanaerobicammoniumoxidizers,oranammoxbacteria,were discovered by Mulder et al. (1995) in a fluidized bed reactor. Theannamox bacteria belong to the phylum planctomycetales. They have averylowgrowthrateof0.072d–1withamassdoublingtimeof11d,whichcanbeanobstacleinprocessstart-up(Jettenetal.,2001).Theoverallpro-cessisillustratedinFigure 13.9.

The followingreactionsarecarriedoutby theannamoxbacteria (vanDongenetal.,2001):

Withoutcellsynthesis:

NH4++NO2

–→N2+2H2O (13.17)

Withcellsynthesis:

NH4++1.32NO2

–+0.066HCO3–→1.02N2+0.26NO3

–+2.03H2O +0.066CH2O0.5N0.15 (13.18)

where0.066CH2O0.5N0.15indicatesnewcells.Advantagesof deammonification are zerooxygendemand, zeroCOD

requirement,andlowsludgeproduction.AnexampleofdeammonificationprocessistheAnnamoxTMprocess.

13.2.1.3.1 AnnamoxTM process

TheAnnamoxprocesswasdevelopedinTheNetherlandsinthelate1990s.Thetermannamoxisanabbreviationforanaerobicammoniumoxidation.TheAnnamoxprocessispreceededbyanitritationprocessthatconverts


+)

1 mol Nitrite( NO2

–)


38% O2 Autotrophicanaerobic

environentAmm

onia

oxid

izers

(e.g.

Nitros

omon

as)

Figure 13.9 Schematic of deammonification (Source: Adapted from Murthy, 2011).


halfoftheammoniumtonitrite,withoutsubsequentconversionofnitriteto nitrate. The oxygen uptake based on initial ammonium concentra-tionis1.72gO2/gN,or38%oftheoxygendemandforoxidationofalltheammoniumtonitrate (Gut,2006).After thisnitritationprocess, theAnammoxprocess (equations13.17and13.18) followswithoutadditionofanyorganicmaterialinaseparatereactor(vanLoosdrechtetal.,2004).

13.2.2 Physicochemical process for nitrogen removal

Physicochemicalprocessesmaybeusedforremovalofnitrogenandsimul-taneousammoniarecoveryfromwastewater.Airstrippersorsteamstrip-pers are used at a number of full-scale installations inEurope. In somecases,ammoniaisrecoveredasanammoniumnitratefertilizerproduct.

13.2.2.1 Air stripping

Theprocessofairstrippinginvolvestheconversionofaqueousammoniumion(NH4

+)togaseousammonia(NH3)andreleasingittotheatmosphere.Theequilibriumbetweenthetwospecieswasoutlinedinequation(13.1)andFigure 13.1.FromFigure 13.1,abovepH11or11.5,morethan99%ofammoniawillbepresentinthegaseousphase.

TheprocessconsistsofpretreatmentofthewastewaterwithlimetoraisethepHabove11.5.Enoughlimehastobeaddedtoprecipitatethealka-linityand raise thepH to thedesired level.Once the conversion togas-eousammonia iscomplete,strippingordegasification isconducted.Oneofthemostefficientreactorsisthecountercurrentspraytower,illustratedinFigure 13.10(Peavy,1985).Largevolumesofairarerequired.Packingmaterialisprovidedtominimizefilmresistancetogastransferandtoaidinformationofliquiddroplets.Airpollutioncontrolmayberequiredforammoniaemissions.Anotherdisadvantageisreductioninefficiencyatcoldtemperatures.Theprocessiseconomicalwhenlimeprecipitationofphos-phorusisalsodesired.

13.3 PHOSPHORUS REMOVAL

Phosphorus iscontributed towastewatermainly fromhumanwastesandfromsyntheticwastessuchasdetergents.Theprincipalformofphosphorusinwastewaterisorthophosphate,togetherwithsomepolyphosphatesandorganically bound phosphorus. Polyphosphates originate from detergentsandcanbehydrolyzedtoorthophosphates.Organicallyboundphosphoruscomesfrombodyandfoodwastesandisbiologicallydegraded/convertedtoorthophosphates.Thephosphorusororthophosphatescanberemovedfromwastewaterbychemicalorbiologicalprocesses.Thesearedescribedbelow.


13.3.1 Chemical precipitation

Chemicalprecipitationinvolvestheadditionofmetalliccoagulantsorotherchemicalstoforminsolublecompoundswithphosphates,andthenremovalofthembyprecipitation.OrthophosphatesconsistofthenegativeradicalsPO4

3–, HPO42–, and H2PO4

–. These can form insoluble compounds withmetalliccations,e.g.withironoraluminum.ThefollowingreactionstakeplacewithchemicalprecipitationatacidicpH:

Fe3++PO43–→FePO4 (13.19)

Al3++PO43–→AlPO4 (13.20)

Coagulants such as ferric chloride or polyaluminum chlorides can beusedforchemicalprecipitation.

LimecanbeaddedtoraisethepHtoabout9.0andformaninsolublecomplexwithphosphates,asshownbelow:

Air outlet

Fan

Drift eliminator

Water inlet

Water out

Air inlet

Fill

Distribution system

Figure 13.10 Counter-current spray tower for air stripping of ammonia.


Ca(OH)2+PO43–→Ca5(OH)(PO4)3+H2O (13.21)

Metallic coagulants and lime consume alkalinity from the wastewater.Thuschemicaldosagesareusuallytwotothreetimesgreaterthanthatpre-dictedfromstoichiometry.Coagulantsmaybeaddedtotheprimaryorsec-ondaryunitsforcombinedremovalwithsolidsandBOD,orseparatelyinatertiaryunit.Applicationinprimaryclarifiersisbeneficial,sinceitresultsin enhanced clarification of BOD and solids. But polymers are requiredforflocculation(Peavy,1985).Applicationinatertiaryunitresultsinthemostefficientuseofcoagulantswiththehighestremovalefficiency.Italsohasthehighestcapitalcostandmetalleakage.Tertiaryunitsforphospho-rusprecipitationcanbedesignedasflocculatorclarifiers,within-linemix-ingofcoagulants.Coagulationandflocculationisfollowedbysettlingtoremovetheprecipitatedcompounds.

13.3.2 Biological phosphorus removal

Biologicalphosphorusremoval(BPR)isaccomplishedbyagroupofbacte-riacollectivelyknownasPAOs(phosphorus-accumulatingorganisms).ThePAOsincorporatelargeamountsofphosphorusintocellbiomass,whichissubsequently removedfromtheprocessbysludgewasting.PAOs includeAcinetobacter, Arthrobacter, Aeromonas, Nocardia, and Pseudomonas(Davis,2011).PhosphoruscontentinPAOscanrangefrom0.2to0.3gP/gVSS,whileinordinaryheterotrophicbacteriaitrangesfrom0.01to0.02gP/gVSS.

Thebasicbiologicalphosphorusremovalprocessconsistsofananaer-obic zone or tank followed by an aeration tank. The anaerobic zone iscalledaselector,sinceitprovidesthefavorableconditionsforgrowthandproliferationofPAOs,withashortHRTof0.5to1.0h.AfractionofthebiodegradableCODis fermentedtoacetateandconsumedbythePAOs.TheyproduceintracellularPHB(poly-hydroxy-butyrate)storageproductsandreleaseorthophosphates.Intheaerobiczone,PHBismetabolizedfornewcellsynthesis.TheenergyreleasedfromPHBoxidationisusedtoformpolyphosphatebondsincellstorage,leadingtoremovaloforthophosphatesfromsolutionandincorporationintopolyphosphateswithinthebacterialcell (Metcalf and Eddy, 2003). Phosphorus is removed from the systemwhenthebiomassiswasted.Maximumspecificgrowthrateof0.95d–1wasobservedforPAOsbyBarkerandDold(1997).

Fromstoichiometry,itisestimatedthatabout10gofbiodegradablesol-ubleCODisrequiredtoremove1gPbythebiologicalstoragemechanism.Thisvalueisbasedonthefollowingassumptions(MetcalfandEddy,2003):(1)1.06gacetate/gbsCODisproducedintheanaerobiczone;(2)cellyieldis0.3gVSS/gacetate;and(3)cellphosphoruscontentofPAOis0.3gP/gVSS. In biological systems, other cations associated with polyphosphate


storage,suchasCa,Mg,andK,mustalsobeavailableinsufficientquanti-tiesforefficientphosphorusremoval.Municipalwastewatersusuallyhavethecationsinrequiredquantities.

13.3.2.1 Selected processes for BPR

The following are descriptions of selected processes used for biologi-cal phosphorus removal. Sequencing Batch Reactors (SBRs) can also beused for combined nitrogen and phosphorus removal,where the reactorsequencesthroughanaerobic/anoxic/aerobicphases.

13.3.2.2 Phoredox

ThetermphoredoxwasusedbyBarnard(1975)todesignateanyBPRpro-cesswithananaerobic/aerobicsequence,asillustratedinFigure 13.11(a).Theanaerobicdetentiontimeis0.5to1h.LowoperatingSRTisusedtopreventnitrificationintheaerobiczone.SRTvaluesrangefrom2to3dat20°C,and4to5dat10°C,topromotephosphorusremovalwithoutsimultaneousnitrification(Gradyetal.,1999).AvariationofthisprocesswithmultiplestageswaspatentedastheA2/OTMprocess.

13.3.2.3 A2OTM process

TheA2OTMprocessisusedforcombinednitrogenandphosphorusremoval.Ananoxiczone isprovidedbetweentheanaerobicandaerobiczones,asillustrated in Figure 13.11(b). Anoxic zone detention time is about 1 h.Chemically bound oxygen in the form of nitrate is introduced into theanoxiczonebyrecirculatingeffluentfromtheaerobiczone.Thisreducestheamountofnitratefedtotheanaerobiczoneinthereturn-activatedsludge.

13.3.2.4 Modified BardenphoTM (five stage)

Thefive-stageBardenphoTMprocessillustratedinFigure 13.11(c)isusedforcombinedcarbon,nitrogen,andphosphorusremoval.Theanaerobic,anoxic,andaerobicstagesprovidephosphorus,nitrogen,andcarbonremoval.Asec-ondanoxicstageachievesadditionaldenitrificationusingnitrateproducedintheaerobiczoneandendogenousorganiccarbon.Thefinalaerobicstageisusedtostripnitrogengasfromsolutionandminimizethereleaseofphospho-rusinthefinalclarifier.ProcessSRTrangesfrom10to20d.

13.3.2.5 UCT process

TheUCTprocesswasdevelopedattheUniversityofCapeTowninSouthAfrica, and hence its name. The standard UCT process is illustrated in


Figure 13.12.Theintroductionofnitratetotheanaerobicstageisavoidedby recycling the activated sludge to the anoxic stage. This improves thephosphorusuptake.Anoxiceffluentrecycletotheanaerobicstageresultsin

(a)

(b)

(c)

E�uent

Secondaryclari�er

Aerobic


In�uent Anaerobic Anoxic

Recycle

Aerobic Anoxic

Sludge(containing P)

E�uent

Secondaryclari�er

Aerobic



Recycle


E�uent

Secondaryclari�er

In�uent Anaerobic Aerobic

Sludge


Figure 13.11 (a) Phoredox (A/OTM) process, (b) A2OTM process, (c) modified BardenphoTM process. (Source: Adapted from Metcalf and Eddy, 2003).


increasedorganicutilization.Theanaerobicdetentiontimerangesfrom1to2h.Theanaerobicrecyclerateisusuallytwotimestheinfluentflowrate.Thestandardprocesswaslatermodifiedtoprovideasecondanoxictankafterthefirstone.Thisimprovednitrateremovalfortheprocess.

13.4 SOLIDS REMOVAL

Thepresenceofexcesssolidsinwastewatereffluentcancreateproblemsinreceivingbodies,dependingonthetypeofsolids(suspendedordissolved)anditsconstituents.Regulatoryrequirementsmayalsonecessitatetheuseof tertiary treatment for further removal of solids of concern. Granularmediafiltrationisusedforremovaloftotalsuspendedsolids.Processeslikemembranefiltration,activatedcarbonadsorption,and ionexchange canbeusedforremovalofsuspendedanddissolvedsolids.Activatedcarbonadsorptionisalsousedforremovalofodorouscompoundsthatarepro-ducedatwastewatertreatmentfacilities.

13.4.1 Granular media filtration

Granularmediafiltrationhasalwaysbeenapartof conventionaldrink-ingwatertreatment.Theuseoftheprocess inwastewatertreatmenthasincreasedover thepast fewdecades. It isusedasa tertiary treatment toremovetotalsuspendedsolids(TSS)fromthesecondaryeffluent.FiltrationisusedwhentheregulatorylimitforeffluentTSSislessthanorequalto10mg/L(Davis,2011).AsimultaneousreductioninBODisachieved,sinceafractionoftheTSSisbiomass,whichcontributestotheBOD.Deepbedfil-tersareusedfordenitrificationandsolidsremoval.Filtrationwithchemicalcoagulationcanbeusedforsimultaneoussolidsandphosphorusremoval.

(d)

E�uent

Secondaryclari er

Aerobic

Anoxic (nitrate) recycle


Anoxic Recycle



Figure 13.12 Standard UCT process.


Conventional filters that are used at municipal wastewater treatmentplantsareoperatedinadown-flowmode,mainlybygravity.Somepropri-etaryfilterssuchasthedeepbedupflowcontinuousbackwashfilter,pulsedbedfilter,andtravelingbridgefilteruseadditionalmethodsofwastewaterandairflows.Pressurefiltersandvacuumfiltersareusedinindustrialoper-ationsforwastewatertreatment.Thecapitalandoperationcostsofthesearemuchhigherthanconventionalfilters.

A typical filter consists of a tankfilledwith granularmedia,with anunderdrainsystematthebottom.Alayerofgravelisplacedbetweenthemediaandtheunderdraintopreventlossofmediawiththeeffluent.Thewastewater enters at the top of the tank and flows down through themedia.Solidsfromthewastewaterareremovedintheporesofthemediaby adsorption, diffusion, settling, and other mechanisms. The clarifiedeffluentleavesthroughtheunderdrains(Figure 13.13).Overtime,assolidsbuildupinthefilterbed,theefficiencydecreasesandheadlossincreases.Thefilteriscleanedbybackwashingwhentheheadlossreachesapredeter-minedterminallimit.Thewastewaterflowisstopped,andcleanwaterispassedthroughthefilterbedviatheunderdrainsysteminareversedirec-tionat ahighvelocity todislodge the collected solids and remove themfromthebed.

Importantdesignparameters includeflowrates,beddepth,andmediacharacteristics.Typeofmediaandcharacteristics,suchasporosity,effec-tivesize,uniformitycoefficient,andspecificgravityarecarefullyconsid-ered.Typicalfiltrationratesrange from5to20m/hwith terminalheadlossesof2.4to3m(Davis,2011).

Gravel

Sand

Anthracite

Underdrainsystem E�uent

In�uent

Wash watertrough

Figure 13.13 Conventional dual media filter.


Differenttypesofgranularmediacanbeusedinafilter.Monomediaorsinglemediafiltersarehardlyusedanymore,duetoproblemsofclogging.Dual-mediafiltersandmultimediafiltersarecommonlyused.Dual-mediafiltersconsistofalayerofanthraciteatthetopandalayerofsilicasandatthebottom,asillustratedinFigure 13.13.Anthraciteofalargereffectivesizeandsilicasandofasmallereffectivesizeareusedtomakeefficientuseofthebeddepth.Largersolidsaretrappedintheanthracitelayers,whilethesmallersolidstravelthroughthebedandaretrappedinthelowerlayersofsilicasand.Anotheradvantageisthatafterbackwashing,astheparticlessettlebackinthetankbasedontheirterminalsettlingvelocitiesandStokeslaw (equations7.5 and7.10), the silicaparticleswith thehigher specificgravitysettleatthebottom,whiletheanthraciteparticleswiththelowerspecificgravitysettleontop.Thustheoriginalconfigurationismaintainedwithanintermixedlayeratthemiddle.Theratioofdepthsoftheanthraciteandsilicasandlayeristypically2:1or3:1.

Multimediafiltershaveathirdlayerofgarnetsandatthebottom,withanthraciteatthetopandsilicasandinthemiddle.Thistypeoffilterhasahighercapitalcostbuthasahigherefficiencyofsolidsremoval,especiallyofsmallersizedparticles.Thegarnetsandhasahigherspecificgravitycom-paredwiththeothertwomedia,andasmallereffectivesizeisused.Theratioofdepthsoftheanthracite,silica,andgarnetsandlayeristypically5:5:1or6:5:1.SomeofthecharacteristicsofthesethreetypesofmediaareprovidedinTable 13.1.

13.4.2 Activated carbon adsorption

Activated carbon adsorption is used as a tertiary treatment for removalofrefractoryorganiccompoundsandotherinorganicsincludingsulfides,nitrogen,andheavymetals.Refractoryorganiccompoundsareresistanttobiodegradation,andhenceremaininthesecondaryeffluent.Whentheseinclude chemical contaminants of concern, activated carbon adsorptioncanbeusedtoremovethem.Carbonadsorptionisalsousedwhenwaste-water is reused.Pretreatmentofwastewaterbygranularmediafiltration

Table 13.1 Characteristics of different media used in granular media filters

Characteristic Anthracite Silica sand Garnet sand

Specific gravity 1.40–1.75 2.55–2.65 3.60–4.30Porosity 0.55–0.60 0.40–0.45 0.42–0.55Effective size, mm 1.0–2.0 0.4–0.8 0.2–0.6Uniformity coefficient 1.4–1.8 1.3–1.8 1.5–1.8Shape factor 0.4–0.6 0.7–0.8 0.6–0.8

Sources: Metcalf and Eddy (2003); Cleasby and Logsdon (1999).


andchlorination isusuallydoneprior to carbonadsorption, to improveprocessefficiency.

Powderedactivatedcarbon(PAC)orgranularactivatedcarbon(GAC)isused.PACcanbeaddeddirectlytotheaerationtankortothesecondaryeffluent.GACisusedinacolumnthatmaybefixedbedormovingbed.Downflowcolumnsarecommonlyusedintertiarytreatment.Flowcanbebygravityorpressure.Anumberofcolumnscanbeoperatedinseriestoincrease removalefficiencies.Figure 13.14 illustratesadownflowcarboncontactoroperatedinseries.GACcolumnscanbecleanedbybackwashingtosomeextenttolimittheheadlossandreducesolidsbuildup.Whentheadsorptioncapacityisexhausted,thespentcarboncolumncanberegener-atedbyheatingundercontrolledconditions.Sometimesthecarboncolumnhastobedisposedofashazardouswasteandreplacedwithanewone.

Typical design parameters include carbon size, bed depth, hydraulicloadingrate,andemptybedcontacttime.Adsorptionisothermsshouldbedevelopedforthetypeofcarbonandwastewaterfrombenchscalelabora-torytests.ThesecanbecombinedwithavailabledesignparametersfromWEF(1998)todesignaparticularcarbonadsorptionsystem.

13.4.3 Membrane filtration

Membranefiltrationisusedasatertiarytreatmenttoremovesolidsfromwastewater, especially when it is desired to use the effluent for aquiferrechargeorindirectreuse.Membranefiltrationisusedasapretreatmenttoremoveparticulatesolidspriortodissolvedionremovalbyreverseosmo-sis. Membrane processes such as reverse osmosis (RO) and nanofiltra-tion (NF)arewidelyused fordrinkingwater treatment.Theseprocessesoperateatveryhighpressures,usuallyinexcessof500kPa.Lowpressure

In

Out

Granularactivatedcarbon

Figure 13.14 Downflow activated carbon contactor in series.


microfiltration(MF)andultrafiltration(UF)membranesareusedintertiarytreatmentofwastewater.Operatingrangeofpressureforwastewatertreat-mentisfrom70to200kPa(Davis,2011).MFmembraneshaveaporesizerangingfrom0.1to1.0µm,whileUFmembraneporesizecanrangefrom0.005 to0.1µm.These areused for removal of particulates andmicro-organisms.Removalmechanisms include straining,adsorption,andcakefiltration.Overtime,particlesbuilduponthemembranesurface,formingacakethatincreasesthefiltrationefficiencyofthemembrane.Likegranularmediafilters,MFandUFfilters are cleanedbybackwashingwithwaterand/orairscouring.Overtime,chemicalcleaningagentshavetobeused.

Secondaryeffluenthastobepretreatedbeforeitisfedtomembranefil-ters.ThequalityoffeedwatertoMF/UFfiltersmustatleastmeetthestan-dardsforsecondaryeffluent,e.g.BOD5≤30mg/L,TSS≤30mg/L,andfecalcoliforms(FC)≤200/100ml,inordertoachievehigh-qualityeffluentforpotentialreuse(WEF,2006).Chemicalcoagulation,chlorination,andscreeningaresomeofthepretreatmentoptions.

13.4.3.1 Fundamental equations

Membranesareorganicpolymersthataresemipermeabletoselectedconstit-uents.Commonlyusedmembranematerialsincludepolysulfone(PS),poly-ethersulfone(PES),andpolyvinylidenedifluoride(PVDF),amongothers.Forfiltration,themembraneisplacedinatank.Aswastewaterflowsthroughthetankunderpressure,themembranepreventsthecontaminantsfromflowingthroughit.Asaresult,awastestreamwithconcentratedcontaminants(reject orconcentrate)andaclarifiedproductstream(permeate)areproducedaseffluent. This is illustrated in Figure 13.15. The rate at which the perme-ateflowsthroughthemembraneisknownastheflux,expressedinunitsofmass/area·time.Therejectorconcentratehastoundergofurthertreatment.Thishastobeincorporatedintothedesignofacompletetreatmentsystem.

ForthemembranefilterinFigure 13.15,fromcontinuitywecanwrite:

QF=QP+QC (13.22)

where:QF=flowrateoffeed,m3/sQP=flowrateofpermeate,m3/sQC=flowrateofconcentrate,m3/s

Themassbalanceequationforthecontaminantcanbewrittenas

QFCF=QPCP+QCCC (13.23)

where:


CF=contaminantconcentrationinfeed,kg/m3

CP=contaminantconcentrationinpermeate,kg/m3

CC=contaminantconcentrationinconcentrate,kg/m3

Therateofrejection(R)orremovalefficiencyisgivenby

RC C

CF P

F

,% %= − × 100 (13.24)

Moreadvancedmodelshavebeendevelopedtocalculaterejectionbasedonparticlediameterandporesize(MWH,2005).

Thevolumetricfluxofpurewateracrossacleanmembranecanbemod-eledusingamodifiedformofDarcy’slaw(AWWA,2005):

JQ

A

P

R m

= = ∆µ

(13.25)

where:J=volumetricfluxthroughcleanmembrane,m3/m2·hQ=flowrateofpurewater,m3/hA=surfaceareaofmembrane,m2

ΔP=transmembranepressure,kPaµ=dynamicviscosityofwater,kPa·hRm=membraneresistancecoefficient,m–1

Importantdesignparametersincludeflux,rejectionorremovalefficiency,membraneresistance,transmembranepressure,andtemperatureeffects.Avarietyofmodelshavebeendevelopedformembranefluxasafunctionoftime,membranethickness,particleconcentration,etc.Theseincludethe time-dependent models, the blocking filtration laws, and the cakefiltrationlaw,amongothers(MWH,2005).However,pilotplantstud-ies shouldbeconducted todetermineprocessparametersandremovalefficienciesforaspecificwastewaterusingaparticularmembranefilter.

Feed

QF, CF QP, CP

QC, CC

Permeate

Concentrate

Figure 13.15 Membrane filtration.


13.4.3.2 Membrane fouling

The term fouling is used to denote the deposition and accumulation ofparticulatesfromthefeedstreamontothemembrane(MetcalfandEddy,2003).Foulingreducestheefficiencyofthemembrane.Itcanoccurinthreeforms:(1)cakeformationorbuildupofconstituentsonthemembranesur-face,e.g.metaloxides,colloids,bacteria;(2)scalingorchemicalprecipi-tationonthemembrane,e.g.calciumcarbonate,calciumsulfate;and(3)damagetothemembranebyacids,bases,orbacteriapresentinthefeed.

A number of options are available to control membrane fouling. Themost importantone ispretreatmentof feedwater to remove the foulingcompounds. Cartridge filters can be used to remove colloidal particles.Chemicalconditioningof feedwater isusedtopreventchemicalprecipi-tation.Reversible foulingcanbetreatedbybackwashingthemembrane.Chemical cleaning is used to remove scaling. When the membrane effi-ciencyordesiredfluxratecannotberecoveredbytheabovemethods,itistermedirreversiblefouling.Inthatcase,thedamagedmembranehastobereplacedwithanewone.

Recentresearchhasfocusedondevelopingmembranesthataremoreresis-tanttofouling.Applicationofacoatingofnanomaterialsonthemembranehasbeeninvestigatedtoreducefouling.CoatingmaterialsinvestigatedincludeTiO2 (titaniumdioxide),Al2 (alumina), silver, silica, iron,andmagnesium-basednanoparticles,amongothers.Ingeneral,nanomaterialshaveimprovedthehydrophilicity, selectivity,conductivity, foulingresistance,andantiviralpropertiesofmembranes(Suetal.,2011;Luetal.,2009;Zodrowetal.,2007;BaeandTak,2005).Butresearchershavecautionedaboutthelossofnanoma-terialswiththepermeateandemphasizedtheneedforfurtherstudyontheirpotentialenvironmentalandhealtheffects(KimandvanderBruggen,2010).

13.4.3.3 Membrane configurations

Amembraneunitcalledamodulecomprisesthemembranes,pressuresup-port structure for themembranes, and feed inletandpermeate/retentateoutlet ports. The main types of modules used for wastewater treatmentare (1)hollowfiber, (2) tubular, and (3) spiralwound.Thehollowfibermembranemoduleisthemostcommon,whereabundleofhollowfibersisplacedinsideapressurevessel.Thefibershaveanoutsidediameterof0.5to2.0mmandawallthicknessof0.07to0.60mm(WEF,2006).Eachves-selcontainsabundleofhundredstothousandsofhollowfibers.AhollowfibermembranemoduleisillustratedinFigure 13.16.Inatubularmodule,themembraneiscastontheinsideofasupporttube.Anumberofthesetubesarethenplacedinapressurevessel.Aspiralwoundmoduleconsistsofflatmembranesheetsseparatedbyflexiblespacers,rolledintoacircle,andplacedinapressurevessel.Membranescanalsobepressuredrivenorvacuumdriven.


Fourdifferentprocessconfigurationsareusedwithhollowfibermem-branemodules,dependingonthedirectionofflowoffeedwaterandreten-tate: (1) inside-out (dead-end), (2) inside-out (cross-flow), (3) outside-in(dead-end),and(4)outside-in(cross-flow)(Davis,2011).Intheinside-outconfiguration,thefeedwaterflowsintothehollowmembrane,andtheper-meatepassesoutthroughthemembranetotheoutside.Intheoutside-inconfiguration, feed water flows against the walls of the membrane, andthepermeateiscollectedinside.Inthedead-endordirectfeedmode,allofthefeedwaterpassesthroughthemembrane.Inthecross-flowmode,feedwaterispumpedtangentiallytothemembrane.Waterthatdoesnotpassthroughthemembraneisrecirculatedthroughthemembraneafterblend-ingwithfeedwater.

13.4.4 Process flow diagrams

Tertiarytreatmentoptionscanbeusedforadvancedtreatmentofwaste-waterforremovalofspecificconstituentsandsolids.Advancedtreatmentis requiredwhen thewastewater is tobe reused for aquifer recharge, inhigh-pressureboilers,orforindirectreuse.Processflowdiagramsarepro-videdinFigure 13.17,incorporatinganumberoftheprocessesdiscussedinprevioussections.

PROBLEMS

13.1 Whyareadvancedortertiarytreatmentprocessesusedatatreat-ment plant? List the pollutants that are removed through theseprocesses.

Feedwater

Permeate

Concentrate

Pressurevessel

Fiberbundle

Figure 13.16 Hollow fiber membrane module.


13.2 What are the sourcesofnitrogenandphosphorus inwastewater?Listtheformsofnitrogenandphosphoruspresentinwastewater.

13.3 Define the three biological nitrogen removal processes. Draw theschematicdiagramofeachoftheprocesses.

13.4 Provide someexamplesof external carbon sources.Whyare theyusedforbiologicaldenitrificationprocesses?Whatisthelimitationofusingmethanolasanexternalcarbonsource?Listthreetypesofnitrification–denitrificationprocesses.Giveanexampleofeachtypeofprocess.

13.5 A municipal wastewater treatment plant has an activated sludgeprocess forcombinedBODremovalandnitrification, followedbyadenitrificationsystemconsistingofananoxictankandclarifier.Theplantusedmethanolasanexternalcarbonsourcefordenitri-fication.Effluentfromtheactivatedsludgeprocesshasaflowrateof3500m3/dayandNO3–Nconcentrationof22mg/L.Thedeni-trificationtankhasanSRTof6d,andthekineticcoefficientswithmethanolat20°Careasfollows:

μmax=1.5d–1

kd =0.03d–1

Ks =6.5bsCOD/L Y =0.35kgVSS/kgbsCOD

COD equivalent of methanol is 1.5 kg COD/kg methanol. If thedaily dosage of methanol is 300 kg/day, calculate the effluentNO3–Nconcentrationfromthedenitrificationtank.

(a)

(b)

(c)

Micro-�ltration

Reverseosmosis

Reverseosmosis

UVdisinfection(optional)

Secondarye�uent

Used in high-pressure boiler

Membranebioreactor

Reverseosmosis

UVtreatment

Cl2disinfection

Primary e�uent

Filtration UVdisinfection

Carbonadsorption

Ultra-�ltration

Airstripping

Ozonation Cl2disinfection

Secondarye�uent

Figure 13.17 Flow diagrams for advanced treatment options.


Why is limepretreatmentnecessary for the air strippingprocess?Whatarethedisadvantagesofthisprocess?

13.6 Describetheprocessofchemicalprecipitationforremovalofphos-phorus.Whyare the chemical requirementshigher than stoichio-metricrequirements?

13.7 What chemicals can be used to enhance biological phosphorusremoval?Giveanexampleofaprocessusedforbiologicalphospho-rusremoval.

13.8 Whyisfiltrationusedinwastewatertreatment?Listtheadvantagesofusingdual-mediaormultimediafilters.

13.9 What ismembrane fouling?Why is it of concernandhowcan itbecontrolled?Whataretheadvantagesanddisadvantagesofusingnanomaterialsonmembranes?

13.10 Amunicipalwastewaterwithatotaldissolvedsolids(TDS)concen-trationof3000g/m3istobetreatedusingmembranefiltration.Forregulatoryrequirement,theproductwater istohaveaTDSofnomorethan200g/m3.Estimatetherejectionrateandtheconcentra-tionoftheconcentratestream.Assume90%ofthewaterisrecov-eredbythesystem.

13.11 Apilotmembranefiltrationplantissetuptodeterminetheoperationalparametersofanoveltypeofmembrane.Theflowrateofpurewaterthrougha20cm2membrane is0.5mL/min. If the transmembranepressureis2500kPa,calculatethemembraneresistancecoefficient.

REFERENCES

Aspegren,H.,Nyberg,U.,Andersson,B.,Gotthardsson, S., and Jansen, J. (1998)“PostDenitrificationinaMovingBedBiofilmReactorProcess.”Water Science and Technology,vol.38,no.1,pp.31–38

AWWA(2005)Microfiltration and Ultrafiltration Membranes for Drinking Water.AWWAManualM53,AmericanWaterWorksAssociation,Denver,CO.

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323

Appendix

Table A.1 Physical properties of water (SI units)

Temperature °C

Specific weight

λkN/m3

Density ρ

kg/m3

Modulus of elasticity*

E/106 kN/m2

Dynamic viscosity μ × 103 N · s/m2

Kinematic viscosity ν × 106 N · s/m2

Surface tension**

Σ N/m

Vapor pressure

pv kN/m2

05

1015202530405060708090

100

9.8059.8079.8049.7989.7899.7779.7649.7309.6899.6429.5899.5309.4669.399

999.81000.0999.7999.7998.2997.0995.7992.2988.0983.2977.8971.8965.3958.4

1.982.052.102.152.172.222.252.282.292.282.252.202.142.07

1.7811.5181.3071.1391.0020.8900.7980.6530.5470.4660.4040.3540.3150.282

1.7851.5191.3061.1391.0030.8930.8000.6580.5530.4740.4130.3640.3260.294

0.07650.07490.07420.07350.07280.07200.07120.06960.06790.06620.06440.06260.06080.0589

0.610.871.231.702.343.174.247.38

12.3319.9231.1647.3470.10

101.33* At atmospheric pressure.** In contact with air.

324 Appendix

Table A.2 Equilibrium concentrations (mg/L) of dissolved oxygen* as a function of temperature and chloride

Temperature °C

Chloride concentration, mg/L

0 5,000 10,000 15,000 20,000

0 1 2 3 4 5 6 7 8 9101112131415161718192021222324252627282930

14.6214.2313.8413.4813.1312.8012.4812.1711.8711.5911.3311.0810.8310.6010.3710.15 9.95 9.74 9.54 9.35 9.17 8.99 8.83 8.68 8.53 8.38 8.22 8.07 8.92 7.77 7.63

13.7913.4113.0512.7212.4112.0911.7911.5111.24

10.97810.7310.4910.2810.05 9.85 9.65 9.46 9.26 9.07 8.89 8.73 8.57 8.42 8.27 8.12 7.96 7.81 7.67 7.53 7.39 7.25

12.9712.6112.2811.9811.6911.3911.1210.8510.6110.3610.13 9.92 9.72 9.52 9.32 9.14 8.96 8.78 8.62 8.45 8.30 8.14 7.99 7.85 7.71 7.56 7.42 7.28 7.14 7.00 6.86

12.1411.8211.5211.2410.9710.7010.4510.21 9.98 9.76 9.55 9.35 9.17 8.98 8.80 8.63 8.47 8.30 8.15 8.00 7.86 7.71 7.57 7.43 7.30 7.15 7.02 6.88 6.75 6.62 6.49

11.3211.0310.7610.5010.2510.01 9.78 9.57 9.36 9.17 8.98 8.80 8.62 8.46 8.30 8.14 7.99 7.84 7.70 7.56 7.42 7.28 7.14 7.00 6.87 6.74 6.61 6.49 6.37 6.25 6.13

Source: Whipple, G. C., and Whipple, M. C. (1911) “Solubility of Oxygen in Sea Water.” J. Am. Chem. Soc., vol. 33, p. 362. Calculated using data developed by Fox, C. J. J. (1909) “On the Coefficients of Absorption of Nitrogen and Oxygen in Distilled Water and Sea Water and Atmospheric Carbonic Acid in Sea Water.” Trean. Faraday Soc., vol. 5, p. 68.* Saturation values of dissolved oxygen in fresh water and sea water

exposed to dry air containing 20.90% oxygen by volume under a total pressure of 760 mm of mercury.

ISBN: 978-0-415-66958-0

9 780415 669580

90000

Environmental Engineering

As the world’s population has increased, sources of clean water have decreased, shifting the focus toward pollution reduction and control. Disposal of wastes and wastewater without treatment is no longer an option. Fundamentals of Wastewater Treatment and Engineering introduces readers to the essential concepts of wastewater treatment, as well as the engineering design of unit processes for the sustainable treatment of municipal wastewater.

Filling the need for a textbook focused on wastewater, it first covers history, current practices, emerging concerns, and pertinent regulations and then examines the basic principles of reaction kinetics, reactor design, and environmental microbiology, along with natural purification processes. The text also details the design of unit processes for primary, secondary, and advanced treatment as well as solids processing and removal. Using detailed calculations, it discusses energy production from wastewater.

Comprehensive and accessible, the book addresses each design concept with the help of an underlying theory, followed by a mathematical model or formulation. Worked-out problems demonstrate how the mathematical formulations are applied in design. Throughout, the text incorporates recent advances in treatment technologies.

Based on a course taught by the author for the past 18 years, the book is designed for undergraduate and graduate students who have some knowledge of environmental chemistry and fluid mechanics. Readers will get a strong grounding in the principles and learn how to design the unit processes used in municipal wastewater treatment operations. Profes-sionals in the wastewater industry will also find this a handy reference.

Dr. Rumana Riffat is a professor in the Civil and Environmental Engineer-ing Department at George Washington University in Washington, D.C. Her research interests are in wastewater treatment, specifically anaerobic treatment of wastewater and biosolids, as well as nutrient removal.

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Fundamentals of Wastewater Treatment and …...treatment of municipal wastewater. Filling the need for a textbook focused on wastewater, it first covers history, current practices,