Evaluation of Conventional Activated Sludge Compared … · Evaluation of Conventional Activated Sludge Compared to ... settles well to remain in the system ... Session 2_MBR_S. Trussell

Evaluation of Conventional Evaluation of Conventional Activated Sludge Compared to Activated Sludge Compared to

Membrane BioreactorsMembrane Bioreactors

R. Shane Trussell, Ph.D., P.E.R. Shane Trussell, Ph.D., [email protected]@trusselltech.com

Short Course on Membrane BioreactorsShort Course on Membrane Bioreactors3/22/063/22/06

OutlineOutline

•• IntroductionIntroduction•• Process DesignProcess Design•• Effluent Water QualityEffluent Water Quality•• Peak FlowsPeak Flows•• Mixed Liquor PropertiesMixed Liquor Properties•• ConclusionsConclusions

OutlineOutline


•• Biological processes have become the Biological processes have become the preferred municipal wastewater preferred municipal wastewater treatment processtreatment process

•• Activated Sludge Process (ASP) hasActivated Sludge Process (ASP) hasdeveloped into a mature process over developed into a mature process over the past centurythe past century

•• Membrane BioreactorMembrane Bioreactor (MBR) process is (MBR) process is relatively new to wastewater treatment relatively new to wastewater treatment with thewith the concept of direct sludge concept of direct sludge filtration emerging four decades agofiltration emerging four decades ago

IntroductionIntroduction

IntroductionIntroduction

•• Membrane Membrane Bioreactor (MBR)Bioreactor (MBR)–– Modified activated Modified activated

sludge processsludge process–– UF/MF membraneUF/MF membrane

•• Two configurationsTwo configurations–– External (EMBR)External (EMBR)–– Submerged (SMBR)Submerged (SMBR)

Flow Schemes for the MBR and Conventional Activated Sludge Process

Flow Schemes for the MBR and Conventional Flow Schemes for the MBR and Conventional Activated Sludge ProcessActivated Sludge Process

Primary Treated Wastewater


Backwash Water

Backwash Water

Secondary

Clarifier

Secondary

Clarifier

WASTEWASTE

MicrofiltrationMicrofiltrationConventionalConventionalConventional

Aeration BasinAeration Basin

Tertiary

Treated

Wastewater

Tertiary

Treated

Wastewater

Flow Schemes for the MBR and Conventional Activated Sludge Process

Flow Schemes for the MBR and Conventional Flow Schemes for the MBR and Conventional Activated Sludge ProcessActivated Sludge Process

Backwash Water

Backwash Water

Secondary

Clarifier

Secondary

Clarifier

WASTEWASTE

Tertiary

Treated

Wastewater

Tertiary

Treated

Wastewater

MicrofiltrationMicrofiltrationConventionalConventionalConventional




WASTEWASTE

MBRMBR


(Equivalent toa 1-3 mm screen)

Tertiary

Quality

Wastewater

Tertiary

Quality

Wastewater

Submerged MBR (SMBR)Submerged MBR (SMBR)Q



WASTEWASTE

EffluentEffluentAeration BasinAeration Basin

Waste Activated SludgeWaste Activated Sludge

Solids RecycleSolids RecycleQR = 3-5xQ

External MBR (EMBR)External MBR (EMBR)



Solids RecycleSolids Recycle

EffluentEffluentAeration BasinAeration BasinQ

QR = 20-30xQ

Waste Activated SludgeWaste Activated Sludge

OutlineOutline


•• MBRs MBRs combine activated sludgecombine activated sludgetechnology with membrane filtration to technology with membrane filtration to expand the normal operating regionexpand the normal operating region

•• MBRs MBRs can be designed at higher MLSS can be designed at higher MLSS concentrations because they are not concentrations because they are not affected by theaffected by the limitations of gravitylimitations of gravitysedimentation for solidsedimentation for solid--liquid liquid separationseparation

•• SMBRs SMBRs are typically designed for are typically designed for MLSS concentrations 8MLSS concentrations 8--12 12 g/Lg/L

Process DesignProcess Design

•• SMBRs SMBRs operate at 2 tooperate at 2 to >6 times>6 times ASP ASP MLSS concentrationsMLSS concentrations

•• Higher MLSS concentrations translate Higher MLSS concentrations translate into:into:

Advantages of High MLSSAdvantages of High MLSS

––Longer SRTLonger SRTsame HRT, orsame HRT, or––Shorter HRT Shorter HRT same SRTsame SRT

•• For a given HRT, the SMBR process can For a given HRT, the SMBR process can operate at 2 to >6 times the SRT of ASPoperate at 2 to >6 times the SRT of ASP

•• ““TraditionalTraditional”” SMBR design has been to SMBR design has been to operate at operate at ““conventionalconventional”” HRTs HRTs withwith long long SRTs SRTs (i.e. > 20 days)(i.e. > 20 days)

•• Long Long SRTs SRTs have the following advantages:have the following advantages:–– Complete nitrification can occur even in cold Complete nitrification can occur even in cold

climatesclimates–– Reduced biological sludge productionReduced biological sludge production–– Complete oxidation of influent organicsComplete oxidation of influent organics–– Possibility that slow growing microorganisms Possibility that slow growing microorganisms

can degradecan degrade persistent organicspersistent organics

Longer SRT DesignsLonger SRT Designs

•• In general, for a given SRT, the SMBR process can In general, for a given SRT, the SMBR process can treat wastewatertreat wastewater in 1/2 toin 1/2 to 1/4 the HRT of ASP1/4 the HRT of ASP

•• Short Short HRTs HRTs have the following advantages:have the following advantages:–– Reduce overall plant footprintReduce overall plant footprint–– Capital cost savings from reducedCapital cost savings from reduced land and tank volumeland and tank volume

•• Concept of shorter Concept of shorter HRTs HRTs brings about one of the brings about one of the principle limitations of principle limitations of SMBRs SMBRs compared to ASPcompared to ASP–– Minimum SRTMinimum SRT

•• There is a minimum SRT where membrane fouling There is a minimum SRT where membrane fouling becomes rapidbecomes rapid–– A general design guideline is target the minimum SRT A general design guideline is target the minimum SRT

for nitrification plus an additional safety factorfor nitrification plus an additional safety factor–– Some manufacturers have established their own lower Some manufacturers have established their own lower

limit at 12 dayslimit at 12 days

Shorter HRT DesignsShorter HRT Designs

Effect of SRT on SteadyEffect of SRT on Steady--State Fouling RateState Fouling Rate

y = 1.661x2.1977

R2 = 0.9517

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

F/M, g COD/g VSS.d

3 210 5 4MCRT, d

HRT = 4 h

HRT = 1 h

•• Key difference is in solidKey difference is in solid--liquid separationliquid separation–– ASPASP is not sensitive to low is not sensitive to low SRTs SRTs and can and can

successfully operatesuccessfully operate in a in a ““conventionalconventional”” modemode–– SMBRs SMBRs are sensitive to low are sensitive to low SRTs SRTs and compact and compact

designs can result in increased membrane designs can result in increased membrane fouling ratesfouling rates

•• This difference in solidThis difference in solid--liquid separation liquid separation also makes pretreatmentalso makes pretreatment imperativeimperative–– Fine screeningFine screening is an absolute must in is an absolute must in SMBRs SMBRs –– The MBR community hasThe MBR community has been and is still been and is still

learning how importantlearning how important reliable screeningreliable screeningequipment isequipment is


Krampe and

Kauth, 2002

•• Higher MLSS concentrationsHigher MLSS concentrations influence the influence the oxygen transfer efficiencyoxygen transfer efficiency

•• Oxygen transfer from coarse bubble Oxygen transfer from coarse bubble aeration required for membrane agitation aeration required for membrane agitation needs to be consideredneeds to be considered


OutlineOutline


•• Biological process Biological process applied to oxidize applied to oxidize organics and remove organics and remove nutrientsnutrients

•• Principle difference is Principle difference is solidsolid--liquid separation liquid separation mechanismmechanism

•• Membrane provides a Membrane provides a more consistent, higher more consistent, higher quality effluentquality effluent

Effluent Water QualityEffluent Water Quality

Effluent Water QualityEffluent Water QualityMembrane provides an absolute barrier and effluent Membrane provides an absolute barrier and effluent

quality is no longer a concern.quality is no longer a concern.

ND (<ND (<22))<2 to 30<2 to 30BODBOD55, mg/L, mg/L

ND to 100ND to 10010,000 to 10,000 to 100,000100,000

Total Total ColiformColiform, , #/100#/100 mLmL

<0.2<0.22 to >102 to >10Turbidity, NTUTurbidity, NTU

ND (<ND (<22))<<3300TSS, mg/LTSS, mg/L

MBR EffluentMBR EffluentASPASP EffluentEffluent

•• MBR eliminatesMBR eliminates the need the need for monitoring sludgefor monitoring sludgesettleability settleability as an as an operational parameteroperational parameter

»» Effluent quality is not Effluent quality is not dependent on operationsdependent on operations

»» Not necessary to determine Not necessary to determine TSS/VSS concentrations to TSS/VSS concentrations to maintain desired SRTmaintain desired SRT

»» Can use fixed waste rate Can use fixed waste rate SRT=V/QSRT=V/QWASWAS


•• Public Health BenefitPublic Health Benefit–– membranesmembranes increase the distance increase the distance

between reclamation and the risk of between reclamation and the risk of microbial diseasemicrobial disease»»pathogens are removed by size pathogens are removed by size

exclusionexclusion•• not a highly selective chemical or photochemical not a highly selective chemical or photochemical

reactionreaction

»»pathogens can be rejected, not just pathogens can be rejected, not just reducedreduced

–– Results fromResults from operating MBR plants:operating MBR plants:


Indigenous Indigenous ColiphageColiphage

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

0 1000 2000 3000 4000 5000 6000 7000 8000

Hours of Operation

Primary Effluent Reactor #1Reactor #2 Reactor #1 (Non-Detect)Reactor #2 (Non-Detect) Tertiary

RepairedIntegrity

New MembranesReactor # 2

Start -upPeriod

•• MBR Effluent AllowsMBR Effluent Allows Modern Objectives Modern Objectives to beto be RealizedRealized–– Ideal for UV disinfectionIdeal for UV disinfection

»» All particulate matter and suspended solids that All particulate matter and suspended solids that can interfere with UV have been rejected at can interfere with UV have been rejected at membrane barriermembrane barrier

»» High percent High percent transmissivity transmissivity (>70%)(>70%)»» Dose of 80 mJ/cmDose of 80 mJ/cm22 adequate for MBR effluent, adequate for MBR effluent,

while 100 mJ/cmwhile 100 mJ/cm22 required for granular filtered required for granular filtered wastewaterwastewater

–– Ideal pretreatment process for reducingIdeal pretreatment process for reducing TDSTDS»» Suitable for direct feed to ROSuitable for direct feed to RO»» Chloramine Chloramine residual is requiredresidual is required


0

25

50

75

100

125

150

175

200

0 250 500 750 1000 1250 1500 1750 2000

Time of Operation, h

Net

Ope

ratin

g Pr

essu

re, p

si

0

5

10

15

20

25

30

35

40

Tem

pera

ture

, °C

Net Operating Pressure Temperature

Plant shutdow nFeed TDS = 1200 mg/L

Aqua 2000 Bureau II StudyAqua 2000 Bureau II Study[[Filmtec Filmtec BW 30BW 30--4040, low pressure4040, low pressure TFC RO membranes]TFC RO membranes]

11 weeks

OutlineOutline


•• Peak flows are well addressed in ASP, but Peak flows are well addressed in ASP, but can be troublesome for can be troublesome for MBRsMBRs

•• Membranes are designed for a certain Membranes are designed for a certain throughput (design flux)throughput (design flux)

•• MBRs MBRs areare typicallytypically limited to a peaking limited to a peaking factor of 1.5Qfactor of 1.5Q–– Dependent onDependent on design fluxdesign flux (aggressive or conservative)(aggressive or conservative), ,

temperature, and mixed liquor conditionstemperature, and mixed liquor conditions•• ASP is capable of sustaining larger peak ASP is capable of sustaining larger peak

flows (>2.5Q) for longer periods of timeflows (>2.5Q) for longer periods of time–– Possible deterioration in effluent qualityPossible deterioration in effluent quality

Peak FlowsPeak Flows

•• MBR designs for large peak flows consider MBR designs for large peak flows consider the following solutions:the following solutions:–– Additional membrane area for peak flow serviceAdditional membrane area for peak flow service–– Flow equalization tanks (frequently primary Flow equalization tanks (frequently primary effeff))–– ““Flux enhancingFlux enhancing”” polymers orpolymers or coagulant coagulant

additionaddition•• Currently, the most conservative and cost Currently, the most conservative and cost

effective solution is typically some kind of effective solution is typically some kind of flow equalizationflow equalization

•• Flux enhancing polymers and coagulant Flux enhancing polymers and coagulant addition are showing great promise, butaddition are showing great promise, butresearch on longresearch on long--term effects is neededterm effects is needed


Peak flows will become less of a disadvantage Peak flows will become less of a disadvantage for for MBRs MBRs as membrane costs continue to as membrane costs continue to decline and our understanding ofdecline and our understanding of conditions conditions affectingaffecting membrane fouling increasemembrane fouling increase


OutlineOutline


•• Mixed liquor properties are important Mixed liquor properties are important because they determine how easily a sludge because they determine how easily a sludge can be filtered through a membrane, settled can be filtered through a membrane, settled or dewateredor dewatered

•• Differences in solidDifferences in solid--liquidliquid separation apply separation apply different selective pressuresdifferent selective pressures–– ASPASP requires a biology that flocculates and requires a biology that flocculates and

settles well to remain in the systemsettles well to remain in the system–– MBRs MBRs retain all biomass, even single cells in the retain all biomass, even single cells in the

mixed liquormixed liquor

Mixed Liquor PropertiesMixed Liquor Properties

•• Merlo Merlo et al. (2004) revealed some keyet al. (2004) revealed some keyfindings comparing SMBR and ASP under findings comparing SMBR and ASP under steady state conditions forsteady state conditions for 2, 3, 4, 5, 102, 3, 4, 5, 10--d d SRTsSRTs–– SMBR has higher colloidal contentSMBR has higher colloidal content–– SMBR has higher filament concentrationsSMBR has higher filament concentrations–– Both SMBR/ASP particle size distribution Both SMBR/ASP particle size distribution

(excluding colloidal (excluding colloidal -- i.e. >2 i.e. >2 µµm) was controlled m) was controlled exclusively byexclusively by mixing intensity, Gmixing intensity, G

Mixed Liquor PropertiesMixed Liquor Properties

0.0

0.1

0.2

0.3

0.4

0.5

0.6

2-4 4-6 6-8 8-10 10-20 20-40 40-100 100-2000

Freq

uenc

yCMAS Hi

Particle Size DistributionParticle Size DistributionASP Hi ASP Hi vsvs. SMBR. SMBR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

2-4 4-6 6-8 8-10 10-20 20-40 40-100 100-2000

Characteristic Length, µm

SMBR

ASP

OutlineOutline


•• SMBRs SMBRs have advantages compared to ASP have advantages compared to ASP (compact, high quality effluent, high MLSS (compact, high quality effluent, high MLSS concentrations)concentrations)

•• SMBRs SMBRs havehave disadvantages compared todisadvantages compared to ASP (low ASP (low SRT limit, peak flow issues)SRT limit, peak flow issues)

•• Mixed liquor properties are differentMixed liquor properties are different in in SMBRs SMBRs compared to the ASP because of the reactor compared to the ASP because of the reactor conditionsconditions

•• Engineers have been studying mixed liquor Engineers have been studying mixed liquor properties to improve the properties to improve the settleability settleability of ASPof ASP

•• Future of the SMBR processFuture of the SMBR process will be studying will be studying mixed liquor properties that improve filterabilitymixed liquor properties that improve filterability

ConclusionsConclusions

Questions?Questions?

Documents

Evaluation of Conventional Activated Sludge Compared … · Evaluation of Conventional Activated Sludge Compared to ... settles well to remain in the system ... Session 2_MBR_S. Trussell