Decentralised Wastewater Treatmentin Developing Countries

DEWATS

Ludwig Sasse1998

BORDA

Decentralised Wastewater Treatmentin Developing Countries

DEWATS

Ludwig Sasse1998

Bremen Overseas Research and Development AssociationBremer Arbeitsgemeinschaft für Überseeforschung und EntwicklungAssociation Brêmoise de Recherche et de Développement d’Outre MerIndustriestrasse 20, D-28199 Bremen

BORDA

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The Author:

Ludwig Sasse, civil engineer, responsible for biogas and wastewater projects of BORDAsince 1980, editor of the BORDA BIOGAS FORUM, wrote several books and articles onbiogas and wastewater technology and dissemination.

This publication of BORDA was prepared with the financial assistance of

The European Commission, Directorate - General IB/D4, Environment andTropical Forest Sector, Rue de la Loi 200, B - 1049 Bruxelles, Belgium

and

The Free Hanseatic City of Bremen, Senator of Ports, Transport andForeign Trade, State Office for Development Cooperation, Martini Str. 24,D - 28195 Bremen, Germany

The views expressed in this publication are those of the author and do not represent theofficial views neither of the European Commission nor the State Government of Bremen

Printed in Delhi 1998

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CONTENTS

1 FOREWORD2 DEWATS: PROPERTIES,

PERFORMANCE AND SCOPE2.1 Properties of DEWATS

2.2 Performance2.3 Scope

2.4 Implementation

3 DISSEMINATION3.1 The Need for Active

Dissemination of DEWATS

3.2 Preconditions fordissemination

3.3 The Social Aspect

3.4 The Economic Aspect

3.5 The Technical Aspect3.6 The Legal Aspect

3.7 Dissemination Strategy

4 ECONOMICS4.1 Economy of

Wastewater Treatment

4.2 Treatment Alternatives4.3 Parameters for Economic

Calculation

5 PROCESS OF WASTEWATER TREATMENT

5.1 Definition

5.2 Basics of BiologicalTreatment

5.3 Aerobic - Anaerobic

5.4 Phase Separation

5.5 Separation of Solids

5.6 Elimination of Nitrogen

5.7 Elimination of Phosphorus

5.8 Elimination of ToxicSubstances

5.9 Removal of Pathogens

6 ECOLOGY AND SELF PURIFICATIONEFFECT6.1 Surface Water

6.2 Groundwater

6.3 Soil

7 CONTROL PARAMETERS

7.1 Volume

7.2 Solids

7.3 Fat, Grease and Oil

7.4 Turbidity, Colour and Odour

7.5 COD and BOD

7.6 Nitrogen

7.7 Phosphorus

7.8 Temperature and pH

7.9 Volatile Fatty Acids

7.10 Dissolved Oxygen

7.11 Pathogens

8 DIMENSIONING PARAMETERS

8.1 Hydraulic Load

8.2 Organic Load

8.3 Sludge Volume

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9 TECHNOLOGY9.1 Grease Trap and Grit

Chamber9.2 Septic Tank9.3 Imhoff Tank9.4 Anaerobic Filter9.5 UASB9.6 Baffled Septic Tank9.7 Fully Mixed Digester9.8 Trickling Filter9.9 Constructed Wetlands9.10 The Horizontal Gravel

Filter9.11 Vertical Sand Filter9.12 Ponds9.13 Hybrid and Combined

Systems9.14 Unsuitable Technologies

10 SLUDGE DISPOSAL10.1 Desludging10.2 Sludge Drying10.3 Composting

11 REUSE OF WASTEWATER AND SLUDGE11.1 Risks11.2 Groundwater Recharge11.3 Fishponds11.4 Irrigation11.5 Reuse for Process and

Domestic Purposes

12 BIOGAS UTILISATION12.1 Biogas12.2 Scope of Use

12.3 Gas Collection and Storage12.4 Distribution of Biogas12.5 Gas Appliances

13 COMPUTER SPREAD SHEETS13.1 Technical Spread Sheets13.1.1 Usefulness of Computer

Calculation13.1.2 Risks of Using Simplified

Formulas13.1.3 About the Spread Sheets13.1.4 Assumed COD / BOD

Relation13.1.5 Wastewater Production per

Capita13.1.6 Septic Tank13.1.7 Imhoff Tank13.1.8 Anaerobic Filters13.1.9 Baffled Septic Tank13.1.10 Biogas Plant13.1.11 Gravel filter13.1.12 Anaerobic Pond13.1.13 Aerobic Pond13.2 Economic Computer Spread

Sheets13.2.1 General13.2.2. Viability of Using Biogas13.2.3 Annual Cost Calculation13.3 Using Spread Sheets without

Computer

14 APPENDIX

15 LITERATURE

16 KEYWORDS

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This handbook is an outcome of a projecttitled

“Low Maintenance WastewaterTreatment Systems - LOMWATS;

Development of Technologies andDissemination Strategies.”

The project had been financed by the Com-mission of the European Union, with sub-stantial contribution by the State Office forDevelopment Co-operation of the FreeHanseatic City of Bremen from October 1994until April 1998.

The following organisations participated inthe project:

CEEIC (Chengdu) and HRIEE (Hangzhou) fromChina; SIITRAT (New Delhi), MDS (Kanji-rapally) and CSR (Auroville) from India, andGERES (Marseilles) from France. BORDA fromGermany co-ordinated the project.

This book aims at a target group which istypical for decentralised technology imple-mentation. This group consist of people whoare aware of the general problem and knowsomething about possible solutions. How-ever, their knowledge is too general on theone hand, or too specialised on the otherto master the very typical problems whichgo together with decentralisation. This bookwants to provide enough basic knowledgeabout the technology to non-technicalproject managers in order to enable themto adapt the technology locally. The bookwants also to help the technical specialistto understand where technical simplifica-tion is required in order to disseminate thetechnology in its typical decentralised con-

text - and provides tables for dimensioningof treatment plants on the computer. Lastbut not least, the book will assist seniordevelopment planners who need to under-stand the specifics of decentralised waste-water treatment technology sufficiently inorder to select or approve appropriate strat-egies for its dissemination.

Consequently, this book cannot and will notprovide additional information to any of thespecialists in his or her own field. On thecontrary, a specialist may be irked by cer-tain simplifications. This may make goodfor the fact, that practical people are oftenirritated by academic specialists, of boththe technical and non-technical field.

There will be always a need for decentral-ised wastewater treatment in DevelopingCountries. The only realistic approach forthe time being is the use of low mainte-nance technology. However, the abbrevia-tion for it, namely LOMWATS, carries un-justly an image of low standard which couldbecome counter productive to dissemina-tion. Therefore, the name DEWATS will beused, which stands for „DecentralisedWastewater Treatment Systems“. DEWATSincludes only such systems which are con-sidered suitable for decentralised applica-tion and dissemination in the case thatqualified maintenance and operation can-not be expected. Nevertheless, DEWATStechnologies may also be suitable for largecentralised applications. On the other handmay sophisticated technology of consider-able maintenance and steering requirementalso be appropriate in certain decentralisedcases.

1 FOREWORD

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However, even the rather simple DEWATS -technologies are generally not mastered atthe place of decentralised pollution. Thisindicates the biggest disadvantage of de-centralisation, which is the need for know-how and expertise at each of the decentral-ised location. This book intends to improvethe situation. Nonetheless, centralised guid-ance and supervision of decentralised ac-tivities is required which is extremely costly,and therefore, dissemination of DEWATSneeds active promotion.

Acknowledgement

I would like to express my sincere thanksto all who enabled the writing of this book,either through providing funds or throughsharing their rich experience. I want to thankespecially those who made mistakes in thepast and saved me from „falling into theditch“ myself. Unfortunately, it would beunfair to mention their names, however, Iwould like to encourage everybody to openlyreport about own failures, because whenthey have been sincerely analysed they arethe best teaching material for others.

The book in your hand can only be an entrypoint to the wide field of wastewater treat-ment technology. Fortunately, there are otherbooks which describe the various sectorsof the technology more profoundly. You finda long list of books and articles in the an-nex who’s authors deserve my gratitude,however, I would like to mention three booksof quite different type which were of par-ticular help to me:

My favourite is „Biologie der Abwasser-reinigung“ (Biology of Wastewater Treatment)by Klaus Mudrack and Sabine Kunst, be-

cause it explains the biochemical subject insuch a manner that even civil engineerslike myself can follow. I hope the book inyour hand has something of this clarity.

Another book indispensable for a Germancivil engineer is „Taschenbuch der Stadt-entwässerung“ (Pocket book of Sewerage)by the famous Imhoffs, in which one findsgeneral guidance on „simply everything“concerning sewage and wastewater treat-ment. The book has been translated intoseveral languages under various titles andparts of it have been included into booksby local authors.

The last of the trinity is „Wastewater Engi-neering“ by Metcalf and Eddy. Beside beinga comprehensive and voluminous handbookfor engineers that is based on theoreticalknowledge and practical experience, it isalso a textbook for students with examplesfor dimensioning and planning.

I would also like to thank the participantsof a workshop held in Auroville, India inNovember 1997, whom I used without theirknowledge to check the general concept ofthis book. I am further indebted to those towhom I have sent the draft version for com-ments. I am grateful for remarks and infor-mation given by D.P. Singhal, Deng Liangwei,Andreas Schmidt, Gilles B., Dirk Esser andChristopher Kellner. I would also like tothank Mr. Siepen who compiled mountainsof literature for me while working as a vol-unteer for BORDA. My special thanks goesto Mrs. Anthya Madiath who tried hard toclean the text from the most cruel „Ger-manisms“ without hurting my ego too much.

Ludwig Sasse

Bremen, March 1998

1 FOREWORD

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This chapter gives a general overview onDecentralised Wastewater Treatment Systemstechnology (DEWATS) and may be consid-ered being a summary of its essentials. It ismeant for development planners or politi-cians, especially, who tend to not to godeep into technical details before makingtheir decisions.

2.1 Properties of DEWATS

2.1.1 DEWATS

o DEWATS is an approach, rather than justa technical hardware package.

o DEWATS provides treatment for waste-water flows from 1 - 500 m3 per day, fromboth domestic and industrial sources.

o DEWATS is based on a set of treatmentprinciples the selection of which has beendetermined by their reliability, longevity,tolerance towards inflow fluctuation, andmost importantly, because these treatmentprinciples dispense with the need for so-phisticated control and maintenance.

o DEWATS work without technical energy,and thus cannot be switched off inten-tionally (see Fig. 1.).

o DEWATS guarantees permanent and con-tinuous operation, however, fluctuationin effluent quality may occur temporarily.

o DEWATS is not everywhere the best so-lution. However, where skilled and re-sponsible operation and maintenancecannot be guaranteed, DEWATS technolo-gies are undoubtedly the best choiceavailable.

2.1.2 Treatment Systems

DEWATS is based on four treatment sys-tems:

o Sedimentation and primary treatment insedimentation ponds, septic tanks orImhoff tanks

o Secondary anaerobic treatment in fixedbed filters or baffled septic tanks (baf-fled reactors)

o Secondary and tertiary aerobic/ anaerobic treatment in con-structed wetlands (subsurfaceflow filters)

2 DEWAT SPROPERTIES, PERFORMANCE AND SCOPE

Fig. 1.One of too many non-aerating aera-tors [photo: Sasse]

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o Secondary and tertiary aerobic / anaero-bic treatment in ponds.

The above four systems are combined inaccordance with the wastewater influent andthe required effluent quality. Hybrid systems

or a combination of secondary on-site treat-ment and tertiary co-operative treatment isalso possible.

The Imhoff tank is slightly more compli-cated to construct than a septic tank, but

septic tanklongitudinal section

inlet outlet

baffled septic tank

provision for principal longitudinal sectiongas release outlet

inlet

settler

baffled reactor

anaerobic filter

septic tank gas release anaerobic filterinlet outlet

horizontal filter (constructed wetland)

longitudinal section

plantation, preferably phragmites

inlet outlet

rizomes

Fig. 2.Treatment systems considered to be suitable for decentralised dissemination

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provides a fresher effluent when de-sludgedat designed intervals. The Imhoff tank isused preferably when post-treatment takes

place near residential houses, in openponds or constructed wetlands of verticalflow type.

2 DEWATS

Pros and Cons of DEWATStype kind of

treatmentused for typeof wastewater

advantages disadvantages

septic tank sedimentation,sludgestabilisation

wastewater ofsettleable solids,especiallydomestic

simple, durable, little spacebecause of beingunderground

low treatment efficiency,effluent not odourless

Imhofftank

sedimentation,sludgestabilisation

wastewater ofsettleable solids,especiallydomestic

durable, little spacebecause of beingunderground, odourlesseffluent

less simple than septictank, needs very regulardesludging

anaerobicfilter

anaerobicdegradation ofsuspended anddissolvedsolids

pre-settleddomestic andindustrialwastewater ofnarrowCOD/BOD ratio

simple and fairly durable ifwell constructed andwastewater has beenproperly pre-treated, hightreatment efficiency, littlepermanent space requiredbecause of beingunderground

costly in constructionbecause of special filtermaterial, blockage of filterpossible, effluent smellsslightly despite hightreatment efficiency

baffledseptic tank

anaerobicdegradation ofsuspended anddissolvedsolids

pre-settleddomestic andindustrialwastewater ofnarrowCOD/BOD ratio,suitable forstrong industrialwastewater

simple and durable, hightreatment efficiency, littlepermanent space requiredbecause of beingunderground, hardly anyblockage, relatively cheapcompared to anaerobic filter

requires larger space forconstruction, less efficientwith weak wastewater,longer start-up phase thananaerobic filter

horizontalgravelfilter

aerobic-facultative-anaerobicdegradation ofdissolved andfine suspendedsolids,pathogenremoval

suitable fordomestic andweak industrialwastewaterwhere settleablesolids and mostsuspended solidsalready removedby pre-treatment

high treatment efficiencywhen properly constructed,pleasant landscapingpossible, no wastewaterabove ground, can becheap in construction if filtermaterial is available at site,no nuisance of odour

high permanent spacerequirement, costly if rightquality of gravel is notavailable, great knowledgeand care required duringconstruction, intensivemaintenance andsupervision during first 1 - 2years

anaerobicpond

sedimentation,anaerobicdegradationand sludgestabilisation

strong andmediumindustrialwastewater

simple in construction,flexible in respect to degreeof treatment, littlemaintenance

wastewater pond occupiesopen land, there is alwayssome odour, can even bestinky, mosquitoes aredifficult to control

aerobicpond

aerobicdegradation,pathogenremoval

weak, mostlypre-treatedwastewater fromdomestic andindustrialsources

simple in construction,reliable in performance ifproper dimensioned, highpathogen removal rate, canbe used to create an almostnatural environment, fishfarming possible when largein size and low loaded

large permanent spacerequirement, mosquitoesand odour can become anuisance if undersized,algae can raise effluentBOD

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Deep anaerobic ponds and shallow polish-ing ponds are also considered being DEWATS.

Special provisions may have to be madefor industrial wastewater before standard-ised DEWATS designs can be applied.These for example include, an open set-tler for daily removal of fruit waste from acanning factory, buffer tanks to mix vary-ing flows from a milk processing plant,grease traps or neutralisation pits to bal-ance the pH of the influent. In these cases,standard DEWATS are applicable only af-ter such pre-treatment steps have beentaken.

Despite their reliability and impressive treat-ment performance, such well-known andproven systems as UASB, trickling filter, ro-tating discs, etc. are not considered as be-ing DEWATS as these systems require care-ful and skilled attendance.

Most of the treatment processes which areused in large-scale treatment plants despitetheir proven efficiency do not meet theDEWATS criteria and therefore, cannot beincluded. The activated sludge process, thefluidised bed reactor, aerated or chemicalflocculation and all kinds of controlled re-circulation of wastewater are part of thiscategory. Regular or continuous re-circula-tion is partly acceptable under the condi-tion that the pumps that are used cannotbe switched off easily, i.e. separately fromtransportation pumps.

Well designed conventional treatment plants may not meet

DEWATS requirements

Admittedly, this self-imposed restraint overtechnical choices in DEWATS could in prac-

tice impact upon the quality of the effluent.However, inferior quality need not to bewhen there is sufficient space for the plant.There are certain measures at hand to dis-charge effluent of acceptable quality:

o provision of sufficient space at the sourceof pollution

o pre-treatment at source and post treat-ment where sufficient land is available

o pre-treatment at source and post treat-ment in co-operation with others

o accepting an effluent with higher pollu-tion load

o restricting wastewater producing activi-ties at this particular site

o connection to a central treatment plantvia sewage line.

Permanent dilution of wastewater or theinstallation of a mechanised and highly ef-ficient treatment plant remain theoreticaloptions, because experience shows that suchprocesses are chronically afflicted by irregu-lar operation.

2.1.3 Kinds of Wastewater

Septic tanks are used for wastewater with ahigh percentage of settleable solids, typi-cally for effluent from domestic sources.

Anaerobic filters are used for wastewaterwith low percentage of suspended solids(e.g. after primary treatment in septic tanks),and narrow COD/BOD ratio; biogas utilisa-tion may be considered in case of BODconcentration > 1.000 mg/l. (BOD = biologi-cal oxygen demand, COD = chemical oxy-gen demand; both are the most commonparameters for pollution).

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Baffled Septic Tanks are suitable for all kindsof wastewater, however, preferably for suchof high percentage of non-settleable sus-pended solids and narrow COD/BOD ratio.

Constructed wetlands are used for waste-water with low percentage of suspendedsolids and COD concentration below 500 mg/l.

Wastewater for treatment in aerobic pondsshould have a BOD5 content below 300 mg/l.

Facultative and anaerobic ponds may becharged with strong wastewater, however,bad odour cannot be avoided reliably withhigh loading rates.

2.1.4 Area requirement

Depending on total volume, which influ-ences tank depth, nature of wastewater andtemperature, the following values may indi-cate permanent area requirement for set-ting up a treatment plant:

septic tank, Imhoff tank:0,5 m2/m3 daily flowanaerobic filter, baffled septic tank:1 m2/m3 daily flowconstructed wetland:30 m2/m3 daily flowanaerobic ponds:4 m2/m3 daily flowfacultative aerobic ponds:25 m2/m3 daily flow

These values are approximate figures forwastewater of typical strength, however, therequired area increases with strength. Theremight be no waste of land in case of closedanaerobic systems as they are usually con-structed underground. Area for sludge dry-ing beds is not included; this may come to0,1 - 10 m2/m3 daily flow, according tostrength and desludging intervals.

2.2 Performance

2.2.1 Treatment Quality

Treatment quality depends on the nature ofinfluent and temperature, but can basicallybe defined in the following approximate BODremoval rates:

25 to 50 % for septic tanks and Imhofftanks70 to 90 % for anaerobic filters and baffledseptic tanks70 to 95 % for constructed wetland andpond systems.

These values and the required effluent qual-ity decide the choice of treatment systems.For example, septic tanks alone are notsuitable to discharge directly into receivingwaters, but may suit treatment on land wherethe groundwater table is low and odour isnot likely to be a nuisance. Taking a limit of50 mg/l BOD being discharged, the anaero-bic filter in combination with a septic tankmay treat wastewater of 300 mg/l BOD with-out further post treatment. Stronger waste-water would require a constructed wetlandor pond system for final treatment.

There are endless possibilities of cheapertreatment solutions based on local condi-tions. What is required is that all options becarefully considered. Whether long-wayopen discharge channels may deliver therequired additional treatment should alwaysbe taken into consideration. Expert knowl-edge is needed to evaluate such possibili-ties; evaluation should compulsorily includeanalysis of wastewater samples.

Substantial removal of nitrogen requires amix of aerobic and anaerobic treatmentwhich happens in constructed wetlands andponds, only. In closed anaerobic tank sys-tems of the DEWATS-type nitrogen forms to

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ammonia. The effluent is a good fertiliser,but because of that, causes algae growth inreceiving waters and is toxic to fish.

Phosphorus is a good fertiliser, and there-fore dangerous in rivers and lakes. Remov-ing of phosphorus in DEWATS is limited,like in most treatment plants. However, con-structed wetlands could be helpful whenfilter media contains iron or aluminium com-pounds. It should be noted that phospho-rus can be accumulated by sedimentationor fixed in bacteria mass, but can hardly beremoved or transformed into harmless sub-stances.

2.2.2 Pathogen control

Like all modern wastewater treatment plants,DEWAT systems, as well, are not made forpathogen control in the first place. Patho-gen removal rates increase with long reten-tion times, but all high rate plants workproudly on short retention times.

The WHO guidelines and other independ-ent surveys describe transmission of worminfections as the greatest risk in relation towastewater. Worm eggs, helminths, are wellremoved from effluent by sedimentation butaccumulate in the bottom sludge. The longretention times of 1 to 3 years in septictanks and anaerobic filters provide suffi-cient protection against helminths infectionin practice. Therefore, frequent sludge re-moval carries a slightly higher risk.

Bacteria and Virus are destroyed to a greatextent, however they remain in infectiousconcentrations in effluent of anaerobic fil-ters and septic tanks. Nevertheless, thestatistical risk of infection is rather limited.High pathogen removal rates are reported

from constructed wetlands and shallowaerobic ponds. This effect is attributed tolonger retention times, exposure to UV raysin ponds, and various bio-chemical inter-actions in constructed wetlands. Pathogenremoval rates of these systems are higherthan in conventional municipal treatmentplants.

Chlorination can be used for pathogen con-trol. Simple devices with automatic dosingmay be added before final discharge. How-ever, use of chlorine should be limited tocases of high risk, as it would be for hospi-tals during an epidemic outbreak. Perma-nent chlorination should be avoided be-cause it does not only kill pathogens butalso other bacteria and protozoans whichare responsible for the self purification ef-fect of receiving waters.

2.3 Scope

2.3.1 Reuse of wastewater

Effluent from anaerobic units is character-ised by foul smell, even at low BOD values.Irrigation in garden areas should then bet-ter be underground. Effluent from aerobicponds or constructed wetlands is suitablefor surface irrigation, even in domestic gar-dens. However, the better the treatment ef-fect of the system, the lower is the fertilis-ing value of the effluent.

Although most pathogens are removed inaerobic ponds, domestic or agricultural ef-fluent can never be labelled “guaranteedfree of pathogens”. Irrigation of crops shouldtherefore stop 2 weeks prior to harvesting.It is best not to irrigate vegetables and fruits,which are usually consumed raw after flow-ering.

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Treated wastewater can be used for fishfarming when diluted with fresh river wateror after extensive treatment in pond sys-tems. Integrated fish and crop farming ispossible.

2.3.2 Reuse of Sludge

Each treatment system produces sludgewhich must be removed in regular intervals,which may reach from some days or weeks(Imhoff tanks) to several years (ponds).Aerobic systems produce more sludge thananaerobic systems. Desludging should com-ply with agricultural requirements becausesludge although contaminated by pathogensis a valuable fertiliser. Consequently sludgerequires careful handling. The process ofcomposting kills most helminths, bacteriaand viruses due to the high temperaturethat it generates.

2.3.3 Use of Biogas

Conventionally, DEWATS do not utilise thebiogas from anaerobic processes becauseof the cost and additional attendance fac-tors. Devices for collection, storage, distri-bution and utilisation of biogas add to thecost to be recovered from the energy valueof biogas. However, under certain circum-stances the use of biogas may actually re-duce the cost of treatment. Biogas utilisa-tion makes economic sense in the case ofstrong wastewater, and especially whenbiogas can be regularly and purposefullyused on-site. Approximately 200 litres ofbiogas can be recovered from 1 kg of CODremoved. A household normally requires2 to 3 m3 of biogas per day for cooking.Thus, biogas from 20 m3 of wastewater

with a COD concentration of not less than1000 mg/l would be needed to serve therequirements of one household kitchen.

2.3.4 Costs

Total costs, described as annual costs, in-clude planning and supervision costs, run-ning costs, capital costs, and the cost ofconstruction inclusive of the cost of land.As is evident, it is not easy to providehandy calculations on the total cost ofwastewater treatment. The comparison ofcosts is also made difficult, by the factwhile a particular system may be cheaperit may not necessarily be the most suit-able, while other systems might be expen-sive at one location but cheaper at an-other due to differential land prices. How-ever, in general it can be confidently saidthat DEWATS has the potential of beingmore economical in comparison to otherrealistic treatment options. This is true onaccount of the following:

o DEWATS may be standardised for certaincustomer-sectors, which reduces planningcost.

o DEWATS does use neither movable partsnor energy, which avoids expensive butquickly wearing engineering parts.

o DEWATS is designed to be constructedwith local craftsmen; this allows to em-ploy less costly contractors which causeslower capital cost, as well, and later lesserexpenses for repair.

o DEWATS may be combined with naturalor already existing treatment facilities sothat the most appropriate solution maybe chosen.

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o DEWATS has the least possible mainte-nance requirements which spares not onlymanpower for daily attendance but alsohighly paid supervisors or plant manag-ers.

No wastewater treatment is profitable in it-self. However, the indirect gains from treat-ing wastewater might be numerous. Whetherdecentralised DEWATS can compete with thefees and creature comforts that people de-rive from of a centralised sewer connectionwould depend on the local situation. Thefeasibility of using treated water, sludge orbiogas also differs from place to place. In-tegrated wastewater farming merits consid-eration albeit as a completely independentbusiness.

2.4 Implementation

2.4.1 Designing Procedure

If the planning engineer knows his craftand recognises his limitations, designingDEWATS is relatively simple. Performanceof treatment systems cannot be preciselypredicted and therefore calculation of di-mensions should not follow ambitiousprocedures. In case of small and me-dium scale DEWATS, a slightly oversizedplant volume would add to operationalsafety.

Based on local conditions, needs andpreferences plants of varying sizes couldbe chosen to become fixed standard de-signs. On-site adaptation can then bemade by less qualified site supervisorsor technicians.

Individual caseshave to be calcu-lated and designedindividually; thestructural details ofthe standard plantsmay be integrated.A simplified, quasistandardised meth-od has been devel-oped for calculationof dimensions (seechapter 13.1).

Co-operative plantsystems that re-quire interconnect-ing sewerage mustbe designed indi-vidually by an ex-

Fig. 3.DEWATS anaerobic filter under construction. Planning and supervision by CEEIC [photo: Sasse]

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perienced engineer who is able to placeplants and sewers according to contoursand other site requirements.

2.4.2 Wastewater Data

Data indispensable to the calculation andchoice of the right DEWATS design are:

o daily wastewater flow

o hours of major wastewater flow or otherdata describing fluctuations

o average COD values and range of fluc-tuation

o average BOD values or average COD/BODratio

o suspended solids content, percentage ofsettleable solids

o pH

o ambient temperature and temperature ofwastewater at source

In the case of domestic wastewater thisdata is easy to arrive at when the numberof persons and water consumption percapita per day is known. While total waterconsumption might be easy to measure onsite, it is important to consider the amountof water only that enters the treatment sys-tem.

For industrial wastewater other parameterssuch as COD/N or COD/P relation, contentof fat and grease, content of toxic sub-stances or salinity are also likely to be ofconcern. A full analysis may be necessarywhen planning the first such plant. Com-prehensive understanding of the produc-tion process of the industry will help tospecify crucial information required. Cus-tomarily production processes are unlikely

to differ vastly within a defined area of pro-duction, thereby making standardisationfairly easy.

2.4.3 Construction

DEWATS are relatively simple structures thatcan be built by reasonably qualified crafts-men or building contractors with the abilityto read technical drawings. If this were notthe case, almost daily supervision by a quali-fied technician would be required. The con-struction of watertight tanks and tank con-nections would require craftsmanship of arelatively high order. Control of construc-tion quality is of utmost importanceif biogas is to be stored within the reactor.

Technical details of a design, which hasbeen adapted to local conditions, shouldbe based on the material that is locally avail-able and the costs of such material.

Important materials are:

o concrete for basement and foundation

o brickwork or concrete blocks for walls

o water pipes of 3“, 4“ and 6“ in diameter

o filter material for anaerobic filters, suchas cinder, rock chipping, or specially madeplastic products

o properly sized filter material for gravelfilters (uniform grain size)

o plastic foils for bottom sealing of filtersand ponds

Gate valves of 6" and 4" diameter are nec-essary to facilitate de-sludging of tanks regu-larly.

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2.4.4 Maintenance

The more a standard design has beenadapted or modified to fit local conditions,the greater the likelihood of operationalmodification during the initial phase. It istherefore important that the contractor ordesign engineer keeps a close eye on theplant, until the expected treatment resultshave been achieved. Despite faultless im-plementation, it may be necessary to ex-tend such attendance up to as long as twoyears.

Permanent wastewater treatment that doesnot include some degree of maintenance isinconceivable. DEWATS nonetheless reducesmaintenance to the nature of occasionalroutine work. Anaerobic tanks would needto be de-sludged at calculated intervals (usu-ally 1 to 3 years) due to the sludge storagevolume having been limited to these inter-vals. Treatment is not interrupted duringde-sludging. Normally, sludge is drawn fromanaerobic digesters with the help of port-able sludge pumps, which discharge intomovable tankers. Direct discharge into ad-jacent sludge drying beds may be possiblein the case of Imhoff tanks with short de-sludging intervals.

Anaerobic filters tend to clog when fed withhigh pollution loads, especially when ofhigh SS content. Flushing off the biologicalfilm is possible by back washing. This willrequire an additional outlet pipe at the in-let side. In practice, what is usually done isto remove the filter media, wash it andclean it outside and put it back after thiscleaning. This may be necessary every fiveto ten years.

Constructed wetlands gradually loose theirtreatment efficiency after 5 to 15 years, de-pending on grain size and organic load.

The filter media would need to be replaced.The same media may however be re-usedafter washing. During this exercise unlessseveral filter beds were to be provided, treat-ment would have to be suspended. Sur-face plantation has then to be replacedalso; otherwise regular harvesting of coverplants is not required.

Pond systems require the least maintenance.De-sludging may not be necessary for 10 to20 years. Normally, an occasional controlof the inlet and outlet structures should besufficient. Control of wastewater flow maybe required when a foul smell occurs dueto overloading in the hot season. Such prob-lems can be avoided through intelligentinflow distribution and generous sizing ofponds.

2.4.5 Training for Operation

DEWATS is designed such that maintenanceis reduced to the minimum. Daily attend-ance is limited to certain industrial plants.However, there should be someone on-sitewho understands the system. It would bebest to explain the treatment process tothe most senior person available, as he islikely to be the one to give orders to theworkers in case of need. In case the educa-tional qualifications of the on site staff islow, the engineer who designed the plantor the contractor who constructed the plantshould provide service personnel, who maycome to the site once a year or at times ofneed.

Extensive training of staff at the lower levelis generally not necessary and in most casesnot effective due to fluctuation of workers.Hiring professional manpower for after careservice may then be considered the better

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17

solution. The case might be different forhospitals or housing colonies that usuallyhave a permanent staff.

If a large number of DEWATS are to be im-plemented the aim should be to standard-ise the maintenance service. This is likelyto be possible because of local standardi-sation of plant design, with similar charac-teristics for operation.

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3.1 The Need for Active Disseminationof DEWATS

Planning and implementation of DEWATS isnot a very profitable business for engineersand building contractors. Therefore, dissemi-nation of decentralised wastewater treat-ment systems will need to be pushed bypolitical will and administrative support.

The treatment units are relatively small, nev-ertheless complicated and are usually spreadover a scattered area. The required exper-tise for their realisation is truly remarkable.Resultantly, general planning must be cen-tralised and designing standardised in or-der to reduce the overall cost per plant andto maintain the required expertise. On theother hand, any centralisation will involvethe setting up of a superstructure with itsbuild-in tendency to create an expensiveofficer’s pyramid. To achieve implementa-tion of a large number of plants, a strongand omnipresent superstructure seems to berequired. The dog bites in his own tail. How-ever, this superstructure becomes relativelyless expensive if the scale of implementa-tion is large. Therefore, a reasonable, afford-able and sustainable dissemination strategyis essential to balance the desired environ-mental benefits with acceptable social costs.Short-term economic viability of the super-structure cannot necessarily be the yardstickfor choosing a dissemination strategy.

3.2 Preconditions for Dissemination

There are some preconditions commonlyrequired for dissemination of any decen-

tralised technology or technical hardware.These preconditions have to be fulfilledbefore one may start thinking about a dis-semination strategy:

o The hardware to be disseminated is tech-nically sound

o In principle it is feasible to operate andmaintain the hardware on the spot.

o The technology in general is economi-cally and/or environmentally useful.

o The technology is suitable and useful inthe particular local situation.

Dissemination would make sense only ifthese pre-conditions are fulfilled.

There will be no improved wastewater treatment without

technical expertise

DEWATS, as a decentralised technology willalways need local adaptation. Even fullystandardised designs are constructed lo-cally; for example, they have to be con-nected to the source of pollution and haveto be set at proper level to allow free flowof effluent to the receiving water. Implicitin the decentralisation of technology is thedecentralisation of know-how and exper-tise. Centralised guidance and supervisionof decentralised activities is extremelycostly. These services would therefore haveto be kept to the minimum. This would bepossible, only if basic knowledge and aminimum of expertise were locally avail-able.

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Local adaptation of DEWATS is influencedby:

o the technical requirements and solutions,

o the geographical or physical environmentand

o the social and socio-economic circum-stances.

On the other hand, dissemination has tochoose a strategy which observes severalaspects:

o the social aspect,

o the economic aspect,

o the technical aspect and

o the legal aspect

The dissemination strategy that is ultimatelychosen has to include all these aspects.

3.3 The Social Aspect

3.3.1 The Status of Waste Disposal inSociety

The public is growing in its awareness ofwastewater treatment as a result of increas-ing damage and pollution to the environ-ment from wastewater. However, public in-terest appears to be confined to the harm-ful effects of pollution, only. In other words,there is no particular public interest in hav-ing a wastewater treatment plant - and thereappears to be definitely even less interestin maintaining it. The general attitude isone of “ somebody must do something “.

The public needs to realise that that “some-body“ is they, if nobody else really caresabout their problem. The question then iswith regard to the preferred treatment con-

cept. It is unlikely that individual DEWATSwhich must be taken care of by the indi-vidual will be very popular. Even if DEWATSwere to be the only possibility, public will-ingness to participate in the programmewould still be limited. Therefore, concepts,which require the participation of the gen-eral public, are not likely to work too well,and consequently should be avoided when-ever possible.

Throughout the ages anything related towastewater seems to have had very lowpriority. Even in olden times it was alwayspeople of the lowest social status who wereput in charge of waste disposal. Unlike car-penters, masons or other professionals, these“scavengers” as they were called were neverreally interested in upgrading their discrimi-nated skills. Understandably, the knowledgeof wastewater disposal lagged behind otherbasic civic techniques. Strangely that miss-ing interest was found even among royaltywhose otherwise impressive castles had allbut a primitive toilet outside. The well-de-signed and elaborate place of Versailles forinstance, did not have a toilet. Similarly,the genius engineer Leonardo da Vinci de-veloped weapons, bridges, air planes; butin his model city of 1484, treatment of wastewas relegated to just one low-level waste-water canal.

Wastewater engineers are probably the only ones who love

handling wastewater

Nonetheless, the taboo on faeces that ex-ists over centuries is perhaps the most effi-cient sanitary measure, and still is from ahealth point of view a beneficial habit. How-soever, the phenomenal population growth

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the world over has increasingly demandedprofessional attention to wastewater andits disposal in response to which, today wehave a new breed of wastewater engineerswho are not ashamed of their profession.

3.3.2 Organising People

For wastewater treatment to take place in aco-operative context, it is important thatpeople are organised. The objectives of or-ganising people may be manifold:

o collecting investment capital,

o contributing land,

o giving permission for trespassing of sew-ers over private land,

o collective operation and maintenance

o collective financing of services for op-eration,

o use of effluent for irrigation, sludge forfertiliser or biogas for energy.

There are numerous ways in which peoplecan come together. The framework of ac-tion may be the general community admin-istration, a development project, or an NGO,which supports self- help activity. Howso-ever, the local tradition of self organisationand co-operation and the particular imageof the subject “wastewater” will influencethe organisational structure.

How to organise people is a well-researchedsubject. The results and suggestions of allthose books and concepts cannot be pre-sented here in detail. However, great careis necessary to check any proposed methodwhether it is suitable and promising in caseof wastewater treatment and disposal. Mostexperiences of community involvement in

sanitation are related to water supply orlow cost toilet programmes for individualhouseholds, programmes which meet im-mediate felt needs. It must be rememberedthat this is not so with wastewater disposal- and even less so with wastewater treat-ment. People do not want to be botheredwith it. In general, people expect that anony-mous authorities should take care of thisproblem. Public willingness to get involvedin treating their wastewater is low and canonly be expected to increase in case ofsevere crises, or in the likelihood of sub-stantial economic benefit as in the case ofre-using wastewater for irrigation, for in-stance.

3.3.3 Partners for Dissemination

The logical partners in dissemination of waste-water treatment systems beside the pol-luter as customer, are the government ad-ministration at one end, and private enter-prise such as engineering companies, atthe other. In an ideal scenario, the govern-ment would announce a set of by-laws andthen oversee the implementation of thesebylaws. Thereupon, polluters by socialagreement or threat of punishment wouldbe obliged to contact private engineers andcontractors to implement an adequate treat-ment and disposal system. The investmentcapital would come from the individual,from bank loans, and perhaps as a subsidyfrom public funds. This scenario is typicalto industrialised countries.

In developing countries, the scenario mightappear deceptively similar. On the contrary,the comparison is not likely to go beyondthe introduction of bylaws, which are sel-dom enforced, as a result of which pollut-

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ers may not even be aware of their exist-ence. The service of private engineers isalso likely to be far too expensive for thesmall polluter. Altogether, this leads to asituation of complete in-action or to one ofarbitrary abuse of power by some officials.As a result of the failure of the governmentor the private sector, informal or registeredself-help groups step in to carry out thevarious tasks that are normally not theirs.

Public awareness towards the problem ofwastewater pollution has grown tremen-dously in recent years. Politicians and gov-ernment administrations welcome any ini-tiative to help solving the problem throughdecentralised measures. Money is often notthe biggest problem, at least for pilot ordemonstration projects. However, there is ageneral helplessness when it comes to in-dividual implementation, and more so whenit comes to active and well organised dis-semination of DEWATS.

Marketing experts, development consultantsor socially oriented NGOs are the first topresent themselves as partners for dissemi-nation. Other business organisations andcontractors more suited to wide scale dis-semination are rare ‘diamonds in a heap ofsand’. The logical consequence of this di-lemma is to find new local partners for de-centralised dissemination at each place.Since these partners will play the key rolein implementation, dissemination conceptsmust necessarily be shaped to suit them.

Dissemination concepts must suit the local partners

3.4 The Economic Aspect

3.4.1 Decentralisation

Whether decentralised wastewater treatmentis better than centralised treatment is basi-cally a question for theoretical or ideologi-cal discussion. In practice, there is alwayslikely to be a mix of centralised and decen-tralised solutions.

Cost-benefit analyses are help-ful as general policy considera-tions and for choosing themost economic treatment sys-tem in individual cases. How-ever, the problem of a cost-benefit analysis of DEWATS liesin the parameters influencingthe calculation, which are of-ten difficult to project a priori.A time frame of 20 to 30 years- the normal lifetime of a treat-ment plant - should constitutethe basis for calculation. Whileconstruction costs are relatively

Indian National Discharge Standards

discharge into

parameter unitinland

surface water

public sewers

land for irrigation

marine coastal

areaSS mg/l 100 600 200 100pH 5.5 to 9 5.5 to 9 5.5 to 9 5.5 to 9temperature °C < +5°C < +5°CBOD5 mg/l 30 350 100 100COD mg/l 250 250oil and grease mg/l 10 20 10 20total res. chlorine mg/l 1 21NH3-N mg/l 50 50 50Nkjel as NH3 mg/l 100 100free ammonia as NH3 mg/l 5 5nitrate N mg/l 10 20diss. phosphates as P mg/l 5sulphides as S mg/l 2 5

CPCB

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Tab. 1.Indian discharge standards

22

easy to calculate, an estimate of realisticrunning costs would need an in-depthstudy of the technical requirements of thesystem as well as the prevailing social en-vironment.

Further, it would need a fairly precise read-ing into future management structures. Over-heads in form of salaries for the manage-ment, expenditure for the logistic require-ments of operation and maintenance areextremely difficult to foresee, especially inthe case of co-operatives. The cost of trans-porting sludge, for example, can increasemanifold if the neighbouring farmer decidesnot to take the sludge. A new drying or dump-ing place could be so far away so that truckswould have to be hired instead of oxcarts asplanned, sending calculations awry.

A totally centralised system would result inthe lowest plant construction cost pertreated volume of wastewater. On the otherhand, connecting individual sources to thetreatment unit may result in up to five timesthe cost for the required sewerage. Man-agement costs are comparatively low be-cause one highly qualified manager caresfor a large volume of wastewater, respec-tively a large number of users. Maintenancecosts are quite high, instead, because so-phisticated mechanised equipment requirespermanent care.

A semi-centralised system connects severalsmaller treatment units to sewerage ofshorter overall-length. Construction costs arerelatively low, but qualified managementmay be needed for each plant, thus push-ing up the cost.

A fully decentralised system would need anatural environment that is capable of ab-sorbing the discharged wastewater of each

individual plant on-site. Structural costs arelikely to be the lowest for fully decentral-ised systems, especially if slightly sub-stand-ard treatment is accepted. Safe sludge dis-posal must also be possible at site, other-wise the cost of transportation for sludgecollection and disposal must be included.Maintenance and management costs aredispensed with when the user of the plantalso attends to it. However, if proper op-eration on a more sophisticated level is tobe secured, the need for qualified supervi-sion and service structures may arise, whichto a certain extent would need to be or-ganised centrally (for example for collec-tion and disposal of sludge). Regular efflu-ent control is particularly costly in decen-tralised systems.

3.4.2 Treatment Quality

The kind of environmental pollution thatexists tends to justify the strict dischargestandards that prevail. However, standardsthat are extremely high, paradoxically mayworsen, not improve the situation.

The nature of treatment is first of all afunction of the area, for which one mayallow a certain degree of environmentalpollution. If the area where pollution oc-curs is infinite, there may be no necessityfor treatment. Similarly, when pollution isinfinitely small the polluted area is zero.Situations between these two extremesshould be open for setting priorities, mean-ing that the final choice should be opento negotiation. Economic, social and envi-ronmental aspects would each merit dueconsideration to reach the most accept-able compromise with regard to the re-quired treatment quality.

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Permitted discharge quality will also dependon the location of pollution. Transport ofwastewater to sites further away instead ofon-site treatment had been practised sinceman developed settlements. Today, waste-water transportation to remote land or wa-ters is still common. While this is may beacceptable today, the damage to the envi-ronment may become irreversible and intime, the legacy of pollution could hit backat the polluter. In the case of DEWATS dis-semination, the decision to treat wastewateron-site and not just send it away is alreadymade.

In most countries national pollution stand-ards allow for higher pollution loads in ef-fluents of smaller plants, and effluent stand-ards are “softer” when the discharge is ontoland and into waters that are little used.

Discharge standards in developing countrieshave often been borrowed from industrial-ised countries, which are based on highlydiluted municipal sewage. DEWATS in de-veloping countries are meant for public toi-lets, hospitals, schools or smaller commu-nities where it is likely that lesser water isused for household and toilet purposes thanin industrialised countries. The high con-centration of wastewater from water savingtoilets leads automatically to higher BODconcentration of the effluent, even whenBOD removal rates are within the technicalrange of an adequate treatment system. Insuch case, as saving of water is crucial tosustainable development it would not bereasonable to dilute the effluent “artificially”in order to achieve an administratively im-posed concentration. Standards which re-late to absolute pollution loads (instead ofconcentrations) would be more reasonable,at least for small units.

This point is especially important, becauseone of the great advantages of decentrali-sation and on-site treatment is that wastetransport in short sewer pipes does not re-quire highly diluted wastewater. Despitehigher concentrations, the absolute pollu-tion load remains the same. Water savingpolicies could easily accept higher concen-trations at the outlet when other environ-mental factors being favourable; for exam-ple when there is enough land available orthe receiving river carries enough water theyear round.

Furthermore, it makes little sense to installDEWATS of a high treatment quality, whentheir effluent joins an open sewer channelwhich receives other untreated wastewater.In this case, simple, individual septic tanks,which cost less but are albeit less effectivein their performance, would be appropri-ate, because treatment of the main waste-water stream would in any case have to bedone.

3.4.3 Treatment Cost

Thirty to fifty percent of the pollution loadmay be removed with simple technology,such as the septic tank. Another thirty toforty percent might be removed with thehelp of units such as baffled septic tanksand anaerobic filters that are yet simple butfar more effective. Any further treatmentwould require post treatment in ponds orconstructed wetlands (conventional systemsusing artificial oxidation do not belong tothe DEWATS family). The higher the relativepollution removal rate, the higher are theabsolute treatment costs per kg BOD re-moved. Additional treatment devices for re-moval of nitrogen, phosphorous or other

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toxic substances are likely to be unusuallyexpensive.

Technically spoken, DEWATS are able to meetany discharge standard. However, since selfsustainable dissemination of DEWATS islikely to be strongly influenced by invest-ment and operational costs, the choice ofan appropriate treatment standard will notonly determine the dissemination strategybut may be vital for the total success ofDEWATS.

Treatment efficiency of DEWATS may be as high as that of any conventional treatment plant

Environmental experts and experiencedwastewater engineers would need to deter-mine appropriate treatment standards. It iscrucial that authorities that administratepollution control have a understanding ofthe issue and can be flexible in acceptingand legalising deviations from general stand-ards, so long as these deviations are eco-logically acceptable.

3.4.4 Investment Capital

As there is generally no financial return onwastewater treatment there is little genuineeconomic interest to invest in it. Apart fromthe few persons who act out of a sense ofresponsibility for the environment, there israrely anybody who invests in wastewatertreatment voluntarily. Only if and until pol-luters are compelled by law to pay for thepollution they cause will investment capitalbecome available to wastewater treatment,because investment in wastewater treatmentwill be viable compared to imposed fines.

Construction budgets for new buildings andenterprises are likely to include wastewatertreatment costs. Existing units, apart fromthe problem of having the space for con-struction may not be able to raise funds forinstalling a treatment unit at one time. Dis-semination programmes must therefore nec-essarily ensure the availability of creditschemes to polluter’s for wastewater treat-ment.

A sustainable dissemination strategy musttake into account the time it may take apolluter to allocate the required capital (forexample, to wait until the next board meet-ing decides on the matter). Practically thiscould translate into a time lag of a year ormore between the time when the planningengineer invites the contractor to see thesite and make a realistic estimate for con-struction and the availability of funds forthe purpose. Besides the inflation factor thisalso implies that a contractor cannot affordto rely solely on DEWATS for his survival.Under these circumstances it may be diffi-cult to recruit contractors who are perma-nently available to a DEWATS dissemina-tion programme.

3.5 The Technical Aspect

3.5.1 Decentralisation

From an economic point of view, decen-tralisation requires a simplified technologyfor the reason that it will be prohibitivelyexpensive to permanently maintain the nec-essary expertise for sophisticated technol-ogy on a decentralised level.

It may be possible to actualise decentral-ised solutions with the help of completelystandardised designs based on local con-

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struction techniques; in other words, byplacing “black-box” hardware packages onthe “market”. In both cases expertise willbe needed for choosing the right design orthe suitable “black-box”. Expertise will alsobe needed to advise the user in properoperation and adequate maintenance. Suchadvice must have a stable address, whichmight be difficult in case certain operationsbecome necessary after one or two yearsonly, for example in the case of de-sludging.This could also mean that the constructingenterprise would need to be contracted formaintenance and operation on behalf of thecustomer. The expertise available for on-site operation and maintenance would de-cide on the nature of required plant man-agement.

Invaluable experience gained from ruralbiogas dissemination programmes over aspan of 30 years in India, China and sev-eral other countries confirms that each suchproject or programme had first to developan appropriate local design notwithstand-ing the availability of standardised designsfrom other projects /countries. Interestinglyneither India nor China have been able tosustain the dissemination of their small-scale rural biogas programme from a tech-nical point of view without the support of asuperstructure. In most cases it has beendifficult to ensure that the user has suffi-cient expertise to maintain the biogas plantsproperly. None of the programmes have beenable to do without a subsidised after salesservice. This scenario is bound to be trueto other areas of technology disseminationas well.

DEWATS is far more complicated than ruralbiogas plants. The bio-chemical and physi-cal properties of wastewater especially incase of wastewater from industrial sources

are far from uniform. Consequently, the ex-pertise required for design, construction andoperation of DEWATS become all the moreindispensable as compared to rural biogasplants. One of the crucial points is to de-cide whether a standard design is suitable,or how it should be modified.

The availability of the right expertise is ir-replaceable in DEWATS dissemination. It isdecisive for a dissemination programme ofdecentralised plants to clarify the nature ofthe expertise that is to be maintained, atwhat level and for which technology. Miss-ing knowledge and expertise cannot be con-signed to the insignificance of a so-called“social matter”. Professionals and techni-cians are duty bound to ensure that thegoods or structures they provide are tech-nically sound and appropriate. The breakdown of a technical system is not neces-sarily the fault of the customer. It is theduty of the technician to deliver the rightdesign for a given situation.

It is the duty of the technician to deliver an appropriate design which will be realised with an

appropriate technology

A division of labour between different ex-perts is essential. It is the duty of the non-technical “social” staff to feed the techni-cian with information about social mattersand the technician to feed the social ex-perts with information about the technicalnecessities. What is required is collabora-tion between the disciplines, not role con-fusion. It is a known fact that techniciansas a rule do not give enough informationabout technical necessities to social experts.This in all likelihood happens for two rea-

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26

sons: the technicians do not know the tech-nology well enough and/or the social ex-perts are not able to understand the impli-cations of the technical requirements. Theproblem of sub-standard technical knowl-edge is likely to be true of most potentialDEWATS constructors - and promotors.

3.5.2 Construction

The local availability of hardware is a pre-condition for decentralisation. In case of mainstructures, appropriateness of local buildingmaterial plays a decisive role in executingthe construction. The kind of filter materialavailable for instance influences the choiceof the treatment principle. Expertise is neededto modify standard designs, or if necessary,make new designs, which can be constructedwith locally available material. It is also im-portant to decide whether traditional con-struction techniques are suitable for waste-water treatment systems, especially if biogasis supposed to be used. The expert has alsoto decide whether the treatment system fitsthe local geography.

At the level of implementation of small-scaletreatment systems, the qualification of crafts-men is usually low. Masons may not be ableto read and write and thus, structural draw-ings are not useful at site. Structural de-signs would therefore have to be simplifiedor supervisors who are properly qualifiedto read the drawing be present regularly.

Familiarity with the design principles apart,a mastery of structural details will be cru-cial to the proper functioning of the plant.The need for correct execution of the struc-tural details of the design cannot be over-emphasised. Cost or efficiency enhancingmodifications of structural details is ill

advised in the absence of a deep under-standing of the proposed measure. Forexample, a dentated sill is of no use if thetop of teeth, instead of the bottom notches,is kept in level.

3.5.3 Substrate

The quality, quantity and other propertiesof wastewater determine the treatment prin-ciple from a scientific point of view. Thetype of treatment system finally chosen willdepend on the geographical, structural andsocio-economic conditions. Expertise toanalyse and evaluate the wastewater, tochoose the most appropriate treatment sys-tem and to countercheck the design in re-spect to its local suitability will be impera-tive.

In the absence of such expertise at the lo-cal level, standardisation on the basis ofsimilar wastewater in the region will be thenext step. The local expert should then atleast be able to distinguish between “stand-ard” and “other” wastewater. To do this,there must be somebody in the disseminat-ing organisation that can read laboratoryresults or, at least understands the impor-tance of working with the right data in choos-ing the appropriate structure. Furthermore,someone would be required to collect rep-resentative wastewater samples and inter-pret the laboratory results for the techni-cian.

Low maintenance is demanded of decen-tralised wastewater treatment. If structuralmeasures can improve treatment, chemicalssuch as coagulants are „forbidden“. How-ever, the use of chemicals may be unavoid-able if bacterial growth is impaired by nu-trient deficiencies, e.g. an unbalanced phos-

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phorous-nitrogen ratio. Bio-chemical exper-tise is required to decide if such measuresare really necessary.

3.6 The Legal Aspect

3.6.1 The Political Environment

The over all political climate is as impor-tant as the administrative framework. It isimportant to know who the movers are andthe different roles each actor plays. It isalso important to be familiar with the regu-latory framework and more specifically theextent to which rules and regulations areactually enforced. To politicians, inefficien-cies of the legal framework are at worst amoral matter, only, whereas to those imple-menting a DEWATS dissemination strategy,inefficiencies of the socio-political environ-ment are essentially a fundamental plan-ning parameter.

Conventionally, policy is formulated on abedrock of relevant facts. Familiarity withscientific or technical facts ensure that theright policy and the right laws and by-lawsare shaped. Decision-makers should knowthat each step in the treatment process re-moves only a portion of the incoming pol-lution load. It is also important that theyknow that DEWATS is premised on the be-lief that wastewater treatment should bepermanent and that permanency can onlybe guaranteed by simple and robust sys-tems. This implies that permanent waste-water treatment is not possible withoutmaintenance. Nonetheless, in DEWATS,maintenance operations are kept to theabsolute minimum - necessarily.

3.6.2 Political Priorities

Political and administrative preferences leanheavily towards large scale, centralisedwastewater and sewerage systems. Giventhe fact that most wastewater is producedby urban agglomerations, this is understand-able. Domestic wastewater from towns andcities is the largest single source of waterpollution. Industrial wastewater from sub-urban areas comes next. In India it has beenestimated that only 50% of the wastewaterthat finally reaches the river Ganges actu-ally passes through urban wastewater treat-ment plants. The other 50% of this waterflows untreated into the environment.Whether this untreated 50% can be actual-ised into a potential demand for DEWATSdepends on policy making and the serious-ness of its application.

Most governments tend to sacrifice envi-ronmental concerns on the altar of fiscaldemand. The industrialised west has beenno different. The history of wastewater treat-ment in Europe and North America reflectsthe tug of war between the economy andthe environment. Earlier and today, the state-of-the-art of treatment technologies andregulatory framework is the outcome of thisdialectic.

The same seems to be happening in devel-oping countries as well, the only differencebeing in the import of advanced treatmenttechnologies from industrialised countries.Today environmental standards, i.e. dis-charge standards for wastewater are beingbased on the treatment technologies thatare available, and not on the prevailing stateof the economy. This is leading to a strangesituation wherein the strict discharge stand-ards are hardly followed because their ap-plication is too expensive. Thus, the indi-

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vidual polluter gets away by completely ig-noring the problem or by setting up a faketreatment system to please the environmen-tal control officer. Either way, the environ-ment is not protected. On the other hand,were environmental standards to be morerealistic and feasible there is greater likeli-hood of adherence to the law by individualpolluters.

“Undue haste in adopting standards which are cur-rently too high can lead to the use of inappropriatetechnology in pursuit of unattainable or unaffordableobjectives and, in doing so, produces an unsus-tainable system. There is a great danger in settingstandards and then ignoring them. It is often betterto set appropriate and affordable standards and tohave a phased approach to improving the stand-ards as and when affordable. In addition, such anapproach permits the country the opportunity todevelop its own standards and gives adequate timeto implement a suitable regulatory framework andto develop the institutional capacity necessary for

only 60mg/l unfiltered; two 3-day maturation pondswould be needed to get the BOD down to a 30 mg/lunfiltered - equal to an increase in retention time of120%! So the filtered, unfiltered question has majorcost implications. Those who might worry about theeffect of pond algae in a receiving watercourseshould remember that they will produce oxygen dur-ing the day but, more importantly, they will be quicklyconsumed by the stream biota,... So maturation

enforcement.”(Johnson and Horan: “Institutional Developments,Standards and River Quality, WST, Vol 33, No 3, 1996)

It is also important to apply a law in keep-ing with its original intention. But this isonly possible if the technology is fully un-derstood. It is interesting to know that Eng-land at the end of the last century consid-ered case by case assessment of individualpolluters but abandoned the propositionfearing administrative snarls and an unten-able relaxation of discharge standards. Inthe current scenario when decentralisedwastewater treatment is being taken farmore seriously than in the past, such anapproach may still be advisable. In the caseof pond treatment Duncan Mara gives anexample:

“If filtered BOD was permissible then a one-dayanaerobic pond plus a 4-day facultative pond couldreduce the BOD from 300 to 30 mg/l filtered, but to

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ponds are not always required.”(D. Mara, “Appropriate Response” in WQI May/June1997).

Another obstacle to achieving a better de-gree of treatment are unrealistic and overlyambitious master plans that could neverbe successfully implemented in the giventime and resource frame; whereas moremodest, intermediate solutions would bemore likely to succeed. In all fast growingtowns and cities, municipal boundaries havebeen over run by rapidly expanding urbanagglomeration; a fact, that is perhaps noteven reflected in the master plan. On theother hand, an over extended master plancould be well beyond the financial andlogistical strength of the municipality. A moregeneral solution, that includes appropriatedecentralised treatment systems could im-prove the environment considerably, al-though this measure would have to bearthe label of being only “temporary” („tem-porary“ like slums, which by practice be-came „permanent“).

3.6.3 Legal Aspects of Some Sectors ofDEWATS Application

3.6.3.1 Human Settlements

Administrative support to disseminateDEWATS must distinguish between low-in-come areas, middle class housing coloniesand high income “enclaves”. A pragmatic

29

approach would be to pass a “temporary”by-law (or short-term master plan) for aspecific area, which reflects both, the eco-nomic as well as the environmental situa-tion. Making the ultimate degree of treat-ment the yardstick does not work. A morerealistic benchmark ought to take into ac-count the capacity of the administration -financial and logistical - to enforce the fea-sible standards of those “temporary” by-laws. Since local conditions vary from siteto site, the prescription of absolute meas-ures would be self-defeating were adminis-trative guidelines would be more befitting.Such guidelines should take into account:

o Items which can immediately be consid-ered, e.g. treatment systems which canbe implemented by the individual pol-luter or the respective group of pollut-ers at the time of constructing the build-ings.

o Items which will remain valid into thefuture, and will not conflict with futuremaster-plans.

o True temporary items which may have ashorter life time or lower performancethan “everlasting” structures.

The key purpose behind temporary by-lawswould be to prescribe, rather dictate a setof DEWATS measures, than to set stand-ards of discharge quality which have to becontrolled. Durable, anaerobic rough treat-ment systems such as baffled septic tanksor anaerobic filters may be most suitablein such cases. Smaller sewage diameterscould later be used if these pre-treatmentsystems are kept permanently in operation.A centralised maintenance service or con-trol over a decentralised service would beneeded to guarantee that the system works.

In case of high-income enclaves, individualpost treatment with planted gravel filterswould be appropriate when receiving wa-ters are not located too far away. Other-wise, post-treatment is preferably done insemi-centralised units, e.g. ponds, whichmight be cheaper to construct and operate.Sewage lines cannot be avoided in that case.

3.6.3.2 Hospitals, Schools, Compounds,Army Camps, Hotels, etc.

An institution could sometimes be the onlysubstantial polluter in an otherwise cleanand healthy rural environment. In such acase, permanent on-site wastewater treat-ment is the only solution.

The best approach to achieving the highestdegree of environmental protection wouldbe to let realistic assessment of possibletreatment methods guide administrativecontrol. The potential of the most appropri-ate DEWATS should be the basis for settingdischarge standards and for dictating com-pulsory treatment units. Durability and per-manence should rule over the tendency toset the highest theoretical standards of treat-ment performance.

In the case of new installations, only op-tions which fulfil DEWATS criteria stand achance of providing permanent and viableservice. Systems using artificial oxidationtechnologies should not be permitted, sincethe system can be switched off without nega-tive impact on the polluter himself. Thepollution control authorities have to pro-pose DEWATS if the polluter or his planningarchitect is not familiar with that option. Itshould not be difficult to enforce the nec-essary by-laws, since DEWATS is probablythe most economic alternative.

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3.6.3.3 Industrial Estates

Industrial estates are a conglomeration ofenterprises that produce wastewater ofvarying volume and strength. A commontreatment plant for the whole estate maybe the best solution. However, it may bedifficult to convince all the entrepreneursto pay towards a co-operative treatmentsystem, particularly if only few membersdischarge substantial amounts of waste-water.

A co-operative treatment plant may be the right solution, but

people forced to co-operate may not think so

In a decision that favours individual treat-ment systems, the application of generaldischarge standards may turn out to beunjust to certain enterprises, or may evendiscourage certain industries from settlingin the estate. In a scenario where the waste-water output by the majority of the enter-prises is low and only a few industries wouldmerit environmental control, strict dischargestandards enforcement for only those fewmay not lead to an acceptable wastewaterquality at the exit of the estate, since thelarge number of small polluters may havegreater impact than one or two severe pol-luters. A more moderate application of thelaw could be more productive because ofits inherent feasibility. However, it is thedecision of the administration to allow suchexceptions. If discharge standards are strictlyenforced, larger cash rich enterprises mayopt for conventional (non-DEWATS) solu-tions, which are quite likely to succeed ifcontrol and operational management willbe maintained. Fund starved, small-scale

units on the other hand are bound to cheaton pollution control whenever possible dueto financial constrains.

Small-scale industries that have substantialwastewater production need land for finaltreatment systems, such as ponds or con-structed wetlands which depend on naturaloxygen supply via surface area. The requiredland ought to be provided to the individualenterprise or to the estate as a whole at alower rate. New industrial estates are sel-dom connected to sewer lines immediately.Thus, provision of land for such post-treat-ment systems would need to be made rightat the planning stages. If the pollution con-trol officer could agree to apply the legaldischarge standard at the outlet of the totalestate, instead at the boundary of the indi-vidual enterprise, existing estates could useopen drains as natural oxidation ditches forpost treatment.

3.7 Dissemination Strategy

It would be overbearing, or at least prema-ture to pretend to know the disseminationstrategy for DEWATS without having in minda particular situation. Dissemination strate-gies for decentralised technologies must bebased on local facts and actors, explicitly,however under consideration of general fac-tors, such as:

o the requirements of the technology itself(first of all!)

o the legal framework, and

o the conditions for funding of the neces-sary infrastructure, including a likely su-perstructure.

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3.7.1 Components of Dissemination

3.7.1.1 Information

The terminology “awareness building” doesnot appear to do justice to the subject. Whilethere may be a need for awareness build-ing with the administration in some places,the existence of rules and regulations areindicative of awareness in most countries.After becoming aware of their problems,clients want information on solutions. Im-plicit in knowing the technology is knowl-edge of the limitations and conditions un-der which the potential of the technologycan be put to use. Beside direct customers,administrators, and potential implementers(contractors and engineers, etc.), the gen-eral public also needs to be informed.

Low maintenance does not mean NO-maintenance.

3.7.1.2 Regulation

Regulation of discharge standards play animportant role to create the need for treat-ment and to choose the appropriate tech-nology for it. Regulations should be flexibleenough to allow appropriate alternatives with-out putting at risk the environmental needs.

3.7.1.3 Financing

Wastewater treatment is a cost factor andrepresents a substantial investment tomost polluters. While financial incentivesmay accelerate the introduction of newtechnology, their prescription as a gen-eral instrument for implementation is illadvised. Nevertheless, access to soft bankloans is essential, particularly to small-scale enterprises.

3.7.1.4 Implementation

Characterised by a small building volumeas base for calculating engineering fees,DEWATS understandably does not attractengineers. Engineering companies wouldneed to be seriously persuaded to designDEWATS instead of “conventional” treatmentsystems of which components can be boughtready made. It may be worth subsidisingengineering fees as a promotion instrumentuntil at least that time when DEWATS be-comes popular. These incentives are socialinvestments which ought to be calculatedagainst the future gains of a pollution freeenvironment. In the same vein, it may benecessary to pay engineers to train con-tractors in DEWATS construction. One pre-condition for that is that expertise andknowledge about DEWATS is available withthe free-lanced engineers.

3.7.1.5 Operation

Treatment plants that work without a mini-mum of maintenance and supervision arenon-existent. It seems difficult to keep theknowledge about maintenance for sure withthe polluter until maintenance becomesnecessary for the first time. User trainingand professional maintenance would haveto be guaranteed for some years at least,through a contract between the customerand the supplier.

3.7.1.6 Control

Control is the flip side of regulation. If animproved control system cannot be part ofthe dissemination strategy for financial orother reasons, then control must insist onthe implementation of reliable technologyoptions, such as DEWATS.

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3.7.1.7 Reuse of Resources

Treating of wastewater requires a differentexpertise than for the re-use of wastewaterand sludge in agriculture or fishery. Simi-larly the utilisation of biogas also requiresspecial expertise. If the re-use of by-prod-ucts is to be part of the dissemination strat-egy, the engagement of other appropriateagencies or individual experts to attend tothis purpose may become expedient.

3.7.2 The Motors of Dissemination

3.7.2.1 The Government

As governments are primarily responsiblefor environmental protection, by the sameconvention, the responsibility for the dis-semination of DEWATS should essentiallyalso vest with the government. The govern-ment’s major guiding instrument is reliablecontrol of reasonable discharge standards.Nonetheless, a suitable legal framework mayalso include tax exemption, the provisionof subsidies - direct or indirect- and suretyfor bank loans. Direct subsidies, which areperceived to distort economic competition,are no longer in vogue. Consequently it maybe necessary to structure indirect financialsupport to activities such as awarenessbuilding, research, training, or infrastructureto non-governmental organisations (NGOs),private enterprise and professional associa-tions to enhance the scope of dissemina-tion. Project ideas would principally haveto come from these agents.

3.7.2.2 Non-Governmental Organisations

Characteristically, NGO’s play a variety ofroles foremost of which is their innate sup-port to weaker groups of society to fight

private or governmental abuse of their rightsas citizens. Today a large number of NGO’sare committed to environmental justice. Itis not uncommon to find NGO’s protestingagainst the violation of pollution standardsand succeeding in getting the governmentto bring the law to bear against carelessoffenders or to even get the government tochange archaic rules and regulations.

The role of the NGO may be extended toimplementation and technical training aslong as the normal “market” forces consist-ing of engineers and contractors have notbecome involved adequately. However, it isimportant to realise that wastewater treat-ment is not a matter of propaganda but amatter of applied natural science. No NGOcan become an implementing agency with-out permanently involving persons of suffi-cient scientific and technical knowledge.

Wastewater treatment does not happen through propaganda, but through applied natural science.

NGOs start traditionally from acute singlecases and then, with growing experienceand knowledge move towards a more gen-eral approach. NGOs have the inherent po-tential to effectively disseminate DEWATSprovided powerful persons and institutionssupport them in overcoming administrativebottlenecks and in accessing the funds thatare needed for their activities.

3.7.2.3 Development Projects

At the one hand, development projects mayeither create a model reality, and try tomanage it well for demonstration purposes,

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or at the other hand, execute and supportmeasures which directly influence the pre-vailing reality outside and beyond theproject.

For demonstrating a general idea it mightbe effective to create a limited and control-lable model reality wherein everything workswell. However, it would be dangerous tobelieve that the model is a reflection ofreality, because of the expenses involvedthis proposed reality can never come true.

Rural biogas dissemination and other de-velopment programmes have proved thatsuch well-designed projects are not exam-ples of what is really possible. The lavishorganisational structures that such pro-grammes demand are counter productive.The pressure to present a perfect projectsolution has proved not to bring forth asustainable solution for the time after theproject is over.

Decentralised wastewater treatment is a verycomplex subject that is greatly influencedby socio-economic and political circum-stances. A development project is at best amodest contribution towards eliminatingbottlenecks in the overall complex realityof a specific development sector.

Foreign aided development projects that arepartnered by government agencies or localNGO’s may be a suitable instrument to over-come financial and structural shortcomings.The character of the local partner organisa-tion and its need for support determine thenature and character of any aid from out-side.

3.7.3 Approach to Dissemination

3.7.3.1 Individual Implementation

Dissemination of DEWATS means first of all,the construction of as many plants as pos-sible. Any private engineer who designs andinitiates the construction of DEWATS for hiscustomers, provided his plants are well de-signed, well constructed, well operated andwell maintained is the perfect disseminator.However, his efficiency depends on the abil-ity to acquire new customers through thepropaganda about his good service.

Buildings and other structures, includingestablished treatment systems such as sep-tic tanks are conventionally implementedby individual contractors. Designs for sep-tic tanks of different sizes are easily avail-able and their construction does not requiremore than the usual building practice. Build-ing contractors install septic tanks as ef-fortlessly as they put up buildings. Anaero-bic filters or constructed wetlands are notthat easy to disseminate. Rigid standardisa-tion tends to jeopardise the economy andthe design principles of these systems. Con-sequently a deeper understanding and ahigher degree of expertise is required atthe level of direct implementation. This couldbe achieved through training or qualifiedsupervision during construction of these“unusual“ structures. However, both optionswould require a superstructure to executeand finance these services. Baffled septictanks and pond systems could be dissemi-nated with less specialised expertise.

3.7.3.2 Sector-wise Dissemination

Individual implementation would be the bestapproach to dissemination provided stand-ardised designs were to be available, at least

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Some selected domestic wastewater data

examples COD BOD5

COD /BOD 5

SS Flow

g/cap.*d g/cap.*d g/cap*d l/cap*dIndia urban 76 40 1,90 230 180USA urban 180 80 2,25 90 265China pub.toilet 760 330 2,30 60 230Germany urban 100 60 1,67 75 200France rural 78 33 2,36 28 150France urban 90 55 1,64 60 250

BORDA

for those cases that are most common in agiven situation. Any dissemination strategy,which relies on relatively low professionalstandards for implementation, should bestfollow a sector-wise approach. For exam-ple, there could be standardised treatmentsystems for housing colonies of one city,for rural hospitals in a hilly area or for ho-tels and holiday resorts at a scenic lake.Latex sheet processing plants at small rub-ber farms could as well be standardised, socould plants for wastewater from rice millsor canning factories.

Tab. 2.Average data of domestic wastewater at variousplaces

tise. The engineer or contractor is likely towaste a lot of time in visiting such placesand in finding technical solutions in orderto build up the reputation of his enterpriseor simply for want of other customers.

A wastewater treatment plant is NOT just another pair of shoes

3.7.3.3 Marketing

Ultimately technology finds its justificationin the “market“. If DEWATS cannot find amarket it will be out of business. So, be-yond doubt DEWATS must marketed. How-

ever, the marketing conceptof DEWATS has to be basedon the specifics of the tech-nology; it is not comparablefor instance with the market-ing of a brand of ready-madeconsumer goods.

For marketing sports shoessuch as “Nike” or “Adidas” for

instance, one needs the product and a lotof ballyhoo to make the name known tothe target group. The brand name carriesan image that fetches prices far beyond thetotal cost. The shoes are easily transport-able to any spot on planet earth and whenthe shoes are sold, the business is over.

It may not be that easy with other prod-ucts. When “ Toyota” conquered the Euro-pean market, or “Volkswagen” entered theUS-market for selling their cars, the firstthing they did was to set up a network ofservice stations. They knew that no onewould buy a car without having access toprofessional service facilities. Only after theservice network was installed, was the proc-

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Before the propagation of standard designsfor a whole sector, it is essential to buildand operate some plants in order to obtainreliable data for the calculation of dimen-sions and to gain overall experience. Thequestion of who will finance and executethose trials necessitates the installation ofa superstructure for planning and other su-pervisory purposes.

However, the pure sector-wise approach islikely to fail under the sheer weight of de-mand on an expertise, which is still rare.Potential customers, who have other thanthe standard problem, are likely to approachanyone constructing such standardisedplants regardless of sector specific exper-

35

ess of selling started. The customers knowthat if service was neglected the result couldbe unreliable performance and/or low re-sale value. As with sport shoes the price ofcars depends not only on the cost of pro-duction and on expectations of economicbenefit but ultimately it is the image thatthe car lends to its owner that counts.

DEWATS are neither shoes nor cars. DEWATSare different. It does not belong to thesecategories. DEWATS are costly, space con-suming, demand attendance and may evenbe stinking structures, which cannot be pro-duced under controlled factory conditions.Their design is often to be adapted locallyand constructed on-site by craftsmen ofuncertain qualification. The customer doesnot love the plant, as he would love hiscar or even his shoes (only wastewater en-gineers love wastewater plants). All the cus-tomer wants is a service, but he wants notto be bothered with the inner workings ofthe plant. Understandably the marketing ofDEWATS will have to be different.

DEWATS is not likely to have the back-ing of a financially strong company be-hind the product that could invest in pre-paring the market prior to the release ofthe product. The marketing strategy forDEWATS will have to depend first of allon the immediate availability of the prod-uct. To sell DEWATS one needs engineersand contractors being present at remotesites which may be several hours awayfrom their office. Howsoever, advertis-ing, which introduces the product to thepublic could start well before the prod-uct is available. The subject of such anadvertising campaign remains a ques-tion, when nothing is truly ready to besold and the price of the product is notknown, yet.

Marketing experts know that any mar-keting strategy has to start from the tech-nical and economic potential of the com-pany that is selling; and that a market-ing strategy is reaching its target whencustomers actually put money on the

Average qualities of industrial wastewaters

production COD BOD5settleable

solidsCOD / BOD5

other indicators

mg/l mg/l mg/lleather 860 290 1.168 3,0 pH 8,8glues from skin 6.000 3.100 25 1,9glues from leather 1.600 340 75 4,7fish meal 6.100 1.560 20 3,9 pH 8,2paper 820 410 2,0 Cl 6400 mg/lpharmaceutica 1.920 1.000 1,9 pH 6,5 - 9starch - maize 17.600 11.540 25 1,5 N 800 mg/l -dito- (water recycl.) 2.920 1.700 1,7 N 25 mg/lpectine 13.800 5.800 2,4 pH 2; N 700 mg/lvegetable oil 600 350 1 1,7 pH 5-9potato chips 1.730 1.270 820 1,4canned juice 550 800 20 0,7 pH 4,6 - 11,4tinned fish 1.970 1.390 60 1,4 Cl 3020 mg/lbeer 1.420 880 1,6yiest 15.000 10.250 5 1,5 pH 4,8 - 6,5

ATV

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Tab. 3.Average data of industrial wastewater

36

desk for buying the product. Successfulmarketing strategies are premised on buy-ing power and supply capacity, which haveto be known in the local context. The bestsalesmen usually cannot sell a product thatis not available, to a customer that doesnot have the money to buy it.

A product that is not available to be shownto customers is by no means easy to sell.By this logic, the existence of well-function-ing wastewater treatment demonstrationplants are a vital pre-condition for success-ful marketing.

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4.1 Economy of Wastewater Treatment

Wastewater, as the name suggests is awaste, which is left over after using waterfor a specific purpose. Treating wastewaterto get back its original quality is an addi-tional process, which has its price. If waste-water treatment would be profitable in it-self, the perpetuum mobile would havebeen invented. Instead, wastewater treat-ment is by scientific principle a cost fac-tor.

The cost of treatment depends on the de-gree and the kind of water pollution as wellas the degree of purification to be achieved.Treatment costs can be reduced by reduc-ing the pollution, by choosing an appropri-ate degree of treatment, and in some casesby reusing water and sludge and/or by uti-lising the biogas. The recovery of other valu-able raw material for reuse within the in-dustrial production process does rarely takeplace in the case of DEWATS.

The objective need (and legal demand) forwastewater treatment today is a result ofenvironmental pollution that has taken placein the past. The extent to which wastewatertreatment can be justified in economic termswill depend on the parameters that are in-cluded in the economic calculation. Onerather unfamiliar parameter is the valuationof environmental protection. How shouldenvironmental protection be valued remainsa question, however, the issue of whetherwater should at all be treated is passé. Factis that there are laws and by-laws, whichdemand a certain effluent quality of anywastewater discharged into the environment.

This to the polluter means that water mustbe treated at any cost, or not discharged atall. And this is the starting point for eco-nomic considerations.

The first economic question is "Why", only the second question is

"How much"

In the first instance, the economy of waste-water treatment demands the reduction ofunavoidable expense. Economising meas-ures primarily recognise the prevailing eco-nomic environment and adapt the treatmentsystem accordingly; or if possible, createan economic environment that suits andsupports the preferred treatment system.

Treatment cost for a given wastewater areinfluenced by:

o the legal discharge standard,

o the chosen treatment system, and

o the degree of reusing water, sludge andenergy (biogas)

The treatment system to be chosen andwhether the reuse of by-products is advis-able will depend on local prices for build-ing materials and service manpower. Thequestion of reasonable discharge standardshas been dealt with in chapter 3, already. Itis not discussed here.

4.2 Treatment Alternatives

The question at a policy or developmentplanning level relates to the extent to whichwater should be treated and its discharge

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centralised, and whether on-site treatmentand individual discharge into the environ-ment should be available as an option atall. The decision is influenced by environ-mental, social, technical and economic pa-rameters within which the place of finaldischarge is likely to play a decisive role.Alaerts et al suggests a population densityof above 200 to 600 capita/ha for central-ised treatment of domestic wastewater.However, such a general rule of thumb can-not be taken as valid for conglomerationsincluding industries; it demands cautiousapplication.

The following considerations in balancingcentralised against decentralised treatmentsystems are recommended:

o There are clear cut economies of scalefor treatment plants so long as the levelof technology is not changed.

o DEWATS in principle - but not always -are cheaper because they are of lowertechnological standard than conventionaltreatment plants.

o The unavoidable costs of sewer lines incase of centralised systems may threatenthe economies of scale; sewerage maycost up to five times more than the cen-tral sewage plant itself.

o The cost of treatment increases propor-tionate to the degree of treatment.

o Management costs are in principle - butnot necessarily - in direct relation to plantsize or to the number of plants.

There are several organisational alternativesfor treatment and discharge of wastewater.These are:

o Controlled discharge without treatment(ground percolation, surface water dilution).

o Treatment in a centralised plant that isconnected to a combined or separatesewer system.

o Treatment in several medium sized treat-ment plants that are connected to a com-bined or separate sewer system.

o Primary and secondary treatment in de-centralised plants that are connected toa sewer line, that leads to a commonplant for final treatment.

o Completely decentralised treatment withdirect and final discharge, or connectionto communal sewerage.

This book deals with DEWATS as an optionand tries to describe its specificities. It wouldbe beyond the scope of this handbook todeal with the general subject of sanitationconcepts suitable for communities. Enoughwork of excellence has been done and pub-lished by specialist groups in different coun-tries. For example:

Alaerts, G.J., Veenstra, S., Bentvelsen, M.,van Duijl, L.A. at al.:” Feasibility of anaero-bic Sewage Treatment in Sanitation Strate-gies in Developing Countries” IHE ReportSeries 20, International Institute for Hydraulicand Environmental Engineering, Delft 1990.

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Sasse, Otterpohl

Cost of wastewater treatment in relation to plant size (daily flow of wastewater)

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0 80,0wwflow m³/d

ECU/m³

Fig. 4.Cost of treatment per m3 daily flow. Larger plantsrequire a more sophisticated technology which mayincrease treatment costs.

39

Once the decentralised option has beenchosen, it is the individual polluter whoshould decide on the treatment system hethinks is most suitable to his circumstance.However, realistic alternatives based on trueeconomic considerations are rare. The Dan-ish Academy of Technical Sciences writes inits evaluating report 1984:

“...It has been shown that, under certain local cir-cumstances, large variations in economy are to beexpected, but the general conclusion (...) is that theeconomy of the various treatment processes doesnot differ that much. In many cases the costs areapproximately the same. This increases the impor-tance of those factors which cannot be included inan economic survey. Some of these factors are lim-iting factors in the sense that they limit the “free”selection between the various methods. If large ar-eas of land are not available, then oxidation pondsmust be disregarded even if it is the most economi-cally favourable solution. If electricity supply is un-reliable, then activated sludge systems cannot beconsidered. (...) It can be argued that the factorsmentioned above are of purely economical nature,e.g. a reliable electricity supply is merely (!) a mat-ter of economy. However, the costs involved inchanging these factors to non-limiting factors are

be very expensive for instance, to keep aqualified engineer at a remote location foroperating a conventional, albeit small treat-ment plant.

Field experiences indicate that there are fac-tors other than economic, which induce thepolluter to opt for a treatment plant. In mostcases the pressure to comply with dischargestandards that meet the law compel pollut-ers into the decision. However, there couldbe other reasons as well. For example, anentrepreneur would like a treatment plantto present a “clean” factory to his foreignpartners, a housing complex that has nofuture unless wastewater is reused for irri-gation, or a doctor in charge of a teachinghospital who wants to treat the hospitalwastewater to safeguard his reputation. Inall these cases the immediate motivationto go in for wastewater treatment is otherthan economic (notwithstanding the fact thatsome economists consider „everything“ aneconomic question).

After deciding in favour of a treatment plant,the polluter would want to compare differ-ent systems of the same size under similarconditions. He would want to consider thespace he has to spare for the treatmentsystem, the level of maintenance he is will-ing to shoulder, and ultimately if he shouldrather re-organise his water consumption inorder to reduce the pollution load or quan-tity of discharge. The geography of the neigh-bourhood and the prevailing wastewaterdischarge standards and regulations woulddetermine the range of alternatives avail-able to an individual polluter. Similarly theprevailing socio-political framework is alsolikely to have a strong bearing on the deci-sion of the polluter, e.g., are the fees orpenalties imposed based on pollution loads,or are they independent from it.

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so high that there is no point in including such con-siderations here.”(The Danish Academy of Technical Sciences: “Indus-trial Wastewater Treatment in Developing Countries”,1984)

The above citation supports the view thatcomparison of several alternatives is notpossible on a general level, and in mostcases true alternatives for the customerdo not exist. There are many cases, wherea DEWATS concept is the only solution thatpromises some degree of continuous treat-ment. Most often, it is not worth compar-ing DEWATS with non-DEWATS solutions ifobviously the cost and management effortof keeping a conventional system perma-nently in operation is formidable. It could

40

Finally, the ultimate destination of the ef-fluent will also influence the choice of treat-ment system. For instance, the necessity ofnitrogen or phosphorous removal may begreater when the receiving water is an iso-lated lake of particular ecological impor-tance, in which case other treatment op-tions with other influencing factors maybecome valid.

First think of ponds, then of tanks, at last of filters

The following chapter describes the param-eters that influence economic calculation.Those parameters help in deciding on themost suitable system for standardisation fora particular group of polluters in a definedlocal situation. For relatively small plants,however, it is not very likely that the sup-plier would offer comparative economic cal-culations for different treatment systems toa potential customer. If such were to be thecase, the cost of planning would be un-affordable, as several plants would have tobe designed on the table purely for thepurpose of economic comparison. More prac-tically, the potential supplier is more likelyto discuss various alternatives with the po-tential customer (without going into detailedeconomic calculations) based on which thecustomer would then choose the most ap-propriate and convenient solution.

4.3 Parameters for Economic Calculation

4.3.1 Methods of Comparison

Wastewater treatment as a rule does notproduce profit. Resultantly methods of eco-nomic analysis such as cost-benefit or breakeven point, to which profit calculations areimportant, do not fit the economy of waste-

water treatment. On the other hand, theannual cost method, which includes depre-ciation on capital investment and opera-tional costs, appears to be more apt as aneconomic indicator. With this method it iseasy for the polluter to include expensessuch as discharge fees, or income from re-use of by-products on an annual base, toget a comprehensive picture of the eco-nomic implications.

The annual cost method could also be usedfor estimating social costs and benefits. Theeconomic impact of treatment on the envi-ronment and on public health is relatedprimarily to the context in which a treat-ment plant operates. For example, if prop-erly treated wastewater is discharged into ariver that is already highly polluted the yieldfrom fishing will surely not improve. On thecontrary, if all the inflows into the receivingwater were to be treated to the extent thatthe self-purifying effect of the river wouldallow the fish to grow, this would have con-siderable economic impact. This economicimpact of a cleaner river is crucially de-pendent on the total number of treatmentplants installed along the river, and not onlyon the efficiency of one single plant.

A spreadsheet for computerised calculationsis presented in chapter 13.2.

4.3.2 Investment cost

4.3.2.1 Cost of land

For economic calculation the value of landremains the same over years and thus, landhas unlimited lifetime. However, the priceof land is never stable. It usually goes up intimes of growth and may go down in timesof political turbulence. In reality, the actualavailability of land is far more important

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than the price; new land will rarely be boughtonly for the purpose of a treatment plant.The density of population usually determinesthe price of land. Land is likely to cost morein areas with a high population density andvice versa. The choice of treatment systemis severely influenced by these facts.

In reality, the cost of land may or may notbe essential to the comparison betweendifferent treatment systems. Wide differencesin the cost of land notwithstanding, it maycontribute in the range of 80% of the totalcost of construction. It follows that at leastin theory, the choice of sand filters and ofponds will be more affected by the price ofland than compact anaerobic digesters. Inany case, it is most likely that where landprices are high compact tanks - not pondsand filters - will be the natural choice. Alaertset al. assume that ponds are the cheapestalternative when the cost of land is in therange of less than 15 US$/m2 in case ofpost treatment and 3 - 8 US$/m2 in case offull treatment. Such figures nonetheless havealways to be checked locally.

4.3.2.2 Construction cost

Annual costs are influenced by the lifetimeof the hardware. It may be assumed thatbuilding and ground structures have a life-time of 20 years; while filter media, somepipelines, manhole covers, etc. are onlylikely to last for 10 years. Other equipmentsuch as valves, gas pipes, etc., may staydurable for 6 years. Practically it suffices torelate any structural element to any one ofthese three categories.

It is assumed that full planning costs willreoccur at the end of the lifetime of themain structure, i.e. in about 20 years. Inany individual case, the costs of planning

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can be estimated. For dissemination pro-grammes, it may be assumed that planningwill be carried out by a local engineeringteam of sound experience to whom thedesign and implementation of DEWATS is aroutine matter. However, this might not beso in reality. At the contrary, of all costs,engineering costs are likely to be the mostexorbitant and to remain so until such timeas the level of local engineering capacityimproves. An estimation of planning work-days for senior and junior staff forms thebasis of calculation to which 100% may beadded towards acquisition and general of-fice overheads. Transport of personnel forbuilding supervision and sample taking -and laboratory cost for initial testing ofunknown wastewater’s must also be in-cluded.

4.3.3 Running Costs

Running expenses include the cost of per-sonnel for operation, maintenance and man-agement, including monitoring. Cost maybe based on the time taken by qualifiedstaff (inclusive of staff trained on the job)to attend to the plant. The time for plantoperation is normally assessed on a weeklybasis. In reality, the time estimated for in-spection and attendance would hardly callfor additional payment to those staff whoare permanently employed. The case wouldbe different for service personal that is spe-cially hired. Facilities that are shared, as inthe case of 5 to 10 households joining theirsewers to one DEWATS, are likely to be10% cheaper than individual plants. How-ever, operational reliability of such a facil-ity cannot be guaranteed if someone is notspecially assigned to the task of mainte-nance.

42

Cost for regular attendance could be higherfor open systems such as ponds or con-structed wetlands due to the occasionaldamage or disturbance by animals, stormyweather or falling leaves. The cost of regu-lar de-sludging will be higher for tanks withhigh pollution loads, than for ponds whichreceive only pre-treated wastewater. The costof cleaning the filter material is not consid-ered to be running cost as these costs aretaken care off by the reduced lifetime ofthe particular structure. So also the cost ofenergy and chemicals that are added per-manently are not included, as such costsare not typical of DEWATS.

4.3.4 Income from Wastewater Treat-ment

The calculation of income from by-productsor activities related to wastewater treatmentcalls for careful selection of the right eco-nomic parameters. Biogas could be assumedto have economic value as it is seen tosubstitute other fuels, whereas in reality itmay be only an additional source of energyof nil utility and consequently zero economicvalue. Just as the use of water and sludgefor agriculture may require the establish-ment of additional infrastructure and staffto manage its utilisation. As is apparent,economic calculations if not reflective of allthese costs and future implications couldbecome redundant.

Biogas production should be taken into thecalculation only if the biogas is likely to beused. The biogas production available touse may be to the extent of 200 l per kgCOD

removed. The actual gas production is 350 l

methane (500 l biogas) per kg BODtotal

, how-ever a part of the biogas would be dis-

solved in water; the portion increases withdecreasing wastewater strength. Biogas con-tains 60% to 70% methane. 1 m3 methaneis equivalent to approximately 0,85 litre ofkerosene.

The moot question is really not if the useof biogas will improve the profitability ofthe wastewater treatment plant but whetherthe additional investment to facilitate theuse of biogas is economically justified. Ifbiogas were to be used, the storage of thegas would demand additional volume anda gas-tight structure. The gas would thenneed to be transported to the place of con-sumption, requiring pipes and valves. Theproper utilisation of the gas and mainte-nance of the gas supply system will entailadditional management and thereby addi-tional costs. The additional investment tofacilitate the use of biogas is likely to addupto 5% of the cost of long lasting structures

Sasse, Otterphol

Profit on additional investment to facilitate use of biogas in relation to wastewater strength

-15

-10

-5

0

5

10

15

20

25

30

- 500 1.000 1.500 2.000 2.500 3.000 3.500

COD mg/l

4 ECONOMICS

Fig. 5.Cost-benefit relation of biogas utilisation. It is noteconomical to use biogas from low strengthwastewater.

(20 years lifetime), 30% of the cost of in-ternal structures (10 years lifetime) and100% of the cost of equipment (6 yearslifetime). The cost of the additional capitalfor such investments is not to be forgotten.

43

Furthermore the cost of operational attend-ance is likely to be 50% more if the biogaswas to be used. If agriculture and fish-farm-ing were to be attached to the wastewatertreatment plant the economic implicationsare much more complex and therefore muchmore difficult to assess a priori. The sizeand organisation of the farm together withthe marketing of the crops would be impor-tant parameters to consider.

4.3.5 Capital Costs

If the investment capital were to be bor-rowed from the bank on interest, such in-vestment would attract direct capital costs.On the contrary, when one’s own money isinvested which if used otherwise could beprofitable (purchase of raw material for pro-duction, investment in shares or bank de-

4 ECONOMICS

posits, etc.) the cost of this capital is indi-rect. The risk of the investment is anotherfactor that may have to be taken into ac-count and that can make the calculation ofthe cost of capital extremely complex.

In case of wastewater treatment plants sinceother profits are in any case not expected,the investment risk is limited to the techni-cal risk of the reliability of performance.However, if profits from wastewater relatedagriculture is expected, the investment riskcould become expensive. Capital costs areto a certain extent speculative by nature.Nevertheless, the fact that capital costsmoney remains.

For strategic calculations one may considerannual capital costs of 8% to 15 % of theinvestment; exclusive of inflation as infla-tion affects both the creditor and the debtorin equal measure.

44

5.1 Definition

The term ‘treatment’ means separation ofsolids and stabilisation of pollutants. In turn,stabilisation means the degradation of or-ganic matter until the point at which chemi-cal or biological reactions stop. Treatmentcan also mean the removal of toxic or oth-erwise dangerous substances (for exampleheavy metals or phosphorus) which are likelyto distort sustainable biological cycles, evenafter stabilisation of the organic matter. Pol-ishing is the last step of treatment. It is theremoval of stabilised or otherwise inactivesuspended substances in order to clarify thewater physically (for example reducing tur-bidity).

Treatment systems are more stable if eachtreatment step removes only the „easy part“of the pollution load, but sends the lefto-vers to the next step.

5.2 Basics of Biological Treatment

The stabilising part of treatment happensthrough degradation of organic substancesvia chemical processes, which are biologi-cally steered (bio-chemical processes). Thissteering process is the result of the bacte-

rial metabolism in which complex and high-energy molecules are transformed into sim-pler, low-energy molecules. Metabolism isnothing but the transformation from feedto faeces in order to gain energy for life, inthis case for the life of bacteria. That me-tabolism happens when there is a net en-ergy-gain for “driving” the bacteria. It isimportant for the bacteria that energy canbe stored and released in small doses whenneeded (adenosine triphosphate - ATP - isthe universal medium to store and trans-port energy). A few chemical reactions hap-pen without the help of bacteria.

In the main, wastewater treatment is a mat-ter of degradation of organic compounds,and finally a matter of oxidising carbon (C)to carbon dioxide (CO2), nitrogen (N) to ni-trate (NO3), phosphorus (P) to phosphate(PO4) and sulphur (S) to sulphate (SO4).Hydrogen (H) is also oxidised to water (H2O).In anaerobic processes some of the sulphuris formed into hydrogen sulphide (H2S)which is recognisable by its typical “rotteneggs“ smell. The largest amount of oxygen(O2) is required for burning carbon (“wetcombustion”).

The process of oxidation happens aerobi-cally, with free dissolved oxygen (DO) present

Sedimentation Anaerobic digestion Aerobic decomposition Post-sedimentation

removal of easily settleable solids

removal of easily degradeable organic solids

removal of more difficult degradable solids

removal of digested solids and active bacteria mass

Fig. 6. Several steps are required for full treatment.

5 PROCESS OFWASTEWATER TREATMENT

45

in water, or anaerobically without oxygenfrom outside the degrading molecules. An-oxic oxidation takes place when oxygen istaken from other organic substances. Facul-tative processes include aerobic, anoxic andanaerobic conditions, which prevail at thesame time at various parts of the samevessel or at the same place after each other.In anoxic respiration and anaerobic fermen-tation as there is no oxygen available, alloxygen must come from substances withinthe substrate. Anaerobic treatment is neveras complete as aerobic treatment, becausethere is not enough oxygen available withinthe substrate itself.

The chemical reactions under aerobic, an-oxic and anaerobic conditions are illustratedby the decomposition of glucose:

Decomposition via aerobic respirationC6H12O6 + 6O2 Ý CO2 + 6H2O

Decomposition via anoxic respirationC6H12O6 + 4NO3 Ý 6CO2 + 6H2O + 2N2

Decomposition via anaerobic fermentationC6H12O6 Ý 3CH4 + 3CO2

Bacteria need nutrients to grow. Any livingcell consists of C, H, O, N, P and S. Conse-quently, any biological degradation demandsN, P and S beside C, H and O. Trace ele-ments are also needed to form specific en-zymes. Enzymes are specialised molecules,which act as a kind of “key” to “open-up”complex molecules for further degradation.

Carbohydrates and fats (lipids) are composedof C, O and H and cannot be fermented inpure form (Lipids are “ester” of alcohol andfatty acids; an ester is a composition thatoccurs when water separates off ). Proteinsare composed of several amino acids. Each

Principle of the anaerobic process

organic matter + watercarbohydrate proteine lipids

hydrolising bacteria

fatty acids

acetogenic bacteria

acetate hydrogen carbohydrate

matanogenic bacteria

methane + carbohydrate

methane + water

mineral sludge

Karstens / Berthe-Corti

Simplified Principle of the Aerobic Process

organic matter + oxygencarbohydrates proteins

oxidation of carbon

enzymes

citric acid cycledehydration /

hydration

respiration of carbon

enzymes

splitting

water + mineral sludge

The aerobic process is very diverse; the above diagram has been almost unacceptably simplified.

However, it shows that carbohydrates and proteins undergo different steps of decomposition. It also shows the importance

of enzymes for breaking up proteins.

Fig. 7.The anaerobic process in principle

Fig. 8.The aerobic process in principle

5 TREATMENT PROCESS

46

amino acid is composed of a COOH-groupand a NH3-group plus P, S, Mg or othernecessary trace elements. Thus, proteinscontain all the necessary elements and con-sequently, can be fermented alone. A fa-vourable proportion between C, N, P and S(varying around a range of 50: 4: 1: 1) is apre-condition for optimum treatment.

5.3 Aerobic - Anaerobic

Aerobic decomposition takes place whendissolved oxygen is present in water.Composting is also an aerobic process.

Anoxic digestion happens when dissolvedoxygen is not available. Bacteria however,get oxygen for “combustion” of energy bybreaking it away from other, mostly organicsubstances present in wastewater, predomi-nantly from nitric oxides.

Anaerobic digestion happens by breakingup molecules which are composed of oxy-gen and carbon to ferment to carbohydrate.

The aerobic process happens much fasterthan anaerobic digestion and for that rea-son dominates always when free oxygen isavailable. The high speed at which decom-position takes place is caused by the shorterreproduction cycles of aerobic bacteria ascompared to anaerobic bacteria. Anaerobicbacteria leave some of the energy unused.It is this unused energy which is released inform of biogas. Aerobic bacteria use a largerportion of the pollution load for productionof their own bacterial mass compared toanaerobic bacteria, which is why aerobicprocesses produce twice as much sludgeas compared to the anaerobic process. Forthe same reason, anaerobic sludge is lessslimy than aerobic sludge and is therefore

easier to drain and to dry.

Aerobic treatment is highly efficient whenthere is enough oxygen available. Compactaerobic treatment tanks need external oxy-gen which must artificially be supplied byblowing or via surface agitation. Such tech-nical input consumes technical energy.

The anaerobic treatment process is slower.It demands a higher digestion temperaturequasi to make good for the unused nutrientenergy. The anaerobic treatment process issupported by higher ambient temperature.Therefore, it plays an important role forDEWATS in tropical and subtropical coun-tries. Ambient temperature between 15° and40°C is sufficient. Anaerobic digestion (fer-mentation) releases biogas (CH4 + CO2)which is usable as a fuel.

5.4 Phase Separation

The term “phase separation” is used for tworather different affairs. On the one hand itis used for the separation of gas, liquid andsolids in anaerobic reactors. On the otherhand it is used to describe the technicalseparation of different stages of the treat-ment process, either in different locationsor in sequences of time intervals. The latterkind of phase separation becomes neces-sary, when suitable nutrients cannot be pro-vided simultaneously to bacteria that havedifferential growth rates and prefer differ-ent feed. Some bacteria multiply slowly whileothers grow rapidly. As all the enzymes re-quired for degradation are not found in allsubstances, the bacteria take time to pro-duce adequate amounts of the missing en-zymes. As was said before, enzymes act asthe “key which opens the lock of the foodbox for bacteria“.

5 TREATMENT PROCESS

47

Substrates for which enzymes are immedi-ately available are „easily degradable“.Whereas, those substrates for which en-zymes have first to be produced by bacte-rial action are „difficult“ degradable. In anenvironment, which hosts substances thatare both easy and difficult to degrade, thebacterial population, which is responsiblefor easy degradation tends to predominateover the others.

To protect the “weaker” (slower) bacteria, itmay be advisable to artificially separate dif-ferent bacterial populations in phases byproviding each with its own favourable en-vironment. Characteristics of wastewatermust be known and the desired treatmentresults must be decided, before the dimen-sions of the vessels for the different phasescan be selected.

Nature follows its laws. Wishes are not laws of nature.

In case of DEWATS, it is often easiest toprovide longer retention times so that the“slow” bacteria find their food after the“quick” bacteria have consumed their re-quirements. This process is easier to man-age than phase separation; in the case ofsmaller plants, it is also cheaper. However,the efficiency of phase separation in thebaffled septic tank justifies its higher costs.It is definitely far more appropriate thanfeeding the plant with sequencing flow rates,the process of which needs steering andcontrol. Phase separation becomes unavoid-able if different phases require either anaero-bic or aerobic conditions.

Pre-composting of plant residues beforeanaerobic digestion is an example of sim-ple phase separation, where lignin is de-

composed aerobically before anaerobic bac-teria can reach the inner parts of the plantmaterial (lignin cannot be digested anaero-bically because of its “closed” molecularstructure).

In case of nitrogen removal, longer reten-tion times alone do not solve the problembecause the nitrifying phase needs an aero-bic environment, while denitrification re-quires an anoxic environment. Anoxic meansthat nitrate (NO3) oxygen is available, butfree oxygen is not. Anaerobic means thatneither free oxygen nor nitrate-oxygen isavailable. Nevertheless, the aerobic phasecan only lead to nitrification if the retentiontime is long enough for the “slow” nitrify-ing bacterium to act, as compared to the“quick” carbon oxidizers.

5.5 Separation of Solids

Wastewater treatment relies on the separa-tion of solids, both before and after stabili-sation. Even dissolved particles are decom-posed into the three main fractions: water,gases and solids of which the solids willhave to be removed, finally. The choice ofmethod of solids removal will depend onthe size and specific weight of pieces andparticles of suspended solids.

5.5.1 Screening

Screening of larger pieces of solids is con-sidered to be the foremost step in any treat-ment plant. In DEWATS, screening is notadvisable, for the reason that screens re-quire cleaning at very short intervals, i.e.daily or weekly, which needs a safe place inthe immediate vicinity for the screeningsthat are removed. A blocked screen is an

5 TREATMENT PROCESS

48

obstacle that blocks the entrance of theplant, however, DEWATS should allow forthe full amount of wastewater to passthrough the plant without blockage or over-flow. If this fails, it may happen - and infact happens quite often - that the opera-tor „organises“ a trouble free by-pass, whichwould pollute the environment as if a treat-ment plant did not exist. For this reason itseems wise to ovoid the screen and alter-nately to provide sufficient additional spaceto accommodate solids of larger size withinthe first sedimentation chamber.

5.5.2 Sedimentation

Separation of solids happens primarily bygravity, predominantly through sedimenta-tion. Coarse and heavy particles settle withina few minutes or hours, while smaller andlighter particles may need days and weeksto finally sink to the bottom. Small parti-cles may cling together, forming larger flocksthat also sink quickly. Such flocculation hap-pens always when there is enough time andlittle to no turbulence. Consequently, stir-ring hinders quick sedimentation. Sedimen-tation is slow in highly viscose substrate.

Sedimentation of sand and other discreteparticles takes place best in vessels with arelatively large surface. These vessels maybe shallow, since depths of more than50 cm are of no influence to the sedimen-

5 TREATMENT PROCESS

Settling speed of coarse particles (m/h)grain size in mm 1 0,5 0,2 0,1 0,05 0,01 0,005

quartz sand 502 258 82 24 6,1 0,3 0,06coal 152 76 26 7,6 1,5 0,08 0,015SS in domestic wastewater 120 60 15 3 0,75 0,03 0,008

K.+K. Imhoff, pg. 126

Tab. 4.Settling speed of coarse particles. Suspended sludge particles have settlingproperties different from coarse particles.

tation process in thecase of discrete parti-cles.

This is different for finercoagulant particles,where sedimentationincreases with basindepth. That is because

settling particles meet suspended particlesto form flocs which continue to grow largerand larger on their way to the bottom. Aslow and non-turbulent flow - still and un-disturbed water - supports “natural” coagu-lation for sedimentation.

Settled particles accumulate at the bottom.In the case of wastewater, any sedimentalso contains organic substances that startto decompose. This happens in any sludgesedimentation basin and to a lesser extentin grit chambers. Decomposition of organicmatter means formation of gases, firstlycarbon dioxide but also methane and oth-ers. These gases are trapped in sludge par-ticles that float to the top when the num-bers of gas molecules increase. This proc-ess not only causes turbulence; it also ru-ins the success of the sedimentation thathas taken place. The Imhoff tank throughits baffles prevents such gas-driven parti-cles from “coming back” to spoil the efflu-ent. The UASB process deliberately playswith the balance of sedimentation (= downstream velocity) and up-flow of sludge par-ticles (= upstream velocity).

After decomposition and release of gases,the stabilised (mineralised) sludge settlespermanently at the bottom where it accu-mulates and occupies tank volume, unnec-essarily. This is why it must be removed atregular intervals. Since several pathogensespecially helminths also settle well, sedi-

49

mentation plays an important role in hygi-enic safety of domestic or husbandrywastewater.

5.5.3 Floatation

Floatation is the predominant method toremove fat, grease and oil. In advancedconventional wastewater treatment it is alsoused to remove small particles with the helpof fine air bubbles blown in from the bot-tom.

Fig. 9.Partition wall retaining scum. Inlet is at the rightside, water flows below the partition wall into thecompartment at the left side. [photo: Sasse]

5 TREATMENT PROCESS

Most fatty matter can be checked by sim-ple observation tests, similar to settleablesolids. If fats which are detected by labora-tory analysis are not separated by floata-tion they present themselves as colloidswhich can only be removed after pre-treat-ment (after acidification, e.g.).

Unwanted floatation happens in septic tanksand other anaerobic systems where float-

ing layers of scum are easily formed. Accu-mulating scum could be removed manu-ally, or could be left purposely to “seal” thesurface of anaerobic ponds to prevent badodour.

Floatation and sedimentation, can be sup-ported by using slanting multi-laminatedsheets or several layers of slanting pipesthat intensify the separation of solids fromliquids because the surface of floatation orsedimentation is artificially multiplied.

5.5.4 Filtration

Filtration becomes necessary when sus-pended solid particles are to be removedthat cannot be forced to settle or to floatwithin a reasonable time, or that cannot befiltered by “self-flocculation”.

Most filters have a double function: Theyprovide a fixed surface for treatment bacte-ria and they form a physical obstacle forsmaller solid particles. Physical filters re-tain solids which accumulate, unless theyare removed from time to time. Rathercoarse filters, where physical filtration hap-pens with the help of the bacteria film inthe first place, can be cleaned by flushing.Bacteria and suspended solids are flushedaway simultaneously as for example is typi-cally done in trickling filters. Upstream fil-ters may be back-flushed. Sand and finergravel filters after some years, must becleaned by replacing the filter media. Thefilter media is reusable after washing.

Needless to say that filtration is more ef-fective with smaller grain size. Unfortunatelyit is also true that effective filtration re-quires the retention of many solids andtherefore, clogs faster. The permeability and

50

principle of lamella solids separator to improvesedimentation

inletoutlet

lamella

scum

settled sludge

settling particles

durability of filters is always reciprocal toits treatment efficiency. Filter material ofround and almost equal grain size is moreefficient and renders longer service thanfilters of mixed grain size.

Aerobic filters produce more sludge thananaerobic filters and consequently wouldblock faster. However, the filter has a cer-tain self-cleaning effect when given suffi-cient resting time, because the aerobicsludge consists of living bacteria, whichpractise a kind of „cannibalism“ (autolysis)when nutrient supply stops.

5 TREATMENT PROCESS

Fig. 10.Lamella solids separator, lamella may be made ofplastic sheets, concrete slabs or PVC pipes.

5.5.5 Sludge accumulation

Sedimentation and filtration leads to sludgeaccumulation at the bottom of vessels (seechapter 8.3). In course of time, this sludgegets compacted, consequently older sludgeoccupies less volume than fresh sludge.

Sludge removal intervals are therefore animportant design criteria. (see also Fig. 67.)Sludge may be removed continuously incase of modern sewage plants, or after sev-eral years in case of anaerobic stabilisationponds.

5.6 Elimination of Nitrogen

Nitrogen is a nutrient that causes algaegrowth in receiving waters and thereforemust be removed from wastewater beforedischarge. It is also poisonous to fish inthe form of ammonia gases and may also

become poisonous in the formof nitrite. The basic process ofnitrogen removal happens intwo steps, namely nitrificationfollowed by denitrification, withthe result that pure nitrogen dif-fuses into the atmosphere.

Nitrification is oxidation. Nitrateis the most stable form of ni-trogen and its presence indi-cates complete oxidation. Deni-trification is reduction, or inother words the separation ofthat very oxygen from the oxi-dised nitrogen. The pure gase-ous nitrogen that remains is in-

soluble in water, and therefore evaporateseasily. Since nitrogen is the major compoundof air it is ecologically completely harm-less. The evaporating nitrogen from thedenitrification process may cause floatingfoam or scum, similar to the effect seenfrom the gas release by settled anaerobicsludge.

During nitrification NH3 (ammonia) is oxi-dised by a special group of bacteria - callednitrobacter - to NO3 (nitrate). Because nitro-

51

ride, aluminium sulphate or lime fix phos-phates, a fact that can be utilised by se-lecting suitable soils in ground filters. How-ever, removal of phosphorus in root zonefilters has not proved to be as efficient andsustainable as expected and propagated bythe pioneers of these systems.

5.8 Elimination of Toxic Substances

Most heavy metals are toxic or cancerogenicand therefore, should not remain in thewastewater because they harm aquatic lifeof the receiving water, or could enter thehuman nutritious cycle when wastewater orsludge is used in agriculture. Since heavymetals settle easy, their removal is not dif-ficult. Nonetheless, sludge must be dumpedsafely, at guarded sites.

Other toxic substances may be soluble andthus difficult to remove. There are numer-ous methods to eliminate or transform tox-ins into non-toxic matters, which cannot bedescribed here. More specialised literaturemay be consulted.

Noxious Substances Units (NSU) acc. to German Federal Law 11/4942, 1989

noxious substance group1 NSU is equal to

oxidisable matter 50 kg CODphosphorous 3 kg Pnitrogen 25 kg Norganic fixed halogenes 2 kg AOXmercury 20 g Hgcadmium 100 g Cdchromium 500 g Crnickel 500 g Nilead 500 g Pbcopper 1000 g Cudilution factor for fish toxicity 3000 m³

Imhoff pg 298

5 TREATMENT PROCESS

bacter grow slowly, a higher sludge agecaused by longer retention times is neededfor oxidation of nitrogen (= nitrification)than is required for oxidation of carbon (seechapter 7.6).

Shorter retention times are required to causedenitrification under anoxic conditions (ab-sence of free oxygen) because there arenot only a few but several groups of bacte-ria able to denitrify, that is to utilise nitrateoxygen. Remaining or additional organicmatter is required for denitrification.

Incomplete denitrification may lead to for-mation of the poisonous nitrite (NO2), in-stead of nitrate (NO3). This happens be-cause the time left for the bacteria to con-sume all the oxygen is not enough or be-cause there is not enough organic materialleft to absorb the NO3-oxygen. Some none-DEWATS treatment processes recycle nutri-tious sludge to prevent such nutrient defi-ciency. A certain amount of nitrate in theeffluent could also be a source of oxygenfor the receiving water.

In case of DEWATS, nitrate removal normallydoes not receive special attention, in thatadditional technical measures are not taken.

5.7 Elimination of Phosphorus

Bacteria cannot transform phosphorus intoa form in which it looses is fertiliser qualitypermanently. Phosphorus compounds re-main potential phosphate suppliers. Thisimplies that no appropriate biological proc-ess, either aerobic or anaerobic can removephosphorus from wastewater. Phosphorusremoval from water “normally” takes placeby removal of bacteria mass (active sludge)or by removal of phosphate fixing solidsvia sedimentation or flocculation. Iron chlo-

Tab. 5.Rating of noxious substances according to Germanlaw. Mercury is the most dangerous matter of this list.

52

A too high salt content inhibits biologicaltreatment in any case. Less dangerousamounts of salt are not necessarily harm-less as a result of the fact that they cannotbe easily removed. For example, in case ofsaline water used for domestic or industrialpurposes, the water remains saline evenafter treatment. It therefore cannot be usedfor irrigation. It is also not allowed to enterthe ground water table or receiving riversthat carry too little water. Costly ion-exchang-ers may have to be used to break-up thestable mineralised molecules of salts.

5.9 Removal of Pathogens

Wastewater even after treatment must beconsidered as being hygienically unsafe.Underground filtration and large pond sys-tems are relatively efficient in pathogen re-moval but not necessarily to an extent thatwastewater can be called safe for bathing -let alone drinking. However, reuse for irri-gation is possible under certain conditions.

Pathogens are divided into helminth eggs,protozoal cysts, bacteria and viruses.Helminth eggs and protozoa accumulate insediment sludge and are largely retainedinside the treatment system, where theystay alive for several weeks. Most bacteria(and virus) caught in the sludge die aftershorter periods. Those bacteria, which arenot caught in the sludge, that remain sus-pended in the liquid portion are hardly af-fected. This is especially true of high ratereactors, like filters or activated sludgetanks. This means that such bacteriaand viruses exit the plant fully alive.Although the risk of virus infectionfrom wastewater has proved to be low.

Exposure to UV rays has a substantialhygienic effect. The highest rate ofpathogen removal can be expected

from shallow ponds with long retentiontimes, e.g. 3 ponds in a row with HRT of 8- 10 days each. Constructed wetlands withtheir multifunctional bacterial life in the rootzones can also be very effective. However,it is the handling after treatment, whichensures hygienic standards (see also chap-ter 11.).

Using chlorination to kill pathogens inwastewater is only advisable for hospitalsin case of epidemics and other such spe-cial circumstances. It may also be used forinstance, in the case of a slaughterhousetreatment plant that is only a short dis-tance away from a domestic water source.Permanent chlorination is never advisable.It has an adverse impact on the environ-ment: Water is made unsuitable for aquaticlife as chlorine itself has a high chemicaloxygen demand (COD).

Bleaching powder (chlorinated lime) contain-ing approximately 25% Cl is most commonlyused as source of chlorine. Granular HTH (hightest hyperchlorite) containing 60 to 70% Clis available on the market under differentbrand names. Since chlorination should notbe a permanent practice, a chamber forbatch supply, followed by a contact tankof 0,5 - 1 h HRT will be sufficient (Fig. 11.).

Chlorination practicetype of infection, type

of wastewatercountry

dose of chlorine

contact time

total rest chloride

g/m³ h mg/lintestinal pathogens China > 1,0 5tubercular pathogens China > 1,5 7raw wastewater Germany 10 - 30 0,25 tracespost treatment India 3post treatment Germany 2 0,25 tracesodour control Germany 4 0,25 traces

Garg, Imhoff, HRIEE

5 TREATMENT PROCESS

Tab. 6..Comparing the use of chlorine for different require-ments at various places

53

5 TREATMENT PROCESS

Fig. 11.Post treatment chlorination in batch chamber for smallscale application. The bucket is filled with bleachingpowder which is washed out automatically. This plantis acceptable for emergency disinfection of effluentfrom rural hospitals only, because controlled dosingis not possible.

54

Fig. 12.Oxygen recovery after pollution of natural waters.Turbulence increases oxygen intake and reduces timefor recovery.

6.1 Surface Water

The biological self-purification effect of sur-face waters depends on the climate, weatherand on the relative pollution load in water.The presence of free oxygen is a precondi-tion for the self-purification process. Thehigher the temperature, the higher the rateat which the degrading bacteria that areresponsible for purification multiply. At thesame time, the intake of oxygen via sur-

6 ECOLOGY ANDSELF PURIFICATION EFFECT

Ability of surface waters to recover oxygen after pollution

0

0,1

0,2

0,3

0,4

0,5

smallponds

largelakes

slowstreams

normalstreams

faststreams

cascades

reco

very

fact

or k

R

Garg pg 220

face and oxygen solubility drops with in-creasing temperature. Rain and wind, in-crease the oxygen intake capacity. Conse-quently, acceptable pollution loads orwastewater volumes must be dimensionedaccording to the season with the least fa-vourable conditions (e.g. winter or summerin temperate zones, dry season in the trop-ics). It is difficult to reanimate water oncethe self-purification effect has stopped asthereafter, it enters the anaerobic stage.

Oxygen intake via surface contact

0,6

0,7

0,8

0,9

1

1,1

1,2

1,3

1,4

1,5

1,6

5 10 15 20 25 30

temperature in °C

fact

or

a

Fig. 13.Oxygen intake of natural waters reduces with risingtemperature.

Extreme seasonal changes make it difficultto maintain the self-purification effect ofwater throughout the year. However, naturehas a way of helping itself as in the case oflakes and rivers that dry-out in long dryseasons when the remains of organic mat-ter compost and are fully mineralised be-fore the next rains come. Minerals retaintheir fertilising quality even after drying. This

The understanding of the self-purificationability of the natural environment helps todesign DEWATS intelligently. That impliesthat, on the one hand, only harmlesswastewater is discharged, and on the otherhand that nature may be incorporated intothe design for completion of the treatmentprocess.

55

is why sludge at the bottom of dried lakes,canals or rivers is better brought to the fieldsbefore it is washed away into the receivingwater by the first heavy rains and its richnutrient value is lost. However, the contentof toxic matter in sludge should be ob-served.

The most important source of oxygen fornatural water in an ecosystem is oxygenfrom the air, which dissolves in water viasurface contact. Floating fat, grease or oilfilms restrict oxygen transmission from airand in addition, need oxygen for their de-composition.

Nutrients descending from wastewater in-crease algae growth. In a healthy ecosys-tem, algae produce oxygen during the dayand consume part of this oxygen at night. Ifthe algae population were to become un-duly dense, sunlight would not be able topenetrate the dark green water. In conse-quence, the algae would consume oxygenduring the day as well and the supply offree oxygen that is needed for aquatic lifewould decrease.

The degree of pollution and more particu-larly the content of dissolved oxygen (DO),can be assumed from the variety of plantand animal species found in the water. Thecolour of the water of river and lakes is yetanother indicator of the quality of the wa-ter. Green or green-brownish water is in-dicative of high nutrient supply due to al-gae, a reddish-rosy colour indicates faculta-tive algae and is suspicious of a severelack of free oxygen; black is often indica-tive of complete anaerobic conditions ofsuspended matter.

Nitrogen in the form of nitrate (NO3) is themain polluting nutrient. In the form of am-monia (NH3) it is also a major oxygen

consuming toxic substance for which rea-son nitrogen should be kept away from liv-ing waters; notwithstanding that nitrate mayalso function as an oxygen donor in cer-tain instances.

The next most important polluting nutrientis phosphorus, which is present mostly inthe form of hydrogen phosphate (H2PO4).Since phosphorus is often the limiting fac-tor for the utilisation of other nutrients, itspresence in surface waters is dangerous,as even in small doses it may lead to anoversupply of nutrients. Nitrogen that isnormally available in plenty needs 10% ofphosphorus to be of use to plants. Thatmeans phosphorus activates ten times asmuch nitrogen and by that effect may beconsidered the most polluting element toany receiving water. For the same reason iswastewater rich in phosphate a good ferti-liser when used for irrigation in agriculture.

Phosphorus accumulates in closed ecosys-tems, e.g. in lakes. Unlike nitrogen that iseliminated, phosphorus remains potentiallyactive in the residue of dead plants, whichhave consumed the element previously. Forexample, phosphate fixed by iron salts canbe set free under anaerobic conditions inthe bottom sludge, where it is available fornew plant growth. It is for this reason thatcontinuous supply of phosphate into lakesis prohibited. While it may be less danger-ous for flowing waters, it must be realisedthat the river ends somewhere at whichpoint phosphorus will accumulate.

While chloride may be used for disinfectingeffluents from hospitals and slaughter-houses, it must be remembered that chlo-ride does also disinfect the receiving wa-ters thereby reducing its self-purificationability.

6 ECOLOGY

56

It is self evident that toxic substances shouldnot enter any living water. Most toxic sub-stances become harmless in the short term,particularly if they are sufficiently diluted.However, most toxic materials are taken inby plants and living creatures, and in thelong run accumulate in the aquatic life cy-cle. Fish from such waters become unsuit-able for human consumption. Heavy met-als also accumulate in the bottom sludgeof receiving waters where they remain as atime-bomb for the future.

6.2 Groundwater

Groundwater was rainwater before. It is themost important source of water for domes-tic use, irrigation and other purposes. Thesupply of ground water is not infinite. To besustainable, it must be recharged. Ratherthan simply draining used water into riversthat carry it to the sea, it would be better topurify this water and use it to recharge thegroundwater.

Organic pollution of groundwater happensin cases where wastewater enters under-ground water streams directly. A crack-free,three meters thick soil layer above ground-water is sufficient to prevent organic pollu-tion. Pollution by mineralised matters is farmore frequent, as salts like nitrate and phos-phate being soluble in water cannot be elimi-nated by physical filtration when passingsoil or sand layers. Some pathogens mayalso reach the groundwater despite soil fil-tration. Viruses can be dangerous due totheir infectious potential irrespective of theirabsolute number.

Nitrate is easily soluble in water. Therefore,it is easily washed out from soil into ground-water, especially in sandy soil during peri-

ods when vegetation is low (e.g. winterin cold climate). Groundwater will there-fore always contain a certain amount ofnitrate (mostly above 10 mg/l).

Nitrate (NO3) in its self is rather harm-less. For example in the European Un-ion, drinking water may legally containNitrate up to 25 mg/l. However, it is la-tently dangerous because it is capableof changing under certain biological orchemical circumstances to nitrite (NO2).This process can even happen inside thehuman blood where nitrite is fixed at thehaemoglobin, as a result of which thecapacity of the haemoglobin to “trans-port” oxygen is reduced, leading to suf-focation. Babies are especially at risk be-cause of a greater tendency to form ni-trite. This is why the water that is usedfor the production of baby food mustalways contain less than 10 mg/l NO3.

6.3 Soil

Soil pollution can be dangerous becauseof washout effects that harm surface andground water alike. Soil itself can alsobe rendered useless for agriculture dueto pollution. For example, the pH maydrop as a result of incomplete anaerobicdigestion of organic matter. This happensparticularly with clay or loamy soils whenoxygen supply is insufficient due to thephysical closure of the pores in the soilby suspended solids from wastewater ir-rigation.

Mineral salts in small doses are normallynot a problem for wastewater treatment.Nonetheless, using saline wastewater forirrigation over a long period of time maycause complete and irreversible salination

6 ECOLOGY

57

6 ECOLOGY

of the topsoil. Clay and loamy soil with slowdownward percolation is most affected,because when water evaporates the saltremains on top of the soil.

On the other hand, sandy soils may ben-efit from irrigation with wastewater evenwhen the organic load is high, on the con-dition that oxygen can be supplied todeeper soil layers. Well treated wastewaterwhich contains mineralised nitrogen, phos-phorus and other trace elements may im-

prove soil conditions and is environmen-tally safe as long as the application of nu-trients is balanced with its in-take byplants. Application of treated wastewaterthroughout the year regardless of demandmay have adverse effects. This happensbecause the nutrients are washed out intowater bodies at times when plant growthis negligible, with the result that nutrientsare not available to the plants whenneeded.

58

Laboratory analysis is used to understandthe quantity and quality of the pollutionload, the feasibility of treatment, the envi-ronmental impact and the potential of acertain wastewater for biogas production.Some properties can even be seen and un-derstood by experienced observation. De-tailed analysing techniques may be foundin books for laboratory work or comprehen-sive handbooks on wastewater, like Metcalf& Eddy’s “Wastewater Engineering”.

As the quality of wastewater changes ac-cording to the time of the day and fromseason to season, the analysis of data isnever absolute. It is far more important tounderstand the significance of each param-eter and its “normal” range than to knowthe exact figures. Ordinarily, an accuracy of±10% is more than sufficient.

7.1 Volume

The volume or the flow rate of a particularwastewater determines the required size ofthe building structure, upon which the fea-sibility or suitability of the treatment tech-nology is decided. Therefore, this is the firstinformation a wastewater engineer needsto know (see chapter 8.1).

Surprisingly, the determination of flow israther complicated and is often difficult toexecute, due to the fact that flow rateschange during the daytime or with seasons,and that volumes have to be measured in„full size“. It is not possible to take a sam-ple which stand for the whole. In case ofDEWATS, it is often easier and more practi-

cal to measure or inquire into the waterconsumption instead of trying to measurethe wastewater production. The flow ofwastewater is not directly equal to waterconsumption since not all the water that isconsumed ends up in the drain (e.g. waterfor gardening), and because wastewatermight be a mix of used water and stormwater. Stormwater should be segregatedfrom the treatment system as far as possi-ble especially if it is likely to carry sub-stantial amounts of silt or rubbish. Rain-water drains should certainly never be con-nected to the treatment plant. However,ponds and ground filters will be exposedto rain. The volume of water in itself isnormally not a problem since hydraulicloading rates are not likely to be doubledand in fact, a certain flushing effect couldeven be advantageous. Soil clogging (silt-ing) could become a problem, if stormwaterreaches the filter after having eroded thesurrounding area.

For high rate reactors, like anaerobic filters,baffled septic tanks and UASB, the flowrate per hour could be a crucial design pa-rameter. If exact flow data is not available,the hours per day which account for mostof the flow, may be considered. This shouldbe read together with hydraulic retentiontimes in appreciation of the fluctuation in-fluence.

Collected volumes per time can be used tomeasure flow rates. This may be the rise inlevel of a canal that is closed for a periodof time, or the number of buckets filledduring a given period. A good indicator ofactual flow rate is also the time it takes

7 CONTROL PARAMETERS

59

during initial filling until the first tank of atreatment plant overflows.

In larger plants the flow rates are normallymeasured with control flumes (e.g. Parshallflume) where rise in level before a slottedweir indicates the flow.

7.2 Solids

Total Solids (TS) or dry matter (DM) includeall matter, which is not water. Organic totalsolids (OTS) or volatile solids (VS) are theorganic fraction of the total solids. TS isfound by drying the sample, the inorganicfraction is found by burning, where it re-mains as ash. TS minus ash is OTS or VS.Solids may be measured in mg/l or in per-centage of the total volume.

The parameter “suspended solids” (SS) de-scribes how much of organic or inorganicmatter is not dissolved in water. Suspendedsolids include settleable solids and non-settleable suspended solids. Settleable sol-ids sink to the bottom within a short time.They can be measured with standardisedprocedure in an Imhoff-cone, in relation toa defined settling time of 30 minutes, 1 hour,2 hours or one day. Measurement of settle-

Tab. 7.Domestic wastewater derives from various sources.Composition of wastewater depends highly on stand-ard of living and domestic culture.

Total solids components of "modern" domestic wastewater

range min maxsource g/cap*d g/cap.*d

feces (solids, 23%) 32 68ground food wastes 32 82wash waters 59 100toilet (incl. paper) 14 27urine (solids, 3.7%) 41 68

Metcalf&Eddy, pg 164

able solids is the easiest method of waste-water analysis because solids are directlyvisible in any transparent vessel. For thefirst on-site information, any transparentvessel will do (e.g. water bottles - whichshould be destroyed after use for hygienicsafety).

Non-settleable suspended solids consist ofparticles, which are too small to sink to thebottom within a reasonable (technical) time.SS is detected by filtration of a sample.Suspended solids are an important param-eter because they cause turbidity in thewater and may cause physical clogging ofpipes, filters, valves and pumps.

Colloids are very fine suspended solids(< 0,1 ηm) which pass normal filtration pa-per but are not fully dissolved in water (dis-solved solids are single molecules that arespread-out among the water molecules). Ahigh percentage of fatty colloids can be areal problem in fine sand filters.

In domestic wastewater, approximately, theBOD derives to 1/3 (33%) from settleablesolids, to 1/2 (50%) from dissolved solids,while 1/6 (17%) of the BOD derives fromnon-settleable SS (Tab. 12.).

7.3 Fat, Grease and Oil

Fat and grease are organic matters whichare biodegradable. However, since they floaton water and are sticky in nature their physi-cal properties are a problem in the treat-ment process and in nature. It is best toseparate fat and grease before biologicaltreatment.

The fat that remains in treated domesticwastewater is normally low. A fat content ofapproximately 15 - 60 mg/l is allowed in the

7 CONTROL PARAMETERS

60

effluent of slaughter houses or meat process-ing plants for discharge into surface water.Mineral grease and mineral oils like petrolor diesel although they may also be treatedbiologically should be kept away from thetreatment system. Their elimination is notwithin the scope of DEWATS.

7.4 Turbidity, Colour and Odour

Most wastewaters are turbid because thesolids suspended in them break the light.Therefore, highly turbid fluid indicates a highpercentage of suspended solids. Metcalf &Eddy (pg 257) gives the relation betweenturbidity and suspended solids with thefollowing equation

SS [mg/l] = 2,35 × turbidity (NTU), orturbidity (NTU) = SS [mg/l] / 2,35

NTU is the standardised degree of turbidity.This can be determined with the help of aturbiditimeter or by standardised methodsby which the depth of sight of a black crosson a white plate is measured.

Turbidity may cause the algae in surfacewaters not to produce oxygen during day-time, as would otherwise be the case.

The colour is not only indicative of thesource of wastewater but is also indicativeof the state of degradation. Fresh domesticwastewater is grey while aerobically de-graded water tends to be yellow and waterafter anaerobic digestion becomes black-ish. Turbid, black water may be easilysettleable because suspended solids sinkto the bottom after digestion when givenenough undisturbed time to form flocs. Abrownish colour is telling of incompleteaerobic or facultative fermentation.

Wastewater that does not smell probablycontains enough free oxygen to restrictanaerobic digestion or the organic matterhas long since been degraded. A foul smell(“like rotten eggs”) comes from H2S (hy-drogen sulphur) which is produced duringanaerobic digestion, especially at a low pH.Ergo, a foul smell means that free oxygenis not available and that anaerobic diges-tion is still under way. Vice versa, when-ever there is substantial anaerobic diges-tion there will always be a foul smell.

Other odours are related to fresh waste-water from various sources. Experience isthe best basis for conclusions: Dairy waste-water will smell like dairy wastewater, dis-tillery wastewater will smell like distillerywastewater, etc., etc.. However, to „smellthe performance“ of a treatment plant ismost important. A wastewater engineershould be alert and „collect“ various odoursand its causes, to build up a repertoire ofexperience for future occasions.

7.5 COD and BOD

Of all parameters, the COD (Chemical Oxy-gen Demand) is the most general param-eter to measure organic pollution. It de-scribes how much oxygen is required tooxidise all organic and inorganic matterfound in water. The BOD (Biochemical Oxy-gen Demand) is always a fraction of theCOD. It describes what can be oxidised bio-logically, this is with the help of bacteria. Itis equal to the organic fraction of the COD.Under standardised laboratory conditionsat 20°C it takes about 20 days to activatethe total carbonaceous BOD (=BODultimate,BODtotal). In order to save time, the BODanalysis stops after 5 days. The result isnamed BOD5, which is simply called the

7 CONTROL PARAMETERS

61

BOD, in practice. The BOD5 is a certain frac-tion (approximately 50 to 70%) of the abso-lute BOD. This fraction is different for eachwastewater. The ratio of BODtotal to BOD5 iswider with difficult degradable wastewater,and thus, it is also wider with partly treatedwastewater.

COD and BOD are the results of standard-ised methods used in laboratory analysis.They do not fully reflect the bio-chemicaltruth, but are reliable indicators for practi-cal use. Biological oxygen demand is a prac-tical description of that portion which canbe digested easily, e.g. anaerobically. TheCOD/BODtotal vaguely indicates the relationof total oxidisable matter to organic matter,which is first degraded by the most com-mon bacteria. For example, if a substrate istoxic to bacteria, the BOD is zero; the CODnonetheless may be high as it would bethe case with chlorinated water. In general,if the COD is much higher than the BOD (>3times) one should check the wastewater fortoxic or non-biodegradable substances. Inpractice, the quickest way to determine toxicsubstances is to have a look into the shop-ping list of the institution which producesthe wastewater. What kind of detergent isbought by a hospital may be more reveal-ing than a wastewater sample taken at ran-dom.

However, one should know that the COD ina laboratory test shows the oxygen donatedby the test-substance, which is normallyK2Cr2O

7

(potassium dichromate). The testedsubstrate is heated to mobilise the chemicalreaction (combustion). Sometimes KMnO4(potassium permanganate) is used for quickon-site tests. The CODcr is approximatelytwice as much as the CODMn. It should benoticed that the two do not have a fixedrelation that is valid for all wastewaters.

Fig. 14.Definition of oxygen demand. The BOD5 is a part ofthe total BOD, the total BOD may be understood aspart of the COD and the COD is part of the absolutereal oxygen demand. The total BOD may be equal tothe COD; the COD may be equal to the real oxygendemand.

Real total oxygen demandCOD

max. oxygen demand that can be captured by defined chemical analysing method

BODtotal = BODultimate

total biodegradable oxygen demand

BOD5 biodegradable oxygen demand that

can be captured by defined biological analysing method within 5 days

COD and BOD5 are not in any case comparable to each other.

BOD related to time and rate constant k at 20°C

0

50

100

150

200

250

300

0 5 10 15 20

days

BO

D m

g/l k = 0,30

k = 0,15k = 0,10k = 0,08

Fig. 15.BOD removal rates are expressed by rate constants(k) which depend on wastewater properties, tem-perature and treatment plant characteristics. Thecurve shows the BOD removal rates at 20°C. Thevalue after 5 days is known as BOD5.

7 CONTROL PARAMETERS

62

Easily degradable wastewater has a COD/BOD5 relation of about 2. The COD/BOD ra-tio widens after biological, especially anaero-bic treatment, because BOD is biologicallydegradable. COD and BOD concentrationsare measured in mg/l or in g/m3. Absolutevalues are measured in g or kg. A weakwastewater from domestic sources for ex-ample, may have a COD below 500 mg/lwhile a strong industrial wastewater maycontain up to 80.000 mg/l BOD.

7.6 Nitrogen (N)

The different forms in which nitrogen isfound in wastewater is a good indicator ofwhat happens or what has already happenedduring treatment. Nitrogen is a major com-ponent of proteins (albumen). A high per-centage of albuminoid nitrogen indicatesfresh wastewater. During decomposition,when large protein molecules are brokenup into smaller molecules, nitrogen is foundin the form of free ammonia (NH3). How-ever, ammonia dissolves in water and atlow pH level forms ammonium (NH4

+). At apH level above 7, NH4

+ remains as - or trans-forms to - NH3. There is always a mass bal-ance between NH3 and NH4. NH3 evapo-rates into the atmosphere, which in case ofirrigation may lead to unwanted nitrogenlosses. Ammonium further oxidises to ni-trite (NO2

-) and finally to nitrate (NO3-).

From the chemical symbol, it is evident thatammonia (or ammonium) will consume oxy-gen to form nitrate, the most stable endproduct. The albuminoid and the ammonianitrogen together form the organic nitrogenor more specifically: the Kjeldahl-N (Nkjel).The total nitrogen (Ntotal) is composed ofNkjel (non oxidised N) and nitrate-N (oxi-dised N).

Pure nitrogen hardly dissolves in water as aresult of which it evaporates immediatelyinto the atmosphere, a fact that is used toremove nitrogen from wastewater in theprocess of denitrification. Pure nitrogen (N2)is formed when oxygen is separated fromNO3 to oxidise organic mater. Nitrification(under aerobic conditions) followed bydenitrification (under anoxic conditions) isthe usual process of removing nitrogen fromwastewater (see chapter 5.6).

Concentrations toxic to the anaerobic process

toxic metal concentrationmg/l

Cr 28-200Ni 50-200Cu 5-100Zn 3-100Cd 70Pb 8-30Na 5000-14000K 2500-5000

Ca 2500-7000Mg 1000-1500

Mudrak / Kunst pg. 158

7 CONTROL PARAMETERS

Tab. 8.Concentration of toxic substances which inhibitanaerobic digestion

Too much BOD or COD discharged into sur-face water could mean that the oxygenpresent in that water will be used for de-composition of the pollutants, and thus, isnot anymore available for aquatic life. Ef-fluent standards for discharge into receiv-ing waters may tolerate 30 to 70 mg/l BODand 100 to 200 mg/l COD.

Total organic carbon (TOC) is sometimesmentioned. This is an indication of howmuch of the COD relates to the carbon only.In the case of DEWATS, knowledge of BODor COD is sufficient, TOC is of no practicalconcern.

63

For optimum bacteria growth the relationBOD/N should be in the range of 15 to 30 inuntreated wastewater.

Nitrogen is normally not controlled in efflu-ent of smaller plants. Discharge standardsfor effluent of larger plants allow Nkjel-N inthe range of 10 - 20 mg/l.

7.7 Phosphorus

Phosphorus (P) is an important parameterin case of unknown wastewater, especiallyin relation to the BOD, the nitrogen or thesulphur. An approximate relation of BOD/P= 100, or N/P = 5 is required for bacterialgrowth. Bacterial activity will be less whenthere is phosphorus deficiency and thus,removal of COD (BOD) will also be less.

On the other hand, very high phosphoruscontent in the effluent leads to water pollu-tion caused by algae growth. However, phos-phorus removal in DEWATS is hardly worthinfluencing and is therefore the least im-portant parameter to the engineer. Dischargestandards for larger plants allow P in therange of 1 - 5 mg/l.

7.8 Temperature and pH

Temperature is important because bacterialgrowth increases with higher temperature,principally, limits notwithstanding. Due tolow energy gains as a result of “incom-plete” anaerobic decomposition, aerobicprocesses are less sensitive to low tem-peratures than anaerobic processes. This isobvious from the fact that biogas is stilloxidiseable and is therefore an energy-richend product. Temperatures between 25° and35°C are most ideal for anaerobic diges-

tion. 18° to 25°C is also good enough. Inshort, a digester temperature above 18°C isacceptable in principal. The ambient tem-perature in tropical and subtropical zonesis almost ideal for anaerobic treatment andas such is not problematic to DEWATS.

Higher temperatures are also favourable foraerobic bacteria growth, but are disadvan-tageous for oxygen transfer (Fig. 13.). Thecooler the environment the more oxygencan be dissolved in water and thereby, moreoxygen will be absorbed from the air. Thisis the reason why ponds may becomeanaerobic in the height of summer.

The pH indicates whether a liquid is acidicor alkaline. The scientific definition of thepH is rather complicated and of no interestto practical engineering (it indicates the H-ion concentration). Pure water has a pH of7, which is considered to be neutral. Aneffluent of neutral pH is indicative of opti-mum treatment system performance. Waste-water with a pH below 4 to 5 (acidic) andabove 9 (alkaline) is difficult to treat; mix-ing tanks may be required to buffer or bal-ance the pH level. In case of a high pH,ammonia-N dominates, whereas as ammo-nium-N is prevalent in case of low pH.

7.9 Volatile Fatty Acids

Volatile fatty acids (VFA) are used as a pa-rameter to check the state of the digestionprocess. A high amount of VFA always goestogether with a low pH. Fatty acids are pro-duced at an early stage of digestion. Thepresence of too many fatty acids indicatesthat the second stage of digestion whichbreaks up the fatty acids, is not keepingpace with acidification. The reason for thiscould be that the retention time is too short

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64

or the organic pollution load on the treat-ment system is too high. Values of VFA in-side the digester in the range of BODinflowvalues indicate a stable anaerobic process.

7.10 Dissolved OxygenDissolved oxygen (DO) describes how muchoxygen is freely available in water. This pa-

7 CONTROL PARAMETERS

rameter indicates the potential for aerobictreatment and is sometimes used in ad-vanced wastewater treatment. DO is morecommon to judge surface water quality, be-cause it is important to aquatic life. For fishbreading, 3 mg/l DO is the minimum re-quired and it is only sufficient for “groundfishes”. For most others fishes, 4 to 5 mg/lat the minimum is required for survival.

Diseases and symptoms caused by pathogens in wastewaterOrganism Disease / Symptoms

Virus (lowest frequency of infection)polio virus poliomyelitiscoxsackie virus mengitis, pneumonia, hepatitis, fever, common colds, etc.echo virus mengitis, paralysis, encephalitis, fever, common colds, diarrhea, etc.hepatitis A virus invectious hepatitisrota virus acute gastroenteritis with severe diarrheanorwalk agents epidemic gastroenteritis with severe diarrheareo virus respiratory infections, gastroenteritis

Bacteria (lower frequency of infection)salmonella spp. salmonellosis (food poisening), typhoid fevershigella spp. bacillary dysentryyersinia spp acute gastroenteritis, diarrhea, abdominal painvibro cholerae choleracampylobacter jejuni gastroenteritisescherichia coli gastroenteritis

Helminth Worms (high frequency of infection)ascari lumbrocoides digestive disturbance, abdominal pain, vomiting, restlessnessascaris suum coughing, chest pain, fevertrichuris trichiura abdominal pain, diarrhea, anemia, eight losstoxocara canis fever, abdominal discomfort, muscle aches, neurological symptomstaenia saginata nervousness, insomnia, anorexia, abdominal pain, digestive distrubancetaenia solium nervousness, insomnia, anorexia, abdominal pain, digestive distrubancenecator americanus hookworm diseasehymenolepsis nana taeniasis

Protozoa (mixed frequency of infection)cryptosporidium gastroenteritisentmoeba histolytica acute enteritisgiardia lamblia giardiasis, diarrhea, abdominal cramps, weight lossbalantidium coli diarrhea, dysenterytoxoplasma gondii toxoplasmosis

various sources, after EPA, Winblad

Tab. 9. Wastewater transmitted diseases and their symptoms

65

7. 11 Pathogens

The World Health Organisation (WHO) dis-tinguishes between high-risk transmissionof intestinal parasites (helminths eggs), andthe relatively lower risk transmission of dis-eases caused by pathogenic bacteria. Thenumber of helminths eggs and the numberof faecal coliformes are indicative of theserisks. For uncontrolled irrigation less than10.000 e-coli per litre and less than 1helminth egg is permitted by WHO stand-ard. E-coli bacteria are not pathogenic butused as an indicator of faecal bacteria.

Regardless of the number of ova, bacteriaor viruses, wastewater is unsafe to man.Exact pathogen counts are of limited im-portance for DEWATS. Bacterial or helminthcounts may become important when waste-water is discharged into surface waters thatare used for bathing, washing, or irrigation.

Domestic wastewater and effluents from meatprocessing plants and slaughterhouses thatcarry the risk of transmitting blood-bornediseases, like hepatitis, are particularly dan-gerous. Handling and discharge of such ef-fluents may demand special precautions.

7 CONTROL PARAMETERS

66

litres of water applied per square meter ofland is equal to 0.15 m3/m2, which in turn isequal to 0.15 m or 15 cm hydraulic load.

Hydraulic loading rates are also responsi-ble for the flow speed (velocity) inside thereactor. This is of particular interest in caseof up-flow reactors, like UASB or the baf-fled septic tank where the up-flow velocityof liquid must be lower than the settlingvelocity of sludge particles. In such cases,the daily flow must be divided by the hoursof actual flow (peak hour flow rate). Forcalculating the velocity in an up-flow reac-tor the wastewater flow per hour is dividedby the surface area of the respective cham-ber (V = Q/A; velocity of flow equals flowdivided by area). This means that the up-flow velocity increases when a given reac-tor volume is split up into several cham-bers. This is because the flow rate per hourremains the same while the surface area ofthe individual chamber is reduced to only afraction of the area of the total volume. Thenecessity to keep velocity low leads to rela-tively large digester volumes, especially inlarge sized baffled septic tanks.

8.2 Organic Load

For strong wastewater, the organic loadingrate and not the hydraulic loading rate be-comes the determining parameter. Calcula-tion is done in grams or kilograms of BOD5(or COD) per m3 digester volume per day incase of tanks and deep anaerobic ponds.For shallow aerobic ponds the organic load-ing is related to the surface with the dimen-

8.1 Hydraulic Load

The hydraulic load is the most commonparameter for calculating reactor volumes.It describes the volume of wastewaterto be applied per volume of reactor, orper surface of filter in a given time. Themost common dimensions for reactors arem3/m3×d, which means 1 m3 of wastewateris applied per 1 m3 of reactor volume perday. The reciprocal value denotes the hy-draulic retention time (HRT). For example,1 m3 wastewater on 3 m3 of reactor volumegives a hydraulic load of 0.33 m3/m3×d, whichis equal to a hydraulic retention time of 3days (3 m3 volume / 1 m3 of water per day).

The hydraulic retention time (HRT) indicatesa volume by volume relation. It does notfor example, distinguish between sludge andliquid. The hydraulic retention time in a sep-tic tank does not say anything about thefraction of the wastewater which stays longerinside the tank, nor it does say anythingabout the time that the bottom sludge hasfor digestion. In case of vessels filled withfilter media, the actual hydraulic retentiontime depends on the pore space of themedia. For example, certain gravel consistsof 60% stones and 40% of the space be-tween the stones. A retention time of 24hours per gross reactor volume is reducedto only 40%, which gives a net HRT of only9.6 hours.

For groundfilters and ponds, the hydraulicloading rates may be measured by m3/ha×d,m3/m2×d or l/m2×d. These values may alsobe given in cm or m height of water coveron a horizontal surface. For example, 150

8 DIMENSIONING P ARAMETERS

67

sions g/m2 or kg/ha BOD5

(or COD). The per-mitted organic loading rate depends on thekind of reactor, the reactor temperature andthe kind of wastewater. The organic load-ing rate takes care of the time which thevarious kinds of bacteria need for their spe-cific metabolism (often expressed as rateconstant k). Organic loading influences aco-ordinated follow up of different treat-ment steps. Easily degradeable substratecan be fed at higher loading rates, becauseall the bacteria involved multiply fast andconsume organic matter quickly. For diffi-cult degradeable substrate, some of the bac-teria species need a longer time.

At too high loading rates it might be possi-ble that the end products from one step offermentation cannot be consumed by thegroup of bacteria that follow. This mightlead to “poisoning” and collapse of theprocess. For instance, in anaerobic diges-tion, the fatty acids produced in the firststep have to be consumed by the group ofbacteria that follow. Otherwise, the substrateturns sour and final methanisation cannottake place.

At loading rates that are too low, little bac-terial sludge is produced, because the bac-teria “eat each other” for want of feed (au-tolysis). Consequently, incoming wastewaterdoes not meet with sufficient bacteria fordecomposition. Nonetheless, while low or-

ganic loading rates may not destabilise theprocess, it does reduce the overall efficiencyof the treatment process.

8.3 Sludge Volume

The volume of sludge is an important pa-rameter for designing sedimentation tanksand digesters. This is because the accumu-lating sludge occupies tank volume thatmust be added to the required reactor vol-ume. Biological sludge production is in di-rect relation to the amount of BOD removed,which however, depends on the decompo-

8 DIMENSIONING PARAMETERS

Organic loading rates , treatment efficiency and optimum temperature

typical valuesaerobic pond

maturation pond

water hyacinth

pond

anaerobic pond

anaerobic filter

baffled reactor

BOD5 kg/m³*d 0,11 0,01 0,07 0,3 - 1,2 4,00 6,00BOD5 removal 85% 70% 85% 70% 85% 85%temperature optimum 20°C 20°C 20°C 30°C 30°C 30°C

mixed sources

sition process. Aerobic digestion producesmore sludge than anaerobic fermentation.In addition to the biological sludge, pri-mary sludge consists partly of sludge thatis already mineralised.

The standard wastewater literature describessludge volumes from different treatment sys-tems; some of which is not comparable toDEWATS. In conventional sewage treatmentworks, the sludge is removed continuouslyand often under water. This produces a veryliquid sludge with a low total solid contentthat varies between 1 and 5 %. In DEWATS,the sludge remains inside the tank for atleast one year where it decomposes underanaerobic conditions. It also compacts withresting time, under its own weight.

Tab. 10.Organic loading rates and removal efficiencies of various treatment systems

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Literature reports vary widely on the vol-umes of sludge, which accumulate over timein primary treatment tanks. Garg cites 30 lper capita per year for septic tanks in India,while Metcalf & Eddy quote 140 l per capitaper year for Imhoff tanks in the USA whichis comparable to Chinese standard of 0.4 lper capita per day. Reports suggest, sludgevolumes in the range of 360 to 500 litresper capita per annum in settlers of conven-tional treatment systems. Imhoff claims1.8 litre per capita per day for fresh settledsludge that after anaerobic digestion is re-duced to 0.3 litres per day or 110 litres perannum. This volume is further reduced af-ter dewatering to 0.1 litre per capita per day.Inamori for an instance, has found 3.35 kgof sludge solids accumulated per capita peryear in a pre-fabricated septic tank in Ja-pan. Assuming a total solids content of com-pressed anaerobic bottom sludge of 11%,this corresponds closely with the 30 litresreported by Garg which may be further re-duced with increased desludging intervals.

To obtain an approximate figure in relationto the BOD load of any wastewater, these30 litres of sludge per annum are related toapproximately 15 - 20 g BOD removed inthe septic tanks per day. This means 0.005

Solids content of domestic wastewatermineral dry matter organic dry matter total dry matter BOD5

g/cap.*d g/m³ g/cap.*d g/m³ g/cap.*d g/m³ g/cap.*d g/m³settleable

solids20 100 30 150 50 250 20 100

suspendedsolids

5 25 10 50 15 75 10 50

dissolvedsolids 75 375 50 250 125 625 30 150

Total 100 500 90 450 190 950 60 300Imhoff pg 103, 104

Tab. 11.Average distribution of solids of domestic wastewater in Germany

litres of sludge per gram BODremoved accu-mulate in the primary treatment step ofDEWATS which also includes a certain per-centage of mineral settleable particles thatno longer appear in the secondary treat-ment step. Furthermore, not all digestedorganic matter accumulates as settleablesludge in the secondary treatment part. Avalue of 0.0075 litres is taken for oxidationponds because of additional sludge fromalgae.

8 DIMENSIONING PARAMETERS

Properties of sludge from primary sedimentation

specific gravity of

solids

specific gravity of

sludgedry solids

kg/l kg/l g/m³1,4 1,02 150,6

Metcalf & Eddy, pg. 773

Tab. 12.Properties of primary sludge

The above figures are valid for „modern“ do-mestic wastewater as described in Tab. 11.

Sludge accumulation from other wastewaterwith different settling properties and differ-ent relations between organic and mineralmatter content may be calculated in thesame manner as explained above keepingin mind however, their particular properties.

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9.1 Grease Trap and Grit Chamber

In case there is a septic tank provided,DEWATS normally does not require greasetraps or grit chambers for domestic waste-water. Whenever possible it is better toavoid them altogether, because grease andgrit must be removed, at least weekly. How-ever, for canteens or certain industrial waste-waters it may be advisable to separate gritand grease before the septic tank.

The function of grease and grit chamber iscomparable to that of the septic tank, namelylight matter should float and heavy mattershould sink to the bottom. The difference isthat bio-degradable solids should have notime to settle. Therefore, retention times forgrit chambers are short - about 3 minutesonly - and for that reason are masonry struc-tures not appropriate in case of minor flows.

A conical trough allows slow flow at a largesurface for grease floatation and fast flowat the narrow bottom which allows onlyheavy and coarse grit to settle. The watersurface is protected from the turbulence ofthe inflow by a baffle; the outlet is near thebottom.

9.2 Septic Tank

The septic tank is the most common, smallscale and decentralised treatment plant,worldwide. It is compact, robust and in com-parison to the cost of its construction, ex-tremely efficient. It is basically a sedimen-tation tank in which settled sludge is stabi-lised by anaerobic digestion. Dissolved andsuspended matter leaves the tank more orless untreated.

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Fig. 16.Principle design of combined grease trap and grit chamber. Accumulating grease, oil and grit may beremoved daily, at least weekly. If this is not assured, an oversized septic tank is preferable to receive gritand grease.

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Two treatment principles, namely the me-chanical treatment by sedimentation andthe biological treatment by contact betweenfresh wastewater and active sludge com-pete with each other in the septic tank.Optimal sedimentation takes place when theflow is smooth and undisturbed. Biologicaltreatment is optimised by quick and inten-sive contact between new inflow and oldsludge, particularly when the flow is turbu-lent. The way the new influent flows throughthe tank decides which treatment effect pre-dominates.

With smooth and undisturbed flow, thesupernatant (the water remaining aftersettleable solids have separated) leaves theseptic tank rather fresh and odourless, im-plying that degradation has not started yet.With turbulent flow, degradation of sus-pended and dissolved solids starts morequickly because of intensive contact be-tween fresh and already active substrate.However, since there is not enough „calm-ness“ for sedimentation, more suspendedsolids are discharged with the effluent dueto the turbulence. The effluent stinks be-cause active solids that are not completelyfermented leave the tank.

Septic TankLongitudinal Section

inlet outlet

scum

liquid

sludge

In domestic wastewater, a heavy scum con-sisting of matters lighter than water suchas fat, grease, wood chips, hair or any float-ing plastics normally forms near the inlet. Alarger portion of the floating scum consistsof sludge particles which are released fromthe bottom and driven to the top by treat-ment gases. New sludge from below liftsthe older scum particles above the watersurface where they become lighter due todrying. Therefore, scum accumulates andmust be removed regularly, at least everythird year. Scum does not harm the treat-ment process as such, but it does occupytank volume.

A septic tank consists minimum of 2, some-times 3 compartments. The compartmentwalls extend 15 cm above liquid level. Theymay also be used as bearing walls for thecovering slab if some openings for internalgas exchange are provided.

The first compartment occupies about halfthe total volume because most of the sludgeand scum accumulates here. The followingchamber(s) are provided to calm the turbu-lent liquid. They are made of equal sizeand in total, occupy the other half of thevolume. All chambers are normally of thesame depth. The depth from outlet level tothe bottom may be between 1.50 m and2.50 m. The first chamber is sometimesmade deeper.

The size of the first chamber is calculatedto be at least twice the accumulating sludgevolume. The sludge volume depends on theportion of settleable solids of the influentand on desludging intervals (Fig. 67.). Mostcountries provide a National Standard fortank volume per domestic user.

The SS removal rate drops drastically whenaccumulated sludge fills more than 2/3 of

Fig. 17.Flow principle of the septic tank. Most sludge andscum it retained in the first chamber; the secondchamber contains only little sludge which allows thewater to flow undisturbed by rising gas bubbles.

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the tank. This must be avoided; especiallyin cases where the effluent is treated fur-ther in a sand or gravel filter.

“Irregular emptying of septic tanks leads to irre-versible clogging of the infiltration bed; rather thanrenewing the bed, most owners by-pass it and di-vert the tank’s effluent to surface drains.”(Alaerts, G.J., Veenstra, S., Bentvelsen, M., van Duijl,L.A. at al.:” Feasibility of anaerobic Sewage Treat-ment in Sanitation Strategies in Developing Coun-tries” )

For domestic sewage, the accumulatingsludge volume should be calculated at 0.1L/cap× d. When desludging intervals arelonger than 2 years, the sludge volume maybe taken to 0.08 L/cap×d as sludge getscompacted with time (see figure Fig. 67.).

The inlet may dive down inside the tank,below the assumed lowest level of the scumor may be above the water level when theinlet pipe is used to evacuate gas. The ven-tilation pipe for digester gases should endoutside buildings, at a minimum of 2 mabove the ground.

The connection between compartments isdone by simple wall openings that are situ-ated above the highest sludge level andbelow the lowest level of the scum. Thisfor domestic wastewater, would mean thatthe top of the opening is 30 cm belowoutlet level and its bottom is at least halfthe water depth, above the floor. The open-ings should be equally distributed over thefull width of the tank in order to minimise

Fig. 18.The septic tank. Dimensions havebeen calculated for 13 m3 of domes-tic wastewater per day.

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turbulence. A slot spanning the full widthof the tank would be best for reducing ve-locity, which would otherwise cause turbu-lence.

Nothing gets lost; even treated water must go somewhere.

The septic tank is a biogas plant, the biogasof which is not used. However, gas accu-mulates inside the tank above the liquid,from where it should be able to escape intothe air. For this reason, an open fire shouldbe avoided when opening the septic tankfor cleaning.

The outlet has a T-joint, the lower arm ofwhich dives 30 cm below water level. Withthis design, foul gas trapped in the tankenters the sewage line from where it mustbe ventilated safely. If ventilation cannotbe guaranteed, an elbow must to be usedat the outlet to prevent the gas from enter-ing the outlet pipe.

There should be manholes in the cover slab;one each above inlet and outlet and one ateach partition wall, preferably at the inletof each compartment. The manholes shouldbe placed in such a manner to make thedrawing of water samples from each com-partment possible.

Septic tanks were originally designed fordomestic wastewater. They are also suit-able for other wastewater of similar prop-erties, particularly those that contain a sub-stantial portion of settleable solids.

The treatment quality of a septic tank is inthe range of 25% - 50% COD removal. Thisis a rough, primary treatment prior to sec-ondary or even tertiary treatment. Post treat-ment may be provided in ponds or ground

filters. In case of the latter, regular de-sludging of septic tanks is mandatory. Aseptic tank may also be integrated into ananaerobic filter or it could be the first sec-tion of a baffled reactor. Septic tanks aresuitable as individual on-site pre-treatmentunits for community sewer systems, becausethe diameter of sewerage can be smallerwhen settleable solids have been removedon-site.

Starting Phase and Maintenance

A septic tank may be used immediately. Itdoes not require special arrangements be-fore usage. However, digestion of sludgestarts after some days only. Regulardesludging after one to three years is re-quired. When removing the sludge, someimmature (still active) sludge should be leftinside to enable continuous decompositionof newly settling solids. This means, if thesludge is removed by pumping, the pumphead should be brought down to the verybottom. It is not necessary to remove allthe liquid. The sludge should be immedi-ately treated further in drying beds or com-post pits for pathogen control. The sur-rounding of the septic tank should be keptfree of plants in order to prevent roots fromgrowing into the pipe lines and controlchambers.

Calculation of Dimensions

Approximately 80 to 100 l should be pro-vided per domestic user. For exact calcula-tion, or other than domestic wastewater theformula applied in the computer spreadsheet (Tab.23.) may be used.

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9.3 Imhoff Tank

Imhoff or Emscher tanks are typically usedfor domestic or mixed wastewater flowsabove 3 m3/d, where the effluent will beexposed above ground for further treatment,and therefore the effluent should not stink,as it could be the case with a septic tank.The Imhoff tank separates the fresh influ-ent firmly from the bottom sludge.

The tank consists of a settling compartmentabove the digestion chamber. Funnel-likebaffle walls prevent up-flowing foul sludgeparticles from getting mixed with the efflu-ent and from causing turbulence. The efflu-ent remains fresh and odourless becausethe suspended and dissolved solids do nothave an opportunity to get in contact withthe active sludge to become sour and foul.Retention times of much longer than 2 hduring peak hours in the flow portion ofthe tank would jeopardise this effect.

Water pressure increases with depth of vessel.

When sludge ferments at the bottom, thesludge particles get attached to foul gasbubbles and start to float upwards, as in

the septic tank. The up-flowing sludge par-ticles assemble outside the conical wallsand form an accumulating scum layer. Thisscum grows continuously downwards, untilthe slots through which settling particlesshould fall into the lower compartment areclosed. The treatment effect is then reducedto that of a too small septic tank. It is forthis reason that the sludge and scum mustbe removed regularly, at the intervals thesludge storage had been designed for.

Only part of the sludge should be removedso as to always keep some active sludgepresent. Sludge should be removed right fromthe bottom to be sure that only fully di-gested substrate is discharged. If sludge isremoved by hydraulic pressure (gravity), thepipes should be of 150 mm diameter. A hy-draulic head loss of 30 cm to 40 cm must betaken into account. If pipes of only 100 mmare used, the head loss is likely to be morethan 50 cm. When sludge is removed it shouldbe immediately treated further in drying bedsor compost pits for pathogen control.

The inlet and outlet pipe is made as in aseptic tank. Pipe ventilation must be pro-vided, as biogas is also produced in the

Imhoff TankCross Section Longitudinal Section

scum

liquid

sludge

flow tank

Fig. 19.Flow principle of the Imhoff tank. The water flows quick but undisturbed from rising gas bubbles throughthe flow tank; the water does not mix with „ripe“ water or sludge.

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Imhoff tank. Additional baffles to reducevelocity at the inlet and to retain suspendedmatter at the outlet are advantageous. Theupper part of the funnel-shaped baffles isvertical for 30 cm above and 30 cm belowthe water surface. The shape of an Imhofftank may be cylindrical, however the fun-nel remains rectangular in order to leavesome space outside the funnel for removalof scum. The funnel structure may consistof ready made parts from ferro-cement.

Starting Phase and Maintenance

As with the septic tank, a special start-ing phase is not required. Desludging isnecessary at regular intervals not forget-ting to leave some younger bottomsludge behind in the tank. The sludgecan be removed by pumping or hydrau-lic pressure pipes right from the bottom.The liquid may remain inside. Scumshould be removed before the sludge isremoved failing which the scum must be

Fig. 20.Imhoff tank. Dimensions have beencalculated for 25 m3 of domesticwastewater per day.

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lifted from further down. Scum needs to beremoved before it grows to the extent thatit closes the slots between upper and lowercompartments. Should this happen, gasbubbles appearing in rows on the watersurface above the slots indicating that scummust be removed.

Calculation of Dimensions

The upper compartment, inside the funnelwalls, should be designed for 2 h HRT atpeak flow and the hydraulic load should beless than 1.5 m3/h per 1 m2 surface area.The sludge compartment below the slotsshould be calculated to retain 2.5 litresludge per kg BOD reduced per day of stor-age for short desludging intervals. For longerintervals use data of spread sheet (Tab. 24.).Treatment efficiency lies in the range of 25to 50% COD reduction. For domestic waste-water, the upper compartment should havea volume of approximately 50 l per userand the sludge compartment below theslots, should have a volume of approxi-mately 120 litre per user. This rule of thumbmay be valid for a desludging interval ofone year. For more detailed calculation orin case of non-domestic wastewater use theformula of the computer spread sheet.

9.4 Anaerobic Filter

The dominant principle of both the septictank and Imhoff tank is sedimentation incombination with sludge digestion. Theanaerobic filter, also known as fixed bed orfixed film reactor, is different in that it alsoincludes the treatment of non-settleable anddissolved solids by bringing them in closecontact with a surplus of active bacterial

mass. This surplus together with “hungry”bacteria digests the dispersed or dissolvedorganic matter within short retention times.Most of the bacteria are immobile. They tendto fix themselves to solid particles or, e.g.at the reactor walls. Filter material, such asgravel, rocks, cinder or specially formedplastic pieces provide additional surfacearea for bacteria to settle. Thus, the freshwastewater is forced to come into contactwith active bacteria intensively. The largerthe surface for bacterial growth, the quickerthe digestion. A good filter material provides90 to 300 m2 surface area per m3 of occu-pied reactor volume. A rough surface pro-vides a larger area, at least in the startingphase. Later the bacterial „lawn“ or „film“that grows on the filter mass quickly closesthe smaller groves and holes. The total sur-face area of the filter seems to be less im-

Fig. 21.Floating filter balls made of plastic. When bacte-ria film becomes too heavy, the balls turn overand discharge their load. The filter medium hassuccessfully been used for tofu wastewater byHRIEE in Zheijiang Province/China. [photo: Sasse]

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portant for treatment than its physical abil-ity to hold back solid particles.

When the bacterial film becomes too thickit has to be removed. This may be done byback-flash of wastewater or by removingthe filter mass for cleaning outside the re-actor. Nonetheless, the anaerobic filter isvery reliable and robust.

By experience, on an average, 25 - 30% ofthe total filter mass may be inactive due toclogging. While a cinder or rock filter maynot block completely, reduced treatment ef-ficiency is indicative of clogging in someparts. Clogging happens when wastewaterfinds a channelled way through only someopen pores as a result of which the high-speed flow washes the bacteria away. Even-tually, the lesser used voids in the filter getclogged. The end result is reduced reten-tion time within the few open voids. How-ever, a sand or gravel filter may block com-pletely due to smaller pore size.

Anaerobic Filtergas release

Septic Tank gas release Anaerobic Filterinlet outlet

scum

liquid filter

sludge grill

Fig. 22.Flow principle of anaerobic up-flow filter. Suspendedsolids are retained as much as possible in the sep-tic tank. Anaerobic filters may also be designed fordown-flow.

The quality of treatment in well-operatedanaerobic filters is in the range of 70% -90% BOD removal. It is suitable for domes-

tic wastewater and all industrial wastewaterwhich have a lower content of suspendedsolids. Pre-treatment in settlers or septictanks may be necessary to eliminate solidsof larger size before they are allowed toenter the filter.

Anaerobic filters may be operated as downflow or up flow systems. The up flow sys-tem is normally preferred as the risk ofwashing out active bacteria is less in thiscase. On the other hand, flushing of thefilter for the purpose of cleaning is easierwith the down flow system. A combinationof up-flow and down-flow chambers is alsopossible. An important design criterion isthat of equal distribution of wastewaterupon the filter area. The provision of ad-equate space of free water before the filterand the same before the outlet pipe sup-ports equal distribution. Full-width down-flow shafts are preferred to down-flow pipes.The length of the filter chamber should notbe greater than the water depth.

For smaller and simple structures, the filtermass consists of cinder (5 to 15 cm in di-ameter) or rocks (5 to 10 cm in diameter)which are bedded on perforated concreteslabs. The filter starts with a layer of largesized rocks at the bottom. The slabs restapproximately 50 to 60 cm above groundon beams which are parallel to the direc-

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Fig. 23.Anaerobic filter. Dimensions have been calculated for 25 m3 domestic wastewater per day.

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tion of flow. Pipes of at least 15 cm in di-ameter or down-shafts over the full widthallows desludging at the bottom with thehelp of pumps from the top. In case thesludge drying beds are placed just besidethe filter, sludge may also be drawn viahydraulic pressure pipes. Head losses of30 to 50 cm have to be counted with.

The water level of interconnected vessels is equal in all the vessels. However, there is friction loss in

filters!

Starting Phase and Maintenance

Since the treatment process depends on asurplus of active bacterial mass, activesludge (e.g. from septic tanks) should besprayed on the filter material before start-ing continuous operation. When possible,start with a quarter of the daily flow andonly then increase the flow rate slowly overthree months. As this might not be possi-

ble in practice, full treatment performanceis not likely until approximately six to ninemonths later.

As with septic tanks, desludging should bedone at regular intervals. Whenever possi-ble, back-flush the filter before sludge re-moval and cleaning the filter when efficiencygoes down.

Calculation of Dimensions

Organic load limits are in the range of 4 to5 kg COD/m3×d. The hydraulic retention timecompared to the tank volume should be inthe range between 1.5 and 2 days. Use theformula applied in the computer spreadsheet, for exact calculation (Tab. 25.). Fordomestic wastewater, constructed gross di-gester volume (voids plus filter mass) maybe estimated at 0,5 m3 /capita. For smallerunits it may come to 1 m3 /capita.

9.5 UASB

The UASB system is not consid-ered a DEWATS technology. How-ever, an understanding of the prin-ciple on which it functions mayimprove understanding of thebaffled septic tank.

The UASB reactor (UpstreamAnaerobic Sludge Blanket reactor)maintains a cushion of activesludge suspended at the lower

Fig. 24.Down-shaft and down-pipes. Both sys-tems may be applied alternatively inanaerobic filters and baffled septictanks.

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part of the digester. It uses this sludge blan-ket directly as filter medium. Upstream ve-locity and settling speed of the sludge is inequilibrium and forms a locally rather sta-ble but suspended sludge blanket. Aftersome weeks of maturation, granular sludgeforms which improves the physical stabilityand the filter capacity of the sludge blanket.

To keep the blanket in proper position, thehydraulic load must correspond to the up-stream velocity and must correspond to theorganic load. The latter is responsible fordevelopment of new sludge. This meansthat the flow rate must be controlled andproperly geared in accordance with fluctua-tion of the organic load. Generally, in smallerunits, fluctuation of inflow is high, but theregulation of wastewater flow is not possi-ble. In addition, it is not possible to stabi-lise the process by increasing the hydraulicretention time without lowering the up-stream velocity. This is most unfortunate,for otherwise a system which is relativelysimple to build, is rendered unsuitable toDEWATS particularly for relatively weak,domestic wastewater.

UASB

gas-liquid-solid

cross section

gas release

outletgas

liquidseparators

sludge contact zone

inlet

Fig. 25.Flow principle of UASB reactors. Up-streaming waterand gas-driven sludge particles hit the baffles whichcauses separation of gas, solids and liquid.

The fully controlled UASB is used for rela-tively strong industrial wastewaters whereinbiogas is utilised. Slanting baffles (compara-tive to the Imhoff tank) help to separategas bubbles from solids, whereby solids arealso separated from the up-streaming liq-uid. These baffles are called 3-phase sepa-rators.

UASB reactors require several months tomature - i.e., to develop sufficient granularsludge for treatment. Granular sludge is likebig flocs of dust. Similarly, bacterial slimeform chains which coagulate into flocs orgranules. High organic loading in connec-tion with lower hydraulic loading ratesquicken the granulation process in the start-ing phase. To move such sludge granules tothe top requires a much higher velocity thenis required for single sludge particles. Agranular sludge bed therefore remains morestable.

Starting Phase, Maintenance andCalculation of Dimensions

The UASB does not belong to DEWATS.Details of its operation and calculations aredeliberately omitted in this handbook toavert the impression that the UASB can stillbe build and operated under DEWATS con-ditions.

9.6 Baffled Septic Tank

The baffled septic tank may be consideredas the DEWATS version of the UASB sys-tem. It in fact is a combination of severalanaerobic process principles - the septictank, the fluidised bed reactor and theUASB. The baffled septic tank is also knownas „baffled reactor“.

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The up-flow velocity of the baffled septictank, which should never be more than2 m/h, limits its design. Based on a givenhydraulic retention time, the up-flow veloc-ity increases in direct relation to the reac-tor height. Therefore can the reactor heightnot serve as a variable parameter to de-sign the reactor for the required HRT. Thelimited upstream velocity results in largebut shallow tanks. It is for this reason thatthe baffled reactor is not economical forlarger plants. It is also for this reason thatit is not very well known and poorly re-searched.

However, the baffled septic tank is ideal forDEWATS because it is simple to build andsimple to operate. Hydraulic and organicshock loads have little effect on treatmentefficiency.

The difference with the UASB lies in thefact that it is not necessary for the sludgeblanket to float; it may rest at the bot-tom. 3-phase separators are also not nec-essary since a part of the active sludgethat is washed out from one chamber istrapped in the next. The tanks put in se-ries also help to digest difficult degrada-ble substances, predominantly in the rearpart, after easily degradable matters have

been digested in the front part, already.Consequently, recycling of effluent wouldhave a slightly negative effect on treat-ment quality. The baffled septic tank con-sists of at least four chambers in series.The last chamber could have a filter in itsupper part in order to retain eventual solidparticles. A settler for post-treatment couldalso be placed after the baffled septic tank(Fig. 51).

Equal distribution of inflow, and wide spreadcontact between new and old substrate areimportant process features. The fresh influ-ent is mixed as soon as possible with theactive sludge present in the reactor in or-der to get quickly inoculated for digestion.This is contrary to the principle of the Imhofftank. The wastewater flows from bottom totop with the effect that sludge particlessettle against the up-stream of the liquid.This provides the possibility of intensivecontact between resident sludge and newlyincoming liquid.

The DEWATS version does not have a grill.It always starts with a settling chamber forlarger solids and impurities followed by aseries of up-flow chambers. The water streambetween chambers is directed by baffle wallsthat form a down-shaft or by down-pipes

that are placed on parti-tion walls. Although withdown-pipes the total di-gester can be shorter(and cheaper), down-shafts should have pref-erence because of betterdistribution of flow.

Baffled septic tankprovision for principal longitudinal section

gas release

inletscum outlet

liquid

sludge

baffled reactorsettler

Fig. 26.Flow principle of baffled septic tank. Incoming wastewater is forced to pass through active bacteria sludgein each compartment. The settler in front prevents larger solids to enter the baffle section.

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Fig. 27.Baffled Septic Tank. Dimensions have been calculated for 25 m3 domestic wastewater per day.

As already mentioned, the wastewater thatenters a tank should as much as possiblebe distributed over the entire floor area.This is taken care of by relatively short com-partments (length < 50% to 60% of theheight). In case, distance between downpipes should not exceed 75 cm. For larger

plants, when longer compartments are re-quired, the outlets of down pipes (as wellas down shafts) should reach out to thecentre of the floor area.

The final outlet as well as the outlets ofeach tank should be placed slightly belowsurface in order to retain any possible scum.

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fully mixed digester

biogas outlet

position of inlet and outlet

is less important with

homogeneous liquid

inlet of high TS content

outlet

Baffled septic tanks may be equipped with3-phase separators in the form of slantingbaffles in the upper third of the tank, how-ever, this is done rarely.

The baffled septic tank is suitable for allkind of wastewaters, including domestic.Its efficiency increases with higher organicload. There is relatively little experience withbaffled reactors, because the system is onlyused in smaller units. However, it is thehigh efficient answer to the low efficientseptic tank, because simple and efficientoperation goes along with easy construc-tion and low cost. Treatment performanceis the range of 65% - 90% COD (70% - 95%BOD) removal. However, three months ofmaturation should be acknowledged. Sludgemust be removed in regular intervals likewith a septic tank. Some sludge should al-ways be left for continuous efficiency. It isnoticeable that the amount of sludge in thefront portion of the digester is more, thanin the rear compartments.

Starting Phase and Maintenance

Treatment performance depends on theavailability of active bacterial mass. Inocu-lation with old sludge from septic tanks has-tens the achievement of adequate treatmentperformance. In principle it is advantageousto start with a quarter of the daily flow andif possible with a slightly stronger waste-water. The loading rate increases slowly overthree months. This would give the bacteriaenough time to multiply before suspendedsolids are washed out. Starting with thefull hydraulic load from the beginning willseverely delay maturation.

Although desludging at regular intervals isnecessary, it is vital that some active sludgeis left in each of the compartments to main-tain a stable treatment process.

Calculation of Dimensions

The up-flow velocity should not exceed 2.0m/h. This is the most crucial parameter fordimensioning, especially with high hydrau-lic loading. The organic load should be be-low 3.0 kg COD/m3×d. Higher-loading ratesare possible with higher temperature andfor easily degradeable substrate. The HRTof the liquid fraction (i.e. above sludge vol-ume) should not be less than 8 hours.Sludge storage volume should be providedfor 4 l/m3 BODinflow in the settler and 1.4 l/m3

BODremoved in the upstream tanks. Forexact calculation use the formula appliedin the computer-spread sheet Tab. 26.

Fig. 28.Flow principle of the fully mixed anaerobic digester.There is only little sedimentation due to viscoussubstrate. All fractions of the substrate stay for thesame period of time inside the digester. The posi-tion of inlet and outlet is less important with homo-geneous liquid of high TS content. Small baffles maybe provided to avoid short circuit of substrate.

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9.7 Fully Mixed Digester

The fully mixed anaerobic digester corre-sponds to the biogas plant that is used inagricultural households of Developing Coun-tries. It is suitable for rather “thick” andhomogenous substrate like sludge fromaerobic treatment tanks or liquid animalexcreta. For economical reasons, it is notsuitable for weak liquid wastewater, becausethe total volume of wastewater must beagitated and kept for full retention time (15- 30 days) inside the digester. This leads tolarge digester volumes and thus, to highconstruction costs.

“Thick” viscous substrates of more than 6%total solid content do not need to be stirred.A digester for such a substrate may thenbe operated for many years without de-sludging because only grit, but hardly anysludge settles. Moreover, all the incomingsubstrate leaves the reactor after digestion.Scum formation is still possible with cer-tain substrates. Therefore, if pipes are usedfor inlet and outlet they should be placedat middle height. In fixed dome digesters,the outlet is preferably made of a verticalshaft of which the opening starts immedi-ately below the zero-line. This is in order toalways allow portions of scum to discharge.

Fig. 29.Traditional biogas plants as fully mixed anaerobic digester. A: The ball-shaped fixed dome plant withintegrated gas storage and expansion chamber. B: The half-ball-shaped fixed dome plant. C: The floatingdrum plant with water seal. All three plants are designed for 600 litre substrate per day of 4% organic drymatter content, at 25°C and HRT of 25 days. The expected gas production is 8,42 m3/d. Comparing spacerequirement and gas pressure of all three plants indicate that floating drum plants are preferable in case ofhigh gas production rates.

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Because the fully mixed digester is usedfor strong substrate only, biogas produc-tion is high and utilisation of biogas maybe recommended. In this case, the gas col-lector tank and the gas storage tank mustbe gas-tight. The immediate gas outletshould be 30 cm above substrate level.Smaller units may use the fixed dome (hy-draulic pressure) system made out of ma-sonry structure. Larger units, store thebiogas in steel drums or plastic bags (seealso chapter 12.).

The choice of gas storage system will de-pend on the pattern of gas utilisation. Nor-mally, gas production should go togetherwith gas consumption, time-wise and vol-ume-wise. For more details, please refer tothe chapter “Biogas Utilisation” and theabundance of special biogas literature.

Starting Phase and Maintenance

Starting with some active sludge from a sep-tic tank speeds up digestion and preventsthe digester from turning sour. In the rarecase that this should happen, reduce theloading rate until the pH turns neutral. Itmay be necessary to remove sand and gritafter some years.

Calculation of Dimensions

The main parameter is the hydraulic reten-tion time, which should not be less than 15days in hot climate and not less than 25days in a moderately warm climate; a HRTof more than 60 days should be chosen forhighly pathogenic substrate. The gas stor-age volume depends on daily gas use inrelation to daily gas production. The stor-age capacity of gas for household useshould exceed 65% of the daily gas pro-

duction. Gas production is directly relatedto the organic fraction of the substrate. Inpractice it is calculated as a fraction of thedaily substrate that is fed. The fraction usedfor calculation is found by experience, forexample, 1 kg fresh cattle dung diluted with1 litre of water produces 40 l of biogas. Formore elaborate calculation use the formulaapplied in the computer spread sheet Tab. 27.

9.8 Trickling Filter

The trickling filter is not considered to be aDEWATS solution. However, an understand-ing of how it works will better one’s under-standing of the principle of aerobic waste-water treatment.

The trickling filter follows the same princi-ple as the anaerobic filter, in the sense thatit provides a large surface for bacteria tosettle. The main difference between the twosystems lies in the fact that the tricklingfilter works under aerobic conditions. Thisimplies that the bacteria that are immobi-lised at the filter medium must have equalaccess to air and wastewater. Therefore,certain doses of wastewater are charged inintervals to give time for air to enter thereactor during the breaks. Further, watermust be distributed equally over the fullsurface in order to utilise the full filter massefficiently.

Therefore, the trickling filter consists of

o a dosing device

o a rotating sprinkler

o the filter body which is ventilated bothfrom the top and the bottom.

Rocks between 3 to 8 cm in diameter areused as filter medium. The outside of thefilter body is closed to prevent sludge flies

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from escaping into the open. The filter restsabove ground to allow ventilation. The bot-tom slab is at a slope to let water andsludge be rinsed off. The bacterial film hasto be flushed away regularly to prevent clog-ging and to remove the dead sludge. Highhydraulic loading rates (> 0.8 m3/m3×h) havea self-flushing effect. With organic loadingrates of 1 kg BOD/m3×d, 80% BOD removalis possible. Higher loading rates would re-duce effectivity.

equal distribution ofwastewater by

rotating sprinkler

O2 O2 O2

oxygen is drawn-inby vacuum effectduring sprinkling

O2

O2

O2

oxygen is availablefor decompositionduring resting time

wastewater

principle of trickling filter

oxygen is also drawn by chimney effect due todifference in temperature

wastewater is supplied in doses which allows resting time of severalminutes or hours between each dose

O2

Fig. 30.The principle of the trickling filter

Considering a 2 m high trickling filter and awastewater of 500 mg/l BOD, the organic load-ing rate comes to 0.8×24hrs ×0.500 kg/m3

BOD / 2 m height = 4.8 kg BOD/m3×d. A re-moval rate of only 60% BOD may be ex-pected with such a high organic load. Thissimple calculation indicates that wastewaterwould have to be re-cycled almost 5 timesto get the expected treatment quality andthe self-flushing effect. However, the trick-ling filter could be operated with lower hy-draulic loading rates if regular flushing isdone.

Despite fluctuation in the flow of waste-water, the self-flushing (high-rate) trickling

filter is a reliable system. Nonetheless, be-cause it requires a rotating sprinkler and apump to be operated, it is not considereda DEWATS solution.

Starting Phase, Maintenance andCalculation of Dimensions

Details for calculation and instructions foroperation are not given in this handbookin order to avoid the impression that thetrickling filter can be built and operatedunder DEWATS conditions.

9.9 Constructed Wetlands

There are three basic treatment systemswhich may fall in the category of constructedwetlands. These are

o the overland treatment system

o the vertical flow filter, and

o the horizontal flow filter.

For overland treatment the water is distrib-uted on carefully contoured land by sprin-klers. The system requires permanent at-tendance and maintenance. For that rea-son it does not belong to DEWATS.

For vertical filter treatment the wastewateris distributed with the help of a dosing de-vice on two or three filter beds which arecharged alternately. Charging intervals mustbe strictly followed which makes the verti-cal filter less suitable for DEWATS.

The horizontal filter is simple by principleand requires almost no maintenance, how-ever under the condition that it has beenwell designed and constructed. Design andconstruction requires a solid understand-ing of the treatment process and good

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water flow in the horizontal filterplan longitudinal section

plantation, preferablyphragmites

inlet outlet inlet outlet

rizomes

1,0%bottom slope

O2 O2 O2O2 O2O2

internal water level

cross distribution trench collection trench

overflow

Principle of horizontal filter processcontinuous oxygen supply to the upper layers onlymajor role of plants: provide favourable environment

for bacteria diversity

anaerobic and anoxic conditions in the lower layers

submerging forequal distribution of

wastewater

O2 O2 O2

oxygen is drawn-inby vacuum effect

O2

O2

O2

oxygen is availablefor decompositionduring resting time

Sand layer

wastewater

principle of vertical filter process

major role of plants: keeping filter surface porous

knowledge of the filter medium that is tobe used.

Constructed wetlands, especially sand andgravel filters, are by no means a simple

Fig. 31. The principle of the horizontal filter

Fig. 32.The principle of the vertical filter

technology, although they may look like partof nature. Before deciding on filter treat-ment, one should always consider the al-ternative of constructing wastewater pondsinstead. Nonetheless, filter treatment hasthe great advantage of keeping the waste-water below ground.

The horizontal and the vertical filter are twosystems that are principally different. Thehorizontal filter (Fig. 31.) is permanentlysoaked with water and operates partly aero-bic (free oxygen present), partly anoxic (nofree oxygen but nitrate -NO3- present) andpartly anaerobic (no free oxygen and nonitrate present). The vertical filter (Fig. 32.)is charged in intervals (similar to a tricklingfilter) and functions predominantly aerobi-cally. Although the vertical filter requires onlyabout half the area of a horizontal filterand has better treatment qualities, only the

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horizontal filter is considered a DEWATS-technology for the reason that it has nomovable parts and does not require per-manent operational control.

9.10 The Horizontal Gravel Filter

Planted horizontal gravel filters are also re-ferred to as Subsurface Flow Wetlands (SSF),Constructed Wetlands or Root Zone Treat-ment Plants. They are suitable for pre-treated (pre-settled) domestic or industrialwastewater of a COD content not higherthan 500 mg/l. Wastewater must be pre-treated especially in respect to suspendedsolids, due to the fact that the biggest prob-lem in ground filters is clogging. When test-ing wastewater, the sediment after 60 min-utes in an Imhoff cone should not be morethan 1 ml/l, and not more than 100mg SS/lin case of non-settling industrial wastewater.If the COD-value of settleable solids is lessthan 40% the total SS-value, then many ofthe solids are likely to be fat in colloidalform which can reduce the hydraulic con-ductivity of the filter considerably (as maybe the case with dairy wastewater).

The treatment process in horizontal groundfilters is complex. There are several argu-

ments “in the air” concerning the physi-cal process of filtration, the intake of oxy-gen as well as the influence of planta-tion on the biological treatment process.Even if all influencing factors would beknown, it is still their interaction whichis difficult to predict.

There are sophisticated methods to cal-culate the proper dimensions and treat-ment characteristics of different filtermedia, especially in respect to hydraulicproperties. However, such calculationsmake sense only if the required param-eters are known quite exactly. This is al-most never the case. Rules of thumb,intelligently chosen, are more than suffi-cient for smaller sized plants in particu-lar, as is usually the case with DEWATS.Going beyond these figures based on ex-perience is not advisable without previ-ous tests. Safety rules of design are:

o large and shallow filter bed

o wide inlet zone

o reliable distribution of inflow over thefull width of the inlet zone

o round coarse gravel of nearly equalsize as filter medium.

∅ 25 mmpore space 22,1 %

max pore size 2,8 mmspec. surface 143 m²/m³

∅ 5 mmpore space 45,7 %

max pore size 0,6 mmspec. surface 652 m²/m³

∅ 5 mm and 25 mmpore space 23,9 %

max pore size 1,6 mmspec. surface 164 m²/m³

mixed grain sizemixed grain shape

pore space and pore sizeunpredictable

Fig. 33. Influence of grain size and shape on filter properties

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While large grain size with a high percent-age of voids prevents clogging, it also re-duces treatment performance. Clogging iscaused by suspended solids and by newlyformed biological or mineralised sludge fromthe decomposition of organic matter. There-fore, the front portion must have voids thatare small enough to retain enough SS andlarge enough to distribute the filtered SSover a longer distance. Round, uniformgravel of 6 - 12 mm or 8 - 16 mm is best.

Conductivity may be only half with edgedbroken stones compared to round gravelbecause of turbulent flow within irregularpores. In case of mixed grain size, it mightbe advisable to screen the gravel with thehelp of a coarse sieve; use the larger grainsin the front and the smaller grains to therear. Large grains should be chosen in caseof flat or mixed grain shape, for examplewhen using chipping made from brokenstones. Care is to be taken when changingfrom a larger grain size to the smaller, be-cause it has been observed that blockagehappens predominantly at the point ofchange.

A rather flat slope (α < 45°) should joinone-grain size to the other in order to ob-tain a larger connecting area. Particularlywhen grain diameter differs considerably,an intermediate zone consisting of inter-

mediate size may be considered. Mixed grainsizes will not improve hydraulic conductiv-ity! Howsoever, removing fine soil fromgravel by washing is more important thanensuring the exact grain size.

In case the length of the filter bed is morethan 10 m, an intermediate channel for re-distribution of cross-flow could be provided.The distribution channel could serve as astep of terrace of the surface level in caseof high percentage bottom slope (Fig. 34. ).

The relation between organic load and oxy-gen supply reduces with length. This hap-pens because oxygen is supplied evenlyover the total surface area, whereas theorganic load diminishes during treatment.It is therefore most likely that anaerobicconditions prevail in the front part, whileaerobic conditions reach to a greater depthin the rear part. However, only the upper 5to 15 cm can really be considered an aero-bic zone.

Clogged gravel filter can become usefulagain after resting periods of severalmonths. This happens due to the bacteriahaving to go without feed and having tolive on its own bacterial mass in that pe-riod. This process is called autolysis.

Filter clogging results normally in surfaceflow of wastewater. This is usually not

Properties of gravel and sand for ground filters

filter mediumdiameterof grain pore volume

theoreticalconductivity

mm coarse total m/s m/dgravel 4 - 40 30% 35% - 40% 4,14E-03 350sand 0,1 - 4 15% 42% 4,14E-04 35

after Bahlo, Wach pg 57

Tab. 13.Theoretical properties of gravel and sand as filter material, lower values should be applied for wastewaterwhen designing filter beds.

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Fig. 34.Horizontal gravel filter (subsurface flow filter). A: Filterbasin in masonry and concrete structure, finer gravelis used in the rear portion. B: Long filter bed with additional distribution trench in the middle, the trenchis filled with rocks and allows a step in the surface level. C: Detail of collection pipe and swivel arm at theoutlet side. D: Details of inlet and outlet structure for improved distribution of flow in case of wider filterbeds. E: Details of filter basins using foils or clay packing for sealing. Sloped side walls are less costly, butplants will not grow near the rim.

wanted, although it hardly reduces the treat-ment efficiency if flow time on the surfaceis not much shorter than the assumed re-tention time inside the filter (this could bethe case with dense plant coverage). Whenfilters are well protected and far away from

residences there is no harm to let some ofthe wastewater run above the horizontalsurface. Such “overland treatment” producesvery good results especially when the wa-ter is equally distributed and does not fes-ter in trenches.

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The actual retention time regarding voidsspace plays a decisive role in the treatmentprocess. Gravel has 30 - 45% voids, de-pending on size and shape. (The calcula-tion of HRT in the spread sheet of chapter13.1.11 is based on 35% void space, it couldproportionally be reduced if the actual voidspace is definitely more.) Void space caneasily be found out by measuring the wa-ter that can be added to a bucket full ofgravel ( Fig. 35.).

measure volume when putting in water

8-litresmark

Volume of water= 8 litres

8-litresmark

Volume of water= 3,2 litres

measure volume when putting in water

empty bucket and fillin filter medium

Fig. 35.Determining pore space of filter medium at site. Forexample the empty bucket is full after pouring 8litre of water. The bucket filled with gravel absorbs3.2 litre of water: Voids space is 3.2/8 = 0.40 or40%.

For high conductivity large pore size is moreimportant than total pore volume. There-fore it is better to use pre-wetted gravelwhen testing the pore volume; then poresof only capillary size are „closed“ in ad-vance.

In reality, short cuts and reduced volumeby partly clogged areas result in 25% shorterretention times and consequently lead toinferior performance (A.N. Shilton and J.N.Prasad in WST, Vol. 34, No. 3-4 Pg. 421).For this reason, the filter bed should notbe deeper than the depth to which plantroots can grow (30 - 60 cm) as water will

tend to flow faster below the dense cushionof roots. However, treatment performance isgenerally best in the upper 15 cm due tooxygen diffusion from the surface. Therefore,shallow filters are more effective comparedto deeper beds of the same volume.

Uniform distribution of wastewater through-out the filter is decisively dependent onequally distributed supply of water at theinlet and equally distributed reception at

the outlet side. Trenchesfilled with rocks of 50to 100 mm diameter areprovided at both endsto serve this purpose. Aperforated pipe that isconnected to the outletpipe lies below the stripof rocks that form thecollection trench. Theheight of this outlet is

adjustable through a swivel arm fixed to aflexible elbow. The height is adjusted ac-cording to hydraulic conductivity by liftingit until water appears at the surface of thefilter near the inlet. While the top of thefilter is kept strictly horizontal to preventerosion, the bottom slopes down from in-let to outlet by preferably 1 %. Site condi-tions permitting, greater slope is also pos-sible. To prevent erosion, long filters shouldhave a terraced surface instead of a slope(see Fig. 34. (B)).

Percolation of wastewater into the groundis normally not desired. To prevent this fromhappening the bottom of the filter must besealed. While solid clay packing might do,heavy plastic foils are more common. A con-crete basin with straight vertical masonrywalls would allow plants to grow up to theouter rim, which is not possible with thesmooth embankment that plastic foils would

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require (Fig. 34. (E)). In dry climate, treessearch for water and their roots may breakthe walls while trying to grow into the fil-ter. Whenever possible, trees should notbe planted directly at the side of the filterfor the reason that the strong roots of thetrees could well spoil the structure and fallenleaves could seal the filter surface.

Observations in Europe indicate that theperformance of gravel filters diminishes af-ter some years. How long a horizontal filtermight work depends on several factors:grain size and shape of gravel, the natureand amount of suspended solids in thewastewater, the temperature and the aver-age loading rate.

If the filter is drained during resting time,alternate charging could increase treatmentperformance of horizontal filters. To allowsuch alternate feeding, it is advisable todivide the total filter area into several com-partments or beds. Other reports insist onchanging the filter every 8 to 15 years. Thistoo, nevertheless, will depend on the load-ing rate and structural details, of which theimpact is almost impossible to predict inpractice. Weaker wastewater, lower loadingrates and larger grain size of the gravel isassumed to increase the lifetime of the sys-tem.

Ground filters are covered by suitable plan-tation - meaning any plant which can growon wastewater and whose roots go deepand spread wide. To an extent, perform-ance may also depend on the species ofplant chosen. Some scientists claim thatthe micro-environment inside the filter issuch that it creates equilibrium betweensludge production and sludge “consump-tion”. Such equilibrium is only likely withlow loading rates.

Plants are normally not harvested. Phrag-mites australis (reed) the world over is con-sidered to be the best plant because itsroots form horizontal rhizomes that guar-antee a perfect root zone filter bed.(Fig. 36.).There could be other plants to suit otherwastewater. For instance, typha angustifolia(cattails) together with scirpus lacustris (bullrush) has been found most suitable forwastewater from petrol refineries. Mostswamp and water grasses are suitable, butnot all of them have extending or deeproots. The large, red or orange floweringiris (sometimes known as “mosquito lily”)grows well on wastewater. It is a beautifulplant, however suitable for shallow domes-tic gravel beds, only. Forest trees have alsobeen used and are said to be only slightlyless efficient (Kadlec and Knight). Whatever,at least 2 bunches of plants or four sproutedrhizomes should be placed per square me-ter when starting plantation.

BORDA after Bahlo/Wach

Fig. 36.Plant species common for gravel filter plantation

Several experts propose a certain follow upof plant species to improve treatment qual-ity. However, the main role of the plantsseems to be that of a „catalyst“ rather than

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an „actor“. Plants transport oxygen via theirroots into the ground. Some scientists claimthat surplus oxygen is also provided forcreating an aerobic environment while oth-ers have found out that only as much oxy-gen as the plant needs to transform its ownnutrient requirement, is transferred. For ex-ample Brix and Schierup claim that 0,02 gO2/m2×d are provided by the plants to thefilter bed, while 2,06 g/m2×d are used bythe plants themselves. Toxic substances nearthe roots may also be eliminated by oxida-tion. Howsoever, the complex ecosystemthat exists in a gravel filter together withplantation in all respects produces goodand reliable treatment results, which mustdefinitely be aerobic, as well. Some reportsclaim COD reduction rates of over 95%which would not be possible under anaero-bic conditions alone. The uptake of nutri-ents by plants is of relatively little impor-tance, especially when plants are not har-vested.

Starting Phase and Maintenance

Young plant seedlings may not grow onwastewater. It is therefore advisable to startfeeding the plant with plenty of fresh waterand to let the pollution load grow slowlyand parallel to plant growth.

When plants are under full load, the outletlevel is adjusted according to the flow. Wa-ter should not stand on the surface nearthe inlet. If this should happen, the swivelarm at the outlet must be lowered. Optimalwater distribution at the inlet side is im-portant and must be controlled from timeto time. It is necessary to replace the filtermedia when treatment efficiency goes down.Since there is no treatment during the timethat the filter media is being replaced, it isadvantageous to install several parallel fil-ter beds.

Stormwater should neither be mixed withthe wastewater before, nor should outsidestormwater overflow the filter bed, becauseof the fine soil particles which come withthat water. Erosion trenches around the fil-ter bed should always be kept in properfunctioning condition.

Calculation of Dimensions

If percolation properties - the so called hy-draulic conductivity of the filter body - isknown, then the required cross sectionalarea at the inlet can be calculated usingDarcy’s law. To make good for reduced con-ductivity after some time of operation onlya fraction of the calculated figures for clear

Fig. 37.Darcy’s law for calculation of hydraulic conductivity

cross sectional area offilter bed [m²]

flow rate [m³/sec]

hydraulic conductivity [m/sec] × slope [m height/m length]=

QsAc =

kf × dH / dsDarcy's Law

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water should be used for designing the plantThe conductivity given in the spread sheettakes care of that, already. However, not tothe extent of some pessimistic statementswhich claim that only 4% of the clear waterconductivity should beused.

The dimensions of thefilter depend on hydrau-lic and organic loadingand temperature andgrain size of the filtermedium. As a rule ofthumb, 5 m2 of filtershould be provided percapita for domesticwastewater. This wouldmean a hydraulic load-ing rate of 30 l/m2 and an organic loadingrate of 8 g BOD/m 2×d. For comprehensivecalculation use of the formula applied inthe computer spread sheet (Tab.28).

9.11 Vertical Sand Filter

The vertical filter functions like an aerobictrickling filter and consequently must befed at intervals with defined resting timesbetween the doses of charging. In additionto the short intervals that are controlled bydosing devices, longer resting periods ofone or two weeks are also required. This isonly possible with at least two filter bedsthat are fed alternately.

Feeding in doses is necessary for equalwater distribution. The resting times areneeded to enable oxygen to enter the filterafter wastewater has percolated (Fig. 32.).Doses must be large enough to completelyflood the filter for a short while in order todistribute the water evenly over the sur-

face and small enough to allow enough timefor oxygen to enter before the next flood-ing. Correspondingly, the filter material mustbe fine enough to cause flooding and po-rous enough to allow quick percolation.

Fig. 38.Vertical filter during charging. Constructed aboveground by Nature & Technique, D. Esser, for a cheesedairy in southern France. [photo: Sasse]

During the short time of charging, thewastewater is also exposed to the open,which can create ‘bad odour’ in case ofanaerobic pre-treatment. The vertical filteris not propagated as DEWATS largely be-cause keen observation of all the abovementioned points are essential. Aside fromthis disadvantage, the vertical filter is - com-pared to the horizontal filter - the more ef-ficient and more reliable treatment systemsfrom a technical and scientific point of view,reasons for which its main features are de-scribed herein.

The body of the vertical filter consist of afiner top layer, a medium middle layer anda rough bottom layer. The area below thefilter media is a free flow area that is con-nected to the drainpipe. The free flow areais also connected to the open via additional

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While vertical filters can bear a hydraulicload up to 100 l/m2×d (100 mm/m2 = 0.1 m) itis better restricted to 50 l/m2×d. The or-ganic load can go up to 20g BOD/m2×d. Incase of re-circulation, 40g BOD/m2×d ispossible (M&E). In the case of pre-treateddomestic wastewater, the hydraulic load isthe deciding factor. Some engineers usethese values only for the active filter bedswhile others claim that the resting beds mayalso be included. Testing is called for incase of doubt. However, the provision of alarger filter area is always recommended.

Calculation of permeability follows alsoDarcy’s law (Fig. 37.), whereas dH/ds = 1.Therefore flow speed (v = Qs/Ac) is equal tohydraulic conductivity (k).

vent pipes. The fine top layer guaranteesequal water distribution by flow. The mid-dle layer is the actual treatment zone whilethe bottom layer is responsible for provid-ing wide-open pores to reduce the capil-lary forces which otherwise would decreasethe effective hydraulic gradient.

The most usual depth of vertical filters is1 m to 1.20 m. However, if there is enoughnatural slope and good ventilation, verticalfilters can also be built up to a depth of 3metres. Vertical filters may or may not becovered by plantation. In the absence ofplantation, the surface must be scratchedat the beginning of the resting period, inorder to allow enough oxygen to enter. Withdense plantation however, this is avoidableas the stems of plants keep the pores openon the surface.

Several charging points are distributed overthe surface to allow quick flooding of thefull area. Flooding is the only reliablemethod of achieving equal distribution ofwater over the filter. It is not possible toachieve equal distribution by using supplypipes of different diameters or by a propercalculation of the distance of outlet points.This has been tried often enough; new fail-ures are not necessary. Flush distribution isa must.

Dosing of flow can be done with the helpof either self acting siphons, automatic con-trolled pumps or tipping-buckets. The lat-ter might be the most suitable underDEWATS conditions because its principle iseasily understood and the hardware can bemanufactured locally.

Each filter bed has a separate inlet pipe withvalve. Alternately, a straight standing pieceof pipe can be used for closing the outletsof the dosing chamber (Fig. 40.).

Fig. 39.Dosing chamber with tipping bucket for controlledoperation of siphon. The bucket closes the siphonuntil it is filled with water. When loosing its equilib-rium due to the weight of the water, the bucketturns over and opens the siphon. It falls back intohorizontal position to receive new water, which againcloses the siphon for the next flush.

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Starting Phase, Maintenance andCalculation of Dimensions

The vertical sand filter does not belong toDEWATS. Detailed operational instructionshave been deliberately excluded in thishandbook to avoid the impression that thevertical filter can still be build and oper-ated under DEWATS conditions.

9.12 Ponds

Ponds (lagoons) are artificial lakes. Whathappens in ponds closely represents treat-ment processes which take place in nature.In artificial ponds the different treatmentprocesses are often separated. All pondsare ideal DEWATS and should be given

Fig. 40.Distribution chamber for alternate feeding of filterbeds. A piece of straight pipe is put on that outletwhich is to be closed, temporarily.

preference over other systems wheneverland is available. Ponds are preferred be-fore underground gravel filters if an openpond is acceptable to the surrounding. Incase of facultative or anaerobic ponds, thedistance to residential houses or workingplaces should be far enough to avoid nui-sance by mosquito breading, or bad odour.Polishing ponds can be nearer because theuse of fish to control mosquitoes is possi-ble. Fish that belong to Gambusia spp. arecommonly used for mosquito control intropical countries.

Pure pond systems are cheap and need al-most no maintenance, even in larger size.

Ponds may be classified into

o sedimentation ponds (pre-treatmentponds with anaerobic sludge stabili-sation)

o anaerobic ponds (anaerobic stabilisationponds)

o oxidation ponds (aerobic cum facultativestabilisation ponds)

o polishing ponds (post-treatment ponds,placed after stabilisation ponds)

Pond systems that are planned for full treat-ment normally consist of several ponds serv-ing different purposes. For instance, a deepanaerobic sedimentation pond for sedimen-tation cum anaerobic stabilisation of sludge,two or three shallow aerobic and faculta-tive oxidation ponds with longer retentiontimes for predominantly aerobic degrada-tion of suspended and dissolved matter andone or several shallow polishing ponds forfinal sedimentation of suspended stabilisedsolids and bacteria mass. Wastewater pondsfor the purpose of fish farming must beinitially low loaded, and in addition, be di-luted by four to five times with river water.

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Otherwise the pond must be about ten timeslarger than calculated in the spread sheet(Tab. 30).

Artificially aerated ponds are not con-sidered to be DEWATS and are thereforenot dealt with in this handbook. It maybe enough to know that such ponds are

Tab. 15.Design parameters for low loaded anaero-bic ponds in relation to ambient tempera-ture

sedimentation pondinlet longitudinal section outlet high-loaded

anaerobic pondinlet longitudinal section outlet

scum

liquid

low-loaded anaerobic pond sludge

inlet longitudinal section outlet

Fig. 41.Principles of anaerobic ponds. Sedimentation ponds have a HRT of about 1 day, low loaded ponds aresupposed to be odourless because of almost neutral pH, high loaded ponds form a sealing scum layer on top.

Performance of a settling pond of 48 hrs HRT for domestic sewage in Morocco

pollutant dimension inflow outflow rem ratesuspended solids mg/l 431 139 68%COD mg/l mg/l 1189 * 505 58%*BOD5 mg/l 374 190 49%Nkjel mg N/l 116 99 15%P total mg/l 26 24,5 6%fecal coli No/100 ml 6.156.000 496.000 92%fecal strepto No/100 ml 20.900.000 1.603.000 92%nematode ova No 139 32 77%cestode ova No 75 18 76%helminth ova No 214 47 78%

* the high COD/BOD ratio is caused by mineral oil pollution which is also the reason for the COD removal rate being higher than that of the BOD5

Driouache et al, GTZ / CDER, 1997 pg 17

Design parameters for low loaded anaerobic ponds

ambient temper. °C

org. load BOD g/m³*d

efficiency BOD rem. %

10 100 4015 200 5020 300 6023 330 6625 350 7028 380 7030 400 7033 430 70

from Mara 1997

Tab. 14.An example of the high performance of a simple settlingpond

1,5 - 3,5 m deep, usually work with a5 days hydraulic retention time (HRT) andorganic loads of 20 to 30 g BOD/m3×d. Theenergy requirement for aeration is about1 - 3 W/m3 of pond volume. In case of onlylittle scum formation only the surface ofanaerobic ponds may be aerated to reducefoul smell.

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9.12.1 Anaerobic ponds

Anaerobic ponds are deep (2 to 6 m) andhighly loaded (0.1 to 1 kg BOD/m3×d). Be-cause of that they need less surface areacompared to aerobic-facultative oxidationponds. Anaerobic ponds maintain theiranaerobic conditions only through the depthof the pond, therefore a minimum depth of2 m is necessary. It is possible to provideseparate sludge settling tanks before themain pond, in order to reduce the organicsludge load of that pond. Such settling tanksshould have a HRT of less than 1 day, de-pending on the kind of wastewater.

Anaerobic ponds with organic loading ratesbelow 300 g/m3×d BOD are likely to stay atan almost neutral pH. Consequently theyrelease little H2S and therefore, are almostfree of unpleasant smell. Highly loadedanaerobic ponds are particularly bad smell-ing in the beginning until a heavy layer ofscum has been developed. Before suchscum layer has developed, a small upper

layer will remain aerobic; one may then la-bel these ponds facultative-anaerobicponds.

Depending on strength and type of waste-water and the desired treatment effect,anaerobic ponds are designed for hydrau-lic retention time between 1 and 30 days. Itdepends on whether only settled sludge orall the liquid is to be treated. The kind ofwastewater and the type of post treatmentdefines the role of the anaerobic pond. Fordomestic wastewater the anaerobic pondmay function as an open septic tank. Itshould then be small in order to develop asealing scum layer. Treatment efficiency isin that case in the range of 50% to 70%BOD removal, only.

A stinky effluent is the result of a „wrong“retention time. Because, if the retention timeis longer than one day, not only bottomsludge but also the liquid portion starts toferment. On the other hand, if the retentiontime is too short for substantial stabilisation

Fig. 42.Cross sections of anaerobic ponds constructed out of rocks with cement mortar pointing. A and B: The inletportion is made deeper in order to accumulate most of the sludge at a limited surface area. C: Twoanaerobic ponds in series. The first pond may be high loaded (scum sealed), the second pond may be lowloaded (neutral pH).

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of the liquid, then the effluent remains at alow pH and stinks of H2S. Normally, tooshort retention times mean also too highorganic loading rates.

Anaerobic ponds are often used as the firsttreatment unit for industrial wastewater,followed by oxidation ponds, for examplein sugar plants or distilleries. Treatmentefficiency of high-loaded ponds with longretention times is in the range of 70% to95% BOD removal (CODrem 65% to 90%)depending on biodegradability of the waste-water. Several ponds in series are recom-mended in case of long retention times.

Anaerobic ponds are not very efficient totreat wastewater with a wide COD/BOD ra-tio (>3 : 1). Sedimentation ponds with veryshort retention times followed by aerobic /facultative stabilisation ponds give betterresults in that case.

Pond size is also based on the long-termsludge storage volume that in turn relatesto long cleaning intervals. The anaerobicpond can also be used as integrated sludgestorage. In this case, sludge removal inter-vals of over 10 years are possible.

Starting Phase and Maintenance

The start-up does not require special ar-rangements. It should however be knownthat a heavy loaded pond would releasebad odour until a layer of scum seals thesurface. Inlet and outlet structures shouldbe controlled during operation. A drop inthe quality of the effluent is a warning thatthe sludge must be removed. If this is ne-glected, the receiving waters or the treat-ment units which follow the pond will beput into trouble.

Calculation of Dimensions

Retention time and volumetric organic loadare the two design parameters for anaero-bic ponds. A non-smelling pond loaded with300 g BOD/m3×d, for a short HRT of oneday would mean approximately 0,2 m3 percapita for domestic wastewater. For anaero-bic stabilisation of the liquid fraction longerretention times are required which dependon temperature, desired treatment qualityand organic load. Organic loading rateshould not exceed 1 kg BOD/m3×d. For ex-act calculation use the formula applied inthe computer spread sheet (Tab. 29a, 29b.).

9.12.2 Aerobic Ponds

Aerobic ponds receive most of their oxygenvia the water surface. For loading rates be-low 4 g BOD/m2×d, surface oxygen can meetthe full oxygen demand. Oxygen intake in-creases at lower temperatures and withsurface turbulence caused by wind and rain.Oxygen intake depends further on the ac-tual oxygen deficit up to saturation pointand thus may vary at 20°C between 40 gO2 /m2×d for fully anaerobic conditions and10 g O2 /m2×d in case of 75% oxygen satu-ration. (Mudrak & Kunst, after Ottmann1977).

The secondary source of oxygen comes fromalgae via photosynthesis. However, in gen-eral, too intensive growth of algae andhighly turbid water prevents sunlight fromreaching the lower strata of the pond. Oxy-gen „production“ is then reduced becausephotosynthesis cannot take place. The re-sult is a foul smell because anaerobic fac-ultative conditions prevail. Algae are impor-tant and positive for the treatment proc-ess, but are a negative factor when it comes

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to effluent quality. Consequently, algaegrowth is allowed and wanted in the be-ginning of treatment, but not desired whenit comes to the point of discharge, becausealgae increase the BOD of the effluent. Al-gae in the effluent can be reduced by asmall last pond with maximum 1 day reten-tion time. Larger pond area - low loadingrates with reduced nutrient supply for al-gae - are the most secure, but also the mostexpensive measure.

Laboratory results of effluent wastewateroften give a false impression of insufficienttreatment. As nearly 90% of the effluent BODcomes from algae, many countries allowhigher BOD loads in the effluent from pondsas compared to other treatment systems.Baffles or rock bedding before the outlet ofeach of the ponds have remarkable effecton retaining of algae. Intelligent structuraldetails increase the treatment quality con-siderably at hardly any additional cost andmay be seen as important as adequate pondsize.

Treatment efficiency increases with longerretention times. The number of ponds is ofonly relative influence. With the same totalsurface, efficiency increases by splitting one

pond into two ponds by approximately 10 %.Having three instead of two ponds addsabout 4 % and from three to four pondsefficiency may be increased by another 2 %.This shows that more than three ponds arenot justifiable from an economic point ofview, because the same effect can beachieved by just enlarging the surface area.The land required for dams and banks ofan additional pond could better directly beadded to the water area. The first pondmay be up to double the size of the others,if there are several inlet points.

In principle, it is of advantage to have sev-eral inlet points in order to distribute thepollution load more equally and to create alarger area for sedimentation. On the otherhand, it might be advisable to provide aslightly separated inlet zone in order toavoid bulky floating matters littering thetotal pond surface.

The inlet points should be farthest awayfrom the outlet. The outlet should be be-low water surface in order to retain floatingsolids, including algae. Gravel beds func-tioning as roughing filter are advisable be-tween ponds in row and before the finaloutlet.

Aerobic Ponds in Series with Polishing Pond

inlet

outlet

1. main pond 2. main pond 3. main pond polishingpond

Fig. 43.Flow pattern of aerobic-facultative ponds in series

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9 TECHNOLOGY

Erosion of banks by waves could be a prob-lem with larger ponds. Therefore slope shouldbe 1 (vertical) to 3 (horizontal) and prefer-ably covered with rocks or large sized gravel.

Banks and dams could be planted withmacrophytes, such as cattail or phragmites.Dams between ponds should be paved andwide enough to facilitate maintenance.

Fig. 44.Section through a large sized aerobic-facultative stabilisation pond. Banks should be protected againsterosion by waves. A: Inlet; banks should also be protected against erosion by influent. B: Cross section B-B (front view of C). C: Outlet structure with swivel arm to adjust height of pond according to seasonalfluctuation of water volume.

Fig. 45.Details for aerobic stabilisation ponds (basins) of smaller size. A: Inlet structure, concrete flooring,B: Partition wall, compacted clay flooring, C: Outlet structure, foil flooring (protection against use may beadvisable).

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Fish feed on algae; but fish can only live ifthere is enough dissolved oxygen available- 3 mg/l is the absolute minimum for sludgefish. Therefore, the dilution of wastewaterby mixing water from other sources (rivers,already existing lakes) becomes necessaryfailing which, only low organic loading ratesare permissible.

Aerobic stabilisation ponds for reasons ofoxygen intake should be shallow but deepenough to prevent weed growth at the bot-tom of the pond. A depth of 90 cm to 1 min warm climate and up to 1.2 m in coldclimate zones (due to frost) is suitable.Deeper ponds become facultative or evenanaerobic in the lower strata.

Smaller volumes of wastewater, such as fromschools, hospitals, residential houses shouldbetter be pre-treated in Imhoff tanks, sep-tic tanks, baffled reactors or at least sedi-mentation pits, before reaching the aerobicstabilisation pond. Properly operated Imhofftanks that keep water fresh and withoutsmell are preferable. A septic tank wouldbe better if regular desludging of the Imhoff

tank cannot be guaranteed. The effluent willbe “stinky” anyhow. If pre-treatment in pondsdoes not take place, the pond must be pro-vided with a deeper sedimentation zone nearthe inlet. However, bad odour is to be ex-pected. It might be wiser to construct a smallsedimentation pond on which a sealingscum layer will develop. Should the scumlayer become more than 10 cm thick, papy-rus that has a beautification effect couldbe grown on it.

Starting Phase and Maintenance

The pond matures much faster if it is filledwith river water before the first wastewaterenters. With the exception of regular con-trol of inlet and outlet structures, no per-manent attendance is required. However,the performance of the pond should be su-pervised and any disturbance of the waterquality should be carefully investigated tofind the cause. Excessive accumulation ofsludge could result in inadequate treatment.The pond must be emptied and the sludge

must be removed in defined inter-vals, before treatment quality goesdown.

Calculation of Dimensions

Organic surface load and hydraulicretention time are the two designparameters. While minimum hydrau-lic retention times may be between

Fig. 46.Papyrus growing on scum of a small sedi-mentation tank in Auroville / India [photo:Sasse]

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[BORDA after Brüggemann]

Fig. 47.Aquatic plants, commonly used for wastewater treat-ment

The area that is required for a pond is al-most the same regardless of aquatic plants.If the organic loading rate is low, plantshave the advantage of protecting fish usedto control mosquito, from birds. However,some plants, water hyacinth for example,are a disadvantage as they shelter mos-quito larva from fish. Snakes also find shel-ter. High organic loading rates that may bewhy additional treatment by aquatic plantsis sought, do not allow survival of fish formosquito control.

Since aquatic plant systems become a nui-sance if not properly taken care of, theyare not considered as DEWATS. However,aquatic plants make sense if utilised in con-junction with wastewater farming which usesintensive and controlled nutrient recycling,or for the purpose of beautifying residencesthat are nearby.

Starting Phase and Maintenance

Maintenance and operation is mainly an is-sue of agricultural management rather, thanan issue of wastewater treatment. The pondshould be started with fresh, river waterand the pollution load should be applied

Tab. 16.Organic surface loadingon aerobic-facultativeponds

Maximum surface load onoxidation ponds

5 days HRTambient

temper. °Corg.load

BOD g/m²*d10 7,015 11,720 17,723 21,825 24,528 28,430 30,833 33,8

from Mara 1997

5 and 20 days, the maximum organic loaddepends on ambient temperature (Tab. 16.).Sunshine hours are also important but arenot included in the calculation. However,ponds should be slightly oversized in ar-eas with permanent cloud cover. Organicloading should better be below 20 g BOD/m2×d. 2.5 to 10 m2 pond surface may becalculated per capita for domestic waste-water. All values will depend on the type ofpre-treatment and temperature and healthobjectives. For more exact calculation usethe formula applied in the computer spreadsheet (Tab. 30.).

9.12.3 Aquatic Plant Systems

Water hyacinth, duck weed, water cabbageand other aquatic plants can improve thetreatment capacity of pond systems. Heavymetals that accumulate in water hyacinthsare removed when the plants are harvested.Duckweed is a good substitute for algae.Since it is easily retained in a surface baffle,it leaves a cleaner effluent. If not confinedin fixed frames, duckweed is blown by windto the lee-side. Improved treatment efficiencyhowever, is only guaranteed by regular at-tendance and harvesting. Special design fea-tures for harvesting increase the total arearequirement of the treatment system. The

evaporation rate ofaquatic plant sys-tems is 4 timeshigher than that ofopen ponds (in therange of 40 l /m2×din hot climate).

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Fig. 48.Wastewater treatment tanks using aquatic plants inAuroville / India. The plant is placed directly besideresidential houses. Lizards which live among thewater hyacinth take care of mosquito control. Thefirst settling tank is sealed against bad odour by athick layer of scum on which papyrus grows (seeFig. 46). [photo: Sasse]

Fig. 49.The complete treatment chain ofDEWATS-technology

settling anaerobic digestionaerobic and,facultative

degradation

aerobicpolishing, finalsedimentation

septic tank,Imhoff tank

anaerobic filterbaffled septic tank

biogas plant

ground filter,aerobic-facultativestabilisation pond

polishingpond

slowly while the plants coverthe pond. Plants must be har-vested regularly in order toprevent dead plants formingbottom sludge. Duckweed, inparticular should be kept inframes that prevent them frombeing pushed to one side bythe wind. Inlet and outlet struc-tures should be controlled asin normal oxidation ponds.

Calculation of Dimensions

For practical reasons, the same formula thatis used for unplanted oxidation ponds maybe used ( Tab. 30.).

Imhoff tank

aerobic-facultative

stabilisation ponds

anaerobicsedimentation

pond aerobic-facultative

stabilisation ponds

baffled septictank

horizontalground

filter

polishingpond

9.13 Hybrid and Combined Systems

Each technology has particular strong andweak points. Therefore it makes sense tocombine different treatment systems, forexample sedimentation in a settler or sep-tic tank followed by anaerobic decomposi-tion of none-settleable suspended solidsin anaerobic filters or baffled septic tanks.

Fig. 50.Typical combinations for full treatment when usingDEWATS-technology

A

B

C

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9 TECHNOLOGY

Further treatment may require aerobic con-ditions for which ponds or ground filtersmay be chosen. Purpose of treatment andsite conditions define those technologieswhich are appropriate for application.

Apart from combining pre-treatment withpost-treatment technologies, other featuresmay be combined also. As known from thehybrid UASB one may as well combine thebaffled septic tank with the anaerobic filterby adding filters in the last chambers ofthe baffled septic tank (Fig. 51). If floatingfilter medium is available, one may providea thin filter layer at the top of each baffledchamber.

hybrid baffled septic tank with clarifier provision for

gas release

inletoutlet

settler post clarifier

baffled septic tank hybrid reactorwith filter at top

Fig. 51.An example of a hybrid and combined reactor

9.14 Unsuitable Technologies

DEWATS commands low maintenance. Thisimplies that technologies which cannot be“switched on and off” as one likes, is inte-gral to the DEWATS concept. DEWATS areintended to function every day with the ef-ficiency envisaged. Systems, which arehighly efficient but require a great deal ofregular care to function at an acceptablelevel, do not suit the concept of decentral-ised wastewater treatment. To avoid anymisunderstanding: The technologies which

are regarded here as non-DEWATS are byno means inferior treatment systems. Theymay even be used in a decentralised con-cept. However, not without highly qualifiedoperational staff which is closely supervisedby an experienced management.

The technologies that do not fit in withDEWATS are:

o the rotating disc reactor

o the trickling filter

o the activated sludge process

o the fluidised bed reactor

o the sequencing batch reactor

Fig. 52.Rotating disk reactor in a slaughter house in Jakarta,Indonesia. The reactor was temporarily out of usewhen the photograph was taken. [photo: Sasse]

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Those systems which work with compressedair - either for aeration or floatation - orrequire chemicals for treatment are also ex-cluded from DEWATS. The UASB is also notsuitable, despite its simple technology. High

What will not be maintained, does not need to be built

rate trickling filters may be suitable whenthe distribution system functions perma-nently. Similarly, vertical planted filter beds

may be suitable if alternate charging of fil-ter beds is incorporated into the “produc-tion process” of wastewater itself.

The choice of treatment system will dependon the management capacity at site. Gen-eral recommendations could at best onlyrank the treatment systems. Much dependson site conditions. For instance, even hori-zontal ground filters can fail if grain size orsurface area is insufficient. In situationswhere little to no care is to be expected,only ponds, septic tanks and baffled septic

tanks would be worththe investment. But ofcourse, it might be dif-ficult for the planningengineer to tell the cli-ent that he considershim being careless.

Fig. 53.Oxidation ditch operated ata rubber factory in Kerala /India [photo: Sasse]

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10.1 Desludging

All organic degradation processes producesludge. The sludge produced by anaerobictreatment is less than that produced byaerobic treatment. The greater the treatmentefficiency the greater the sludge produc-tion. So as not to occupy reactor volume,sludge must be removed at intervals of 1/2to 3 years in tanks and in 1 to 20 years inponds. Sludge from industrial processesmay be removed in intervals of one to sevendays, only. In such case, the sludge is notstabilised and should be treated further inanaerobic digesters or must be compostedimmediately. Sludge, which is not fully sta-bilised, should not be dried in the openbecause of bad odour and the nuisance fromflies.

Bottom sludge from domestic and hus-bandry wastewater is highly contaminatedby worm eggs and cysts and should be han-dled hygienically. From a technical point ofview, desludging of septic tanks, baffledseptic tanks and anaerobic filters can bedone with buckets, by pumping or by hy-draulic pressure. What is important is thatonly the oldest sludge is removed and thatthe active sludge which is composed of liv-ing bacteria is left behind in the tank. Withuse of buckets - beside the obvious healthrisk - there is always the danger of remov-ing the active sludge together with the old-est sludge, as the old sludge is always foundbelow the active sludge.

Pump heads must be pushed down to thebottom of the tank in order to reach theoldest sludge. The effluent at the pump

outlet should always be visible in order tocheck and control the colour and consist-ency of the sludge. When the colour of theeffluent becomes too light, pumping mustbe stopped for a while to give the sludgetime to flow to the mouth of the pump onceagain. Free flow rotary pumps that allowsand, grit and smaller solid particles to passthrough without blockage are best fordesludging.

10 SLUDGE DISPOSAL

Fig. 54.Flexible pipes for de-sludging. Sludge is dischargedby hydraulic pressure when lowering the pipe. [photo:Sasse]

Old bottom sludge, because it compactswith time through its own weight, can berather “thick”. Consequently, desludgingpipes which work by hydraulic pressure must

107

be of large diameter. The diameter of adesludging pipe should not be below 100 mm;150 mm would be better still. The hydraulichead loss is in the range of 15 to 20%. Fora 2,50 m long pipe, the outlet must be0.35 to 0.50 m below the normal wastewateroutlet. The desludging pipe is best equippedwith a gate valve, which has a free openingof the full diameter. It is also possible to fitflexible pipes at the upper end of the downpipe. This would ensure that sludge flowsout when the flexible outlet is lowered. How-ever, pipes of such large diameter are notvery flexible, requiring that the connectionbetween the flexible hose and the rigid pipeis strong and durable. Flexible desludgingpipes must be closed and locked when notin use and handle of valves must be re-moved in order to prevent mischief by chil-dren.

10.2 Sludge Drying

Sludge has a total solid con-tent of 2 to 10%. This meansthat it is rather liquid and can-not be transported easily withsimple equipment. Apart fromthis, sludge is contaminatedand occupies large volumes forstorage. Therefore, it would bebetter to dry sludge before fur-ther use or final dumping. Only sludge thathas stabilised - free from bad odour - shouldbe dried in open beds. Anaerobic sludgedries best.

It is best to dry sludge in the immediatevicinity of the plant from which it has beenremoved. In case of domestic wastewaterthis could mean that drying places wouldthen be directly near residential houses. This

is not a problem if the sludge is spread asfertiliser on flowerbeds, as a thin layer ofsludge dries immediately. The slight foulsmell once a year may in most cases beacceptable, nor should the somewhat theo-retical health risk be overestimated as inthe case of a pet strolling through the flow-ers. However, drying sludge in sand bedsat residential sites is problematic since theusual sludge depth of 20 cm would needseveral weeks to dry. It would disturb chil-dren when playing in the vicinity and drawthe ire of those residents who are particu-lar about a clean environment. Therefore,sludge may have to be transported by tank-ers to a suitable drying place.

Drying itself is no problem on special dryingbeds in hot and dry climate but needs someconsideration in case of moderate tempera-ture, frequent rain or high humidity. Onelayer of sludge should not exceed 20 cm,

five loads per year from five different plantsmay be possible at the same bed. Thismeans 1 m3 of sludge needs about 1 m2 ofdrying surface. However, at least 5 m2 areneeded per m3 of sludge when sludge fromonly one plant is dumped at a time.

10 SLUDGE DISPOSAL

Fig. 55.Sludge drying beds constructed by University ofChiang Mei and GTZ in Thailand [photo: Sasse]

108

from an average DEWATS can only add to,but not fully substitute other fertiliser, evenfor a small farmer. However, compost hasits value as fertiliser and soil conditioner,and therefore should be honoured as a re-source, and not disregarded as a waste.

Composting requires soil or dry organicmatter, for example straw, to be mixed within order to reach 50% TS content. Such hightotal solids content is needed for a looselypacked consistency which is required forventilation (aeration), while the moisture isneeded to provide a suitable environmentfor aerobic bacteria.

Compost is best prepared by pouring liq-uid sludge on dry organic matter in severallayers of 10 to 20 cm. The heap is thencovered with soil. The compost should beturned over several times during matura-tion in order to distribute active bacteriaall within the substrate, and to provide oxy-gen to deeper layers. Rising temperature isthe best sign for the aerobe decompositionprocess. Temperature drops after matura-tion, which might be after three months upto one year under farm conditions.

A properly heaped compost produces a re-action temperature of up to 70°C. The hightemperature over several weeks of matura-tion has a sterilising effect on pathogens,including helminths and ova. However, caremust be taken during preparation of com-post.

Compost is needed at a certain time froman agricultural point of view. Desludgingmust be done before that date, in order toallow time for composting. Annual de-sludging is convenient for the farmer, butmay not be convenient for the plant opera-tor. Longer desludging intervals produce amore safe sludge, than shorter intervals and

Drying beds consist of about 30 cm coarseaggregate (>50 mm diameter) or gravel,covered with 10 to 15 cm coarse sand. Drain-age pipes are imbedded in the bottom layer.The bottom must be sealed when contami-nation of ground water would be possible.Drying beds with slightly sloped surface areeasier to drain than completely horizontalbeds. Sludge drying beds should be roofedat places of frequent rain.

Fig. 56.Principle cross section of a sludge drying bed

10.3 Composting

Composting of sludge is advisable for hy-gienic reasons, especially in case of domes-tic and husbandry wastewater. Improvingfertiliser quality may be a by-product, buthas normally no primary importance to theclient. In respect to DEWATS the questionis first how to get rid of the sludge.Composting is a good method for that, be-cause compost is not only hygienically safer,it is also better to handle by small farmers,because it can be carried by buckets.

The amount of compost one may get peryear from 20 m3 of domestic wastewaterper day is in the range of 25 m3, whichshrinks to approximately 12.5 m3 after matu-ration. This volume lasts for less than aquarter acre, which indicates that compost

10 SLUDGE DISPOSAL

30 cm gravel

15 cm sand

splash plate for dumping

drainrocks

30 cm freeboard

principle cross section of sludge drying bed

109

thus, may be recommended from a hygi-enic point of view. Howsoever, the opera-tor of the treatment plant has to organisethe whereabouts of his sludge and shouldcome to an agreement with either the farmerdirectly or with a transporting enterprisewhich collects the sludge at suitable times.

If composting is not possible but sludge isto be used fresh on agricultural land, thensludge must be disposed in trenches whichare covered by 25 cm of soil, at least.

10 SLUDGE DISPOSAL

110

Fresh, untreated domestic and agriculturalwastewater contains over one million bac-teria per millilitre, thousands of which arepathogens - both bacteria and virus. Eggsof worms are found in the range of 1000per litre. Epidemical statistics reveal thathelminthic (intestinal worm’s) infectionpresents the most common risk from irriga-tion with untreated wastewater. The risk ofbacterial infection comes followed by therisk of virus infection, which is the lowest.Although the removal rates in anaerobicsystems are usually over 95%, many patho-gens remain even after treatment. The ef-fluent from oxidation ponds is less patho-genic.

The World Health Organisation (WHO) rec-ommends that treated wastewater for un-restricted irrigation should contain lessthan 10.000 fecal coliforms per litre (1000/100 ml), and less than 1 helminth egg perlitre. This limit should be observed strictlysince the risk of transmitting parasites is

relatively high.

In this regard, there are a few mecha-nisms and rules to be learnt andpracticed. Eggs and larvae settle withsludge within which they may remainalive for many weeks. This explainstheir high concentration in the sludge.Nonetheless, eggs and larvae do notsurvive high temperatures; they die.It is for this reason that sludge shouldbe composted before use. Spread-ing sludge out on the field and ex-posing it to the sun is another way

11. 1 Risks

The risk that the use of wastewater for pur-poses of irrigation can mean to soil is de-scribed in chapter 6.. However, as was saidbefore, fresh or pre-treated wastewater ifhandled properly can be a valuable agricul-tural input.

Wastewater is never safe water

Wastewater is never hygienically safe. Properhandling of wastewater and sludge is theonly successful preventive health method.The farmer who uses wastewater for irriga-tion must consider the risk to his own healthand to the health of those who consumethe crops grown by him. He must thereforecheck whether the wastewater he uses forirrigation is suitable to the crops or pas-ture ground he intends to water.

11 REUSE OFWASTEWATER AND SLUDGE

Tab. 17.Survival of pathogens

Common survival times of pathogens

in sludge and water in soil on plantspathogens 10-15°C 20-30°C 1)

< days < days < days < daysvirus 100 20 20 15

bacteriasalmonella 100 30 20 15cholera 30 5 10 2fecal coli 150 50 20 15

protozoeamoebae cyst 30 15 10 2

wormsascari ova 700 360 180 30tape worm ova 360 180 180 301) not exposed to direct sun light EAWAG

111

of killing the unwanted organisms. In thelater case, nitrogen is lost and the fertiliservalue is reduced.

Pathogenic bacteria and viruses are notgreatly effected in anaerobic filters or sep-tic tanks because they remain in the treat-ment plant for only a few hours beforethey are expelled together with the liquidthat exits the plant. Post treatment in ashallow pond that ensures exposure to thesun reduces the number of bacteria con-siderably.

Those farmers who use sewage water forfarming or sludge as a fertiliser are exposedto certain permanent health risks. Thesehealth risks are controlled within organisedand specialised wastewater farming orwithin commercial horticulture, because ofcertain protective measures that are taken,such as the use of boots and gloves by theworkers and the transportation of thewastewater in piped systems. However, suchprecautions are very unlikely in small-scalefarming. Plants are either watered individu-ally with the help of buckets or trench irri-gation is used. The flow of water is usuallycontrolled by small dykes which are puttogether by bare hand or bare foot wherebydirect contact with pathogens is hardlyavoidable.

A shallow storage pond to keep water stand-ing for a day or more before it is used mayminimise the number of pathogens, butwould hardly reduce the indirect health risk.It is also likely that children will take toplaying, ducks will come to swim and ani-mals may start to drink from such an ar-rangement. Fencing may help. The morefoolproof preventive measure may be apermanent repetitive health education pro-gramme that reminds users of the dangersand precautionary measures to be taken.

WHO guide lines for wastewater use in agriculture

cathegory reuse conditions treatment required

A

irrigation of crops to be eaten

uncooked, sports fields, public parks

series of stabilisation ponds

B

irrigation of ceral- industrial- and fodder crops,

pasture and trees

10 days retention in stabilisation

ponds

C

localised irrigation of crops of

cathegory B, no contact by workers

or public

at least primary sedimentation

WHO 1989

11 REUSE

Consumers of crops grown by such meansand animals that graze on pastures thatare irrigated with wastewater are also en-dangered. Since bacteria and virus are killedby a few hours, or at most a few days ofexposure to air, wastewater should not bespread on plants which are eaten raw (e.g.lettuce) for at least two weeks prior to har-vesting. India has prohibited the use ofwastewater irrigation for crops that are likelyto be consumed uncooked.

Since bacteria and virus stay alive muchlonger when wastewater percolates into theground, root crops like potatoes or carrotsexcept for seeds or seedlings should notbe irrigated with wastewater.

11.2 Groundwater Recharge

Recharge of groundwater is probably thebest way to reuse wastewater particularlysince the groundwater table tends to loweralmost everywhere. Wastewater had beenfreshwater, and freshwater drawn from wellshas been groundwater before. Sustainable

Tab. 18.WHO-guide lines for wastewater use in agriculture

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development is directly related to the avail-ability of water from the ground. Thus, re-charging of this source becomes absolutelyvital to human civilisation. The main ques-tion is how far the wastewater needs treat-ment before it may be discharged to theground. For that please refer to chapter 6.

11.3 Fishponds

Wastewater is full of nutrients which can di-rectly be used by algae, water plants andlower animals which then could become fishfeed. But fish need also oxygen to breath,which must be dissolved in water in pureform of O2 (4 mg/l for carp species, > 6 mg/lfor trout species). Since free oxygen is neededfor degradation of the organic matter presentin wastewater, it cannot be expected to bein sufficient supply for the survival of fish.Therefore, pre-treated wastewater must bemixed with freshwater from rivers or lakes,otherwise wastewater ponds must becomeso large that oxygen supply via pond sur-face overrules the oxygen demand of theorganic load.

Organic load on fishponds should be be-low 5g BOD/ m2×d before 5 times dilutionwith freshwater. This implies that if thechances of dilution are non-existent, theorganic load may be 1 g BOD/ m2×d.

If possible, there should be several inletpoints in order to distribute organic mattermore equally where it comes into contactwith oxygen quickly. It had been mentionedbefore that turbulent surface increases theoxygen intake, and cooler temperature in-creases the ability to store free oxygen inwater. However, it is not worthwhile tryingto increase oxygen intake by special shapedinlet structures or similar measures. In a

stage where oxygen deficiency can only belittle, oxygen absorption is also little.

The pH should be 7 - 8. Fish culture is notpossible when wastewater may be toxic orpolluted by mineral oils, temporarily or per-manently.

Mixing of wastewater with fresh watershould not happen before the fishpond. Oth-erwise wastewater nutrients would initiateheavy growth of fungi, algae and other spe-cies without being consumed by fish. Whenstarting a fishpond, it first should be filledwith fresh water, wastewater is added later.

When using natural lakes for wastewaterbased fishery it should be known, whetherthe lake is legally considered being part ofthe treatment system or already part of theenvironment in which wastewater is dis-charged. With other words, it must be clearwhether discharge standards must be ob-served at the inlet or whether the effluentof the lake will do.

Kind and condition of fishes are an indica-tor of water quality. Carp species can livein water with lower oxygen content and aremost common in wastewater based fish cul-ture. Tilapia has become the most common“Development Project-Fish” and is alsogrowing well in wastewater ponds. Tenchspecies often have difficulties to survive,because they take feed from the groundand get problems with anaerobic bottomsludge. It is advisable to empty the pondsonce every year in order to remove sludgeor at least to expose the bottom sludge tooxygen for stabilisation.

Fish ponds are normally more turbid thanother ponds, because fish swirl up sludgefrom the ground. Trout species survive sur-prisingly well, despite higher turbidity, when

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113

Sewage sludge as fertiliseraverage nutrient content in

kind of nutrient

1000 kg of sewage sludge

(10% TS)

1000 kg of farm yard manure

grams gramsN 5,5 17,5

P2O5 17,5 17,5K2O 0,75 65

S (total) 12,5 25MgO 30 15

Cu (total) 1,2 0,03Zn (total) 1,5 0,15Mn (total) 0,6 0,4Mo (total) 0,01 0,001B (total) 0,03 0,035

Mudrak / Kunst, pg. 162

oxygen content is sufficient. However, itshould be clear that more specified knowl-edge on fish species, fish production andmarketing is needed than can be read fromthis chapter. More information is availablefrom regional offices of fishery departmentsand should be requested from there beforestarting a wastewater fish farming system.

Fishponds have a hydraulic retention timeof 3 to 10 days and a depth of 0.5 - 0.8 m.Net fish production is about 500 kg/ha(50 g /m2), 900 to 1200 kg/ha are said tobe harvested from Calcutta’s municipalityfish farm. There is also the possibility ofraising fish in 2.5 - 3 m deep ponds wheredifferent kind of fish live in different strataof the pond. An almost unbelievable 12.000kg/ha are claimed to have been harvestedin Brazil in such ponds per annum. Higherfish population produces more sludge whichreduces the amount of free oxygen. Whetherwastewater based fishery becomes a viable

business depends on market price of fishand operational cost for fishery. Fingerlingsmust be kept separate because fish, whenset free, should have 350 g life weight inorder to be too heavy for fishing birds.Losses can reach 50% when fish ponds be-come an ecological niche which attracts fishhunting birds.

Fish lose the foul taste of wastewater whenkept for some days in fresh water beforeconsumption. This reduces also the risk ofpathogen transfer. Fishermen during harvestshould be aware that fish live in wastewaterwhich bears always a certain, albeit small,health risk.

11.4 Irrigation

Treated domestic or mixed communitywastewater is ideal for irrigating parks andflower gardens. Irrigation normally happensthere in the evening or early morning whennobody is bothered, even not by the slightlyfoul smell of anaerobic effluent. Nonethe-less, irrigation of public parks is often le-gally forbidden.

For an irrigation rate of 2 m per year (20.000m 3/ha) which is commonly required in semi-arid areas, even well treated wastewaterwith as low concentrations as 15 mg/l oftotal nitrogen and 3 mg/l total phosphorusprovides 300 kg N and 60 kg P per ha viairrigation without additional cost; at thesame time the respective amount of ground-water is saved. Not only water but energyfor pumping is saved also. In areas of plentyof rainfall less water is applied for irriga-tion and use of pre-settled but otherwisefresh wastewater may be more appropriatewith respect to fertiliser. With 0.1 m per year(1,000 m3/ha) of fresh wastewater for irriga-

11 REUSE

Tab. 19.Fertiliser value of sewage sludge

114

tion, some 60 kg nitrogen, 15 kg phospho-rus and a similar amount of potassium couldbe applied per ha. However, domesticwastewater in modern households some-times lacks potassium which might need tobe added to mobilise nitrogen and phos-phorus.

This booklet deals with wastewater, it can-not provide sufficient information on gen-eral or specific local questions of agricul-ture or nutrient requirements of differentcrop. Each farmer has to find out his ownmethod and his own way of using efficientand safe quantities of water. The practicalfarmer knows which nutrients are neededfor which crop and a studied agriculturistwould also know from the result of waste-water analysis whether the composition ofnutrients and trace elements suits the pro-posed plantation. He will also know fromthat analysis whether too much of toxic el-ements are remaining in the water (toxicelements might be there if the COD is muchhigher than the BOD). Such tests are advis-able when using industrial or hospitalwastewater for the first time. The responsi-ble person of the wastewater source isobliged to inform farmers about toxic orotherwise dangerous substances in the ef-fluent, for example radioactive elementsfrom x-ray laboratories.

Original saline water will remain saline evenafter intensive treatment. Copper and othermetals, especially heavy metals accumulatein the soil. Long term application of suchwater will spoil the soil forever.

11.5 Reuse for Process and DomesticPurposes

Pathogenic wastewater, this is from domes-tic sources, slaughter houses or animal sta-bles should better not be reused for otherpurposes, except for irrigation. Partly treatedorganic wastewater (this is more or less allwastewater from DEWATS-treatment) shouldas well better not be reused as processwater in industries or as flushing water intoilets. Reuse of wastewater means alwayssome traces of organic matter or toxic sub-stances present or even accumulating. Re-use means as well longer retention timesin a closed system which might facilitateanaerobic processes within pipes and tankswhich will cause corrosion. There is also atheoretical risk of biogas explosion.

To suppress organic decay one may haveto add lime, which might form lime stoneinside the system, or other inhibiting sub-stances which would make proper final treat-ment of wastewater costly. For example,even the first washing water in a fruitprocessing plant or in a potato chip plantmight contain already too much organicmatter for any reuse without adding limeto oppress fermentation.

The chance of re-circulation of parts of thewater to serve the production process islimited, especially when the wastewater en-gineer and the production engineer havelimited theoretical knowledge. Pollution con-tent and degree of possible treatment, aswell as demand of water for consumptionand the wastewater flow over a given pe-riod of one day (or one season) must beinvestigated. Construction of intermediatewater stores and installing additional pumpsmay become necessary, as well. Reuse ofwastewater is an option which sounds very

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115

reasonable in the context of sustainable de-velopment. However, the problems whichgo along with it do not allow to recom-mend any reuse, in general.

Reuse of industrial wastewater which is onlyslightly polluted and where pollution mightnot even be of organic nature, is a com-pletely different matter. For example, presswater in a soap factory may be reused formixing the next load of soap paste. All wa-

11 REUSE

ter consuming modern industries have re-duced their water consumption considerablyin the last years. In most countries, amongstthem India and China, water consumptionlimits are obligatory for many industrial proc-esses; such as sugar refineries, breweries,canning factories, etc.. Saving water in theprocess is always the better alternative, in-stead of reusing water which had been care-lessly wasted and polluted.

116

Biogas

All anaerobic systems produce biogas. 55%- 75% of m ethane (CH4), 25% - 45% ofcarbon dioxide(CO ) plus traces of H2S, H,NH3 go to form biogas. The mild but typicalfoul smell of biogas is due to the hydro-sulphur, which after it is transformed intoH2SO3, is also responsible for the corrosivenature of biogas. The composition rate ofbiogas depends on the properties ofwastewater and on the design of the reac-tor, i.e. the retention time. Theoretically, therate of methane production is 350 l per kgremoved BODtotal. In practice however, meth-ane production should be compared to 1 kgremoved COD of which values are closer tothe removed BODtotal than to the removedBOD5 (see Fig. 14.). By doing so, one as-sumes that during anaerobic digestion only

biodegradable COD is removed, which isinvolved in the production of methane. Inreality, the gas production rates are lowerthan this because a part of the biogas dis-solves in water and cannot be collected ingaseous form. It is also common to relatebiogas production to organic dry matter incase of very strong viscous substrate, 300- 450 l biogas per kg DM can be expected.

The calorific value of methane is 35.8 MJ/m3

(9.94 kwh/m3). The calorific value of biogasdepends on the methane content. Hydrogenhas practically no role. As a rule of thumb,1 m3 biogas can substitute 5 kg of firewoodor 0.6 l of diesel fuel.

Methane damages the ozone layer of theatmosphere. From an environmental pointof view it should be burnt to become harm-less. Only CO2 and water remain on burn-ing methane. Since the residual CO2 doesnot stem from fossil sources, it is harmless

to the atmosphere.

12.2 Scope of Use

Biogas may be used in burnersfor cooking or in combustionengines to generate power. Theuse of biogas will depend onthe regularity of biogas supplyand its availability to meet theminimum requirement of a par-ticular use. If biogas cannot beutilised, it should be releasedin the air by safe ventilation.It is not meaningful to collect,store and distribute biogas

12 BIOGAS UTILISATION

Tab. 20.Potential biogas production from some selected in-dustrial processes

Biogas production from industrial processes

industryCOD per product

COD removal

relative gas production

methane content

kg / to %m³ CH4 / kg CODin

%

beet sugar 6-8 70-90 0,24-0,32 65-85starch - potato 30-40 75-85 0,26-0,30 75-85starch wheat 100-120 80-95 0,28-0,33 55-65starch - maize 8-17 80-90 0,28-0,32 65-75melasses 180-250 60-75 0,21-0,26 60-70distillery - potato 50-70 55-65 0,19-0,23 65-70distillery - corn 180-200 55-65 0,19-0,23 65-70pectine 75-80 0,26-0,28 50-60potato processing 15-25 70-90 0,24-0,32 70-80sour pickles 15-20 80-90 0,28-0,28 70-75juice 2-6 70-85 0,24-0,30 70-80milk processsing 1-6 70-80 0,24-0,28 65-75breweries 5-10 70-85 0,24-0,33 75-85slaughter 5-10 75-90 0,26-0,32 80-85cellulose 110-125 75-90 0,26-0,33 70-75paper / board 4-30 60-80 0,21-0,28 70-80

ATV, BDE,VKS

117

when it does not meet any real purpose, inwhich case it would not be used anyhow.

The extraction of carbohydrate (CO2) be-fore the utilisation of biogas is not essen-tial. However it might be advisable to re-move an unusually high H2S content withthe help of iron oxide. Biogas flows througha drum or pipe filled with iron oxide (e.g.rusted iron borings). The oxygen reacts withthe hydrogen to form water, while sulphurand iron (or sulphide of iron) remain. Theiron may be reused after again becomingrusty due to exposure to air.

The minimum requirement of biogas for ahousehold kitchen is approximately 2 m3/d;below which meaningful use is rare. Approxi-mately 20 to 30 m3 of domestic wastewaterare required daily to produce the minimumamount of gas. From an economical point ofview, biogas utilisation from wastewaterbecomes meaningful if the strength of thewastewater is at least 1000 mg/l COD andthe regular daily flow is 20 m3. For moreinformation see chapter 4.

The best way of using biogas is for heatproduction. Biogas burners are simple inprinciple and can be self-made from con-verted LPG-burners. There are many fieldsin which biogas can be used: cooking inhouseholds and canteens or drying andheating as part of industrial processes, aresome examples. The best use of biogaswould be as fuel for the very process thatproduces the wastewater.

It is also possible to use biogas for directlighting in gas lamps. It is only meaningfulto do so in the absence of electricity sup-

ply, or frequent power cuts that justify theinvestment. The light from a biogas lampcannot compete in quality and comfort withan electric light.

Biogas may be used as fuel in diesel en-gines and Otto motors. As the ignition pointof biogas is rather high, it does not ex-plode under pressure of a normal dieselengine. Therefore, approximately 20% ofdiesel must be used for ignition, togetherwith biogas. Diesel engines are most suit-able as they can be run only on diesel incase of irregular biogas supply. Further, theslow flame speed of biogas is better suitedto the slow revolving diesel engine thanOtto motors. Biogas would not have enoughtime to burn completely with engines thatrun with more than 2000 revolutions perminute.

It would be self-defeating to produce anduse biogas for the purpose of meaninglessdemonstration. The only exception may beto aid natural science teaching in second-ary schools.

12.3 Gas Collection and Storage

Biogas is produced within wastewater andsludge, from where it rises in bubbles tothe surface. The gas must be collected abovethe surface and stored until it is ready foruse. Even when gas production is regular,the accumulation of useable gas is irregu-lar. Gas bubbles cause turbulence whichleads to the explosive release of gas in achain reaction. Stirring of substrate, espe-cially stirring of sludge, has a similar effect.As a result of this effect, gas productionfluctuates by plus/minus 25% from one dayto the other. The volume of gas storagemust provide for this fluctuation.

12 BIOGAS UTILISATION

What will not be used, does not need to be built

118

The volume of gas in stock changes ac-cording to gas production and the patternof gas consumption. With rigid structures,the volume of the storage tank eitherchanges with changing volume of gaspresent, or the gas pressure increases alongwith the stored volume. In fixed domeplants, and with flexible material such asplastic foils, both the volume and the pres-sure fluctuates. There are two main sys-tems for rigid materials, namely:

o the floating drum and

o the fixed dome

For flexible material there are two variants,as well, namely

o the balloon, and

o the tent above water

The floating drum (Fig. 29. (C)) is a tankthat floats on water, the bottom of which isopen. The actual storage volume changestogether with the amount of gas availableand the drum rises above the water accord-

ing to gas volume. The drum is normallymade out of steel. To avoid corrosion, ma-terials such as ferro-cement, HDP and fi-breglass have also been tried. As a rule,only very experienced workshops have beensuccessful with such material. Most findleakage a problem. The gas pressure is cre-ated by the weight of the drum (the weightis to be divided by the occupied surfacearea to calculate the pressure). A safety valveis not required as surplus gas is releasedunder the rim when the drum rises beyonda point.

The fixed dome principle ( Fig. 29. (A) + (B))has been developed for biogas digestersfor rural households as an alternative tothe corrosion problem of the floating drum.The fixed dome plant follows the principleof displacing liquid substrate through gaspressure. The gas pressure is created bythe difference in liquid level between theinside and outside of the closed vessel. Incase of very high gas pressure, the outletpipe functions as a safety valve. Therefore,

the inner level of the outletpipe must be higher thanthat of the inlet.

In biogas plants of a rela-tively high gas productioncompared to the volume ofsubstrate, an expansionchamber is needed to sus-tain gas pressure during

Fig. 57.Floating drum plant. The drumsare lifted for re-painting whichallows a view on the double ringwall of the water jacket. Con-structed by LPTP and BORDA fora slaughter house in Java / Indo-nesia. [photo: Sasse]

12 BIOGAS UTILISATION

119

use. In case of wastewater, where the vol-ume of water is relatively large comparedto the volume of gas production, an expan-sion chamber may be not required becausethe in-flowing wastewater replaces thewastewater, which has been pushed out bythe gas. For this reason, the correspond-ence of the time of gas consumption withthe time of intensive wastewater inflow isimportant. An expansion chamber is requiredin cases when there is little to no wastewaterflow during gas consumption. An expansionchamber is not required when the simulta-neous volume of consumed gas is less thanvolume of wastewater inflow.

The surface area of an anaerobic treatmenttank is relatively large compared to theamount of biogas produced. Consequently,fluctuation in liquid level due to variation ingas volumes in the upper part of the reactoris respectively small. All the same, it may

influence the design, espe-cially the level of baffles toretain floating solids.

Biogas is an end product ofdecomposition and there-fore, has very fine moleculesthat can pass through thesmallest crack and the fin-est hole. Gas storage mustbe constructed as gas tightas a bicycle tube for exam-ple. The usual quality of con-crete and masonry is notsufficiently gas-tight. Bricksare porous and concrete hascracks. Therefore, bricks and

concrete must be well plastered by applyingseveral layers and adding special compoundsto the mortar in order to minimise shrinkingrates. Several layers of plaster help to covercracks of one layer with the next layer ofplaster, hoping that cracks in different lay-ers do not appear at the same spot.

Fig. 58.This plant had been constructed by LPTP and BORDAfor cattle dung in a meat factory in Central Java

12 BIOGAS UTILISATION

Typical prescription for gas-tight plaster in fixed dome biogas plants1st layer cement - water brushing2nd layer cement plaster 1 : 2,53rd layer cement water brushing

4th layercement plaster 1 : 2,5 with

water proof compound

5th layercement - water brushing

with water proof compound

6th layercement plaster 1 : 2,5 with

water proof compound

7th layercement - water finish with

water proof compoundCAMARTEC, BORDA

Tab. 21.Prescription for gas-tight plaster. The method hadbeen developed by CAMARTEC/GTZ in Arusha / Tan-zania and is successfully applied since 1989 in manycountries.

120

A structure under pressure cannot developcracks, therefore the structure of the gasstorage should be under pressure when-ever possible. This is the reason why anaero-bic reactors should have arched ceilings,where heavy soil covering creates the re-quired pressure. Normally, baffled septictanks and anaerobic filters are rectangularin shape. Since it is difficult and expensiveto make these structures gas-tight, andconsidering the fact that gas production ishighest in the first part of the reactor, itmay be reasonable to collect gas from thefirst chambers only. These chambers mustbe completely gas-tight; rear chambers mustbe ventilated separately.

Tent systems ( Fig. 60.) are mostly used incase of anaerobic ponds. Balloons may beconnected to any anaerobic tank reactor.Balloons and tent systems require the samematerial. These materials must be gas-tight,UV-resistant, flexible and strong. PVC is notsuitable. The weakest points are the seamsand more so the connections between thefoil and the pipes. To secure gas tightness,foils of tent plants are fixed to the solidstructure below the liquid level. Foil cover-ing may also be fixed to frames floating onthe wastewater. Balloons should be laid ona sand bedding or be hung on belts or gir-dles. It may be necessary to protect themagainst damage by rodents. The gas pres-

Fig. 59.Baffled septic tank with biogas utilisation. Only biogas from the settler and the first two baffled chambersis used. They are arched in order to guarantee a gas-tight structure. The tanks which store biogas areseparated from the three chambers at the rear of which biogas is not collected. The design is based on 25 m3

daily wastewater flow, 4000 mg/l COD and a necessary gas storage volume of 8 m3.

12 BIOGAS UTILISATION

121

sure must be kept under control to fit thepermissible stress of the material, especiallyat joints. The provision of a safety valvewhich functions as a water seal on gas pres-sure should solve this problem.

Balloon and tent systems, unless securelyfenced and protected against stones or rub-bish being thrown on them by children, arenot suitable for domestic plants.

12 BIOGAS UTILISATION

Fig. 60.Tent gas storage above a liquid manure tank. Biogasplant constructed by SODEPRA and GTZ at a cattlepark in Ferkessedougou / Ivory Coast. [photo: Sasse]

12.4 Distribution of Biogas

Normal water pipe installation technologymay also be used for biogas distribution inDEWATS. However, ball valves should re-place gate valves. All parts should be rea-sonably resistant against corrosion by sul-phuric acid. Joints in galvanised steel pipesshould be sealed with hemp and grease orwith special sealing tape. Joints of PVC pipesmust be glued; the glue must be spreadaround the total circumference of the pipe.

Biogas always contains a certain amountof water vapour, which condenses to waterwhen gas cools down. This water must bedrained; otherwise it may block the gas flow.Drain valves or automatic water traps toavoid blockage must be provided at thelowest point of each pipe sector. Pipes mustbe laid in continuous slope towards thedrain points; straight horizontal pipesshould not sag.

Gas pressure drops with growing length ofpipe, and more so with smaller pipe diam-eter. Diameter of pipes must be larger whenthe point of consumption is far off. Longdistances are generally not a problem. Thedistance should be kept as short as possi-ble for economic reasons. Connecting stovesor lamps with a piece of flexible hose to themain distribution pipe has the advantage inthat equipment can be moved without dis-connecting the pipe. It also allows for con-densed water to be drained.

In the case of fixed dome plants, a U-shapedgas pressure meter (manometer) could beinstalled near the point of consumptionwhere it is difficult to see the amount ofgas available.

Fig. 61.Pressure gauge out of transparent flexible pipes andwater trap to collect condensed vapour which de-velops in gas pipes due to changing temperature.Water must be drawn from the trap when gas-flamesstart to flicker.

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12.5 Gas Appliances

In principle, biogas can be used as any othergaseous fuel, for example in refrigerators,incubators, or water heaters. Nonetheless,use in stoves, lamps and diesel engines ismost common.

Biogas needs a certain amount of air toburn - on an average one cubic meter ofgas requires 5.7 m3 of air for complete com-bustion, one quarter of what LPG wouldneed. Therefore, LPG burners have smallerjets; consequently the relative air intakecompared to biogas burners is greater. Theair intake that is needed for combustion isregulated by the difference of jet diameterto mixing pipe diameter. For open burners,which draw primary air at the jet and somesecondary air at the flame port, the ratiobetween jet diameter and mixing pipe di-ameter may be taken as 1 : 6. For lamps,where secondary air supply is lower, thisfigure may be 1 : 8.

When converting LPG equipment to biogas,the jet must be widened, to say 1/6 of thediameter of the mixing pipe of a burner.These ratios are the same for all gas pres-sures. There is no need to regulate the airintake when gas pressure changes. How-ever, air requirement is greater when meth-ane content is higher. The difference is toosmall to be of practical importance. Since

flame speed of biogas is relatively low,biogas flames tend to be blown off whengas pressure is high. It may be advisableto increase the number or size of orifices atthe flame port in order to reduce the speed.It is also possible to reduce the flow byplacing an obstacle at the flame outlet; forexample, a pot set on the burner.

It is trickier to regulate the air - gas mixturein lamps that use textile mantles, becausethe hottest part of the flame must be di-rectly at the mantle in order to cause themineral particles to glow. If the flame burnsinside the mantle, the pressure might betoo low and the primary air may be toomuch. If on the other hand, the flame burnsoutside the mantle, then primary air wouldbe too little and the pressure might be toohigh. Since the composition of biogas alsoplays a role, general recommendations forlamp design are not easy to give. Practicaltesting is the only solution.

Diesel engines always have a surplus of airand proper mixing is not required. The gasis connected to the air supply pipe after theair filter. Mixing of air and gas is improvedwhen gas enters the air pipe by cross flow.Dual fuel engines are started with 100% die-sel; biogas is added slowly when the engineis hot and under load. The amount of biogasis regulated by hand according to experi-ence. The engine usually starts to splutter

when there is too much gas. Whenthe engine runs smoothly, it isregulated like a pure diesel en-gine with the help of the throttle.For generating 1 kwh electricity,approximately 1.5 m3 biogas and0.14 l diesel are required (GTZ).

Fig. 62.Design parameters for gas appliances. The relation between jet diameter and mixing pipe diameter is mostimportant for performance and efficiency, irrespective of gas pressure. Other parameters are less crucial or caneasier be found by trial and error, e.g. like number and diameter of orifices or length of mixing pipe.

12 BIOGAS UTILISATION

jet ∅ djmixing pipe ∅ = 6 x dj for burners

(mixing pipe ∅ = 8 x dj for lamps)

gas supply

flame port with orifices ∅ 2.5 mm

principle design parameters for biogas appliances

123

13.1 Technical Spread Sheets

13.1.1 Usefulness of ComputerCalculation

The purpose of this chapter is to ensurethat the engineer can produce his or herown spread sheet for sizing DEWATS in anycomputer programme that he or she is fa-miliar with. The exercise of producing one’sown tables will compel engineers to deepentheir understanding of design.

The curves that have been used as thebasis for calculation in the formulas ap-plied in the computer spread sheets mayalso be of interest to those who may notuse a computer (chapter 13.3 is addressedto them, especially). As these curves visu-alise the most important relations betweenvarious parameters, they will enhance un-derstanding of the factors that influencethe treatment process. It should be no-ticed that the graphs have been developedon the base of mixed information. There-fore, the ways of calculation follow notalways the same logic.

Computerised calculations can be veryhelpful, particularly if the formulas and theinput data are correct. On the other hand,what may look impressive may as well be‘rubbish in’, ‘rubbish out’. Nevertheless,assuming correct input data, the compu-ter spread sheet gives a quick impressionof the plant’s space requirement and thetreatment performance that can be ex-pected. Ready-to-use computer spreadsheets are especially helpful to those whodo not design DEWATS on a daily basisand who would otherwise need to recol-

lect the entire theory for sizing a plantbefore starting to design.

13.1.2 Risks of Using SimplifiedFormulas

The formulas used in the spreadsheet havebeen constructed for use by practitionerswho are less bothered by theoretical knowl-edge. Nevertheless, the formulas are basedon scientific findings which had been sim-plified under consideration of practical ex-perience.

Even if the formulas were to be 100% cor-rect, the results would not be 100% accu-rate since the input data are not fully reli-able. The accuracy of the formulas in gen-eral is likely to be greater than the accu-racy of wastewater sampling and analys-ing. There are many unknown factors in-fluencing treatment efficiency and any „sci-entific“ handbook would give a possiblerange of results. This book, howsoever „sci-entifically“ based, is made for practicalpeople who have to build a real plant outof physically real and rigid building mate-rial. The supervisor cannot tell to the ma-son to make a concrete tank „about 4,90mto 5,60m long“, they have to say: „makeit exactly 5,35 m“. The following spreadsheets are of the same spirit. Anybody whouses more variable methods of calculationalready, does not belong to the targetgroup of this book and is free to modifyformula and curves according to his or herexperience and ability (the author wouldappreciate any information on improvingthe spread sheet).

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124

Since the formulas represent simplificationsof complex natural processes, there is acertain risk that they do not reflect the re-ality adequately. However, the risk ofchanges in the reality that is assumed isgreater. For example, the expansion of afactory without enlarging the treatment sys-tem is obviously of greater influence thanan assumed BOD of 350 mg/l, when in real-ity it is only 300 mg/l.

o In respect to using wrong figures forsludge accumulation in septic tanks, sedi-mentation ponds, Imhoff tanks andanaerobic reactors, the biggest risk isthat shorter desludging intervals maybecome necessary.

o In case of anaerobic reactors, severeunder-sizing could lead to a collapse ofthe process, while over-sizing may re-quire longer maturation time at the be-ginning.

o Incorrect treatment performance of pri-mary or secondary treatment steps couldbe the cause of over- or undersized post-treatment facilities. This would mean un-necessarily high investment costs, or thenecessity of enlarging the post treatmentfacilities.

o Undersized anaerobic ponds stink, butslightly oversized ponds may not developsufficient scum as a result of which theseponds stink as well.

o There is no harm in over-sizing aerobicponds, bearing in mind that aerobicponds, which are too small, may alsosmell badly.

o The biggest risk lies in clogging of fil-ters, in both anaerobic tanks and gravelfilters. However, the risk is more likelydue to inferior filter material, faulty struc-

tural details or incorrect wastewater datathan incorrect sizing.

In general, moderate over sizing reducesthe risk of unstable processes and inferiortreatment results.

13.1.3 About the Spread Sheets

The spreadsheets that are presented in thishandbook are on EXCEL, version 5.0. Anyother suitable programme may also be used.Please note that all calculations have beenmade on the German version of the com-puter programme. Therefore, decimal fig-ures are expressed by a comma and Thou-sands are expressed by a point. For exam-ple 1.100,5 means „one thousand one hun-dred point five“.

There might be differences in the syntax offormulas, for example 32 (second power of3) may be written =POWER(3;2) or =3^2,similar square root of 9 could be =SQRT(9)or =9^1/2, cubic root of 27 would be=power(27;1/3) or =27^1/3. Some programmesmay accept only one of the alternatives.

The spreadsheets are based on data that isnormally available to the planning engineerin the context of DEWATS. For example,while the measurement of the BOD5 andthe COD may be possible at the beginningof planning, it is unlikely that the BOD5 willbe regularly controlled later on. Therefore,calculations are based on COD or, the re-sults of BOD-based formula have been setin relation to COD, and vice versa. The termBOD stands in the following for BOD5.

Some of the formula used in the spread-sheets is based on curves, which had beenobtained from scientific publications, hand-books and the author’s and his colleagues

13 COMPUTER SPREAD SHEETS

125

experience. The formulas basically definetrends. For example, it is well known thatthe removal efficiency of an anaerobic re-actor increases when the COD/BOD ratio isnarrow. Such curves have been simplifiedto a chain of straight lines to allow thereader to easily understand the formulasand to adjust their values to local condi-tions if necessary. The number of data onwhich these curves are based are some-times too insignificant to be of statisticalrelevance. However, they have been usedand adjusted in accordance with practicalexperience.

The formulas are simple. Beside basic ar-ithmetical operations, they use only onelogical function, namely the „IF“-function.For example,

if temperature is less than 20°C; then hydraulicretention time is 20 days; if not, then it is 15 days

in case the temperature is less than 25°C;otherwise (this means, if temperature is over

25°C) the HRT is 10 days.

Assuming the temperature is shown in cellF5 of the spread sheet, the formula is writ-ten in the cell for calculating the retentiontime as:

=IF(F5<20;20;IF(F5<25;15;10)).

The formula has been kept simple, so thatthe user may modify it according to experi-ence or better knowledge. For example, itmay be that with a certain substrate, theHRT should be 25 days at 20°C, 23 days at25°C and 20 days above 25°C and for safetyreasons, 10% longer retention time is added.Then the formula would read as:

=110%*IF(F5<20;25;if(F5<25;23;20)).

Values between the defined days may becalculated by using the famous “rule ofthree”, of which there are plenty examplesin the tables that follow. The slope of a

straight line is expressed in its tangent andthe height of a certain point is found bymultiplying the length with the ratio of theslope, i.e. total height divided by total length(Fig. 63.).

2,8

7,8

10,5

12 18,5

Hp = (12 * (10,5 - 2,8) / 18,5) + 2,8 = 7,8

Hp

P

0

Fig. 63.The graphical expression of the „rule of three“

13 COMPUTER SPREAD SHEETS

In case the reader is not familiar with work-ing in EXCEL, it would be better not tomodify formulas but to manipulate the re-sults by entering „modified“ data. For ex-ample, if values of spread sheet results aregenerally lower or higher than found byexperience, dimensioning could be adjustedby entering lower or higher temperaturevalues, or shorter or longer retention times.One could also multiply wastewater volumesor COD concentrations by a safety factorbefore starting the calculation. In any caseall cells of the spread sheet should belocked, except the ones written in bold fig-ures.

When the user prepares his or her owntable the columns (A; B; C; D...) and therows (1; 2; 3; 4; 5...) should not be writ-ten, because this would change the celladdresses of the formulas. Anyhow, cellsand rows are shown on the mask of themonitor. When copying the formulas be-low, the cell name before the equals sign

126

should not be written. For example E6=D5/E5 is to be written in cell E6 as =D5/E5.

The italic figures are mostly guiding figuresto show usual values, or they indicate lim-its to be observed. The bold figures arethose which have to be filled in by hand,the other figures are calculated. Columnswhich are labelled „given“ contain datawhich reflect a given reality, for example,wastewater flow volume or wastewaterstrength. Columns which are labelled „cho-sen“ contain data which may be modifiedto optimise the design, for example hydrau-lic retention time or desludging intervals.All other cells contain formulas and shouldbe locked, in order to avoid accidental de-letion of formulas. Cells which are labelled„check“ or „require“ are to be observedwhether the chosen and given values fitthe chosen or calculated design.

13 COMPUTER SPREAD SHEETS

BOD/COD removal efficiency compared to COD removal

1

1,05

1,1

1,15

1,2

1,25

50 55 60 65 70 75 80 85 90COD removal in %

Fig. 64. Change of COD/BOD ratio during anaerobictreatment. The samples have been taken by SIITRATfrom anaerobic filters, most of them serving schoolsin the suburbs of Delhi / India.

simplified curve of ratio of efficiency of BOD removal to COD removal

1

1,05

1,1

1,15

35% 45% 55% 65% 75% 85% 95%COD rem. efficiency in %

Fig. 66. Changes of COD/BOD ratio during anaerobictreatment of domestic wastewater. The samples havebeen taken by SIITRAT. The few sample points ofhigh COD/BOD ratio (to the right of the graph) stemfrom a post-treatment plant and are not really com-parable to the majority of samples.

Fig. 65. Simplified curve of Fig. 64. which is used inthe spread sheet formulas.

COD/BODout to COD/BODin compared to COD/BODin

1,0

1,1

1,2

1,3

1,4

1,5

1,6

1,7

1,8

1,9

1,5 2,0 2,5 3,0 3,5COD/BOD of inflow

fact

or

13.1.4 Assumed COD / BOD Relation

The COD/BOD ratio widens during biologi-cal treatment because the BOD reflects onlythat part which consumes oxygen in a bio-logical process while the COD representsall oxygen consumers. The removed BOD ispercentage-wise a smaller portion of theCOD than it is of the BOD. The COD/BODratio widens more when biological degra-dation is incomplete, and is less wide whentreatment efficiency reaches almost 100%.

127

A B C D E F G1 Wastewater production per capita

2user

BOD5 per user

water consump. per user

COD / BOD5

ratiodaily flow of wastewater

BOD5

concentr.COD

concentr.

3 given given given given calcul. calculated approx.4 number g/day litres/day mg/l / mg/l m³/day mg/l mg/l5 80 55 165 1,90 13,20 333 6336 range => 40 - 65 50-300

13.1.5 Wastewater Production per Capita

The spread sheet (Tab. 22.) helps to definedomestic wastewater on the basis of thenumber of persons and the wastewater theydischarge. BOD and water consumption fig-ures vary widely from place to place, andtherefore should be inquired very carefully.

Formulas used on the spread sheet „waste-water per capita“

E5=A5*C5/1000

F5=A5*B5/E5

G5=D5*F5

13.1.6 Septic Tank

The size of septic tanks is standardised inmost countries. However, in case of DEWATSthe wastewater that is used may not be thenormal domestic wastewater. The spreadsheet (Tab. 23.) will help to design the sep-tic tank accordingly.

Volume, number of peak hours of flow andpollution load are the basic entries. Start-ing from these data the “entrance param-eters” are the desludging interval and theHRT because the former decides the digestervolume to store the accumulating sludgeand the later, because it decides on thevolume of the liquid.

To observe the fact that sludge compactswith time, the formulas in the spread sheetsare based on graph Fig. 67.

COD removal rates in settlers and septictanks depend to a great deal on the amountof settleable solids, their COD content andthe intensity of inoculation of fresh inflow.

Tab. 22. Spread sheet for calculation of wastewater per capita

The contact between fresh incoming sub-strate and active sludge in Imhoff tanks isnearly zero, while in sedimentation pondswith a deep inlet, it is intensive. This facthas been taken into the formula by divid-ing the parameter “settleable SS per COD”by an experienced factor of 0.50 - 0.60.The general tendency is shown in graphFig. 68 .

13 COMPUTER SPREAD SHEETS

reduction of sludge volume during storage

0%

10%

20%30%

40%

50%

60%

70%80%

90%

100%

0 20 40 60 80 100 120

months

slud

ge v

olum

e

Fig. 67.Reduction of sludge volume during storage

128

Formulas of spread sheet „septic tank“

C5=A5/B5

H5=G5/0,6*IF(F5<1;F5*0,3;IF(F5<3;(F5-1)*0,1/2+0,3;IF(F5<30;(F5-3)*0,15/27+0,4;0,55)))The formula relates to Fig. 68. The number0,6 is a factor found by experience.

I5=(1-H5)*D5

J5=(1-H5*J6)*E5

13 COMPUTER SPREAD SHEETS

E6=D5/E5

J6=IF(H5<0,5;1,06;IF(H5<0,75;(H5-0,5)*0,065/0,25+1,06;IF(H5<0,85;1,125-(H5-0,75)*0,1/0,1;1,025)))The formula relates to Fig. 65.

D11=2/3*H11/B11/C11

F11=D11/2

H11=IF(H12*(E5-J5)/1000*A11*30*A5+C5*F5<2*A5*F5/24;2*A5*F5/24;H12*(E5-J5)/1000*A11*30*A5+C5*F5)+0,2*B11*E11The formula takes care that sludge volumeis less than half the total volume.

I11=(E11+G11)*C11*B11

J11=(D5-I5)*A5*0,35/1000/0,7*0,5350 l methane is produced from each kgCOD removed.

H12=0,005*IF(A11<36;1-A11*0,014;IF(A11<120;0,5-(A11-36)*0,002;1/3))The formula relates to Fig. 67.

A B C D E F G H I J

1 General spread scheet for septic tank, input and treatment data

2daily waste

water flow

time of most waste

water flow

max flow at peak hours

COD inflow

BOD5

inflow

HRT inside tank

settleable SS / COD

ratio

COD removal

rate

COD outflow

BOD5

outflow

3 given given calcul. given given chosen given calcul. calcul. calcul.4 m³/day h m³/h mg/l mg/l h mg/l / mg/l % mg/l mg/l5 13,0 12 1,08 633 333 18 0,42 35% 411 2096 COD/BOD 5 -> 1,90 12 - 24 h 0,35-0,45 domestic BODrem.-> 1,06

7 dimensions of septic tank

8

de-sludging interval

inner width of septic tank

minimum water

depth at outlet point

inner length of first chamber

length of second chamber

volume incl.

sludge

actual volume of

septic tank

biogas 70%CH4;

50% dissolved

9 chosen chosen chosen requir . chosen requir. chose n requir . chec k calcul.10 months m m m m m m m³ m³ m³/d11 12 2,50 2,00 3,13 3,10 1,56 1,55 23,46 23,25 0,7212 sludge l/g BODrem. 0,0042

Tab. 23. Spread sheet for calculation of septic tank dimensions

COD removal in settlers

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0 5 10 15 20 25 30 35 40

settling time in hours

fact

or

Fig. 68.COD removal in settlers

129

Imhoff Tank

Except for the fact that sedimentation ismore effective in the Imhoff tank, the othertreatment properties are comparable to thatof any settler. However, if wastewater hasto remain „fresh,“ it cannot at the sametime, come into close contact with activesludge. Therefore, BOD removal from theliquid is close to zero. Because sedimenta-tion is more, the COD or BOD removal iscomparable and only reflected in the factor0.50 of cell H5.

13 COMPUTER SPREAD SHEETS

cross section longitudinal sectioninlet outlet

all values are

2,15 2,00 inside measurements

2,50 3,10 1,55

Fig. 69.Illustration to spread sheet for calculation of septic tank dimensions

0,55 1,30 0,25

0,30

rectang. 0,30

(circular) 0,08

1,41 2,28

rectang. 0,57

(circul.) 0,51

2,24 2,82

flow tank

sludge

Volume, number of peak hours of flow andpollution load are the basic entries. Start-ing from these data, the “entrance param-eters” are - similar to that of the septictank - HRT and desludging intervals.

Fig. 70.Illustration to spread sheet for calculation of Imhoff Tank dimensions

130

A B C D E F G H I J

1 General spread sheet for Imhoff tank, input and treatment data

2daily waste

water flow

time of most waste

water flow

max flow at peak hours

COD inflow

BOD5

inflow

HRT inside flow

tank

settleable SS / COD

ratio

COD removal

rate

COD outflow

BOD5

outflow

3 given given calcul. given given chosen given calcul. calcul. calcul.4 m³/day h m³/h mg/l mg/l h mg/l / mg/l % mg/l mg/l5 25,00 12 2,08 633 333 1,50 0,42 27% 460 2376

COD/BOD 5 -> 1,90

domestic: 0,35-0,45 COD/BODrem 1,067 dimensions of Imhoff tank

8

de-sludging interval

flow tank volume

sludge volume

inner width of flow tank

space beside

flow tank

total inner width of Imhoff tank

inner length of Imhoff tank

sludge height

total depth at outlet

biogas 70%CH4;

50% dissolved

9 chosen calcul. calcul. chosen chosen calcul. calcul. calcul. calcul. calcul.10 months m³ m³ m m m m m m m³/d11 12 3,13 3,61 1,30 0,55 2,24 2,82 0,57 2,28 1,0812 sludge l/g BODrem. 0,0042

Formulas of spread sheet „Imhoff Tank“

C5=A5/B5

H5=G5/0,5*IF(F5<1;F5*0,3;IF(F5<3;(F5-1)*0,1/2+0,3;IF(F5<30;(F5-3)*0,15/27+0,4;0,55)))The formula relates to Fig. 68. The number0,5 is a factor found by experience.

I5=(1-H5)*D5

J5=(1-H5*J6)*E5

E6=D5/E5

J6=IF(H5<0,5;1,06;IF(H5<0,75;(H5-0,5)*0,065/0,25+1,06;IF(H5<0,85;1,125-(H5-0,75)*0,1/0,1;1,025)))The formula relates to Fig. 65.

B11=C5*F5

C11=A5*30*A11*C12*(E5-J5)/1000

F11=D11+E11+0,25+2*0,07All formulas of dimensions relate to thegeometry of the Imhoff tank as shown inFig. 70.

G11=B11/(0,3*D11+(D11*D11*0,85/2))

H11=C11/F11/G11

I11=H11+0,85*D11+0,3+0,3

J11=(D5-I5)*A5*0,35/1000/0,7*0,5350 l methane is produced from each kgCOD removed.

C12=0,005*IF(A11<36;1-A11*0,014;IF(A11<120;0,5-(A11-36)*0,002;1/3))The formula relates to Fig. 67.

Tab. 24.Spread sheet for calculation of Imhoff Tank dimensions

13 COMPUTER SPREAD SHEETS

131

13.1.8 Anaerobic Filters

Volume of flow and pollution load are thebasic entries. Starting from that data the„entrance parameter“ for the anaerobic fil-ter is the hydraulic retention time. The per-formance of anaerobic filters is based onthe curve which shows the relation betweenhydraulic retention time and percent of CODremoval. The curve (Fig. 71.) is based onCOD 1500 mg/l and 25°C. Their values arefurther multiplied by factors reflecting tem-perature (Fig. 72.), wastewater strength(Fig. 73.) and specific filter surface ( Fig. 74.).

The void space of filter medium influencesthe digester volume which is required toprovide sufficient hydraulic retention time.Gravel has approximately 35% void spacewhile special plastic form pieces may haveover 90%. When filter height increases withtotal water depth, consequently, the impactof increased depth on HRT is less with gravelthan with plastic form pieces. When filterheight shall remain the same, the distancefrom filter bottom to digester floor must beincreased.

anaerobic filter, CODrem in relation to HRT, CODin 1500 mg/l; 25°C

40%

45%

50%

55%

60%

65%

70%

75%

80%

0 20 40 60 80 100HRT in hours

CO

Dre

m

anaerobic reactors, CODrem relative to temperature

0,40

0,50

0,60

0,70

0,80

0,90

1,00

1,10

10 15 20 25 30 35temperature in °C

fact

or

Fig. 72.COD removal relative to temperature in anaerobicreactors

anaerobic filter, CODrem in relation to wastewater strength

0,80

0,85

0,90

0,95

1,00

1,05

1,10

0 500 1000 1500 2000 2500 3000 3500 4000

COD in mg/l

fact

or

13 COMPUTER SPREAD SHEETS

Fig. 74.COD removal relative to filter surface in anaerobicfilters

Fig. 71.COD removal relative to HRT in anaerobic filters

Fig. 73.COD removal relative to wastewater strength inanaerobic filters

anaerobic filter, CODrem in relation to specific filter surface

0,80

0,85

0,90

0,95

1,00

1,05

1,10

0 50 100 150 200 250

specific filter surface in m²/m³

fact

or

132

A B C D E F G H I J K L

1 General spread scheet for anaerobic filter (AF) with integrated septic tank (ST)

2daily waste

water flow

time of most waste

water flow

max. peak flow per

hour

COD inflow

BOD5

inflowSSsettl. /

COD ratio

lowest digester temper.

HRT in septic tank

de-sludging interval

COD removal septic tank

BOD5

removal septic tank

BOD / COD

remov. factor

3 given given calcul. given given given given chosen chosen calcul. calcul. calc.4 m³/day h m³/h mg/l mg/l mg/l / mg/l °C h months % % ratio5 25,00 12 2,08 633 333 0,42 25 2 36 25% 26% 1,066 COD/BOD 5 -> 1,90 0,35-0,45 (domestic) 2h

7 treatment data

8

COD inflow in

AF

BOD5

inflow into AF

specific surface of

filter medium

voids in filter mass

HRT inside AF reactor

factors to calculate COD removal rate of anaerobic filter

COD removal rate (AF

only)

COD outflow of

AF

COD rem rate of total

system9 calcul. calcul. given given chosen calculated according to graphs calcul. calcul. calcul.10 mg/l mg/l m²/m³ % h f-temp f -strenght f-surface f-HRT % mg/l %11 478 247 100 35% 30 1,00 0,91 1,00 69% 70% 142 78%12 80 -120 30-45 24 - 48 h

13 dimensions of septic tank

14

BOD / COD rem.

factor

BOD5 rem rate of total

system

BOD5

outflow of AF

inner width of septic tank

minimum water

depth at inlet point

inner length of first chamber

length of second chamber

sludge accum.

Volume incl.

sludge

actual volume of

septic tank

15 calc. calcul. calcul. chosen chosen calcul. chosen calcul. chosen calc. requir. calcul.16 ratio % mg/l m m m m m m l/kg BOD m³ m³17 1,10 85% 49 1,75 2,25 1,69 1,70 0,85 0,85 0,00 10,00 10,0418 sludge l/g BODrem.

19 dimension of anaerobic filter biogas production check !

20

volume of filter tanks

depth of filter tanks

length of each tank

number of filter tanks

width of filter tanks

space below

perforated slabs

filter height (top 40

cm below water level)

out of septic tank

out of anaerobic

filtertotal

org.load on filter volume COD

maximum up-flow velocity inside

filter voids

21 calcul. chosen calcul. chosen requir. chosen calcul. assump: 70%CH4; 50% dissolved calcul. calcul.22 m³ m m No. m m m m³/d m³/d m³/d kg/m³*d m/h23 31,25 2,25 2,25 3 2,69 0,60 1,20 0,97 2,10 3,07 1,57 0,9824 max!! <4,5 < 2,0

Tab. 25.Spread sheet for calculation of anaerobic filter dimensions

Formulas of spread sheet „anaerobic filter“

C5=A5/B5

J5=F5/0,6*IF(H5<1;H5*0,3;IF(H5<3;(H5-1)*0,1/2+0,3;IF(H5<30;(H5-3)*0,15/27+0,4;0,55)))The formula relates to Fig. 68. The number0,6 is a factor found by experience.

K5=L5*J5

L5=IF(J5<0,5;1,06;IF(J5<0,75;(J5-0,5)*0,065/0,25+1,06;IF(J5<0,85;1,125-(J5-0,75)*0,1/0,1;1,025)))The formula relates to Fig. 65.

D6=D5/E5

A11=D5*(1-J5)

B11=E5*(1-K5)

F11=IF(G5<20;(G5-10)*0,39/20+0,47;IF(G5<25;(G5-20)*0,14/5+0,86;IF(G5<30;(G5-25)*0,08/5+1;1,1)))The formula relates to Fig. 72.

G11=IF(A11<2000;A11*0,17/2000+0,87;IF(A11<3000;(A11-2000)*0,02/1000+1,04;1,06))The formula relates to Fig. 73.

13 COMPUTER SPREAD SHEETS

133

septic tank gas release anaerobic filter septic tank anaerobic filterinlet outlet

0,40

2,55 2,25 1,20

0,60

1,70 0,85 2,25 2,25 2,25 1,75 2,69

0,25 0,25 0,25

11,88 3 chambers included in total length

5

H11=IF(C11<100;(C11-50)*0,1/50+0,9;IF(C11<200;(C11-100)*0,06/100+1;1,06))The formula relates to Fig. 74.

I11=IF(E11<12;E11*0,1612+0,44;IF(E11<24;(E11-12)*0,07/12+0,6;IF(E11<33;(E11-24)*0,03/9+0,67;IF(E11<100;(E11-33)*0,09/67+0,7;0,78))))The formula relates to Fig. 71.

J11=IF(F11*G11*H11*I11*(1+(D23*0,04))<0,98;F11*G11*H11*I11*(1+(D23*0,04));0,98)The formula considers improved treatmentby increasing the number of chambers andlimits the treatment efficiency to 98%

K11=A11*(1-J11)

L11=(1-K11/D5)

A17=IF(L11<0,5;1,06;IF(L11<0,75;(L11-0,5)*0,065/0,25+1,06;IF(L11<0,85;1,125-(L11-0,75)*0,1/0,1;1,025)))The formula relates to Fig. 65.

B17=L11*A17

C17=(1-B17)*E5

F17=2/3*K17/D17/E17

H17=F17/2

J17=0,005*IF(I5<36;1-I5*0,014;IF(I5<120;0,5-(I5-36)*0,002;1/3))The formula relates to Fig. 67.

Fig. 75.Illustration to spread sheet for calculation of anaerobic filter dimensions

K17=IF(OR(K5>0;J5>0);IF(J17*(E5-B11)/1000*I5*30*A5+H5*C5<2*H5*C5;2*H5*C5;J17*(E5-B11)/1000*I5*30*A5+H5*C5);0)The formula controls that sludge volume isless than half the total volume and allowsto omit any settler.

L17=(G17+I17)*E17*D17

A23=E11*A5/24

C23=B23

E23=A23/D23/((B23*0,25)+(C23*(B23-G23*(1-D11))))

G23=B23-F23-0,4-0,05

H23=(D5-A11)*A5*0,35/1000/0,7*0,5350 l methane is produced from each kgCOD removed.

I23=(A11-K11)*A5*0,35/1000/0,7*0,5350 l methane is produced from each kgCOD removed.

J23=SUM(H23:I23)

K23=A11*A5/1000/(G23*E23*C23*D11*D23)

L23=C5/(E23*C23*D11)

13 COMPUTER SPREAD SHEETS

134

baffled septic tank, BODrem in relation to HRT

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 5 10 15 20 25 30

HRT in hours

BO

Dre

m

13.1.9 Baffled Septic Tank

Volume, number of peak hours of flow andpollution load are the basic entries. Start-ing from these data, the “entrance param-

eter” for designing a baffled septic tank isthe up-flow velocity (cell I17). However, theperformance of the baffled septic tank de-pends as well on the retention time whichcannot be reduced by simply changing thedepth of the up-flow chambers, be-cause up-flow velocity will then increase.To achieve the desired effluent quality, it isbetter to add another chamber because treatment efficinency increases withnumber of chambers (see formula of cell H17).The influence of temperature is less severethan with other anaerobic tanks. Too weak wastewater does not produce enough sludge forcontact of bacteria and incoming wastewater. Fig. 78 takes care of this effect. An additionalcurve is used used to prevent organig overloading overloading caused by too strong wastewater Fig. 77

13 COMPUTER SPREAD SHEETS

Fig. 76.BOD removal relative to HRT in baffled septic tanks

baffled septic tank, BODrem in relation to organic load

0,60

0,65

0,70

0,75

0,80

0,85

0,90

0,95

1,00

0 5 10 15 20

Kg BOD/m³*d

fact

or

Fig. 77.BOD removal affected by organic overloading inbaffled septic tanks

baffled septic tank, BODrem in relation to wastewater strength

0,8

0,85

0,9

0,95

1

1,05

1,1

1,15

0 500 1000 1500 2000 2500 3000 3500 4000

BOD in mg/l

fact

or

Fig. 78.BOD removal in baffled septic tanks related to waste-water strength

135

13 COMPUTER SPREAD SHEETS

A B C D E F G H I J K

1 General spread sheet for baffled septic tank with integrated settler

2daily waste

water flow

time of most waste

water flow

max peak flow per

hour

COD inflow

BOD5

inflowCOD/BOD

ratio

settleable SS / COD

ratio

lowest digester temp.

de-sludging interval

HRT in settler

(no settler HRT = 0)

COD removal rate in settler

3 avg. given max. given given calcul. given given chosen chosen calcul.4 m³/day h m³/h mg/l mg/l ratio mg/l °C months h %5 25 12 2,08 633 333 1,90 0,42 25 18 1,50 23%6 COD/BOD 5 -> 0,35-0,45 1,5 h

7 treatment data

8

BOD5

removal rate in settler

inflow into baffled reactor

COD / BOD5

ratio after settler

factors to calculate COD removal rate of baffled reactor

COD rem, 25°, COD

1500

theor. rem rate acc. to factors

COD rem.rate

baffle onlyCOD out

9 calcul. COD BOD5 calcul. calculated according to graphs calcul. calcul. calcul.10 % mg/l mg/l mg/l/mg/l f-overload f-strength f-temp f-HRT % % mg/l11 24% 489 253 1,94 1,00 0,84 1,00 1,02% 0,84 72 7012 1,06 <-COD /BOD rem.factor COD / BOD removal factor-> 1,08513 dimensions of settler baffled septic tank

14total COD rem.rate

total BOD5

rem.rateBOD5 out

inner masonry measurements chosen acc. to

required volume

sludge accum.

rate

length of settler

length of settler

max upflow velocity

number of upflow

chambers

depth at outlet

15 calcul. calcul. calcul. width depth calcul. calcul. chosen chosen chosen chosen16 % % mg/l m m l/g COD m³ m m/h No. m17 79% 63% 172 2,00 1,50n 0,0037 2,39 2,40 1,8 5 1,5018 1,4-2.0 m/h19 dimensions of baffled septic tank status and gp

20length of chambers should not exceed

half depth

area of single upflow

chamber

width of chambersactual upflow velocity

width of downflow

shaft

actual volume of

baffled reactor

actual total HRT

org. load (BOD5)

biogas (ass: CH4

70%; 50% dissolved)

21 calcul. chosen calcul. calcul. chosen calcul. chosen calcul. calcul. calcul. calcul.22 m m m² m m m/h m m³ h kg/m3*d m³/d23 0,75 0,75 1,16 1,54 2,00 1,39 0,25 15,00 14 0,84 3,5224 HRT reduced by 5% for sludge

TIP: If removal rate is insufficient; increase number of upflow chambers to keep upflow velocity low.

Tab. 26.Spread sheet for calculation of baffled septic tank dimensions

principal longitudinal section cross sectionprovision for the actual numbers of baffled tanks chosen is 5

gas release

inlet

30 outlet

1,65

settler 1,50 1,50 1,50 1,50

baffled settler

baffled reactor0,75 0,75 0,75 0,75 2,00 (settler)

2,40 0,25 0,25 0,25 0,25 2,00 (baffled reactor)

total length incl. walls in m: 9,51

20

Fig. 79.Illustration to spread sheet for calculation of baffled septic tank dimensions

136

Formulas of spread sheet „baffled septictank“

C5=A5/B5

F5=D5/E5

K5=G5/0,6*IF(J5<1;J5*0,3;IF(J5<3;(J5-1)*0,1/2+0,3;IF(J5<30;(J5-30)*0,15/27+0,4;0,55)))The formula relates to Fig. 68. The number0,6 is a factor found by experience.

A11=K5*A12

B11=D5*(1-K5)

C11=E5*(1-A11)

D11=B11/C11

E11=IF(J23<6;1;1-(J23<6)*0,28/14)The formula relates to Fig. 77.

F11=IF(C11<150;C11*0,37150+0,4;IF(C11<300;(C11-150)

*0,1/150+0,77;IF(C11<500;(C11-300)*0,08/200+0,87;IF (C11

<1000;(C11-500)*0,1/500+0,95;IF(C11<3000;(C11-1000)*

0,1/2000+1,05;1,15))) The formula relates to Fig. 78.

G11=IF(H5<15;(H5-10)*0,25/5+0,55;IF(H5<20;(H5-15)0,11/5+0,8;IF(H5<25;(H5-20)*0,09/5+0,91;IF(H5<30;(H5-25)*0,05/5+1;(H5-30)*0,03/5+1,05)))The formula relates to Fig. 78 b.

H11=IF(J17=1;0,4;IF(J17=24;0,7;IF(J17=3;0,9;(J17-3)*0,06+0,9)))The formula realtes to Fig. 76

J9=E11*F11*G11*H11*I11

J11=WENN(J9<0,8;J9;WENN(J9(1-0,37((J9)-0,8))<0,95;J9*(1-0,37*((J9)-0,8));0,95))The formula limits unrealistic BOD removalrates

K11=(1-J11)*C11

A12=IF(K5<0,5;1,06;IF(K5<0,75;(K5-0,75)(K5-0,5)*0,065/0,25+1,06;IF(K5<0,85;1,125-(K5-0,75)*0,1/0,1;1,025)))The formula relates to Fig. 65.

K12=IF(A17<0,5;1,06;IF(A17<0,75;(A17-0,5)*0,065/0,25+1,06;IF(A17<0,85;1,125-(A17-

13 COMPUTER SPREAD SHEETS

0,75)*0,1/0,1;1,025)))The formula relates to Fig. 65.

A17=1-K11/E5

B17=A17*K12

C17=(1-B17)*D5

F17=0,005*IF(I5<36;1-I5*0,014;IF(I5<120;0,5-(I5-36)*0,002;1/3))The formula relates to Fig. 67.

G17=IF(A11>0;IF(F17*(E5-C11)/1000*30*I5*A5+J5*C5<2*J5*C5;2*J5*C5;F17*(E5-C11)/1000*30*I5*A5+J5*C5);0)/D17/E17The formula takes care that sludge volumeis less than half the total volume, the set-tler may be omitted.

A23=K17*0,5

C23=C5/I17

D23=C23/B23

F23=C5/B23/E23

H23=(G23+B23)*J17*K17*E23

I23=H23/(A5/24)/105%

J23=C11*C5*24/H23/1000

K23=(D5-K11)*A5*0,35/1000/0,7*0,5350 l methane is produced from each kgCOD removed.

13.1.10 Biogas Plant

The biogas plant as it is known in ruralhouseholds of India functions more or lessas a fully mixed reactor, where cattle dungis thoroughly mixed by hand with water.The substrate, even as effluent is very vis-cous, little sludge settles as a result andfor many years no sludge is removed at all.The same rural biogas plant in China re-ceives a substrate which is a mixture ofhuman excreta, pigs dung and water, how-ever by far not as homogeneous as com-monly found in India. Other wastewater, for

137

13 COMPUTER SPREAD SHEETS

example from slaughter houses may haveagain different properties. It is thereforedifficult to find dimensions for all kind of„strong“ wastewater for which a biogasplant could be suitable. The followingspread sheet should be used with certainreservation and formulas may need to beadapted locally.

The spread sheet, however, reveals the in-fluencing factors. The formulas are basedon the following assumptions:

o Solids which settle within one day ofbench scale testing represent 95% of allsettleable solids.

o There is a mixing effect inside the di-gester due to relatively high gas produc-tion which does not allow additionalsludge to settle. Any additional sludgewill only make good for the loss in vol-ume by compression. Thus, the accumu-lating sludge volume is the same vol-

gas production in relation to HRT

00,10,20,30,40,50,60,70,80,9

1

0 5 10 15 20 25 30

HRT in days

fact

or

ball shaped digester half round digestergas inlet gas 3,19

inlet outlet outlet

0,25 0,25

0,60 1,34

0,90 VG 0,74

2,251,80

half round-> 4,50ball-> 3,60

1,07

0,92 0,25

1,20 3,19

3,28 3,13

1,50 1,38

plan of fixed dome digesters floating drum digester

volume of expansion chamber is equal to volume of gas storage

bottom may be flat, conical or bowl shaped

inlet

outlet

ume which is calculated from the oneday of bench scale testing.

o All settleable and non-settleable solidswill digest within hydraulic retentiontimes typical for sludge reactors

o 95% of their BOD is removed after 25days and 30°C, this is equivalent to 400 lof biogas produced from 1 kg of organicdry matter

Fig. 80.Gas production of fixed dome biogas plants in rela-tion to HRT

Fig. 82.Illustration to spread sheet for calculation of biogas plant dimensions

138

A B C D E F G H I J K L

1 General spread sheet for biogas plants, input and gas production data

2

daily flowTS (DM) content

org. DM / total DM

org. DM content

solids settleable within one

day

HRTlowest

digester temper.

ideal biogas

product. at 30°C

gas production factors

total gas product.

methan content

3 given given assumed calcul. tested chosen given given calcul. acc. to graphs calcul. assumed4 m³/d % ratio % ml/l d °C l/kg org DM f-HRT f-temp m³/d ratio5 0,60 6,0% 67% 4,0% 20 25 25 400 0,97 0,90 8,42 70%6 200-450

7 values for all digester shapes for all fixed dome plants

8

non-dissolv. methan prod.

approx. effluent

COD

de-sludging interval

sludge volume

liquid volume

total digester volume

gas storage capacity

gas holder volume

VG

free distance above

slurry zero line

outlet above zero

diameter of left shaft

diameter of expans. chamber

9 assumed calcul. chosen calcul. calcul. calcul. given calcul. chosen chosen chosen calcul.10 ratio mg/l months m³ m³ m³ ratio m³ m m m m11 80% 7.943 12 4,32 15,0 19,3 65% 5,5 0,25 0,60 1,20 3,1912 minimum 0,60 m

13 cylindrical floating drum plant ball shaped digester

14radius of digester

width of water ring

wall thickness of water

ring

radius of gas holder

theor. height of

gas holder

theor. depths of digester

actual height of

gas holder

actual depth of digester

volume of empty space above

zero line

radius ball shape

actual digester radius (ball)

actual net volume of digester

15 chosen chosen chosen calcul. calcul. calcul. calcul. calcul. calcul. requir. chosen check16 m m m m m m m m m³ m m m³17 1,50 0,25 0,12 1,38 0,92 3,13 1,07 3,28 0,34 1,77 1,80 20,5618

19 ball shaped digester half-ball shaped digester

20

lowest slurry level below zero line (fill in trial until "calcul."

match "target")

gas pressure

ball shaped

volume of empty space above

zero line

radius half round shape

actual digester radius (half

round)

actual net volume of digester

lowest slurry level below zero line (fill in trial until "calcul."

match "target")

gas pressure half-ball

21 trial !! calcul. target calc. calcul. requir . chosen check trial !! calcul. target calc.22 m m³ m³ m w.c. m³ m m m³ m m³ m³ m w.c.23 0,90 5,89 5,81 1,50 0,43 2,23 2,25 20,01 0,74 5,91 5,90 1,3424 1,50 max. 1,50 max.

13 COMPUTER SPREAD SHEETS

gas production in relation to temperature

00,10,20,30,40,50,60,70,80,9

1

0 5 10 15 20 25 30

temperature in °C

fact

or

Tab. 27.Spread sheet for calculation of biogas plant dimensions

Fig. 81.Gas production of fixed dome biogas plants in rela-tion to temperature

Formulas of spread sheet „biogas plants“

D5=B5*C5

I5=IF(F5<10;F5*0,75/10;IF(F5<20;(F5-10)*0,19/10+0,75;(F5-20)*0,06/10+0,94))The formula relates to Fig. 80

J5=IF(G5<5;0;IF(G5<10;(G5-5)*0,4/5;IF(G5<25;(G5-10)*0,5/15+0,4;(G5-25)*0,1/5+0,9)))The formula relates to Fig. 81

139

( )0 02

11 11 2 17419

3,/

,+

+ +F H I

13 COMPUTER SPREAD SHEETS

K5=H5*I5*J5*A5*D5

B11=1,1*((1000*K5*L5/A11/0,35)/(0,95*I5*J5))*(1-0,95*I5*J5)/A5The formula finds the influent COD and cal-culates the COD removal by assuming 350lmethane per kg COD removed; the addi-tional 10% stand for the anorganic CODwhich is not removed.

D11=30*C11*A5*E5/1000

E11=F5*A5

F11=D11+E11

H11=K5*G11

L11=2*SQRT((H11/J11-(K11/2)*(K11/2)*PI())/PI())The mathematical expression is:

D17=A17-B17/2

E17=H11/(D17*D17*PI())The mathematical expression is:H11/(D172 × π)

F17=(F11-POWER(A17-B17-C17;2)*PI()*E17)/(A17*A17*PI())+E17The mathematical expression is:

G17=E17+0,15

H17=F17+0,15

I17=3,14*I11*I11*(K17-I11/3)

J17=0,02+POWER((F11+H11/2+I17)/4,19;1/3)The theoretical digester volume is taken asthe volume below the zero line plus halfthe gas storage; 0,02 m are added for plas-ter. The mathematical expression is:

; 4,19 is 4/3π

2

1111

112

2

×

−

×

HJ

Kπ

π

( )F

A B C EA

E1117 17 17 17

17²17

2

−− − × ×

×+

ππ

L17=4,19*(K17-0,02)*(K17-0,02)*(K17-0,02)-I17-H11/2

B23=PI()*(I11+A23)*(I11+A23)*(K17-(I11+A23)/3)The volume above the lowest slurry levelis found by trial and error; p is expressedas PI().C23=I17+H11

D23=A23+J11

E23=3,14*I11*I11*(G23-I11/3)

F23=0,02+POWER((F11+H11/2+E23)/2,09;1/3)The mathematical expression is:

; 2,09 is 2/3π

H23=2,09*(G23-0,02)*(G23-0,02)*(G23-0,02)-E23-H11/2

J23=PI()*(I23+I11)*(I23+I11)*(G23-(I23+I11)/3)The volume above the lowest slurry level isfound by trial and error; π is expressed asPI().

K23=E23+H11

L23=I23+J11

( )0 02

11 11 2 232 09

3,/

,+

+ +F H E

13.1.11 Gravel Filter

Volume, number of flow and pollution loadare the basic entries. Starting from thesedata, the „entrance parameter“ is the de-sired effluent quality(BODout, cell E5). Thehydraulic retention time and temperaturehave the greatest influence on treatmentperformance. The  HRT depends on desiredBOD removal rate (Fig. 84.). The curve istbased on 25°C and 35% pore space. Thepore space inside the filter defines the „real“HRT, and the type of plantation plays alsoa certain role. However, more influencingfactors may be near to 1.0 and, more im-portantly, the information needed to definethese factors in any case, are most prob-ably not available at site.

140

13 COMPUTER SPREAD SHEETS

In practice, the limiting factors are the or-ganic load and the hydraulic load. The limitfor hydraulic load is approximately 100 l/m2

(0.1 m *d). However, this value could bemuch higher when using coarse filter me-dia with guaranteed conductivity. A horizon-tal filter should not receive more than 10 gBOD/m2*d, because oxygen supply via thesurface is limited. This value of 10 g, com-

planted gravel filter, 90% BOD rem.

0

10

20

30

40

50

60

70

80

90

10 15 20 25 30 35

temp. °C

HR

T d

ays

Fig. 83.HRT relative to temperature in gravel filters, basedon 90% BOD removal

Planted gravel filter, 35% pore space; 25°C

0,40

0,50

0,60

0,70

0,80

0,90

1,00

1,10

1,20

1,30

1,40

70% 75% 80% 85% 90% 95%BODrem

fact

or o

n H

RT

Fig. 84.Influence of desired BOD removal rates on HRT ofgravel filters, based on 35% pore space at 25°C

A B C D E F G H I J K L

1 General spread sheet for planted gravel filter, input and treatment data

2average

flowCOD in BOD5 in

COD/BOD ratio

outfl. BOD5

BOD5

rem. rateCOD rem. COD out

min. annual Temp.

HRT factor acc.

to k20=0,3

HRThydraulic conduct.

Ks

3 given given given calcul. wanted calcul. calcul. calcul. given calcul. calcul. given4 m³/d mg/l mg/l mg/l / mg/l mg/l % % mg/l C° via graph days m/d5 26 410 215 1,91 30 86% 84% 66 25 0,86 11,20 2006 BOD rem.factor via graph-> 1,025 Ks in m/s=> 2,31E-037 dimensions results

8

HRT in 35% pore

space

bottom slope

depth of filter at

inlet

cross section

area

width of filter basin

surface area

required

length of filter basin

chosen width

length chosen

actual surface

area chosen

hydr. load on chosen

surface

org. load on chosen

surface

9 calcul. chosen chosen calcul. calcul. calcul. calcul. chosen chosen check ! calcul. calcul.10 days % m m² m m² m m m m² m/d g/m² BOD11 3,92 1,0% 0,60 37,27 62,1 559 9,0 62,5 9,0 563 0,046 9,912 ^information only 0,3-0,6 m max BOD 5 150 g/m² always > 62,1 max. loads=> 0,100 10

Tab. 28.Spread sheet for calculation of dimensions of horizontal gravel filters

pared to 20 g for aerobic ponds are low.This is so because the gravel filter worksmore like a plug flow system, the organicload in the front part is much higher thanin the rear part and oxygen supply is infe-rior in the lower part, also. This is the rea-son why cross section area at the inflowside is also related to organic loading (cellE12).

141

13 COMPUTER SPREAD SHEETS

Fig. 85.Illustration to spread sheet for calculation of dimensions of horizontal gravel filters

Formulas of spread sheet „gravel filter“

D5=B5/C5

F5=1-E5/C5

G5=F5/G6

H5=B5*(1-G5)

J5=IF(F5<0,4;(F5*0,22/0,4);IF(F5<0,75;(F5-0,4)*31/35+0,22;IF(F5<0,8;(F5-0,75)*9,5/5+0,605;IF(F5<0,85;(F5-0,8)*12,5/5+0,7;IF(F5<0,9;(F5-0,85)*17,5/5+0,825;(F5-0,9)*30/5+1)))The formula refers to Fig. 84.

K5=J5*IF(I5<15;82-(I5-10)*37/5;IF(I5<20;45-(I5-15)*31/5;IF(I5<25;24-(I5-20)*11/5;IF(I5<30;13-(I5-25)*6/5;7))))The formula refers to Fig. 83.

G6=IF(F5<0,5;1,06;IF(F5<0,75;(F5-0,5)*0,065/0,25+1,06;IF(F5<0,85;1,125-(F5-0,75)*0,1/0,1;1,025)))The formula refers to Fig. 65.

L6=L5/86400

A11=K5*35%

D11=IF(A5/L5/B11<A5*C5/E12;A5*C5/E12;A5/L5/B11)The formula compares hydraulic load tomaximum organic load in cell E12.

E11=D11/C11

F11=IF(A5*C5/L12>A5*K5/C11;A5*C5/L12;A5*K5/C11)The formula compares permitted hydraulicload with organic load in cell L12

G11=F11/E11

J11=H11*I11

K11=A5/J11

L11=K11*C5

H12=E11

plan longitudinal section

plantation, preferably phragmites

62,5 inlet outlet

rizomes

h= 0,60 1,0%bottom slope

9,0 0,50 9,0 0,50

13.1.12 Anaerobic Pond

Anaerobic ponds may be built for sedimen-tation only, with very short retention times,as highly loaded ponds when heavy scumformation may be expected to seal the sur-face, or as relatively low loaded ponds whichare almost odourless because of neutral pHvalues. The spread sheet may be used forall three categories, Therefore the hydrau-lic retention time is the „entrance param-eter“. Ponds with long retention times (loworganic loading rates) may be divided intoseveral ponds in a row. The front portionmay be separated to support developmentof scum at ponds with short retention times.The organic load of the effluent may be themajor design criteria which can best be in-fluenced by the HRT. Ambient temperatureis important and should not be chosen toohigh for want of smaller ponds. It is as-sumed that temperature has no influenceon COD removal at short retention times ofless than 30 hours.

142

Cell G11 should be observed and comparedwith F11 when the pond is near residentialhouses.

13 COMPUTER SPREAD SHEETS

A B C D E F G H I J

1 General spread sheet for anaerobic and sedimentation ponds

2daily flow COD in BOD5 in

COD / BOD5

HRTsettleable SS / COD

ratio

ambient temp.°C

BOD5 removal factors

3 given given given calcul. chosen given given calculated acc. to graphs4 m³/d mg/l mg/l ratio h mg/l / mg/l °C f-HRT f-temp f-number5 260 2000 850 2,35 72 0,42 25 57% 100% 100%6 domestic-> 0,35-0,45

7 treatment data

8

BOD5

removal rate

BOD / COD

remov.

COD removal

rateCOD out BOD5 out

org. load BOD5 on total vol.

odourless limit of

org. load

de-sludging interval

sludge accum.

sludge volume

9 calcul. calc. calcul. calcul. calcul. calcul. calcul. chosen calcul. calcul.10 % factor % mg/l mg/l g /m³*d g /m³*d months l/g BOD m³11 57% 1,08 53% 943 366 171 263 60 0,0023 5121213 dimensions biogas potential

14

water volume

depth of pond

total area of pond

width of ponds

total length of pond

number of ponds

length of each pond

if equal

methan content

non-dissolv. methan prod.

potential biogas

product.

15 calcul. chosen required chosen calcul. chosen calcul. assumed assumed calcul.16 m³ m m² m m number m ratio ratio m³/d17 780 2,0 646 6,00 107,67 1 107,67 70% 50% 68,6718

Tab. 29a. Spread sheet for calculation of dimensions of anaerobic sedimentation pond (short HRT). In thatexample the pond is extremely long and narrow to allow development of scum in the highly loaded frontportion. A partition wall in the front third could support the effect. Would the pond be more square in size,there would be no highly loaded areas, but as well no sealing scum layer. Both options are possible.

The biogas potential is also calculated todecide whether a closed anaerobic tank withbiogas collection should be built instead.

COD removal factor of digestion in anaerobic ponds

00,10,20,30,40,50,60,70,80,9

1

0 120 240 360 480 600 720

HRT in hours

fact

or

Fig. 86. Influence of HRT on COD removal of non-settled solids in anaerobic ponds

COD removal factor of sedimentation

0

0,1

0,2

0,3

0,4

0,5

0,6

0 120 240 360 480 600 720

HRT in hours

fact

orFig. 87. Influence of HRT on COD removal of settledsolids in anaerobic ponds

143

13 COMPUTER SPREAD SHEETS

A B C D E F G H I J

1 General spread sheet for anaerobic and sedimentation ponds

2daily flow COD in BOD5 in

COD / BOD5

HRTsettleable SS / COD

ratio

ambient temp.°C

BOD5 removal factors

3 given given given calcul. chosen given given calculated acc. to graphs4 m³/d mg/l mg/l ratio h mg/l / mg/l °C f-HRT f-temp f-number5 260 2000 850 2,35 480 0,42 25 92% 100% 108%6 domestic-> 0,35-0,45

7 treatment data

8

BOD5

removal rate

BOD / COD

remov.

COD removal

rateCOD out BOD5 out

org. load BOD5 on total vol.

odourless limit of

org. load

de-sludging interval

sludge accum.

sludge volume

9 calcul. calc. calcul. calcul. calcul. calcul. calcul. chosen calcul. calcul.10 % factor % mg/l mg/l g /m³*d g /m³*d months l/g BOD m³11 98% 1,03 96% 88 17 36 263 60 0,0023 8811213 dimensions biogas potential

14

water volume

depth of pond

total area of pond

width of ponds

total length of pond

number of ponds

length of each pond

if equal

methan content

non-dissolv. methan prod.

potential biogas

product.

15 calcul. chosen required chosen calcul. chosen calcul. assumed assumed calcul.16 m³ m m² m m number m ratio ratio m³/d17 5.200 2,5 2.432 20,00 121,62 2 60,81 70% 50% 124,2918

inlet longitudinal section outlet cross section

alternately split into several ponds 2,00

107,67 6,00

side walls are likely to be

slanting

Tab. 29b.Spread sheet as Tab. 29a but used for calculation of dimensions of anaerobic fermentation pond (long HRT)

Formulas of spread sheet „anaerobic andsedimentation pond“

D5=B5/C5

H5=IF(E5<1;F5/0,6*(0,3*E5);IF(E5<3;F5/0,6*(E5-1)*0,1/2;IF(E5<30;F5/0,6*((E5-3)*0,15/27+0,4);IF(E5<120;E5*0,5*(1-0,55*F5/0,6)/120+0,55*F5/0,6;IF(E5<240;(E5-120)*0,25*(1-0,55*F5/0,6)/120+0,5*(1-0,55*F5/0,6)+0,55*F5/

Fig. 88.Illustration to spread sheet for calculation of dimensions of anaerobic ponds (figures of Tab.29a.

0,6;IF(E5<480;(E5-240)*0,19*(1-0,55*F5/0,6)/240+0,55*F5/0,6+0,75*(1-0,55*F5/0,6);(E5-480)*0,06*(1-0,55*F5/0,6)/240+0,55*F5/0,6+0,94*(1-0,55*F5/0,6)))))))The formula refers to Fig. 86. and Fig. 87.Below 30 hours HRT is the COD removalfactor influenced by settling properties (F5/0,6), longer retention times influence alsonon-settled solids.

144

13 COMPUTER SPREAD SHEETS

I5=IF(E5<30;1;IF(G5<20;(G5-10)*0,39/20+0,47;IF(G5<25;(G5-20)*0,14/5+0,86;IF(G5<30;(G5-25)*0,08/5+1;1,1))))The formula refers to Fig. 72. COD removalby sedimentation (HRT <30 hours) is notinfluenced by temperature.

J5=IF(E5<24;1;IF(F17=1;1;IF(F17=2;1,08;IF(F17=3;1,12;1,13))))

A11=IF(H5*I5*J5<0,98;H5*I5*J5;0,98)

B11=IF(A11<0,5;1,06;IF(A11<0,75;(A11-0,5)*0,065/0,25+1,06;IF(A11<0,85;1,125-(A11-0,75)*0,1/0,1;1,025)))The formula refers to Fig. 65.

C11=A11/B11

D11=B5-(C11*B5)

E11=C5-(A11*C5)

F11=A5*C5/(A17+J11)

G11=75%*IF(G5<10;100;IF(G5<20;G5*20-100;IF(G5<25;G5*10+100;350)))The formula refers to the rule of thumb givenby Mara, reflected in Tab. 15.

I11=0,005*IF(H11<36;1-H11*0,014;IF(H11<120;0,5-(H11-36)*0,002;1/3))The formula refers to Fig. 67.

J11=30*A5*(C5-E11)*I11*H11/1000

A17=A5/24*E5

C17=(J11+A17)/B17

E17=C17/D17

G17=E17/F17

J17=A5*(B5-D11)*0,35/1000/H17*I17The formula assumes 350 l methane per kgCOD removed.

145

13.1.13 Aerobic Pond

Volume of flow and pollution load are thebasic entries. Starting from these data, the“entrance parameter“ is the wanted effluentquality (BODout, cell F5). The HRT necessaryto achieve a certain BOD removal rate de-pends on temperature. The curve (Fig. 91.)shows this relationship for a 90% BOD re-moval rate. Fig. 90. shows how HRT changeswith changing treatment performance, de-fined as BOD removal rate at 25°C.

Sludge production may be high in aerobicponds due to dead algae sinking to thebottom. According to Suwarnarat 1,44 gTS can be expected from 1 g BOD5. Assum-ing a 20% total solids content in com-pressed bottom sludge and a 50% reduc-tion of volume due to anaerobic stabili-sation, almost 4 mm of bottom sludge pergram BOD5 / m3xd organic load would ac-cumulate during one year. At loading ratesof 15 g BOD5 / m3xd approximately 6 cm ofsludge are expected per year. However, thesludge volume has not been taken intothe calculation because the surface areaplays the major role for dimensioning.

10 7,517,5 14

30 37

Suwarnarat, pg. 45

oxidation ponds, max org load according to temperature (and solar energy)

0

5

10

15

20

25

30

35

40

10 15 20 25 30

temperature in °C

BO

D5

/ m²*

d

Fig. 89.Maximum organic load relative to temperature onaerobic-facultative oxidation ponds. The influenceof sun-shine hours has been included.

13 COMPUTER SPREAD SHEETS

Fig. 92.Illustration to spread sheet for calculation of dimen-sions of aerobic-facultative ponds

plan of pond system

inlet

9,00 1. pond 2. pond 3. pond polishing

pond 5,00

9,55 9,55 9,55 4,44

aerobic pond, 25°C

0,20

0,40

0,60

0,80

1,00

1,20

1,40

1,60

70% 75% 80% 85% 90% 95%

BOD rem.

HR

T fa

ctor

Fig. 90.Influence of desired BOD removal on HRT in aero-bic-facultative ponds, based on 25°C

aerobic pond, 90% BOD rem.

0,0

5,0

10,0

15,0

20,0

25,0

30,0

35,0

40,0

10 15 20 25 30 35

temp. °C

HR

T in

day

s

Fig. 91.Influence of temperature on BOD removal in aero-bic-facultative ponds, based on desired 90% BODremoval

146

13 COMPUTER SPREAD SHEETS

A B C D E F G H I J K M

1 General spread sheet on aerobic - facultative ponds, input and treatment data

2

daily flow COD in BOD5 inCOD / BOD5

min. water temp.

BOD5 out (wanted)

BOD rem. COD rem COD outBOD5 rem factor for

HRTHRT

de-sludging interval

3 given given calcul. calcul. given chosen calcul. calcul. calcul. calcul calcul. chosen4 m³/d mg/l mg/l mg/l / mg/l °C mg/l % % mg/l % days months5 20 500 170 2,94 20 30 82% 78% 108 0,59 12,9 126 0,05 -1,0

7 dimensions of aerobic - facultative ponds polishing pond 1 day HRT total

8

accum. sludge volume

permit. org. load

BOD5

actual org. load

(BOD5)

depth of ponds

total pond area

number of main ponds

width of ponds

length of each pond

area of polish pond

width of polish. pond

length of polish. pond

area of all ponds

9 calc. calcul. calcul. chosen calcul. chosen chosen calcul. calcul. chosen calcul. calcul.10 m³ g/m²*d g/m²*d m m² No m m m² m m m²11 6,3 19,3 13,2 0,9 258 3 9,00 9,55 22 5,00 4,44 79612 0,00624 l/g BOD 0,9-1,2 m

Formulas of spread sheet for calculation of„aerobic pond“

D5=B5/C5

G5=1-(F5/C5)

H5=G5*1/IF(G5<0,5;1,06;IF(G5<0,75;(G5-0,5)*0,065/0,25+1,06;IF(G5<0,85;1,125-(G5-0,75)*0,1/0,1;1,025)))The formula refers to Fig. 65.

I5=B5-H5*B5

J5=IF(G5<0,8;(G5-0,7)*0,05/0,1+0,37;IF(G5<0,9;(G5-0,8)*0,54/0,1+0,46;(G5-0,9)*0,48/0,05+1))The formula refers to Fig. 90.

K5=J5*IF(E5<15;39-(E5-10)*10/5;IF(E5<20;29-(E5-15)*7/5;IF(E5<25;22-(E5-20)*6/5;IF(E5<30;16-(E5-25)*4/5;12))))The formula refers to Fig. 91.

A11=30*A5*(C5-F5)*A12*L5/1000

B11=IF(E5<17;(E5-10)*7,5/7,5+7,5;(E5-17)*23/13+14)The formula refers to Fig. 89.

C11=A5*C5/(F11*G11*H11)

E11=IF(IF(F11=1;1;IF(F11=2;1/1,1;IF(F11=3;1/1,14;1/1,16)))*(A11+A5*K5)/D11>C5*A5/B11;IF(F11=1;1;IF(F11=2;1/1,1;IF(F11=3;1/1,14;1/1,16)))*(A11+A5*K5)/D11;C5*A5/B11)The first part of the formula considers theinfluence of dividing the total pond area intoseveral ponds. The second part comparespermitted organic load with calculated HRT.

H11=E11/F11/G11

I11=A5/D11

K11=I11/J11

L11=I11+F11*E11

A12=0,0075*IF(L5<36;1-L5*0,014;IF(L5<120;0,5-(L5-36)*0,002;1/3))The formula refers to Fig. 67.

Tab. 30.Spread sheet for calculation of dimensions of aerobic-facultative ponds

147

13 COMPUTER SPREAD SHEETS

13.2 Economic Computer Spread Sheets

13.2.1 General

This chapter intends to help the reader toproduce his or her own tool for calculatingannual costs of DEWATS. Since economiccalculations always incorporate the un-known future, they are never exact. How-ever, it would be reckless to invest inDEWATS without prior economic evaluation.The computer spreadsheet helps to calcu-late the annual costs, which include capitalcost, operational cost and maintenance.Expected income from biogas or sale ofsludge for fertiliser may be deducted. Touse the spread sheet, the following datahave to be collected:

o Planning cost, including transport to siteand laboratory cost for initial wastewateranalysis.

o Investment costs of buildings, site workand equipment

o Assumed maintenance and operationalcost

o Rate of interest (minus inflation rate),and

o Wastewater data to calculate possiblebenefits and to compare cost per amountof treated wastewater

13.2.2. Viability of Using Biogas

Whether the use of biogas is economicallyviable in itself, depends on whether thenecessary additional investment to facili-tate storage, transport and utilisation ofbiogas can be recovered by the income frombiogas in a reasonable time. The paybackperiod is considered to be an adequate in-dicator of viability.

Formulas of spread sheet „viability ofbiogas“

B4=6,5%*A4For rough calculation it is assumed that ad-ditional construction costs are 6,5% of origi-nal costs; which include cost for makingreactor roof gas-tight, for additional volumeto store gas, and for gas distribution andsupply pipes.

D4=50%*C4To guarantee permanent gas supply, addi-tional care has to be taken at site. Thisadditional effort is assumed to be plus 50%of normal operational cost.

F4=B4/(E4-D4)Negative values show that costs will neverbe recovered.

A B C D E F1 Economic viability of using biogas

2Investment cost without

use of biogas

additional constr.cost to facilitate use

of biogas

operational cost without

use of biogas

additional operational cost to use

biogas

income from biogas

pay back period of additional

cost

3 l.c. l.c. l.c./year l.c./year l.c./year years4

307.000 19.955 250 125 3.650 5,7

Tab. 31.Spread sheet for calculation the viability of necessary measures to facilitate biogas utilisation

148

13 COMPUTER SPREAD SHEETS

13.2.3 Annual Cost Calculation

A B C D E F G H I J K

1 Calculation of annual costs of DEWATS2 planning and site supervision cost investment cost total annual cost

3salaries for

planning and supervision

transport and allowance for

visiting or staying at

site

cost for wastewater

analysis

total planning cost includ. overheads

and akquisition

cost of plot incl. site

preparation

main structures of

20 years durability

secondary structures of

10 years durability

equipment and parts of

6 years durability

total investment cost (incl. land

and planning)

total annual cost

(including land)

total annual cost

(excluding land)

4 l.c. l.c. l.c. l.c. l.c. l.c. l.c. l.c. l.c. l.c. l.c.5 1.200 650 500 2.350 150.000 295.000 9.000 3.000 459.350 74.359 62.3596 wastewater data annual capital costs

7daily

wastewater flow

strength of wastewater

inflow

COD/BOD ratio of inflow

strength of wastewater

outflow

rate of interest in % p.a. (bank rate minus inflation)

interest factor q = 1+i

on investment

for land

on main structures of

20 years lifetime (incl planning fees)

on secondary structures of

10 years lifetime

on equipment of

6 years lifetime

total capital costs

8 m³/d mg/l COD mg/l / mg/l mg/l COD % l.c. / year l.c. / year l.c. / year l.c. / year l.c. / year9 20 3.000 2 450 8% 1,08 12.000 30.286 1.341 649 37.17910 operational cost income from biogas and other sources explanat.

11

cost of personal for operation,

mainten. and repair

cost of material for operation,

mainten. and repair

cost of power (e.g. cost for

pumping)

cost of treatment

additives (e.g. chlorine)

total operational

cost

daily biogas production

(70% CH4, 50% dissolved)

price 1 litre of kerosene

(1m³ CH4 = 0,85 l kerosene)

annual income from biogas p.a.

other annual income or savings

(e.g.fertiliser, fees)

total income per annum

l.c. = local currency;

mg/l = g/m³;

12 l.c. / year l.c. / year l.c. / year l.c. / year l.c. / year m³/d l.c./litre l.c./year l.c./year l.c./year13 100 100 50 0 250 12,75 2,69 7.347 0 7.347

Tab. 32.Spread sheet for economic calculation of DEWATS (based on annual costs).

I9=G5*(POWER(F9;10))*(F9-1)/(POWER(F9;10)-1)the mathematical expression is:

J9=H5*(POWER(F9;6))*(F9-1)/(POWER(F9;6)-1)the mathematical expression is:

K9=SUM(G9:J9)+E13-J13

E13=A13+B13+C13+D13

F13=A9*(B9-D9)*0,35*0,5/0,7/1000The formula assumes 350 l biogas per kgCOD removed

H13=F13*70%*G13*0,85*360

J13=H13+I13

GF F

F5

9 9 19 1

10

10×× −

−( )

HF F

F5

9 9 19 1

6

6×× −

−( )

Formulas of spread sheet „annual costs ofDEWATS“

D5=SUM(A5:C5)

I5=SUM(D5:H5)

J5=SUM(G9:K9)+E13-J13

K5=SUM(H9:K9)+E13-J13

F9=1+E9

G9=E5*E9

H9=(F5+D5)*(POWER(F9;20))*(F9-1)/(POWER(F9;20)-1)This and the following formulas are finan-cial standard operations; the mathematicalexpression is:

( )( )

F DF F

F5 5

9 9 19 1

20

20+ ×× −

−

149

13 COMPUTER SPREAD SHEETS

13.3 Using Spread Sheets withoutComputer

Not everybody uses a computer. Some maynot even have access to a computer. How-ever, computer formulas may also be use-ful to those who usually work with a pocketcalculator. The explanations that follow areespecially given for such persons. The ta-ble (Tab. 33.)for calculating the septic tankmay be used as example:

A computer table is described in columnsA.....X, AA...AX, etc. and in rows 1.....>1000.Each table consists of cells that have anaddress. For example, the first cell in thetop left corner has the address A1 (columnA, row 1). On the table below, cell J10 readsm3/d for example and cell D5 reads 633. CellI11 reads 23,25. This figure is the result of a

formula hidden ‘under’ it. On the computer,the formula appears in the headline everytime one hits the cell. These formulas canalso be used without a computer in con-nection with the various graphs. One hasto realise that the computer writing differsfrom normal mathematical writing in somepoints. Most important is that for examplethe usual 4/3x2 is written on the computeras =4/3/2, and the usual 4x2/3 may be writ-ten either 4*2/3 or 4/3*2.

Cell A5 and all other bold written figurescontain information to be collected and donot comprise formulas. The cells with hid-den formulas are these:

C5=A5/B5this is 13,0 [m3/d] / 12 [hours] = 1,08[m3/hours]

A B C D E F G H I J

1 General spread scheet for septic tank, input and treatment data

2daily waste

water flow

time of most waste

water flow

max flow at peak hours

COD inflow

BOD5

inflow

HRT inside tank

settleable SS / COD

ratio

COD removal

rate

COD outflow

BOD5

outflow

3 given given calcul. given given chosen given calcul. calcul. calcul.4 m³/day h m³/h mg/l mg/l h

mg/l / mg/l % mg/l mg/l5 13,0 12 1,08 633 333 18 0,42 35% 411 2096 COD/BOD 5 -> 1,90 12 - 24 h 0,35-0,45 domestic BODrem.-> 1,06

7 dimensions of septic tank

8

de-sludging interval

inner width of septic tank

minimum water

depth at outlet point

inner length of first chamber

length of second chamber

volume incl.

sludge

actual volume of

septic tank

biogas 70%CH4;

50% dissolved

9 chosen chosen chosen requir. chosen requir. chosen requir. check calcul.10 months m m m m m m m³ m³ m³/d11 12 2,50 2,00 3,13 3,10 1,56 1,55 23,46 23,25 0,7212 sludge l/g BODrem. 0,0042

Tab. 32.Sample spread sheet which is used to help to understand computer formulas

150

13 COMPUTER SPREAD SHEETS

H5=G5/0,6*IF(F5<1;F5*0,3;IF(F5<3;(F5-1)*0,1/2+0,3;IF(F5<30;(F5-3)*0,15/27+0,4;0,55)))this is (0,42 [mg/l / mg/l] / 0,6 [a given fac-tor found by experience] ) multiplied by thevalue taken from Fig. 68 at 18 hours HRT(shown in cell F5). The calculation is there-fore:(0,42/0,6) × 0,495 = 0,35 = 35% (which isshown in cell H5)

I5=(1-H5)*D5(1-0,35)x633 = 411 (shown in cell I5)

J5=(1-H5*J6)*E5(1-0,35x1,06)x333

E6=D5/E5633 / 333 = 1,90

J6=IF(H5<0,5;1,06;IF(H5<0,75;(H5-0,5)*0,065/0,25+1,06;IF(H5<0,85;1,125-(H5-0,75)*0,1/0,1;1,025)))This formula refers to Fig 65. Since cell H5(the removal rate) is 35%, the value of cellJ6 is found in the graph where it shows1,06

D11=2/3*H11/B11/C11((2/3) x 23,46 ) / (2,50 x 2,00) = 3,13

F11=D11/23,13/2 = 1,56

H11=IF(H12*(E5-J5)/1000*A11*30*A5+C5*F5<2*A5*F5/24;2*A5*F5/24;H12*(E5-J5)/1000*A11*30*A5+C5*F5)+0,2*B11*E11The formula refers via cell H12 to Fig. 67,cell H12 must be calculated first. The for-mula H11 says, that the total volume mustbe at least twice the sludge volume. Onehas to check whether the total volume mustbe calculated via the hydraulic retention timeor via the double sludge volume. The totalvolume is the sludge volume, which is0,0042 × (333-209) × 12 × 30 [days/month]×13,0/1000 plus the volume of water whichis 1,08 × 18 = 21,88 m3. This is compared

to 2 × 13,0 × 18 / 24 [hours/day], which is19,50 m3 Therefore 21,88 is bigger and mustbe used. In addition there is the volume of20 cm of scum added, which is 0,2 × 2,50× 3,10 = 1,55. The total volume is 21,88 +1,55 = 23,43 (the computer is more exactand says 23,46 m3 in cell H11

I11=(E11+G11)*C11*B11(3,10 + 1,55) × 2,00 × 2,50 = 23,25 m3

J11=(D5-I5)*A5*0,35/1000/0,7*0,5(633 - 411) × 13,0 × 0,35 × 0,5 / (1000× 0,7)= 0,72 m3

H12=0,005*IF(A11<36;1-A11*0,014;IF(A11<120;0,5-(A11-36)*0,002;1/3))The last formula refers to Fig. 67.. Thedesludging interval is 12 months (cell A11)which gives a value in the graph of approx.80% which is to be multiplied by sludgeproduction figure of 0,005. The calculationis therefore: 0,8 × 0,005 = 0,004 (the com-puter calculates exactly 0,0042).

151

Geometrical formulasrectangel A= a b×rectangular prism A= ( )2 × × + × + ×a b a c b c V= a b c× ×

trapezium A=a c

h+

×2

trapeziform prism V= ( )ha b c d a b c d

3× × + × + × × ×

circle A=π × r 2C= 2× ×π r

cylinder A(mantle)= 2 × × ×π r h V=π × ×r h2

sphere (ball) A= 4 2× ×π r V=43

3× ×π r

spherical segment A= 2× × ×π r h V= π × × −

h r

h2

3

cone A(mantle)= π × ×r s V=π × ×rh2

3law of pythagoras a b c2 2 2+ = sides of 90° triangle: 3 / 4 / 5

tan 45° = 1tangent a / b tan 30° = 0,577

tan 60° = 1,732tan 90° = ∞

velocity v Q A= / Q v A= × ; A Q v= /

14 APPENDIX

Geometric formulas

152

Conversion factors of US-unitsitem US-unit SI-unit US/SI-unit SI/US-unit

length in cm (10mm) 2,540 0,394ft (12in) m (100cm) 0,305 3,281yd (3 ft) m 0,914 1,094

mi (1.760yd) km (1.000m) 1,609 0,621area in² cm² 6,452 0,155

ft² m² 0,093 10,764yd² m² 0,836 1,196

acrehectar

(10.000m²)0,405 2,471

mi² km² 2,590 0,386volume in³ cm³ 16,387 0,061

ft³ liter 28,317 0,035ft³ m³ 0,0283 35,314

gallon litre 3,785 0,264yd³ (202gal) m³ 0,765 1,308

acre-foot m³ 1.233,5 0,001force / mass lb N 4,448 0,225

oz g 28,350 0,035lb (16oz) kg (1000g) 0,454 2,205

ton (short) (2000lb)

t (1000kg) 0,907 1,102

ton (long) (2240lb)

t (1000kg) 1,016 0,984

pressure in H2O Pa (N/m²) 204,88 0,005lb/in² kPa (kN/m²) 6,895 0,145lb/ft² Pa (N/m²) 47,88 0,021

flow rate gal/min l/s (86,4m³/d) 0,0631 15,850gal/d l/s 0,0000438 22825gal/d

(1.440gal/min)m³/d

(0,0116l/s)0,00379 264

energy + Btu kJ 1,055 0,948power hp-h MJ 2,685 0,373

kWh kJ 3.600 0,00028Ws J 1.000 0,001hp kW 0,746 1,341

temperature °F °C 0,56(°F-32) 1,8(°C)+32°F °K 0,56(°F+460) 1,8(°K)-460

14 APPENDIX

153

A B C D E F G H I1 Energy requirement and cost of pumping

2flow rate

main flow h/d

flow rate per hour

pump hight

assumed head loss

efficiency of pump

required power of

pump

cost of nergy

annual energy

cost3 m³/d h m³/h m m η kW ECU/kWh ECU4 26 10 2,6 10 3 0,5 0,18 0,15 100,85

Formulas of spread sheet for cost of pumpingC4=A4/B4

G4=9,81*(D4+E4)*C4/F4/3600

I4=B4*G4*365*H4

A B C D E F G H I J1 Flow in partly filled round pipes

2pipe Φ

flow height

flow areamoisted area/m

hydraulic radius

sloperough ness

flow speed

flow

3 chosen given calcul. calcul. calcul. chosen estimat. calcul. calcul. calcul.4 d h/d A U rhy s rf v Q Q5 m m/m m² m m % m/s l/s m³/h6 0,1 0,15 0,00074 0,080 0,0093 1,0% 0,35 0,21 0,153 0,557 0,1 0,25 0,00154 0,105 0,0147 1,0% 0,35 0,31 0,478 1,728 0,1 0,35 0,00245 0,127 0,0194 1,0% 0,35 0,40 0,969 3,499 0,1 0,5 0,00393 0,157 0,0250 1,0% 0,35 0,49 1,932 6,9610 0,1 0,75 0,00632 0,210 0,0302 1,0% 0,35 0,58 3,641 13,11

Formulas of spread sheet for flow in partlyfilled pipes (after Kutter’s short formula)

C6=0,295*(A6/2)^2All figures - as here 0,295 - are geometricalconstants, referring to the flow height inrelation to the diameter of the pipe

D6=1,591*(A6/2)

E6=C6/D6

H6=(100*SQRT(E6)/(G6+SQRT(E6)))*SQRT(E6*F6)

I6=C6*H6*1000

J6=I6*3,6

C7=0,614*(A7/2)^2

D7=2,094*(A7/2)

E7=C7/D7

H7=(100*SQRT(E7)/(G7+SQRT(E7)))*SQRT(E7*F7)

I7=C7*H7*1000

J7=I7*3,6

C8=0,98*(A8/2)^2

D8=2,532*(A8/2)

E8=C8/D8

H8=(100*SQRT(E8)/(G8+SQRT(E8)))*SQRT(E8*F8)

I8=C8*H8*1000

J8=I8*3,6

C9=1,571*(A9/2)^2

D9=3,142*(A9/2)

E9=C9/D9

H9=(100*SQRT(E9)/(G9+SQRT(E9)))*SQRT(E9*F9)

I9=C9*H9*1000

J9=I9*3,6

C10=2,528*(A10/2)^2

D10=4,19*(A10/2)

E10=C10/D10

H10=(100*SQRT(E10)/(G10+SQRT(E10)))*SQRT(E10*F10)

I10=C10*H10*1000

J10=I10*3,6

14 APPENDIX

154

14 APPENDIX

The above graph shows the results of set-tling tests in a jar test under batch condi-tions (SS = settleable solids, TS = totalsolids; COD is measured as CODKMnO4).The curve might be different in through-flow settlers. The more turbulent the flow,the lesser is the removal rate of settleablesolids. The more old and new wastewatermixes the higher might be the BOD andCOD removal rates.

Sedimentation and Floatation

time SS TS BOD COD0 0 0 0 0

0,25 60 32 15 100,5 85 45 20 12

0,75 90 57 21 151 93 62 23 16

1,5 98 68,5 26 182 100 73 28 203 100 77 32 234 100 79,5 35 265 100 80 35 28,56 100 80 35 29,67 100 80 35 30

Removal rates in settling tests of domestic wastewater

0102030405060708090

100

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6 6,5 7

time in h

% o

f tot

al SS

TS

BOD

COD

after Sierp, in Imhoff pg.133

The performance of a settler is sufficientwhen the effluent of domestic wastewatercontains less than 0,2 ml/l settleable sludgeafter 2 h jar test.

Flocculent sludge has a settling velocitybetween 0,5 and 3 m/h.

The velocity in a sand trap should not ex-ceed 0,3 m/s [1000 m/h]. The minimum crosssection area is then:

Area [m 2] = flow [m3/s] / 0,3 [m/s], or

Area [m2] = flow [m3/h] / 1000 [m/h]

The general formula for floatation and sedimentation is:

watersurface mwaterflowvelocity

m hm h

[ ²][ ³ / ][ / ]

=

where velocity is the slowest floatation or settling velocity of the particles.

Settling and floatation time can be observed in a glass cylinder. The formula is then:

velocity m hheighttime

mh

[ / ][ ][ ]

=

where velocity is settling or floatation velocity, height is height of cylinder, and time isthe observed settling or floatation time.

155

Abwassertechnische Vereinigung e.V. Editor, “Lehr-und Handbuch der Abwassertechnik”, Vol. I -VI, 3rdEdition, Wilhelm Ernst und Sohn, Berlin, München,Düsseldorf, 1982

Alaerts, G.J., Veenstra, S., Bentvelsen, M., van Duijl,L.A. at al., “Feasibility of Anaerobic Sewage Treat-ment in Sanitation Strategies in Developing Coun-tries” IHE Report Series 20, International Institutefor Hydraulic and Environmental Engineering, Delft,1990

Bachmann, A., Beard, V.L. and McCarty, P.L., „Per-formance characteristics of the anaerobic baffled re-actor“, Water Science and Technology, IAWQ, 1985

Bahlo, K., Wach, G., „Naturnahe Abwasserreinigung- Planung und Bau von Pflanzenkläranlagen”, 2ndedition, Ökobuch, Staufen bei Freiburg, 1993

Batchelor, A. and Loots, P., „A Critical Evaluation ofa Pilot Scale Subsurface Flow Wetland: 10 Years Af-ter Commissioning“, Water Science and Technology,Vol. 35, No. 5, IAWQ 1997

Bischof, W. (Hosang/Bischof), “Abwassertechnik”, 9thedit., B.G. Teubner Verlag, Stuttgart, 1989

Blum, D. and Feachem, R.G., „Health Aspects ofNightsoil and Sludge Use in Agriculture andAquaculture“ IRCWD Reprot No. 05/85, (reprint 1997),EAWAG / SANDEC, Dübendorf CH, 1997

Boller, M., „Small Wastewater Treatment Plants - achallenge to Wastewater Engineers“, Water Scienceand Technology, Vol. 35, No. 6, IAWQ 1997

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Cooper, P.F (edit.), „European Design and Opera-tions Guidelines for Reed Bed Treatment Systems“(revised document), EC/EWPCA Emergent HydrophyteTreatment Systems Expert Contact Group, Swindon,UK 1990

Cross, P., Strauss, M., „Health aspects of nightsoiland sludge use in agriculture and aquaculture“,IRCWD, Dübendorf CH, 1986

De Vries J., “Soil filtration of wastewater effluentand mechanism of pore clogging”, JWPCF, vol.44,1972

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158

aerobic facultative see aerobic

aerobic ponds 9.12.2

aerobic process 5.3

aerobic respiration 5.2

air/gas mixture 12.5

albuminoid nitrogen 7.6

amino acids 5.2

ammonium 7.6

anaerobic fermentation 5.2

anaerobic filter 9.4

anaerobic ponds 9.12.1

anaerobic process 5.3

anoxic conditions 5.4

anoxic respiration 5.2

aquatic plants 9.12.3

area requirement 2.1.4

autolysis 5.5.4, 9.10

baffled septic tank (baffled reactor) 9.6

bio-chemical process 5.1

biogas plant 12.3,13.1.10

biogas production 12.1

biogas, pressure gauge 12.4

black box 3.5.1

BOD - bio-chemical oxygen demand 7.5

BOD removal in aerobic ponds 13.1.13

BOD removal in baffled tanks 13.1.9

BOD removal in gravel filters 13.1.11

BOD/N ratio 7.6

BOD/P ratio 7.7

BOD5 7.5

C/N/P/S-ratio 5.2

carbohydrates 5.2

chlorination practice 5.9

COD - chemical oxygen demand 7.5

COD removal in anaerobic filters 13.1.8

COD removal in anaerobic ponds 13.1.12

COD removal in settlers 13.1.6

COD/BOD ratio 7.5

colloids 7.2

compost 10.3

constructed wetland 9.9

cost of treatment, annual costs 4.2, 13.2.3

cost-benefit on biogas 4.3.4,13.2.2

Darcy’s law 9.10

de-nitrification 5.6

DEWATS 2.1.1

difficult degradable 5.4

discharge standards 3.6.2

discharge standards, India 3.4.2

diseases, water borne 7.11

dissemination, pre-conditions 3.2

domestic wastewater 3.7.3.2

dosing chamber 9.11

easily degradable 5.4

enzymes 5.2

expertise 3.5ff,3.7.1.4

facultative ponds 9.12.2

fertiliser value 11.4

filtered BOD 3.6.2

fish ponds 11.3

flow rate 7.1, 8.1

gas storage 12.3

glucose 5.2

grain size 5.5.4, 9.10

H2S release in ponds 9.12.1

head loss 9.3,9.4

KEYWORDS

159

KEYWORDS

health guide lines 11.1

heavy metals 5.8

hybrid reactor 9.13

hydraulic load 8.1

hydraulic retention time 8.1

industrial wastewater 3.7.3.2

Kjeldahl-N 7.6

lamella separator 5.5.3

lemna 9.12.3

maintenance 2.4.5, 3.5.1,3.7.1.5

marketing 3.7.3.3

metabolism 5.2

methane 12.1

nitrate 5.2, 6.2

nitrification 5.6

nitrite 6.2

Nkjel-kjeldahl nitrogen 7.6

noxious substance units 5.8

odour 7.4

organic loading rates 8.2

organic total solids 7.2

oxidation ditch 9.14

oxidation ponds 9.12.2,13.1.13

oxygen intake via surface 6.1

oxygen recovery 6.1

papyrus 9.12.2

pathogens 7.11

pathogens, survival time 11.1

pH 7.8

phase separation 5.4

phosphorous 5.7

phragmites 9.10

ponds 9.12

projects 3.7.2.3

rate constant 8.2

rate of interest 13.2

retention time 8.1

rotating disc reactor 9.14

rule of three 13.1.3

sand, gravel properties 9.10

screen 5.5.1

scum 5.5.3

sedimentation ponds 9.12.1

septic tank 9.2, 13.1.6

settling, sedimentation 5.5.2

sludge accumulation 8.3

sludge drying bed 10.2

sludge production 5.3

sludge properties 8.3

sludge removal 9.3, 10.1

sludge volume 13.3.2

social acceptance 3.3

solids in domestic wastewater 7.2, 8.3

specific filter surface 13.1.8

stormwater 7.1, 9.10

suspended solids 7.2

temperature 7.8

tipping bucket 9.11

toxic concentrations 7.5

trace elements 5.2

treatment quality 2.2.1

treatment steps 5.1, 8.2

velocity 8.1, 9.6

vertical filter 9.9, 9.11

viability biogas 13.2.2

void space 9.10

volatile fatty acids 7.9

wastewater per capita 13.1.5

wastewater strength 13.1.9

waterhyacinth 9.12.3

160

This book is for development planers and project managers who intent to disseminatedecentralised wastewater treatment systems. The book helps to understand the scopeand requirements of the technology.

This book is for engineers who need a tool for designing wastewater treatment plants.The book helps to decide about the most suitable treatment system and enables theengineer to produce calculation spread sheets for dimensioning and annual cost calcula-tions on the computer programme he or she is familiar with.

The book is for anybody who needs to understand the reason for wastewater treatmentand the basic principles of the various treatment processes. The book explains the mostcommon parameters of wastewater analysis.