Resources-Conservation-and-Recycling

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Resources,ConservationandRecycling55 (2011) 1232–1251

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Resources,ConservationandRecycling

journalhomepage:www.elsevier.com/locate/resconrec

Review

SustainableoptionsofposttreatmentofUASBefﬂuenttreatingsewage:Areview

AbidAliKhana,∗,RubiaZahidGaura,V.K.Tyagia,AnwarKhursheeda,BeniLewb,InduMehrotraa,A.A.Kazmia

DepartmentofCivilEngineering,IITRoorkee,NH58,Uttrakhand247667,IndiaTheVolcaniCenter,InstituteofAgricultureEngineering,BetDagan50250,Israel

article

info

abstract

Articlehistory:

Received19February2010

Receivedinrevisedform16May2011Accepted17May2011

Keywords:

Highratemicro-aerobictreatmentsystemsNBMS

PostsettlingtreatmentstepPolishingmethodsSewage

UASBposttreatment

Theupﬂowanaerobicsludgeblanket(UASB)processisreportedtobeasustainabletechnologyfordomes-ticwastewaterstreatmentindevelopingcountriesandforsmallcommunities.However,theinabilityofUASBprocesstomeetthedesireddisposalstandardshasgivenenoughimpetusforsubsequentposttreat-ment.InordertoupgradetheUASBbasedsewagetreatmentplants(STPs)toachievedesiredefﬂuentqualityfordisposalorforreuse,varioustechnologicaloptionsareavailableandbroadlydifferentiatedasprimarypost-treatmentfortheremovaloforganicandinorganiccompoundsandsuspendedmatter;secondarypost-treatmentfortheremovalofhardlydegradablesolublematter,colloidalandnutrients;andpolishingsystemsforremovalsofpathogens.Hence,thispaperdiscussesthedifferentsystemsforthetreatmentofUASBreactorefﬂuenttreatingsewage.Additionally,acomparativereview,aneconomicevaluationofsomeoftheemergingoptionswasconductedandbasedontheextensivereviewofdifferentintegratedcombination,i.e.UASB-differentaerobicsystems,atreatmentconceptbasedonnaturalbio-logicalmineralizationrouterecognizedasanadvancedtechnologytomeetallpracticalaspectstomakeitasustainableforenvironmentalprotection,resourcepreservationandrecoveringmaximumresources.

Contents1.2.

Introduction............................................................................................................................................1233PosttreatmentoptionsofUASBreactor’sefﬂuent....................................................................................................12352.1.CharacteristicsofefﬂuentofUASBreactortreatingdomesticwastewater...................................................................1235

2.1.1.BOD,CODandTSS....................................................................................................................12352.1.2.NandP................................................................................................................................12352.1.3.Reducedcompounds..................................................................................................................12352.1.4.Microbialpathogenicindicators......................................................................................................1236

2.2.Postsettlingsystems...........................................................................................................................1236

2.2.1.Conventionalpostsettlingmethods..................................................................................................12362.2.2.Flotationmethods....................................................................................................................1238

2.3.Physicalandbiologicalmicro-aerobicmethods(includingremoval/orrecoveryofdissolvedgases)........................................12382.4.Highratebiologicalaerobicmethods(includingnitriﬁcation–denitriﬁcationsteps).........................................................12392.5.Lowrateprimaryposttreatmentsystems(includingvalorization/orremovalofnutrients).................................................12422.6.Finalpolishingsteps............................................................................................................................1244Discussion/summary...................................................................................................................................12453.1.Solutionsforsustainabilityandenvironmentalprotection....................................................................................12463.2.Selectionofsustainabletechnology............................................................................................................1246Conclusions.............................................................................................................................................1249References..............................................................................................................................................1249

∗Correspondingauthor.

E-mailaddresses:abidkdce@iitr.ernet.in,dee.abid@gmail.com(A.A.Khan).

A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

1233

1.Introduction

Anaerobictreatmentofdomesticwastewaterisnotanewcon-ceptandfromtimeimmemorialseptictanks,soakpits,cesspool,etc.havebeenused.Sincethesesystemscanonlypartiallytreatthesewageand,theefﬂuentstillcontainshighconcentrationoforganicmatter,suspendedsolidsandnutrients,theinterestforsewagetreatmentswitchedovertoaerobictreatmentsystems.Therearenumerousaerobictreatmentsystemssomeofwhichinclude,acti-vatedsludgeprocess(ASP),ﬂuidizedbedreactors(FBR),tricklingﬁlter(TF),aeratedlagoonsandoxidationponds.Regardlessofthegoodtreatmentperformanceandlowlandrequirementofaerobicsystems;thesemethodssufferfromplentifuldrawbacksascom-paredtoanaerobictreatmentsystems(alsosummarizedinTable1)(Lettinga,2008):

•Energyintensive;

•Productionofhighandpoorlystabilizedsludge(60–70%ofincomingCODisconvertedtobiomass);

•Highinvestmentandoperational/maintenancecost;•Complexinfrastructure.

Though,mostoftheabovementioneddrawbacksarenotasso-ciatedwithoxidationpondsbuthighlandareaisneededwhichisveryuneconomicalindenselypopulatedcountrieslikeIndia.Therefore,thesementioneddrawbacksmaketheanaerobicsys-temssuitableforruralareasanddevelopingcountries.

Withtheadventofhighrateanaerobicsystemssuchasup-ﬂowanaerobicsludgeblanketreactor(UASB),anaerobiccontactprocess,anaerobicﬁlter(AF)orﬁxedﬁlmreactorsandﬂuidizedbedreac-tors,whichpromoteagoodcontactbetweentheinﬂowwastewaterandthemicro-organismsathighconcentrationandconsequentlyhighorganicmatterremovalatshortretentiontimes,thestrategyforthetreatmentofsewagewasshiftedbacktoanaerobicprocesswhichhastheadvantagesoflowcost,energyrecoveryintheformofbiogas,operationalsimplicity,lowenergyconsumption,andlowproductionofdigestedsludge.In1970s,duetotheenergycrisisandrelativelylessexpensivetreatmentconcept,theUASBprocesswasrecognizedasoneofthemostfeasiblemethodforthetreatmentofsewageindevelopingtropicalandsub-tropicalcountrieslikeIndia,BrazilandColombiawhereﬁnancialresourcesaregenerallyscarce.

Since1980,thediscussionontheapplicabilityofUASBpro-cessforthetreatmentofsewagehasbeenpresentedbyLettingaandco-researchers(Lettingaetal.,1980,1981,1993;LettingaandHulshoffPol,1986;Seghezzoetal.,2002;vonSperlingandChernicharo,2005;Lettinga,2008)andtheresultsindicatedthatabout70%chemicaloxygendemand(COD)removalcanbeachievedinwarmclimatescountries(Schellinkhoutetal.,1985;Souza,1986;Siddiqi,1990;Khan,2011).Presentlyabout30UASBbasedSTPswereinstalledinIndiasincelate1980sandmorethan20areunderconstruction(MoEF,2005and2006).

Therefore,singlestepUASBprocessundoubtedlyexperiencedasanattractiveoptioninwarmclimateregions.However,thetreatmentefﬁciencydecreaseswiththedecreaseintemperature,reachinga50%CODremovalat15◦C(Elmitwallietal.,2001;SinghandViraraghavan,2002;Lewetal.,2003).TheUASBreactorperformanceatlowtemperaturescanbeimprovedbychang-ingitsconﬁgurationlikeincorporatingsettlerabovetheGLSS(gas–liquid–solidseparator),oraddingtheAFatthetopoftheUASBreactor,makingitasahighrateanaerobichybridreactorandextended(staged)typesofUASB-systems,viz.UASB-reactorscom-pletedwithanAForsupplementedwithadditionalsludgedigesteroperatedatoptimaltemperatureforstabilizingsludge‘extracted’fromtheUASBreactor,whichfollowingitsstabilizationinthedigesterpartiallywillbereturntotheUASBreactorinordertokeepthemethanogenicactivityatasufﬁcientlyhighlevel.Theper-formanceofUASB-Digestersystemfororganicmatterremovalwasobservedsubstantiallybetterattemperatureof15◦CascomparedtosinglestepUASBreactoratlowtemperature(Mahmoud,2002).Thetwo-stepUASBsystemwasstudiedbyvariousauthors(SayedandFergala,1995;Halalsheh,2002;Seghezzo,2004).However,resultsshowedsimilarperformanceofthetwo-stepreactorincom-parisontoaone-stepsystem,duetolowerremovalefﬁcienciesinthesecondstage,whichwasattributedtolowsludgeretentiontime(SRT).TypicalproblemsofhighlyloadedUASBreactorslikesludgeﬂotationandwashoutofactivebiomasswereobserved,mainlyattemperaturesbelow20◦C.Wang(1994)evaluatedatwostagesys-temcomposedofUASB-EGSB(expandedgranularsludgebed)forsewagetreatmentatlowtemperature.Theprimaryobjectiveoftheﬁrst-steptreatmentwastheremovalandpartialhydrolysisofsus-pendedCODandthesecond-stepprocessobservedtoconvertthedissolvedCODtoenergyrichmethanegas.

ChernicharoandMachado(1998)studiedpilotscalesystemcomposedofthreeunitsviz.;416LUASBreactoroperatedat6hand4hhydraulicretentiontime(HRT)followedbytwoanaero-bicﬁltersinupﬂowanddownﬂowmodesoperatedinparallel.Theanaerobicﬁltershadatotalcapacityof102L(32Lofpackingmate-rial),operatedatHRTvaryingfrom24to1.5h(upﬂowvelocitiesvariedfrom0.06to1.44m/h).Theselectionofupwardanddown-ﬂowmodeofAFsoperationwastoidentifytheextentofremovaloforganicmatterduetophysicalmechanismsofsedimentationandﬁltrationthatarepredominantinupwardmodeorbiochemi-calstabilizationmechanismsofCODremoval.Thedownﬂowmodewasmeanttoinduceattachedgrowthasabioﬁlmthatfavoursthebiochemicalstabilization.TheUASBreactorperformedwellalmostachievingabove80%removalofCOD.TheresultsdepictedthattheAFsadditionallypromotedtheremovalwhichimprovedtheoverallefﬁciencyofthesystemwithintheambitofonlyanaero-bicregime.TheoverallCODandbiologicaloxygendemand(BOD)removalvariedfrom85to95%andtheconcentrationofﬁnalefﬂu-entCODrangedfrom60to90mg/LandtheBODandSSvalueswerelessthan40and25mg/L,respectively.However,theauthorssug-gestedthatUASB-AFsystemcouldbeanoptionforthetreatmentofdomesticsewageindevelopingcountries,sincethesystemcouldbeoperatedatanHRTof6hforUASBand3–4hforAFresultinginverycompactandlowcosttreatmentbesidesthistherewasnoenergyconsumption.

Similarly,Elmitwallietal.(1999)andLewetal.(2004)com-paredtheperformancesofahybridUASB-ﬁlterandaclassicalUASBreactorforthetreatmentofdomesticwastewateratdifferentoper-ationaltemperatures(28,20,14and10◦C)andloadingrates.ForeachtemperaturestudiedaconstantCODremovalwasobservedaslongastheupﬂowvelocitywaslowerthan0.35m/hinbothreactors.However,atlowertemperatureof14and10◦CtheUASBreactorshowedabetterCODandTSSremovalthanthehybridreac-tor.

AgainElmitwallietal.(2003)studiedacompositesystemoftwostepanaerobicsystemfollowedbyaerobicsystemconsist-ingofanaerobicﬁlter,anaerobichybridreactorandtricklingﬁlter(AF+AH+TF)atlowtemperature.TheoperatingconditionssuchasHRTandtemperatureweremoreorlesssimilartoothertwostepsystems.Thetreatmentperformanceoftwostepanaerobicsystem(AF+AH)wasimprovedfrom63%to85%byaddingtheTF.Thetreatedefﬂuentofthisstagedsystemcanbereusedforrestrictedirrigationandnutrientcanberecycled.

Mahmoudetal.(2004)investigatedacombinedUASB-CSTR(completelystirredtankreactor)digestersystem,wheretheaccu-mulatedsludgefromtheUASBreactorwasthendirectedtoaCSTR-digester,operatingat35◦Cforfurthertreatment.ResultsshowedthattheUASB-CSTRdigesterhadabetterperformancethanasinglestageUASBreactorat15◦C.Theremovalefﬁcienciesfortotal,suspended,colloidalanddissolvedCODwere72,74,74and

1234A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

Table1

Drawbacksofaerobicwastewatertreatmentsystemsasﬁrstbiologicaltreatmentstepcomparedtotheapplicationofanaerobicwastewatertreatmentasﬁrststep.

Drawbacksofaerobicsystemoveranaerobicsystems1.Energydemandinginsteadofenergyproducing

2.Productionofhugeamountsof(generallypoorlystabilizedandquitevoluminous)excesssludge

3.Highlandrequirementsforthetotaltreatmentsystem(consistingofprimarysettlers,theAeWT-systems,secondarysettlers,sludgethickeners,sludgestabilizationmethods,sludgedewatering,andsludgedryingbeds)4.Highinvestmentandoperation/maintenancecosts(morelaborintensive)

5.Useoftechnicallyrathercomplexmechanicalequipmentwithrelativelyshortlifetime6.Morecomplexinoperationandmaintenance,i.e.higherdependencyonspecialists

7.Needofamuchmorecomplexinfra-structure,suchaspowersupply,consequentlyahighvulnerability

8.Formationofrecalcitrantorganiccompounds(e.g.humicacids)frominessencewellbiodegradablecompoundsduetoexposureofoxygen9.Poordegradationofcompoundssuchasazo-dyes,PAC’s,nitro-aromatics10.Occasionallyseriousmal-odornuisanceproblemsAdvantagesofaerobicsystemoveranaerobicsystems

11.Theefﬂuentqualityofaerobicsystemsisbetterthananaerobicsystems12.CanproduceEfﬂuentofnon-potablereusequality13.NutrientRemovalPossible

14.Noimmediateoxygendemand

15.Aesthetically(Color,Turbidity)betterquality16.Noodorrelatedproblems

Source:AdaptedfromLettinga(2008).

62%,respectively.TheperformanceoftheUASB-digestersystematlowtemperaturesissimilartotheremovalobservedat28◦C.TheproblemofsolidsaccumulationobservedatlowtemperaturescanbehandledbyincorporatingasludgedigesterwithUASBreactor.

Inallcases,changesinconﬁgurationoftheUASBsystemsuchasadditionoftheAFanddigestersystemimprovedtheCODremovalatlowtemperatures(lowerthan20◦C),however,theefﬂuentcom-positionwasverysimilartothecompositionobservedforsingeUASBreactorsoperatingat25–28◦C(Table2).

Further,duetogrowingconcernovertheimpactofsewagecon-taminationofriversandlakesandincreasingscarcityofwaterintheworldalongwithrapidpopulationincreaseinurbanareasgivesreasontoconsiderappropriatetechnologiesforsewagetreatmentsothatsewagecanbetreateduptoreusestandard.

Theanaerobicprocessesconstitutethecoremethodinthenaturalbiologicalmineralization(NBM)treatmentconceptfortreatmentoflowandhighstrengthwastewaters.Highrateanaer-obictreatmentsystems,suchasUASBprocesscombinedwiththecomplementaryNBMsystem,deﬁnitelyrepresentapossi-bleroutetosustainableenvironmentalprotectioninspiriteventowardsamoresustainablesociety.StillresearchersrealizedgreatchallengingimprovementsinNBMroute/ﬁeldinordertogetsuf-ﬁcientconﬁdenceamongpolicy/decisionmakers,engineers,etc.Themaximumreuse/orrecoveryofresourcescanbeachievedthroughanaerobicpre-treatment(UASB)followedbythetreatmentconceptsappliedbasedonthenaturalbiologicalmineralizationsequence/orroute(NBMS).Moreover,atthesametimelessenergyconsumptioncanberealizedincomparisonwithpurelyaerobictreatmentsystems.

Thusbyfar,theUASBprocesshasbeendemonstratedasarobusttechnologyforsewagetreatment,mainlyfordevelopingcountriesand/orsmallcommunities(Lettingaetal.,1980,1993;HulshoffandLettinga,1986;vanHaandelandLettinga,1994;VerstraeteandVandevivere,1999;Arceivala,2001;GnanadipathyandPolprasert,1993;SousaandForesti,1996;Foresti,2001;Chernicharo,2006;SchellinkhoutandCollazos,1992;Seghezzoetal.,1998;vonSperlingandChernicharo,2005;Tandukaretal.,2006a,b).More-over,theUASBreactortreatingdomesticwastewatercanproducetwomainresources,whichcanberecoveredandutilized:methaneandtheefﬂuent.

ThevaluablebyproductmethaneproducedduringCODremovalcanberecovered(from28%to75%)andtransformedintoenergy(Mendozaetal.,2009).Inenergyterms,1m3ofbiogaswith75%methanecontentisequivalentto1.4kW-helectricity.Thebiogascanbeusedtorundualfuelgeneratorsorstreetlighting(Arceivala

andAsolkar,2007).AccordingtoArceivalaandAsolkar(2007)approximately23.5%methanegaswasobserveddissolvedinUASBefﬂuents,therefore,therecoveryofdissolvedmethanegasisdis-cretionaryandmaynotbeacceptableincaseofsewagetreatmentduetohighexpenditurecostandcomplexityofrecoveryarrange-ments.However,themethanegasevolvedtotheheadspace(gasphase)canbeofmuchimportanceandeasilycollected.Forhighstrengthindustrialwastewaterstherecoveryofdissolvedmethanegasisfavoredinviewoftheglobalwarminganditsfuelvalue.More-over,athightemperaturethesolubilityofgaseouscompounds,e.g.methanegasdecreases.Therefore,theissueofgasrecoveryespe-ciallydissolvedmethanegasmustbecarefullyreviewedforeachindividualcaseintermsofeconomicsanddesirability.

Inspiteofwellprovenadvantages,theefﬂuentofUASBreactordoesnotcomplywiththeefﬂuentdischargestandardsestab-lishedbyvariousenvironmentalagencies(Chernicharo,2006;Tandukaretal.,2006a,b).TheefﬂuentofUASBreactortreatingsewagecontainshighnutrientandpathogensincludingsufﬁcientlyhighamountofresidualorganicmatter.Thetreatedefﬂuentforreusemustmeetcertaincontrolssuchaspathogensconcentra-tionsfollowtheWHO(1989)standards.Topreventthepollutionofreceivingwaterbodiesandmakeituptoreusestandards,stringentdischargelegislationsmustberequired.

Todevelopanationalefﬂuentstandard,itisnecessarytocon-sideravarietyoflocalgeographical,socio-economic,dietaryandindustrialconditions(WHO,1989).Thenationalstandardsthere-fore,differappreciablyfromtheWHOguidelinevaluesaswellasbetweendifferentcountries.Forexample,efﬂuentdischargestandardsinIsraelaremorestringentthaninIndia:standardsguidelinesforTSSinIsraelare10mg/LwhereasinIndiatheval-uesare100mg/Lfordischargeintowaterbodies.Moreover,theFlemishstandardsforefﬂuentdischarge(fordomesticwastewa-ter)intosurfacewaterbodies,whichfollowstheEuropeanUnionenvironmentallegislationsismore/lessstringentthantheIsraelistandard,(BOD=25mg/L;TSS=35mg/L).Despitethedifferencesinguideline,thesevaluesarehardlyachievedthroughsingleanaero-bicprocesses.Thus,UASBreactoralonerendersanefﬂuentwhichisnotsuitableforagriculturere-useand/ordischargeintowaterbodies.However,itcanbeusedasaﬁrststepinwastewatertreatmentmainlyfororganicmatterremovalespeciallyinregionswhereﬁnancialresourcesarescarce.Toprotectthereceivingwaterbodiesandtoreusetreatedwaterforrestrictedaswellasunre-strictedirrigation,itisnecessarytofurthertreatefﬂuentfromUASBreactor,i.e.apost-treatmentsystem.ThemainroleoftheUASBpost-treatmentsystemsistoattaintheefﬂuentdisposalguidelines

A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

1235

Table2

Treatmentperformanceoflab&fullscaleUASBreactorstreatingsewage.

Country

Capacity

Temp.(◦C)

HRT(h)

Inﬂuent(mg/L)

Efﬂuent(mg/L)

Removalefﬁciency(%)

Reference

COD

BOD291240167214210150––300195––

TSS

COD

BOD

TSS

COD

BOD

TSS–––7375–89737369–6965–90–

JapanJapanIndia–IndiaBrazil–

Colombia–

Brazil

NetherlandsNetherlands

–

1148L5MLD–

5MLD106L110L35m33.7m3106L120L6m3

––25–

20–3121–2512–1823–2424–26208–2020

6610864.7185.210–1841218

600532590463560265465

430–520660424500550

333––174420123154

200–250–188––

222197201125140133163170178170225165

1537960395359––6661––

–––47105334265–59––

6363667374–78506566736060–9070

5367648275–8561–807869––

Tandukaretal.(2007)Tandukaretal.(2005)Draaijeretal.(1992)Goncalvesetal.(1998)Arceivala(1995)Vieira(1988)

Monroyetal.(1988)

Schellinkhoutetal.(1988)NobreandGuimarães(1987)VieiraandSouza(1986)Lettingaetal.(1983)Lettingaetal.(1981)

withacompleteremovaloforganicmatter;aswellasremovalofconstituentslittleaffectedbytheanaerobictreatment,suchasnutrients;reducedinorganic(sulﬁde,ferrous,etc.)compounds,andpathogenicorganisms(viruses,bacteria,protozoaandhelminthes)alongwithresourcesrecovery,and/orvalorizationofmineralizedcompounds.

Thecompleteremovaloforganicpollutantscouldbepossibleifthesewagecanbetreatedviaasequentialanaerobic,micro-aerobicandfullyaerobicbiodegradationofthepollutantsonthebasisofprocessesproceedingaccordingtothebiologicalC–NandS-cycle,togetherwithassociatedchemicalandphysicalprocessesFig.1.Thistreatmentconceptenablesconserving/orrecoveryofusefulbyproductsintheformoffertilizers,soilconditionersandrenewableenergy.

Therefore,theobjectiveofthisreviewpaperistosumma-rize,highlightandevaluatedifferentposttreatmentoptionsfortheefﬂuentofUASBreactorstreatingdomesticwastewaterinanattempttofulﬁllthe‘NaturalBiologicalMineralizationRoute’(NBMR),conceptoftreatment.Theinformationthus,gatheredcanbeappliedtoidentifyappropriatealternativetoexistingposttreatmenttechniquestoupgradetheefﬂuentqualityfromexist-ingUASBbasedSTPs.Alsoitprovidesajustiﬁcationforfurtherresearch.

2.1.CharacteristicsofefﬂuentofUASBreactortreatingdomesticwastewater

TheefﬂuentqualityofUASBreactorwasmeasuredintermsofBOD,COD,TSS,nutrientssuchasNandP,reducedcompounds,i.e.sulﬁdesandmicrobialpathogens.

2.1.1.BOD,CODandTSS

Theefﬂuentbiologicaloxygendemand(BOD)ofmostoftheanaerobictreatmentsystemssuchasanaerobicponds,UASBreac-tors,septictanksandImhofftanktreatingsewagewithoutanyposttreatmentsystemhasbeenreportedtovaryfrom60to150mg/L(Chernicharo,2006).TheCODandtotalsuspendedsolids(TSS)oftheanaerobicallytreatedmunicipalwastewaterrangesbetween100to200and50to100mg/L,respectively(Forestietal.,2006).Theprocessefﬁciencyvarieswithtemperature,strengthandcomposi-tion,e.g.fractionofindustrialwastewaterinﬁltratedanddiurnalﬂuctuations.Theefﬂuentsolublemineralizedcompoundssuchasammonia,phosphateandsulﬁdesalsocertainlyvariedwiththesefactors.Thetreatmentperformancedecreaseswithadecreaseintemperature(Lewetal.,2003,2004;Elmitwallietal.,2001;Wang,1994).TheperformanceofUASBreactors(COD,BODandTSSinﬂuent,efﬂuentandremoval)treatingsewageatdifferenttemperaturesissummarizedinTable2.

2.PosttreatmentoptionsofUASBreactor’sefﬂuent

Varietyofposttreatmentconﬁgurationsbasedonvariouscom-binationswithUASBwasreportedintheliterature.Thetreatmentperformanceofdifferentcombinationsofanaerobic(UASB)andaerobicposttreatmentsystemwassummarizedinTable3.Amongthese,tricklingﬁlter;TF,submergedaeratedbio-ﬁlter;SABF,rotat-ingbiologicalcontactor;RBC,wetlands,sequencingbatchreactor;SBR,chemicallyenhancedprimarytreatment;CEPTandzeolitecolumn,dissolvedairﬂotation;DAF,aerationsystemhavebeeninvestigatedatlaboratoryandpilotscale.Fewsystemslikepolish-ingpond,activatedsludgeprocess;wetlandsandaerobiclagoonsbasedonpilotandfullscaleapplication.

ThespeciﬁcaspectoftheposttreatmentofefﬂuentofUASBreactortreatingsewagewouldbethecompleteremovalofpathogenstoprotecthealthofhumanbeingsandthehighestremovalofCODinordertoprotectenvironmentwithrecov-eryofenergy,i.e.methaneandcertaincompoundssuchasNH4+–N,NO2−–N,NO3−–NandPO4–P.Therefore,theselectionofappropriatesustainabletechnologyshouldbebasedontheval-orizationofwastewaterbyrecoveringandreusingby-products,usesimpletechnologiesandconceptswhichcouldbeselectedbasedonthethoroughevaluationoftheefﬂuentqualityofUASBreactor.

2.1.2.NandP

Littleornonutrientremovalmaybeexpectedinananaero-bicsystemtreatingdomesticwastewater,asreportedbyseveralauthors(Lettingaetal.,1981;Forestietal.,2006;Moawadetal.,2009).Thereasonofthelownutrientremovalisthatduringtheanaerobicprocess,organicnitrogenandphosphorousarehydrolyzedtoammoniaandphosphate,whicharenotremovedfromthesystemandinconsequence,theirconcentrationincreasesintheliquidphase.Theconcentrationofammonianitrogenandphosphorousinanaerobicallytreatedmunicipalwastewaterhavebeenreportedtorangefrom30-50and10-17mg/Lrespectively(Forestietal.,2006).

2.1.3.Reducedcompounds

Sulfurcompoundsexistassulﬁdesinanaerobicsystemsefﬂuenttreatingdomesticwastewater.Theefﬂuenttotalsulﬁdescon-centrationgreatlydependsoninﬂuentsulfateconcentrationandsulfatereducingbacterialactivityinsidethereactor.Generally,thesulﬁdeconcentrationaround7–20mg/LwasobservedintheUASBefﬂuenttreatingsewageanditincreasestheefﬂuentoxygendemand(Walia,2007;Khan,2011).Further,thechemicalandbio-chemicaloxidationalsodependsonsulﬁdesconcentrationalongwithotherreducedspeciesFe2+andmercaptans.althoughlowfer-rousionconcentrationhasbeenobservedintheanaerobicefﬂuent

1236A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

CO2 CH4 H2S CO2 CO2 Organic wastes Red, (AnDi, SuDeNitr) And Degr. Solution with N- and P- compounds, mainly nutrients Micro- AeDegr AeDegr. viz. nitrification, mineralisation Solution with mineralized compounds S Soil cond itioner Fig.1.NaturalBiologicalMineralizationRoute(NBMR)oforganicmatteradaptedfrom(Lettinga,2008).

ofsystemstreatingdomesticwastewater.However,ferrousions

additiontoinﬂuentwasstudiedtoenhanceCODremoval.Vlyssidesetal.(2007)investigatedtheeffectofferrousiononbiologicalactiv-ityofUASBreactor.TheadditionofferrousioninducesastableandoutstandingconversionrateofCODandwasprovedtoenhancethebiologicalactivityofUASBreactor;otherwisetheferrousionsresultedduetoreducedenvironmentifsewagewastreatedbyUASBreactor.

2.1.4.Microbialpathogenicindicators

AlthoughUASBsystemsarenotdesignedforpathogenicremoval,fecalcoliformsreductionisaroundoneorderofmag-nitude(fromaround108to107);whilehelmintheggsremovalefﬁciencyhasbeenreportedtobe60–90%(Chernicharoetal.,2001;vonSperlingetal.,2002;Chernicharo,2006;vonSperlingandMascarenhas,2005).

Therefore,inordertomakethesituationidealforsustainabletreatmentthehighrateanaerobictreatmentsystemsespeciallyUASBrectormustbecombinedwithinnovativeposttreatmentsys-temsbasedonNMBsequence.Severalposttreatmentsystem/orcombinationofanaerobicpretreatment(i.e.UASBreactor)andfol-lowedbyaerobicsystemswereinvestigatedatlaboratoryandpilotscalelevelsforthetreatmentofsewage.ApplicationofdifferentaerobicsystemswithUASBreactorfortreatmentofsewagehasgivenlessemphasisonmakingthemsustainableand/ortoachievetherecoveryofresources.However,mostofthesecombinationswerefoundviableoptionforthetreatmentofefﬂuentofUASBreactor.

Thediscussionfortheselectionofthesustainabletechnologyforthepolicymakers,engineers,contractors,consultantsandauthor-itiesofthepublicsanitation(PuSansector)hasbeenpresentedindiscussion/summarypartofthisreviewpaper.

Further,inordertopromotethemasmoresustainableforthetreatmentofsewageandtoimprovetheirtreatmentperformance,thesesystems/combinationswerecategorizedbasedontheirappli-cationtoremovethesuspendedsolidswithorwithoutchemicalcoagulants,solubleorganicandinorganicmatter,andremovalofreducedcompoundssuchasferrousionsandsulﬁdesandrecoveryofmethane.

Themajorcategorieswere(I)conventionalorinnovativepostsettlingsystemsandmodernﬂotationmethodswithorwith-outchemicalcoagulants,etc.fortheremovalofsuspendedsolids

andsolubleorganicandinorganiccompoundslikephosphateortermedasprimaryposttreatmentoptions;(II)applicationofphys-icalmethodstoremoveandrecoverdissolvedmethanefromtheefﬂuent,whichisveryimportantissuefortheresearchers,engi-neersandscientists;(III)highrateprimarybiologicalmicro-aerobicmethodsfortheremovalofhighlyreduced(malodors)compoundslikesulﬁdesandvolatileorganicS−compounds,Fe2+andcolloidalmatter;(IV)highrateaerobicsystemsfornitriﬁcation,whencom-binedwithdenitriﬁcationstep;(V)Lowrateprimarytreatmentsystemsincludingthesystemsmeantforcultivationofbiomass,e.g.removingandvalorizingnutrients;(VI)polishingstepsforhighrateremovalsofpathogens.Theposttreatmentsystemsthus,cat-egorizedcaneitherbeusedsinglyorsequentially.

2.2.Postsettlingsystems

TheUASBefﬂuentcontainhighlystabilizedsuspendedmat-terwhichcanberemovedbymicro-aerationandsettlingprocess.Therefore,propermethodsofremovalofsuspendedsolidsareneeded,however,presentlyatfullscale,onlySTPsnaturalsettlingmethodsarewidelyused.Moreover,naturalsettlingmethodisoftenslowandinefﬁcientandsometimesenhancedbyadditionofchemicalwhichcouldeasilyremovethecolloidalandﬁnelydissolvedsolidsandseparatedbyphysicalaeration.Furthertherecoveryofresourcesintermsofphosphatesandtreatedefﬂuentifusedforirrigationpurposesmakingitanidealforsustainableoption.

2.2.1.Conventionalpostsettlingmethods

Overlandﬂowsystem(OFS)isaclassicalexampleofafullscalenaturalsysteminuseforUASBefﬂuentposttreatment.ThesystemwasextensivelystudiedinBrazilunderPROSABandisoperatedintwophases,characterizedbyconstantandtransienthydraulicregime,respectively(Chernicharoetal.,2001).Threeslopes(physicallyidentical)forwastewateroverlandﬂowconsti-tutedthepost-treatmentsystem.AverycommonweedspeciesnamedBrachiariahumidicolawasusedasvegetativecoverontheslopes.Thisweedisknownforitshighrateofnutrientabsorptionandhighresistanceagainstﬂooding.

TheapplicationofUASBefﬂuentatlowﬂowratefrom0.4to0.5m3/mhtooverlandﬂowsystempresentedagoodperformance.Theﬁnalefﬂuentconcentrationofthecombinedsystemshowed

A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

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)100)28(99o1tn(eod))))ma111i)))h00)8cc))0707709)9s00a2)90b)9a490090)2)02701)NM2((..ll0909(1090200(4d0120212.l0((((02.0daal2((0a0....2(nttla0n..(ll2lllt2(a(.2aaee.lt(s.aae(aaatl..tttlae.ooog)atetlilataeeeealtrrraaaain11icteedtesetnhhhlr0eahtntarreaaeovleccce2ekrstcefaairladdeueagaashhhwknaiiiirnnp(aﬁfeynkaavuSccwmcnenlrrrSnleeeaiRAeryao-aaolauoehhnohPPTCSEMMMTSGKhCCCvK)9.99–99(50))))L59)8)1m)99995)999999×0)9.......))09999999994.9992919999999(((((((((/–(N533233422543P000000000000M111111111111××××××××××××(030800458140C............1–41514–379–4––8–F5)L)/))9))gm4888599(76)(((4((0P567124(.....T00–––01––1–3–––––––)L))/g)m9)0))7))95750716((5(8)4(23(5(9(6((N5...5748(.8207T0–––214621–32––––––)L/gm()N)89))9)))–900()80284(5702091H((138(1(–(N.044.8(2.40–––2106102–––––1––.)e9)g)aL84)92w/g))––8)5)))))))37()em)42))1038446s8.816677783(9099998((518g88(99(90((4(1(((((05nS––(4(30872(70.08.332737Sit24>21130T15–192<<114aerts))m8)3e858ts)–9–yL4–7)s/7573g)))(8(9tnm))7)9))))))20(9–)e(188172341439)20156m*D99(9(899998(419188((0(8((((((88(tanO.–––((5757029066347800807eoC41>2154442453766955irttatrston)pe)c43Bn)9)8SoL–5–Ac/093Ugt))8–5dnm)16).)))))(5()e(59226))7367829euD8(9999699965(68t((aﬂO0(((99(((8(rf22244((84–0–.1.67482gEB3–>12–1295–1921<4–2.estensiesunrhotootniitrsceaaaldrvunemaremtepfcaoclsdstneostdesysibrsysceﬂdenynt–nwodreetclwnasetiopnmmlnoopdedxdlﬁooeieigtﬁeﬁcﬂitrgricezteotancedregbdarﬁsf+liufryeetnnelioeBuhrwesstktﬁlraalpSFgiscSldRCamokeraAAaFlSonouicaePhsvteHBBrbunUDocSpcdDSReasbirnvSaota+++++++++dtaoAﬂ3e+r++mBBBBe+++++gTBBBBBBBteemtePSSSSSSSSSSSaBBBBBrltbaEAAAAAAAAAAArSSSSS%naeICUUUUUUUUUUUeAAAAAaUUUUU*TrTaveragevaluesofBODfrom48to62mg/L;CODfrom98to119mg/LandSSfrom17to57mg/L.Thecombinedsystemremoved2–3log-unitsofFCtherebyreducingtheresidualFCofefﬂuenttoaround8.4×104to2.4×105MPN/100mL.Inaddition,asigniﬁcantremovalofhelmintheggswasobservedwithanaverageefﬂuentconcentrationof0.2egg/L.However,theﬁnalefﬂuentqualityoftheoverlandﬂowsystemwasinterferedbythetransientﬂowregimeandthehighconcentrationsofsolidsandorganicmatterintheUASBreactorefﬂuent.Forthesesituations,thelengthoftheslopewassuggestedtobekeptabove35m.

Cavalcantietal.(2001)studiedinBrazilthefeasibilityofasingle,ﬂow-throughpolishingpondsubdividedinlanesbyusingbafﬂesforthepost-treatmentofUASBreactorefﬂuent.Thepolishingpondwasconstructedtomaintainplugﬂowregimeinordertoraisethefecalcoliformremovalefﬁciencyofthesystemincontrasttoconventionalwastestabilizationpondswheretheorganicmat-terremovalwasconsideredasthedesignparameter.Twodistinctretentiontimesof5dand15dweremaintainedinthepond.Theresultsobtainedfromthestudyshowedthatattheretentiontimeof5d,theaverageBOD,CODandTSSvalueswerereducedto68,188and68mg/L,respectively.AtanHRTof15dtheseconcentrationslowereddownto24,108and18mg/L,respectively.Theresultsoftheremovalofpathogenicmicrobialindicatorswerealsoencour-agingwiththecompleteremovalofhelmintheggsatanHRTof3d.TheefﬂuentFCconcentrationatanHRTof15dwasobservedverycloseto1000MPN/100mLtoconformtheWHOguidelineforunrestrictedirrigation.

AgaininBrazil,vonSperlingandMascarenhas(2005)inves-tigatedtheperformanceoffourshallow(depth=0.40m)polishingpondsinseriesforthetreatmentofUASBefﬂuentatatotalhydraulicretentiontimeof7.4d(1.4–2.5daysineachpond)duringphaseI.InphaseII,thesefourpondswereoperatedinparalleleachhavingtwopondsinserieswithdoubledepthoffourpondsinseries.Basedontheresults,thetreatmentefﬁciencyinphaseIandphaseIIwassim-ilarandnothigh,withaﬁnalefﬂuentaverageconcentrationofBODandCODwere44and170mg/L,respectively.TheﬁnalefﬂuenthadgeometricmeanFCconcentrationslowerthan1000MPN/100mL.ThemeanoverallFCremovalefﬁciencywasremarkablyhigh(6.42logunits,or99.99996%),totalnitrogenconcentrationof10mg/Lintheefﬂuent,werefoundcompatiblewiththedischargestan-dardsof15mg/L(70%removal)forurbanwastewatersfromtheEuropeanCommunity.Theammonianitrogenconcentrationinefﬂuentfromcombinedsystemwas7.3mg/L(67%removal).How-ever,phosphorusremovalwasonly28%(efﬂuenttotalphosphorus(TP)concentrationof2.8mg/L).

Severalstudiesonintegratedanaerobic-aerobicsystemswerecarriedoutinBrazil.AnotherstudyonUASB-polishingpondsys-temwasdonebyvonSperlingetal.(2005)treatingsewagefortheremovalofE.coliandhelmintheseggs.Thedepthofthepondsandaverageretentiontimeinpondsvariedfrom0.40to2.00mand2to21days,respectively.Theshallowpondsinseries,evenwithlowretentiontimes,wereabletoproduceefﬂuentscomply-ingwiththecoliformsWHOguidelinesforunrestrictedirrigation(lowerthan1000MPN/100mL).Allpolishingpondsystemswereabletoproduceefﬂuentswithouthelmintheseggs,whatisincom-pliancewiththeWHOguidelinesforunrestrictedandrestrictedirrigation(≤1egg/L,arithmeticmean).Thus,theefﬂuentqualityofthepolishingpondsinseriessatisﬁedtheefﬂuentpathogendisposalstandards.But,higherlandrequirement,poornitrogenremovalandodorproblemmakingthemdisadvantageousfortheposttreatmentofUASBefﬂuentstreatingsewage.

AccordingtoKhan(2011)inIndiathereisapracticetouseﬁnalpolishingunits(i.e.FPUs)totreattheUASBefﬂuentandtheseFPUsaredesignedatanHRTof1d.However,thetreatmentperfor-manceoftheFPUswasinsigniﬁcantandmerelyrunningassettlingtanks,withaverylimitedalgalactivity.TheBODandTSSremoval

1238A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

isgenerallyfoundlessthan50%.Duetoverylimitedalgalactivity,coliformremovalalsorestrictedtogenerally1–2logunit,how-ever,helmintheggsareremovedcompletely(vonSperlingetal.,2005).Khan(2011)investigatedthe38millionliterperday(MLD)UASBfollowedbyFPUsystemforthetreatmentofsewageatSaha-ranpur,Indiaandobservedthatthehelmintheggsreducedfrominﬂuent(sewage)40–70eggs/Lto0–0.4eggs/Lintheﬁnalefﬂuentafterpond.

2.2.2.Flotationmethods

Dissolvedairﬂotation(DAF)clariﬁeswastewaterbyremovingﬂoatingsuspendedmattersuchasoil,fatsorsolids.Penetraetal.(1999)inBrazil,studiedtheperformanceofabenchscaledissolvedairﬂotationtreatingefﬂuentfromaUASB–coagulationsystem.Theferricchloride(FeCl3)doseof65mg/LatpHbetween5.3and6.1providedinDAFsystemeffectivelyreducedCODupto91%.TheremovalofTP,TSS,turbidityandtotalsulﬁdes(TS)was94–97%.TheefﬂuentCODfromthecombinedUASB-coagulation-DAFsys-temwasfoundtorangefrom20to50mg/L.TSSwasaround4mg/L.SimilaryanotherresearcherRealietal.(2001)inBrazilstudiedthebenchscaleDAFforthetreatmentofefﬂuentofpilotscaleUASBreactortreatingsewageandfoundthatabout73%CODremoval,86%phosphorousremovaland98%turbidityremovalcanbeachievedinDAFusingFeCl3asacoagulant(dosagesbetween30and65mg/L)alongwith0.4mg/Lofnonionicpolymer.However,thenitrogenremovalwasverypoor.Theuseofchemicalwasthemajordraw-backofthissystemmakingitalesssustainableoptionduetohighcostofchemical.Thissystemcanbeusedwithainterestofresourcerecoveryintermsofnitrogen,phosphorousandsulfurifefﬂuentisusedfortheirrigationpurposes.

Thecoagulationandﬂocculationmethodofposttreatmentfortheefﬂuentof38MLDUASBreactortreatingsewagewasstud-iedbyPrakashetal.(2007)inIndia.BOD,COD,TSSandFCoftheUASBefﬂuentwereintherangeof38–55,109–256,65–110mg/Land4.3×104to9.3×106MPN/100mL,respectively.Theoptimumdosesofalum,polyaluminiumchloride(PAC),ferricchlorideandferricsulfate,determinedthroughaseriesofjartestswerefoundtobe20mg/L(asAl)foralum,24mg/L(asAl)forPACand39.6mg/L(asFe)forFeCl3and17.6mg/L(asFe)forFeSO4,respectively.AllcoagulantseffectivelyreducedtheefﬂuentBODandSStolessthan20mg/Land50mg/L,respectively.However,fecalcol-iformsconcentrationscouldnotbereducedtoapermissiblelimitof1000MPN/100mLforunrestrictedirrigation.Theﬁnalconcen-trationoffecalcoliformofUASBreactorefﬂuentonlyreducedto2300MPN/100mL.Chlorinationatthedosingrateof1–2mg/Lwithcontacttimeof30minwasfoundsuitabletomeetthestan-dards.Further,higherdosesofchlorine,i.e.3mg/LwassuggestedtoremovealltheFCnumbersfromthesampleaftercoagulationwithabovementionedoptimumalumandPACdoses,butitwas4mg/Laftercoagulationwithironcoagulants.

Aiyuketal.(2004)inGhent,Belgium,proposedanintegratedcoagulation–ﬂocculation–UASB-zeolitecolumnconceptforalow-costtreatmentofdomesticwastewater.Inthisintegratedtreatmentsystem,domesticwastewaterwasinitiallysubjectedtochemicallyenhancedprimarytreatment(CEPT)usingFeCl3asacoagulantandapolymertoremovesuspendedmaterialandphosphorus,fol-lowedbyUASBtreatmenttoremovesolubleorganics.TheefﬂuentofUASBreactorwastreatedbyregenerablezeolitestoremovetotalammonianitrogen.Theionexchangesystemwasregeneratedbybiologicalnitriﬁcation.

Thecoagulant(FeCl3)doseof50mg/Lwasselectedfromaseriesofjartest.TheCEPTpretreatmentonanaverageremoved73%COD,85%SSand80%PO43−.However,thecoagulation/ﬂocculationstep,i.e.CEPTofthisintegratedsystemproducedthicksludgecontaining8.4%solids.Aftercoagulation/ﬂocculationstep,thepretreatedwastewatercontaininglowtotalCOD(approximately

140mg/L)wasintroducedtoanUASBreactorof2.1Lvolume(0.05minternaldiameterwith0.9mheight).Thereactorwasoperatedwithavolumetricloadingrateof0.4gCOD/Ld(HRT,10h)and0.7gCOD/Ld(HRT,5h).Thezeolite(asapost-treatmentcartridge)provedmostbeneﬁcialasitremovedalmost100%NH4+.Thelargeinterstitialspacesinlatticesofzeolitecon-tributedtotheremovalofNH4+throughionexchange.Theintegratedcoagulation–ﬂocculation–UASB-Zeolitesystemeffec-tivelydecreasedtheinﬂuentTSSandCODupto88%andmorethan90%,respectively.Thenitrogenandphosphorusweredecreasedto99%and94%,respectively.Pathogenindicators(fecalcoliform)werereducedto105cfu/Lfrom107cfu/Loftheinﬂuent.TheﬁnalefﬂuentcontainedlowCODandTSSvaluesaround50and35mg/L,respectively,whichcanbeusedforcropirrigationordischargedinstreams.Theauthorshavefurthersuggestedexploringthepossibilityofrecycling/reusingordisposalofthesidestreamsgen-eratedshouldbeexploredfurtherandevaluatedinfutureresearch,togetherwiththeenergyrecoverypotentialoftheCEPTsludge.

2.3.Physicalandbiologicalmicro-aerobicmethods(includingremoval/orrecoveryofdissolvedgases)

TheUASBefﬂuentcontainsreducedorganicandinorganicspeciesanddissolvedmethanegaswhichcanberemovedbymicro-aeration.Micro-aerationimpliesaerationoftreatedefﬂuentforabout30min.Theroleofmicro-aerationistostripoffandtooxi-dizethereducedspeciessuchassulﬁdesandferrousions,whichexertimmediateoxygendemandandremainingeasilybiodegrad-ableorganicpollutantsandtoremovethedissolvedmethanegas.Generally,thesesystemshaveveryshortHRTandtheamountofexcesssludgegeneratedisnegligible.Thesimplephysicalmicro-aerationcanbesufﬁcientlyremoveorstripoffthedissolvedsulﬁdesormethanefromtheUASBefﬂuent.Howevertheremovalofsus-pendedsolidsisinsigniﬁcantfromthisprocess.

Walia(2007)investigatedtheaerationofUASBefﬂuentbydif-fusers,surfaceaeratorsandcascadeonbenchscaleinRoorkee,India.Theoutcomeofthestudyhasgivenaconceptualmodelwhereinitdemonstratedthattheaerobicnatureoftheefﬂuentdependsondissolvedoxygen(DO),ORPandBOD.AnaerobicUASBefﬂu-entbecomesaerobicifitsBODisreducedtolessthan30mg/LandminimumvaluesofDOandORPareobserved,4–5mg/Land120–135mV,respectively.Basedonexperimentalresultsempiri-calcorrelationsbetweenBOD,ORPandDOhavebeendevelopedandtheresultsindicateda50%reductioninBODoftheUASBefﬂuentatHRTof∼100min.Subsequentlytreatmentofsewageina60LpilotscaleUASBreactorfollowedbyacontinuousdif-fusedaerationsystemandthefullscaleplant(111MLDcapacity;UASB+aeration+FPU)wasinvestigatedbyKhan(2011)inIndia.TheUASBreactorwaskeptat8hHRTwhichisidenticaltofullscaleUASBreactorsfunctioninginIndia.TheHRTofcontinuousaerationsystemwasmaintainedat15,30and60min.Duringaer-ationateachdetentiontimebulkliquidDOof5-6(high)and1–2(low)mg/Lweremaintained.TheﬁnalefﬂuentconcentrationsofCOD,BODandTSSoperatingunderhighDO(5–6mg/L)obtainedat30minHRTwere40–60,25–35,30–40mg/L,respectivelyand30–50,18–30,20–30mg/Lat60minHRT.Thecombinedreduc-tionefﬁciencyoftheintegratedUASB-CASsystematHRTof30and60minrangedfrom80to85%COD,85to90%BOD,65–75%TSS.Thetreatedefﬂuent,however,stillcontainedsigniﬁcantnum-berofFC,whichwasgreaterthanthepermissiblelimitforbathingandunrestrictedirrigationassetbyWHO(1989).TheremovalofNH4–Nandtotal-PwasinsigniﬁcantatanyofthemaintainedHRTinaerationofUASBefﬂuent.TheIntegratedUASB-continuousaer-ationsystem(CAS)forsewagetreatmentcouldberecognizedasasustainableandcosteffectiveoptionasthecombinedHRTofthesystemis(8hforUASB)+(0.25–1.0hforaeration)8.25–9.0h.The

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resultsinferredthatperformanceofexistingUASBbasedSTPscanbeupgradedbyinstallingcontinuousaerationsystemthroughﬁneporediffuserandtheenergyproducedbyUASBreactorintermsofbiogascouldbeusedtooperatetheaerationsystem.

Forthelasttwodecadesprogresshasbeenmadeontheuseofhighratemicro-aerobicmethodsfortheremovalorrecoveryofdissolvedsulﬁdescontainedinanaerobicefﬂuents.Besides,sulﬁdepurgingintotheatmosphere,micro-aerationcanalsobeutilizedforbiologicaloxidationofsulﬁdesintoelementalsulfur,whichoffersanexcellentpotentialforreuseandithasbeenshowntobeacosteffectivealternative(Valleroetal.,2003;Chuangetal.,2005;Chenetal.,2010).Theprocessisgenerallyfocusedonthetreat-mentofbiogas,off-gas,naturalgasorlowstrengthwastewaters,likeinthecaseofanaerobicefﬂuents.Inaddition,micro-aerationintotheanaerobicsystemmaybeanoptionforincreasehydrogensulﬁdestrippingandmethaneproduction(vanderZeeetal.,2007).Buismanetal.(1990)developedalow-cost,high-ratebiotechno-logicalaerobicprocessfortheoxidationofsulﬁdeintoelementalsulfurbyagroupofcolorlesssulfurbacteria,wherethesulﬁdeoxi-dationratewasdependentontheoxygenlevel.Thebioﬁlmonareticulatedpolyurethanewasmoresuitabletoproducesulfatethanafreecellsuspensionofbiomass,forthesamegivenoxygenandsulﬁdeconcentrations.Forefﬁcientachievementofelemen-talsulfur,highsulﬁdeloadsorlowoxygenconcentrationsmustbeapplied(Stefessetal.,1996).Valleroetal.(2003)utilizedthemicro-aeratedreactorsfortheoxidationofsulﬁdestoelementalsulfurfromtheliquidphaseofanaerobicallytreatedsewage.Theresultswereencouragingandpartialconversionofsolublesulﬁdes(HS−)intocolloidalelementalsulfurwasobserved.

Analternativetothebiologicaloxidationofsulﬁdeisthechemi-caloxidationofaqueoussulﬁdetoelementalsulfurbyferricsulfateatlowpH,whichyieldselemental,orthorhombic␣-sulfur(deSmulandVerstraete,1999).Theprocesscanbecoupledtoamembrane-assistedextraction(e.g.permeablesilicon)ofH2Soutoftheliquid.Aftertheremovalofthesulfurfromtheferricsolution,theferricsolutioncanberegeneratedbyaeration.Excellentremovalofsul-ﬁdehigherthan99%wasobservedduetoimmediateformationofelementalsulfur.Anotheroptionistheformationofinsolublemetalprecipitatesbytheadditionofanexogenoussourceofheavymet-als(e.g.Fe2+)(JohnsonandHallberg,2005).However,thisprocessiseconomicallyandenvironmentallynotviableandnevertheless,thereareotheralternativewaystopromotetheformationofmetalprecipitatesfor,theremovingofsulﬁdefromanaerobicefﬂuents.Tothebestofourknowledge,however,thishasnotbeentestedfordomesticsewageanaerobictreatmentsofar.

Theproducedelementsulfurformstransparentglobulesofupto1micro-meterindiameter,whicharedepositedinsideoroutsidethebacteria.Animportantissueistherecoveryofthecol-loidalsulfurparticles.Janssenetal.(1999)studiedthepropertiesofthecolloidalsulfurparticlesanddevelopedanup-sidedownconesexpanded-bedbioreactorforspatiallyseparationoftheaer-ationandoxidationphases.After50daysofoperation90%ofthesludgesettledatavelocitygreaterthan25m/handcouldbeeasilyremoved.Althoughtheresultsareveryencouraging,morestud-iesontheapplicationofhighmicro-aerobicsystemsforcolloidalmatterremovalarenecessary.

Oneofthemostpromisingtechnologiesforsulﬁderemovalfrombiogasesisatwo-stepprocesswheregaseoussulﬁdeisdissolvedintotheliquidintheﬁrststep,followedbysulﬁdeoxidationtoelementalsulfur.AlthoughlittleresearchhasbeenconductedonthesubjectChuangetal.(2005)treatedasulfate-richwastew-aterinaUASBfollowedbyaﬂoatedbedmicro-aeratedreactor.TheﬂoatedbedwasoperatedatshortHRT(2.8h)andduringlong-termsteadystateoperationresultsshowedthatalmostallsulﬁdes(>96%)wasoxidizedtoelementalsulfurandsulfate.AnnachhatreandSuktrakoolvait(2001)observedasulﬁdeconversionhigher

than90%atsulﬁdeloadingratesof0.13–1.6kgS/m3/dandatDOslowerthan0.1mg/Lsulfurwasthemajorendproduct.

Thesimplestmethodofsulﬁdeoxidationistheintroductionofmicro-aerobicconditionsintheanaerobicreactor.Despitethetoxicityexertedbyoxygenagainstobligatoryanaerobes,itsmod-erateintroductionisnotexpectedtohaveaharmfulimpacttothebiomass,mainlytothelimitedpenetrationdepthofoxygeninbioﬁlm.vanderZeeetal.(2007)determinedtheairinjectedtosulﬁderatiotobe8–10:1(O2:Sinmolunits),whichwassufﬁcienttoreducethebiogasH2Scontenttoundetectablelevels.Elementsulfurandsulfatewerethemainproducts.

Matsuuraetal.(2010)investigatedatwostageDown-ﬂowHangingSponge(DHS)systemfortheposttreatmentofUASBefﬂu-entinNagaoka,Japaninordertorecoverthedissolvedmethanegas.Mostofthedissolvedmethane(99%)wasrecoveredbythetwostagesystem,whereasabout76.8%ofinﬂuentdissolvedmethanewasrecoveredbytheﬁrststageoperatedat2hHRT.ThesecondclosedDHSreactorwasusedforoxidationoftheresidualmethaneandpolishingoftheremainingorganiccarbons.TheremovalofCODandBODintheﬁrststagewasinsigniﬁcantastherewasnoairsupply;however,highremovalswereexpectedinthesecondstageduetosufﬁcientsupplyofair,whichisquicklyoxidizetheresidualdissolvemethaneintheupperreactorportionbeforebeingemittedtotheatmosphereasoff-gas.

2.4.Highratebiologicalaerobicmethods(includingnitriﬁcation–denitriﬁcationsteps)

Thepoorlybiodegradablesolublematter,hazardouscom-poundsormicropollutantsincludingammonia–nitrogenandphosphorousinefﬂuentofUASBreactorsometimesaredifﬁculttoremovebymicro-aerobicorsimplesettling.Therefore,secondaryposttreatmentisrequiredaftertheﬁrststep,micro-aerobicorset-tlingtreatmentmethods.Anumberofsecondaryposttreatmentprocesseshavebeencategorizedintomethodsresponsiblefortheremovalof(i)poorlybiodegradablesolublematterincludingmicropollutantandhazardousmaterial,(ii)ﬁnelydispersedorganicmat-ter,i.e.colloidalandpathogensremovaland(iii)ammonia–Nandphosphorous.

Theremovalofresidualbiodegradablecarbon,ammonianitro-genandphosphorouscanalsobeachievediftheefﬂuentofUASBistreatedbyhighrateaerobicbiologicaltreatmentmethods.

Thesequencingbatchreactor(SBR)isaﬁllanddrawtypemod-iﬁedactivatedsludgeprocess,wherefourbasicstepsofﬁlling,aeration,settlinganddecantationtakeplacesequentiallyinabatchreactor.TheoperationofSBRcanbeadjustedtoobtainaerobicandanoxicphaseswithinthestandardcycles(DrosteandMasse,1995;Surampallietal.,1997;Wildereretal.,1999).TheUASBfollowedbySBRsystemwasmainlyinvestigatedinBrazil,MiddleEastregionandinIndiaforthesewagetreatmentunderambientconditions.SousaandForesti(1996)proposedacombinedsystemcom-posedofanaerobic-aerobicprocessesconsistingofaUASBreactorfollowedbySBR.Thesystemperformancewasevaluatedthrougha4LbenchscaleUASBreactorandtwoSBRsof3.6Leach.TheUASBreactorwasfedwithpartiallymixedsyntheticsubstrateinsewagewhiletheSBRreceivedefﬂuentofUASBreactor.Thecom-binedsystemwasoperatedcontinuouslyfor38weeksatcontrolledtemperatureof31±1◦C.TheUASBreactorwasoperatedat4hHRTthroughoutthestudy.TheSBRsweremaintainedat4hcyclesinthefollowingsequenceofﬁll(0.10h),reaction(1.9h),sedimen-tation(1.6h),discharge(0.25h)andidle(0.15h).Thecombinedsystemremoved∼85%TKNthroughnitriﬁcation.TheCODremovalinUASBreactorwasaround86%whileinSBRitwasaround65%oftheremaining.Thecombinedsystemsremoved95%COD(residualefﬂuentCODwas20mg/L).Theperformanceofcombinedsys-1240A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

temwas96%intermsofTSSremoval(residualefﬂuentTSSwas9mg/L)and98%intermsofBODremoval(residualefﬂuentBODwas6mg/L).

TorresandForesti(2001)studiedtheeffectofaerationontheperformanceofSBRtreatingUASBefﬂuent.TheUASBreactorwasoperatedataconstantHRTof6hwhiletheSBRperformancewasmonitoredat24,12,6and4hcyclescorrespondingtoaerationtimes(AT)of22,10,4and2h,respectively.TheoverallremovalefﬁcienciesofCODandTSSwere91%and84%,respectivelyandobservedindependentofaerationtimegiveninSBR.However,thenutrientsremovalwasfoundtobedependentonaerationtime.TKNremovalofapproximately90%wasachievedforaerationtimeequaltoormorethan10h;completenitriﬁcationoccurredforaerationtime>4h;signiﬁcantphosphateremoval(72%)occurredonlyattheaerationtimeof2h.

Moawadetal.(2009)alsoinvestigatedtheperformanceofthecombinedUASB-SBRsystemunderdifferentoperatingconditionsforthetreatmentofdomesticwastewater.TheretentiontimeintheUASBwaschangedfrom4to3handtheaerationtimeintheSBRcyclevariedfrom2to5h,andthento9h.Theyobserved94–98%removalofCOD,BODandTSSforthethreeruns.TheresidualCOD,BOD,andTSSwere26,5.8and5.0mg/L,respectively.Completenitriﬁcationofammoniawasachievedafter5haerationintheSBR.Theaveragepercentageremovalofphosphorusreachedupto65%.IncreasingtheHRTintheSBRfrom2to9hcausedasig-niﬁcantimprovementinFCremovalasthegeometriccountofFCwasreducedto7.5×102MPN/100mLintheefﬂuentofthe3rdrun(HRT9h).TheresultsoftheabovestudiesofcombinedUASBandSBRsystemrevealedthattheadditionofSBRwithUASBreactorcouldbeafeasibleoptionforthetreatmentofsewage.

Again,Khanetal.(2011)investigatedtheperformanceofapilotscaleintegratedUASB-CFIDsystemfortreatmentofsewage.TwodifferentvariantofSBRviz.processContinuousFlow-IntermittentDecantSequencingBatchReactor(CFID)andIntermittentFill-IntermittentDecantSequencingBatchRector(IFID)wereinvestigatedforabout18monthsinconjunctionwithUASBreactoratambientenvironment.InphaseI,theUASB-CFIDsystemwasoperatedatanHRTof8hinUASBreactorwhileitwasvariedinCFIDfrom20,8and6hwithdifferentdissolvedoxygen(DO)regimeof4.0to5.0and<0.5mg/L,2.5–3.5and<0.5mg/Land2.5–3.5and<0.5mg/L,respectivelyateveryHRT.TheBODandTSSremovalefﬁciencyofcombinedUASB-CFIDsystemwasupto90%and90%,respectively.TheFCreductionwasachievedmorethan99%inanintegratedsystem.ItwasobservedthataveragereactorMLVSSconcentrationreducedaround30%atDOof2.5–3.5mg/Lshowinghighdegreeofmineralization.Insecondphase,aninte-gratedUASBfollowedbyIFIDsystemforthetreatmentofsewagewasevaluatedfortheremovaloforganicsandnutrientformorethansixmonthsatambientconditions.TheHRTinUASBreac-torwasmaintainedconstant,i.e.8h.TheIFIDwasoperatedat6hHRTatDOconcentrationrangedbetween2.5and3.5mg/L.ResultsrevealedthattheremovalofBOD,CODandTSSwereachieved90,95and90%,respectively,inIFID.DuringhigherorganicloadingconditionsandlowSRT,theremovalofphosphorouswassignif-icantlyhigherthanthatoflowerorganicloadingsandhigherSRT.ThesuitablerangeofCOD:Pwas105–160effectivelyremovingthephosphate.TheTNremovalwassufﬁcientlygood.

Also,classicalactivatedsludgeprocess(ASP)canbeappliedtotreatUASBefﬂuents.Apilotplantcomprisingofa416LUASBreactorfollowedbya23Lactivatedsludgesystemwasmonitoredfor261dbyvonSperlingetal.(2001),treatingactualmunicipalwastewaterfromalargecityinBrazil.Theaerationwasdonebydiffusedairanddissolvedoxygenwaskeptaround2.0mg/L.Thewholestudywasdividedintoﬁvephases.Constantinﬂowandvari-ableHRTwasstudiedinphasesIandIIandsubsequentlyconstantHRTandvariableinﬂowinphaseIII,reductioninvolumeofUASB

settlerinphaseIVand20%sewagebypassedtoactivatedsludgewasinvestigatedinphaseV.TheUASBreactorremoved69–84%CODandASPremoved43–56%oftheremainingCODresultingin85–93%removal.Theperformanceofthecombinedsystemintyp-icalphasesII,IIIandIVwasverygood.TheoverallefﬂuentﬁlteredCODonanaveragereducedto50,56,58and128mg/LinphasesI–IV.DataontheefﬂuentBODhowever,isnotavailable.TheﬁnalefﬂuentSSconcentrationwas13–18mg/L.Therefore,UASBandASPconﬁgurationwassuggestedtobeabetteralternativeforwarm-climatecountriesthantheconventionalactivatedsludgesystemalone,consideringthetotallowhydraulicdetentiontimeof7.9h(4.0hUASB;2.8haerobicreactor;1.1hﬁnalclariﬁer),offeringtheadvantagesintermsofsavingsinenergyconsumption,absenceofprimarysludgeandpossibilityofthickeninganddigestingtheaerobicexcesssludgeintheUASBreactoritself.

Tawﬁketal.(2010)investigatedalaboratory-scaleintegratedUASBreactorfollowedbyamovingbedbioﬁlmreactor(MBBR)forsewagetreatmentatthreedifferentcombinedHRT’sof13.3h(8+5.3),10h(6+4)and5.0h(3+2)undertemperaturerangeof22–35◦Cfor290dinEgypt.TheworkingvolumesofUASBreactorandMBBRwere10and8.0L,respectively.Thecylindricalcarriermediaof1.85cmdiameterand1.8cmlongmadeofpolyethylenewasusedinMBBR.Itsspeciﬁcgravityandeffectivespeciﬁcsurfaceareawere0.95and363m2/m3,respectively.Thedissolvedoxy-genwasmaintainedat2.0mg/Lthroughouttheexperiment.TheperformanceoftheintegratedUASB-MBBRsystemwasmonitoredintermsofCODfractionsandFC.AtanHRTof5–10h,anoverallreductionof80–86%fortotalCOD;51–73%forcolloidalCODand20–55%forsolubleCODwasachieved,respectively.Theremovalefﬁciencieswereincreasedupto92,89and80%,fortotal,colloidalandsolubleCODrespectivelybyincreasingtheHRTto13.3h.How-ever,theremovalefﬁciencyofsuspendedCODinthecombinedsystemremainedunaffectedwhenincreasingthetotalHRTfrom5to10handfrom10to13.3h.ThisindicatedthattheremovalofsuspendedCODwasindependentoftheHRT.FinalefﬂuenttotalCODatthreedifferentHRTswere54,95and142mg/L,respec-tively.TheﬁnalaverageFCcountswere8.9×104,4.9×105and9.4×105MPN/100mL,correspondingtooveralllog10reductionof2.3,1.4and0.7,respectively.ThemainmechanismsobservedfortheremovalofFCwereadsorptionintothemediaandpredationbyhighermicrobessuchasprotozoaandmetazoa.

TheremovalofammonianitrogenwasalsoinvestigatedinMBBR.Theresultsrevealedthattheremovalofammonianitro-gengreatlydependsonorganicloadingrate(OLR).About62%ofammonianitrogenwasremovedatOLRof4.6gCOD/m2/daybuttheremovalefﬁciencydecreasedby34and43%atthehigherOLR’sof7.4and17.8gCOD/m2/day,respectively.Thenitrogenmainlyreducedbyassimilationintobiomassanddenitriﬁcationinanoxiczoneinthebioﬁlm.ThesludgeproducedbyMBBRshowedpoorset-tleability,however,thecombinedsystemstillproducedlesssludgecomparedtoconventionalASP.Theauthorsreportedthattheinte-gratedUASB-MBBRsystematanHRTof8and5.3haretechnicallyfeasibleforsewagetreatment.

DHS(Down-ﬂowHangingSponge)reactorwasdevelopedbyHidekiHaradaandhisresearchgroupatNagaokaUniversityofTechnology,Japan,asanaerobicpost-treatmentforUASBefﬂuent.InDHScubesprovidedalargespeciﬁcsurfaceareatoaccommodatemicrobialgrowthundernon-submergedconditions.Thewastewa-tertrickledthroughthespongecubessuppliesnutrientstoresidentmicroorganisms;oxygenissuppliedthroughnaturaldraughtofairinthedownstreamwithoutanyequipment,i.e.noexternalaer-ationrequirement.Moreover,nowithdrawalofexcesssludgeisnecessary.TheDHSevolvedfromthehangingspongecubespro-cessbasedonextensivestudiescarriedoutindownﬂowandupﬂowmode.Agrawaletal.(1997)fortheﬁrsttimeevaluatedtheperformanceofcombinedUASBandDHSsystemattwodif-A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

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ferenthydraulicloadingratesof30L/dduringwinter(7◦C)and60L/dinsummer(30◦C)forthetreatmentofrawsewageinJapan.TheUASBreactorvolumewas47.1LandtheDHShadthreefallsof2mheighteach.Eachhangingfallcontained120spongecubesof1.5cminsizediagonallylinkedthroughnylonstring.Initially,toinoculate,thespongeswerekeptfor48hinexcesssludgeofanactivatedsludgeplanttreatingsewage.TheUASBreactorefﬂuentwasthenallowedtopassthroughthesethreefalls.Theﬁnalefﬂu-entcontainedanegligibleamountofSSandtotalCODwasonlyintherangeof10–25mg/L.Lownitriﬁcationrateswereobservedevenatthelowtemperatureof10◦C,however,whentemperatureroseabove24◦Ccompletenitriﬁcationwasachieved.

Machdaretal.(1997)alsostudiedanUASBandDHSsystemasanovelsewageanaerobic–aerobictreatmentfordevelopingcoun-tries.Thecombinedsystemachieved94%CODremovalandnearlytotalSSandBODremovalattheoverallHRTof8.3h(7hinUASBand1.3hinDHSunit).Also,theDHSreactorwascapableofstabi-lizingTKNthrough73–78%nitriﬁcation.Inanotherstudy,Machdaretal.(2000)observedthatthecombinedUASB+DHSsystemsuc-cessfullyachieved94–97%BODremoval,81–84%CODremoval,and63–79%TSSremovalatanoverallHRTof8h(6hforUASBand2hforDHSunit)producingaﬁnalefﬂuentof10mg/LBOD,58mg/LCODand46mg/LTSS.Tandukaretal.(2005,2006a,b,2007)eval-uatedtheperformanceofapilot-scaleDHSmodiﬁedbychangingthearrangementofcubestoenhancethedissolutionofairintothewastewaterandtoavertthepossiblecloggingofthereactorespe-ciallyduringsuddenwashoutfromtheUASBreactor.ThecombinedprocesswasoperatedattheHRTof6hinUASBand2hinDHSforaperiodofover600days.Theefﬁciencyofthecombinedsystemwas96–98%forBODremoval,whichwasreducedto6–9mg/L.CODandSSremovalrangedfrom91to98%and93–96%,respectively,reaching46and17mg/L,respectivelyintheﬁnalefﬂuent.Simi-larly,FCremovalwas3.5log-units,withanefﬂuentcountof103to104MPN/100mL.NitriﬁcationanddenitriﬁcationinDHSaccountedfor72%TKNremoval,whereasefﬂuentTKNwas11mgN/L.A60%ammoniumnitrogenremovalwasobservedforanefﬂuentconcen-trationof9mgN/L.Tawﬁketal.(2008)studiedthecombinedUASBandDHSsystemoperatedatdifferentHRTs,16,11and8h.ItwasobservedthatincreasingthetotalHRTfrom8to16hsigniﬁcantlyimprovedtheCODandBODremoval,asexpected.ThecombinedsystemachievedasubstantialTSSreductionwiththeincreaseinHRT,from89.5%at8hHRTto94%at16hHRT.

Therotatingbiologicalcontactor(RBC)offersmanyadvantagessuchaslowpowerrequirementsincomparisontootherinten-siveaerobicsystems,longbiomassretentiontime,lowsludgeproduction,goodprocesscontrolandcapableofworkingatﬂuc-tuatingﬂows.FormorethanadecadeDr.TawﬁkandProf.LettingaperformedextensivestudiesondifferentpatternsofRBCsystemtreatingtheefﬂuentofUASBreactorforsewagetreatment.TheapplicationofpilotscalestudieswereperformedinEgyptandNetherlandsunderambientconditions.Tawﬁketal.(2002a)stud-iedatwo-stageRBCsystemconnectedinseriesandobservedthattheCODremovalefﬁciencyincreasedatlongerHRTandlowerorganicloadingrates(OLR).ThehighestperformancewasobservedatanHRTof10handOLRof6.4gCOD/m2/day.Theresultsindicatedthattwo-stageRBCsystemprovideforabetterefﬂuentqualityoverasinglestageRBC.MostoftheCODreductionwasobservedinﬁrst-stage,whileinsecond-stagenitriﬁcationproceedsefﬁciently.Thenitriﬁcationremovalefﬁciencywas92%atanammonialoadingor1.1g/m2/day.TheE.coliremovalwas99.5,99and89%foranHRTof10,5and2.5h,respectively.

InNetherlands,Tawﬁketal.(2002b)evaluatedathree-stageRBCsystematdifferentHRTs(1.25+1.25+0.5=3hand4+4+2=10h)withthecorrespondingOLR(47,25and1.8gCOD/m2/dand15.3,6.5and0.5gCOD/m2/d)underambienttemperaturerangedfrom11to17◦C.TheefﬂuentconcentrationatanoverallHRTof10hwas

43mgCOD/L,2.2mgNH4–N/Land2.0×103MPN/100mLE.coli.IncreasingtheoverallHRTfrom3to10hledtoareductioninE.colicountby99.9%whichfurtherimprovedto50%bymaintaining10hHRTwith50%recirculationoftheﬁnalefﬂuent.TheE.colicountinﬁnalefﬂuentofRBCwasstillhigherthanthelimitssetbyWHO(1989)thatcanbeusedforrestrictedirrigationbutcannotbereuseinunrestrictedirrigation.

Inanotherstudy,Tawﬁketal.(2003)examinedtheremovaloforganicmatter,ammoniaandE.colibyUASB-RBCsystemunderdif-ferentOLRandtemperature(11,20and30◦C)conditions.TheRBCsystemwasoperatedataconstantHRTof2.5handfedwithsewageandUASBefﬂuentwithmeanCODvaluesof527±160(sewage)and276±38mg/L(UASBefﬂuent)atvariableratiostoreachOLRsof47,27,20and14.5gCOD/m2/d.TheprimaryobjectiveofthisstudywastooptimizetheRBCsystemfortheposttreatmentofUASBefﬂuenttreatment.Theresultsshowedclearlyagoodper-formanceofthesystemwhenoperatedatthelowerOLRsof27,20and14.5gCOD/m2/dforalltemperaturesstudied.TheefﬂuentCODvalueswere100,85and72mg/LfortheOLRof27,20and14.5gCOD/m2/d,respectively.Moreover,ahighammoniaremovalandlowE.coliresidualvalueswereobservedforthehighopera-tionaltemperatureof30◦Cascomparedtothelowertemperaturesof11and20◦C.Theefﬂuenthowever,didnotcomplywithWHOguidelinesforunrestrictedirrigation.

Tawﬁketal.(2005)comparedtheperformanceofUASBcom-binedwithasinglestageRBCandwithatwo-stageRBC.Moreover,thenitriﬁedefﬂuentwastreatedfornitrogenremovalusingathreestagessystemconsistingofanattachedgrowthanoxicup-ﬂowsubmergedbio-ﬁlterfollowedbyasegmentaltwostageRBC.TheemphasiswasgiventoassesstheimpactofbiodegradableCOD(bCOD)ontheremovalefﬁciencyofdifferentCODfractions,E.coli,ammoniaandapartialnitrateremovalforthedifferentreactorcombination.

Thetwo(singlestage)RBC’swereoperatedataconstantHRTof2.5handtemperatureof21◦CbutatdifferentOLR’s,10and14gbiodegradableCOD/m2/day.TheresultsshowedsigniﬁcantlylowresidualvaluesofCOD,ammoniaandE.coliintheﬁnalefﬂuentatOLRof10gbCOD/m2/day.

Theperformanceofasingle-stageversustwo-stageRBCsystematlowtemperatureof12◦Cwasevaluated.Bothsystemswereoper-atedatthesameOLRof18gbCOD/m2/dayandatHRTof2.5h.TheresultsshowedthattheCODfractions,ammoniaandE.colicontentintheﬁnalefﬂuentofthetwo-stageweresigniﬁcantlylowerthaninthesinglestage,72,9.0and3.2×104MPN/100mL,respectively.

Thenitriﬁedefﬂuentofthetwo-stageRBCwasrecycledtotheanoxicup-ﬂowsubmergedbio-ﬁlterfornitrogenremoval.TheresultsshowedthattheintroductionofananoxicreactorasaﬁrststagecombinedwithrecirculationofthenitriﬁedefﬂuentofthesecondstageRBCwasaccompaniedwithaconversionofnitrateintoammonia,atleastincasethecontentofbCODintheUASBefﬂuentwaslow.Insuchasituationtheammonianeedstobenitri-ﬁedtwotimes,whichobviouslyshouldbeavoided.ThereforeinsuchsituationsofatoohighqualityanaerobicefﬂuentintermsofbiodegradableCODcontent,theintroductionofaseparateanoxicreactorfordenitriﬁcationasﬁnalpost-treatmentstepcannotberecommended.ThereactorperformanceissummarizedinTable3.

Thetricklingﬁlterconsistsofaﬁxedbedofrocks,grav-els,slag,polyurethanefoam,sphagnumpeatmosses,orplasticmediaoverwhichsewagehttp://en.wikipedia.org/wiki/Sewageorotherwastewaterﬂowsdownward.Aerobichttp://en.wiktionary.org/wiki/aerobicconditionsaremaintainedbysplashing,diffusion,eitherbyforcedairﬂowingthroughthebedornaturalconvec-tionofairiftheﬁltermediumisporous.Theprocessmechanisminvolvessorptionoforganicpollutantsbythelayerofmicrobialslime.Diffusionoftheairwithwastewateroverthemediafurnishesdissolvedoxygenhttp://en.wikipedia.org/wiki/Oxygenwhichthe

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slimelayerrequiresforthebiochemicaloxidationoftheorganiccompoundsandreleasescarbondioxidegas,waterandotheroxi-dizedendproducts.ChernicharoandNascimento(2001)studiedtheapplicabilityofpilotleveltricklingﬁlter(TF)forpolishingofefﬂuentofUASBreactorfor16monthscontinuouslyinBrazil.ThevolumeofUASBreactorwas416LoperatedatanaverageHRTof4handthetricklingﬁlter(TF)volumewas60Lwithblastfurnaceslagof4–6cminsizeusedasmedia.TheoperationalconditionsinUASBreactorwaskeptconstantthroughoutthestudyperiodwhiletheTFwasoperatedathydraulicandorganicloadingratesvary-ingfrom3.4to30.6m3/m2dand0.3to3.9kgBOD/m3dineightphasesaslowrate,intermediateandhighrateTF.TheperformanceofUASBreactorwasconsistentwithabove70%efﬁciencyintermsofBODandCODremoval.TheaverageCODandBODremovalefﬁ-cienciesofcombinedsystemwerevariedfrom74to88%and80to90%,respectively.Theﬁnalefﬂuentvalueswererangedbetween60and120mgCOD/L,lessthan60mgBOD/Landbelow30mgSS/L,respectivelyatlowandintermediaterateTF.Thesystemprovedveryefﬁcientunderlowloadingconditions.ThehighrateTFwork-ingathighorganicloadingconditionswasnotefﬁcienttoremovetheBOD,COD&SS.Incontinuationofthisstudy,againPontesetal.(2003)investigatedtheperformanceofUASB-TFsystemusingcom-bineddomesticsewageandexcesssludgeproducedfromTFasinﬂuent.Nodetrimentaleffectwasobservedduetofeedingofaero-bicexcesssludgetoUASBreactor.Oncontrary,theresultsindicatedthattheperformanceintermsofCODwasincompliancewiththedischargestandardssetbyBrazilianenvironmentallegislation.TheresultsofthisstudyshowedthattheTFcanbepromotedastheposttreatmentoptionforthetreatmentofUASBefﬂuentforloworganicandhydraulicratesathightemperatureintropicalcountries.

Suminoetal.(2007)investigatedthefeasibilitystudyofpilotlevel17m3UASBfollowedbyaeratedﬁxedbedreactor(AFB)underambientconditions.Thecombinedsystemfollowasequenceofdenitriﬁcation(DN)reactor,up-ﬂowanaerobicsludgeblanket(UASB),aeratedﬁxedbedreactor(AFB)andsettlingunitstreatingsewage.ThesystemwasinstalledatamunicipalsewagetreatmentplantsiteinHigashi-Hiroshima,Hiroshima,Japan.TheDNandAFBreactorscontainedspongesheetsmediaﬁxedtoboththesurfacesoftheboardsorientedvertically.Theairwassuppliedthroughthebot-tomoftheAFBreactor.GranularsludgeobtainedfromfoodwastetreatmentplantwasusedastheinoculumfortheUASBreactorandactivatedsludgefromasewagetreatmentplantwasusedastheinoculumfortheAFBreactor.TherecirculationofSSfromsettlingtankwasmadetodenitriﬁcationtank(DN).Thepoly-aluminumchloridePACwasinjectedtoABFthroughapumpforphosphorousremoval.Thewholesystemwasstudiedformorethan300daysunderconstantHRTof24hinthreedifferentseason’sviz.summer,autumnandwinter.TheperformanceofthecombinedsystemwasfoundsatisfactorywithﬁnalmeanefﬂuentvaluesofsolubleCODwere54,66and65mg/Lintherespectiveseason’s,whilethemeantotalsolubleBODvalueswere11,18and25mg/Lforthecorre-spondingperiods.Theinformationonnitrogenandphosphorousremovalandindicatorsofpathogenswasnotincludedinthisstudy.

Thesubmergedaeratedbioﬁlters(SABF)iscomposedofﬂoatingporousmedia,throughwhichwastewaterandairﬂowsfromthebottomofreactorandliquidﬂowsinupﬂowordownﬂowmode.Thedevelopmentofthin,homogeneousandactivebioﬁlmlayerhasbeenobservedasthemainmechanismofbioﬁlterstoremovethesolubleorganiccompoundandsuspendedsolidsfromthewastew-ater.Besidesservingassupportmediumformicroorganisms,thegranularmaterialalsoworksasaneffectiveﬁlter(vonSperlingandChernicharo,2005).Goncalvesetal.(1998)investigated46LUASBreactorand6.3LSABFsystemfordomesticsewagetreatmentinBrazil.TheﬂoatingandtotallysubmergedgranularmediumintheSABFwasmadeofS5typepolystyrenespheresof3mmdiameter,1200m2/m3speciﬁcsurfacearea,0.04densityand0.50mheight.

Theairco-currentwithwastewaterwasinjectedintheSABFbot-tomandcrossedthebedinupstreamﬂowmode.TheUASBreactorwasinitiallyoperatedat8hhydraulicretentiontimeandsubse-quentlyreducedto6and4h.The4hHRTinUASBreactorwasmaintainedtoinvestigatetheperformanceofreactorunderbreak-downsituation.VieiraandGarcia(1992)andvanHaandelandLettinga(1994)recommendedHRT<5hinUASBasdeterminantparameterinordertokeepanadequatemethanogenicactivityinUASBreator.However,theperformanceofUASBreactorwasstableandnoticedthattheefﬁciencyofreactorwassimilaratallHRTs.TheﬁnalmeanremovalefﬁciencyofthecombinedsystemintermsofSS,BODandCODwere94,96and91%,respec-tively.Thecorrespondingconcentrationintheefﬂuentwere10,10and49mg/L,respectively.AboutsixyearlaterGoncalvesetal.(1999)studiedthecombinedUASB-SABFsysteminBrazilandobservedsimilarresults.ExperimentsconductedwithUASBreac-toroperatedatanHRTof6hwithoutsludgerecirculationandthebio-ﬁlteratHRTof0.5h,theaverageremovalefﬁcienciesofSS,BODandCODwere95,95and88%,respectively.Theﬁnalefﬂu-entqualityobservedwiththefollowingcharacteristics;suspendedsolids=10mg/L,BOD=10mg/LandCOD=50mg/L,althoughtheefﬁciencyoftheUASB-SABFsystemwassatisfactoryintermsoforganicmatterremoval,buttheremovalofthepathogenicmicroorganismswasverylow.Kelleretal.(2004)investigatedthecombinedUASB-SABFsystemfollowedbyConventionalandUVdisinfectionsystemtoenhancetheefﬁciencyofthesystemtoremovethepathogenicmicroorganisms.Theresultsrevealedthatthe84%ofCOD(ﬁnalefﬂuentCOD,78mg/L),86%ofBOD(ﬁnalefﬂuentBOD,26mg/L)and86%ofTSS(ﬁnalefﬂuentTSS23mg/L)removalwasachieved.TheremovalofE.coli,salmonellaeandcol-liphagesreducedtoverylowdensitiesintheﬁnalefﬂuentofthesystem.TheassociationofUASB-SABFconﬁrmedtheviabilityofthesystemwithexcellentﬁnalefﬂuentqualityofthesystem.TheUVdisinfectioncaneffectivelydestroythepathogenicmicroorganismsuptoreusestandardcriteria.

2.5.Lowrateprimaryposttreatmentsystems(includingvalorization/orremovalofnutrients)

Low-rateprimarytreatmentsystemsareanaerobic,aerobicandfacultativetypeespeciallypondssuchaswastestabilizedponddesignedtohandlewastewatersthatarehighinsuspendedsolidsand/orfats,nutrients,variableinﬂowand/orcharacteristics,ortreatedunderlessidealconditions,suchaslowtemperatures.Theorganicloadingsarelow(typically1.0–3.0kg/m3d)andhydraulicretentiontimesrelativelylong(typicallygreaterthan2days),arerobustandstableallowingforsigniﬁcantdigestionofinﬂuentsolidsandwastesludge.Thesetreatmentsystemsrequirelessmanpower,lowoperatingandmaintenancecostandnegligibleenergyrequire-ment.ManyresearchersinvestigateddifferentpondssinglyorinseriesfortheposttreatmentofUASBefﬂuentstreatingsewageundertemperateclimateregionssuchasBrazilandIndia.

Pilotscaleintegratedduckweedpond(DP)andalgaepond(AP)systemforthepost-treatmentofUASBreactorefﬂuentwasinves-tigatedbyVanderSteenetal.(1998)inIsrael.RemovalofnitrogenfromtheUASBefﬂuentwastheprimegoalofthisstudy.Thesystemconsistedof10pondsinseries,arrangedin3stagesof2DPfollowedby3APand5DPinsecondandthirdstages.TheUASBefﬂuentcon-tainedhighnutrientstherefore;theﬁrststageofDPwasincludedtoincreasetheduckweedproductionandsettlingofsolidsaswell.Thesecondstagewasmeantfortheremovalofpathogens.Thenutri-entconversionandremovalofalgaeandpolishingofefﬂuentwasmainlyachievedinﬁnalstage.Thesystemwasmainlydesignedinseriestoachievetheconceptofplugﬂowregimeknowntobemoreefﬁcientforpathogensremoval.

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TheremovalofammonianitrogeninintegratedDPandAPsys-temtakesplaceviauptakebytheduckweed,sedimentationofparticleswithorganicnitrogen,volatilizationofNH3andnitriﬁ-cation/denitriﬁcation.Theauthortriedtoindicatetheimportanceofeachprocessratherthanexactcontributionofeachprocess.Basedonnitrogenbalance,about59–66%ofNH3wasremovedbyvolatilizationfromshallowDP,probablythemostimportantnitrogenremovalmechanisms.Theammoniauptakebyduckweedcontributedto18%ofthetotalnitrogenremoval.Sedimentationwasoflessimportanceas90%ofnitrogeninthepondinﬂuentwasinammonium(NH4+)form.Thetotalcontributionofremovalfromsedimentationwasabout6%.Approximately3%ofincomingnitro-genwasnitriﬁedwithoutsubsequentdenitriﬁcation.Insigniﬁcantdenitriﬁcationwasduetoconsistentaerobicconditionprevailinginponds.ThepHhasgreatimportanceinpondsfortheremovalofnitrogen.

Theﬁnalefﬂuentofthecombinedsystem(UASB-integratedDP&APsystem)hadahighqualityandsatisﬁedthebacterialstan-dardsforunlimitedirrigationsetbyWHO(1989).Thelayoutoftheintegratedsystemwasadjustedtoachievehighalgalremovalalongwithremovalofbacterialpathogens.TheconcentrationofCODandTSSintheﬁnalefﬂuentwaslowduetoalgaeremovalinthesecondduckweedstage.

Theconstructedwetland(CW)systemforwastewatertreatmentacceptedasatechnicallyandeconomicallyfeasiblealternativeforsmallcommunities(Okurutetal.,1999).Sousaetal.(2001)investi-gatedthedemonstrationscalewetlandsystemforthetreatmentofefﬂuentofUASBreactorfortheremovalofresidualorganicmatter,suspendedsolids,nutrients(nitrogenandphosphorus)andfecalcoliformsinBrazil.TheUASBreactorof1500Lcapacitywasoper-atedatvariedHRT(3and6h)whiletheefﬂuentoftheUASBreactorwastreatedinfourparallelunitsofartiﬁcialwetlandseachof10mlongand1.0mwidewithcoarsesandunderdifferenthydraulicandorganicloads.Macrophytes(Juncussp.),wereplantedinthreeCW’s,whereasoneCWwasoperatedasacontrolunitwithoutplants.ResultsdidnotrevealanyeffectofvariedhydraulicloadappliedaswellaspresenceofplantsinCW’sontheremovalefﬁciencyofCODandphosphate.CODwasmainlyremovedviabiosorptionwhereasphosphorouswasremovedbysorption,precipitationandassimilationbythebio-ﬁlmdevelopedonsandgrains.

TheTKNremovalefﬁciencyvariedfrom59%to87%inwetlandscontainingmacrophytes.Thetwobasicfactorsfortheremovalofnitrogeninwetlandscontainingmacrophyteswereobservedasassimilationbyplantsandmicroorganismspresentinwetlandsandprobablynitriﬁcationduetouptakeofoxygenfromatmospherebytheplants.Thepresenceofmacrophytesenhancedthenitro-genremovalefﬁciencysigniﬁcantly.ThehighestremovalefﬁciencyoccurredintheunitwithlowesthydraulicloadcorrespondingtoHRTof10d.ThepresenceofmacrophytesdidnotappeartohaveanyeffectontheremovalofTKN.Theremovalefﬁciencyoffecalcoliformswasobservedveryhighinwetlandswithmacro-phytesthanwetlandunitswithoutplantsbutthedifferenceinremovalefﬁciencywasnotsigniﬁcantinwetlandwithandwithoutmacrophytes.Theincreaseinhydraulicloadreducedtheremovalefﬁciency.

Theaquaticmacrophytebasedtreatmentsystemssuchasduck-weedpondscanbeusedtorecoverthenutrientsandtransformedthemintoeasilyharvestedprotein-richby-products.TheUASBefﬂuentsarehighlyrichinnutrientsandshouldthereforebepost-treated,notonlytoremove;buttorecoveraswell.Therefore,duckweedpondscanbeusedforpolishingoftheefﬂuentofUASBreactortreatingsewagethroughnutrientsrecovery.Duckweedpondsarecoveredbyﬂoatingmatofmacrophytes,thusprevent-inglightpenetrationintothepond.Thehighgrowthrateofthemacrophytepermitsregularharvestingofthebiomassandhencenutrientsareremovedfromthesystem.Theproducedbiomasshas

economicvalue,becauseitcanbeappliedasfodderforpoultryandﬁsh.

El-Shafaietal.(2007)evaluatedtheperformanceofacombinedUASBandDuckweedPonds(3pondsinseries)systemforthetreat-mentofdomesticsewageinEgypt.TheefﬂuentfromUASBreactorof40Loperatingat6hHRTwasfedtoduckweedpondsof1m2sur-faceareaand0.48mdepth.TheHRTofeachpondwas5dprovidingtotalHRTof15dinallponds.Theduckweedpondswereinocu-latedwithL.gibba,obtainedfromalocaldrain,containing600gfreshduckweedperm2.Thesystemremoved93%COD,96%BODand91%TSSduringwarmseason.Residualvaluesofammonia,TKNandtotalphosphoruswere0.41mgN/L,4.4mgN/Land1.1mgP/L,withremovalefﬁcienciesof98%,85%and78%,respectively.Thesystemachieved99.998%fecalcoliformsremovalduringthewarmseasonwithﬁnalefﬂuentcontaining4×103cfu/100mL.Duringwinter,thesystemefﬁcientlyremovedCOD,BODandTSSbutnotthenutrientsandfecalcoliforms.Thecoliformcountintheefﬂuentwas4.7×105cfu/100mL.Theauthorsreportedthatthefecalcol-iformsremovalinduckweedpondswasaffectedbythedeclineintemperature,nutrientavailabilityandduckweedharvestingrate.IncontinuationwiththeexperimentonpondsVanderSteenetal.(1999)integratedduckweedandstabilizationpond(SP)forthepost-treatmentofUASBreactorefﬂuentmainlyfortheremovalofbacterialpathogensandfurtherpolishingtheefﬂuentqualityinIsrael.Thesystemconsistedof10shallowpondsinseries,arrangedin3stages.InitiallytheefﬂuentofUASBreactorfedtoﬁrststageconsistedof2duckweedpondssubsequentlytosecondstagecon-sistedof3stabilizationpondsandﬁnally5duckweedpondsinthethirdstage.Severalresearcherselucidatedtheremovalmechanismofpathogensinstabilizationpondsandorganicmatterremovalinduckweedponds.TheuseofSPandDPindividuallyfortheposttreatmentofUASBefﬂuenthadmanydrawbacksnamely;verylowTSSandBODremovalinshallowSPduetothepresenceofalgaeintheefﬂuent,whilepoorbacterialpathogensremovalinDPduetopreventionoflightpenetration.Therefore,themainpurposeofintegratingtheDPandSPwastoconverttheabove-mentionedshortcomingstotheirrespectiveadvantage.Eachpondof0.29mdepth,0.24m2surfaceareaandaneffectivevolumeof63Lweremaintainedatanoverallretentiontimeof4.2dinallthreestages.Pondsreceived300–600W/m2averageintensityofglobalradiationduringdaytime.TheBOD,CODandTSSoftheefﬂu-entfromUASBreactorwere23±13,126±81and35±30mg/L,respectively.TheremovalefﬁciencyofcombinedDPandSPsystemintermsofBOD,CODandTSSwere60±32,54±24,and57±29%,respectively.TheoverallBODremovalofcombinedUASBandinte-gratedDPandSPsystemwasabout90%;approximately75%BODremovalinUASBreactorandanadditional60%ofremaininginintegratedDPandSPsystem(fromUASBefﬂuent).TheaverageﬁnalefﬂuentBOD,CODandTSSconcentrationsofcombinesys-tem(UASB-integratedDPandSP)were9,55and15mg/L.Thefecalcoliformremovalintheintegratedsystemwasapproximately99.9to99.99%.Coliformcountreducedto330–5000MPN/100mLfrom105to106MPN/100mLpresentintheinﬂuent.

Khan(2011)studieddifferentfullscalewastestabilizationponds(WSP)treatingefﬂuentofUASBreactortreatingsewageinIndia.ThemainoperatingparameterofthesepondswastheHRT.TheHRTofthesepondsvariedfrom1to2days.TheperformanceoftheWSPdependsonmanyfactorssuchastemperature,dissolvedoxygenandpH.TheremovalofBOD,CODandSSweremainlyduetothesettlingofsolidsorparticulateBOD.ThemainmechanismofremovalofBODandSSobservedsedimentation.Theanalysesof8sewagetreatmentplantshavingUASBfollowedbyWSPweremoni-toredatvaryingtemperaturerangeof20–40◦C.TheaverageHRTofthesepondswereabout1day.Insigniﬁcantremovalofnutrientwasobserved.Theremovalofpathogenswasalsoverypoor.OnedayHRTdidnotsufﬁcientforthealgalgrowth.Thepercentageremoval

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ofBOD,CODandSSinWSPwerevariedfrom20to30%,25to35%and30to40%,respectively.TheﬁnalefﬂuentBODandSSvariedfrom40to50and40to50mg/L,respectively.

2.6.Finalpolishingsteps

Toachievenearlycompleteremovalofpathogens,colorandhaz-ardouscompounds,theUASBefﬂuentneedtopolishaftergivingﬁrststepmicroaerationtreatmentorsecondaryposttreatmentsuchashighrateaerobictreatmentbeforereusingforintendedpurposeordischargingitintoreceivingwaterbodies.Thus,thefol-lowingmethodsdiscussedasunderthatwereresponsiblefortheremovalofpathogens,color,etc.

Tesseleetal.(2005)invesitgatedtheperformanceofcom-binedUASBandtwostageﬂotationandUVdisinfection(TSF-UV)systemtreatingsewageinBrazil.ResultswerecomparedwithUASB-stabilizationpond(SP)systemwhichwasrunningparalleltoUASB-TSF-UVsystem.Thetreatmentsystemwasoperatedatahydraulicloadofupto50m3/d.InTSF-UVsystem,theﬁrststageunitwasaﬂocculation–ﬂotation(FF)processreferredasF1whichremovedthesuspendedsolidsonthebasisofaeratedﬂocsfor-mationinthepresenceofhighmolecularweightpolymerunderhighshear.Insecond-stageﬂotation,i.e.DAFreferredasF2phos-phateionswereremovedbyprecipitationandcoagulationwithFe3+(FeCl3)whichalsoseparatingtheresidualﬁnesolids.

Thetwostageﬂotationunithadtheadvantageofseparatingthebiomassandsludgethatcontainedthephosphateandhydox-ide.TheairﬂowinFFprocesswas3.0NL/minwhileDAFairﬂowratewas0.9–1.2NL/min.Thehydraulicloadingratewaskeptabout49m3/m2.hatanHRTof2mininDAFwhichwashigherthancon-ventionalDAFrateofabout6–10m3/m2h.AfterF2,theefﬂuentwasdisinfectedwithlowpressureUVlampoperatedatatheoreticalvalueof25mJ/cm2.TheresultspresentedthatthecombinedUASB-TSF-UVprocessismoreefﬁcientthanUASB-stabilizationpond(SP)system.TheﬁnalefﬂuentcontainedlowCOD<20mg/Landverylowturbidity<1.0NTUandair/watersurfacetensionisashighasthatoftapwaterwhiletheammoniaremovalwasinsigniﬁcant.Thecoliformswerewellbelowtheemissionstandards.

VariousresearchersinvestigatedtheeffectivenessofslowsandﬁlterfortertiarytreatmentofsewageatlaboratoryandpilotscaleandfoundthatSSFwascapableofremovingBODupto86%,SSupto68%,turbidityupto88%andtotalcoliformsover99%(Ellis,1987;Suhail,1987;A1-Sawaf,1986;A1-Adham,1989;Gersbergetal.,1989)inmiddleeastregion.Allthesestudieswerefocusedonefﬂuentpolishingofaerobicallytreatedsewage.However,lim-iteddataisavailableontheapplicabilityofslowsandﬁlterfortreatmentofUASBefﬂuentandrecently,Tyagietal.(2009)studiedthefeasibilityofabenchscaleslowsandﬁlterof0.10mdiameterand0.54mmediaheightinIndia.Thisstudywasperformedintotwophases;inphaseI,theﬂowwasoptimizedwhileinphaseII,theperformancewasevaluatedthoroughlyatoptimizedﬂowrateof1.0L/hor0.14m/h.Thesandﬁltercolumnoperatedformorethan60dathydraulicloadingrateof0.14m3/m2handfoundtobemosteffectiveinremovingturbidity(91.6%),TSS(89.1%),COD(77%),BOD(85%),TC(99.95%)andFC(99.99%).Theaverageval-uesofCOD,BODandSSinSSFefﬂuentwere27,12and20mg/L,respectively.ThemechanismsoftheremovalofBODandSSinSSFwerestrainingandattachmenttomedia.Theremovalofcoliformswasobservedwithintheupperlayersofthesandbedmainlyduetolowﬁltrationrate,effectivesizeofthesandandbiologicalprocessesinthelayeraccumulatedabovethesandsurface(schmutzdecke).ThehighgrowthofbiomassinupperlayeralsocontributedtohighremovalefﬁciencyofBOD,SSandFC.Thefecalcoliformsintheﬁltratewerelessthan1000MPN/100mL,standardsetbyWHO(1989).However,SSFwaseffectiveupto7daysatahydraulicloadof0.14m3/m2hascomparedtothehydraulicloadsof0.19m3/m2h

and0.26m3/m2h.Hence,slowsandﬁltrationcouldalsobeaneffec-tivetechnologyfortheposttreatmentofUASBreactorefﬂuent.However,themajordrawbackofSSFwasfrequentcleaningandmaintenancerequirement.

ThreeshallowpolishingpondsinseriesfortheUASBefﬂuentandonecoarserockﬁlterforthepolishingofthepondefﬂuentwerealsostudiedasanoptionfordifferenttypesofreuse(vonSperlingandAndrada,2006)inBrazil.Thetotalhydraulicretentiontimeofthesystemwasof12.7d(UASB:0.3d;pond1:3.1d;pond2:3.1d;pond3:4.2d;ﬁlter1:2.0d).TheoverallBODandSSremovalefﬁcien-cieswere90and85%,respectively.MeanefﬂuentconcentrationsofBOD,SS,andE.coliwere27,26mg/Land450MPN/100mL,respectively.Therefore,theefﬂuentcouldbeusedforunrestrictedagricultureandrestrictedurbanandindustrialusesWHO(1989).TheE.coliremovalwasobserved5.68logarithmicunits(99.9998%).TheabovearrangementwithminormodiﬁcationwasvalidatedbyvonSperlingetal.(2008)inasmallfull-scalewastewatertreat-mentsystemcomprisingofUASBreactorfollowedbythreepolishingpondsandonecoarserockﬁlter.Therockﬁlter,madeofcommer-cialcrushedstone(diameters3–8cm)wasinsertedinsidethelastpolishingpond.MostoftheorganicmatterwasremovedinUASBreactor,whereaspondsinserieswereresponsiblefortheremovalofcoliformandammonia.Thecoarseﬁlterdecreasedthealgalcon-centrationintheefﬂuent,thusleadingtocomplementaryBODandSSremoval.Systemworkedefﬁcientlyatanoveralldetentiontimeof13days.Coliformremovalof5.70logunitswasobserved.TheﬁnalaverageefﬂuentBOD,COD,SS,ammonia,E.colioftheefﬂu-entwere39,109,41,10mg/Land540MPN/100mL,respectively.Thepositiveaspectsofthissystemweresimplicity,absenceofmechanization,lowpowerrequirementsandlowchemicalcon-sumption,therebymakingiteconomicallyviable.However,thelargearearequirement(approximately2–3m2/inhabitant)wasthemajordisadvantage.Nevertheless,thetreatmentconﬁguration,attotalhydraulicretentiontimeoflessthan13days,ismorecompactthanmostothernaturaltreatmentsystems.

Recentlylargenumberofmembranetechnologieswasinvesti-gatedforsecondaryandtertiarytreatmentofsewage.Therefore,inordertoachievethequalityoftreatedefﬂuentuptoreusestan-dardfromUASBreactor,YingYuetal.(2009)evaluatedthepilotscalecrossﬂowmembraneﬁltrationsystemforpolishingtheUASBefﬂuenttreatinglowstrengthsewageinSingapore.ThepilotscaleUASBreactor(volume=34L)wascoupledwithasidestreammem-branemodulehavingacentrifugalpumptofeedtheefﬂuentofUASBreactorintothemembraneﬁltrationunit.TheHRTofUASBreactorwasreducedfrominitial10hto5.5hafter119daysofoper-ationandkeptconstantthroughoutthestudyperiod.Thepreciseandconstantholdingtankwasusedpriortomembraneﬁltrationmoduleunitinordertofeedconstantpermeateﬂowrate.ResultsclearlyrevealedhighperformanceofUASBreactorfortotalsolidsremovalatHRTof10hwhich,however,signiﬁcantlyreducedfrom91.1to83.6%atHRTof5.5h.AfterachievingstableconditionsinUASBreactor,theaverageTOCremovalefﬁciencyofUASBreactorwasobtained65%whichincreasedto81%bytreatingtheefﬂuentofUASBreactorthroughmembraneﬁltrationatHRTof10h.But,theperformanceofthissystemintermsofTOCremovalfurtherincreasedto73and85%respectivelyatanHRTof5.5h.Thismightbeduetotheincreasedup-ﬂowvelocitywhichprovidesbettercon-tactanddistributionofwastewaterwithmembrane.Butfoulingofmembranelimitsitsuseforthestatedpurpose.Therefore,exten-sivestudieswererequiredregardingitcontrollingfactorssuchasmembranetubediameterandcrossﬂowvelocity.

YingYuetal.(2010)againproposedmembraneﬁltrationforthepost-treatmentoftheefﬂuentofUASBreactorinSingapore.ThesystemcomprisedofUASBreactorandmembraneﬁltration.TheUASBreactorwithworkingvolumeof30Ldividedintotwoparts,i.e.asludgezoneandamembranezone.Agas/liquidseparator

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wasinstalledatthetopofthesludgezonetoseparatethebio-gasfromtheliquidsuspension.Twoﬂat-sheetmembranemodules(0.22␮m,PVDF,0.1m2)weredirectlysubmergedintotheuppermembranezoneofthereactorabovegas/liquidseparator.Themod-ulesofﬂatsheetmembraneweresubmergedintotheUASBreactortoasabarriertoretainthesuspendedsolidspresentintheefﬂuentofUASBreactoratintermittentpermeationandairspargingoperat-ingconditions.ThewholesystemwasoperatedataconstantHRTof12h(basedonpreviousstudy)atatemperatureof35◦Candnosludgewasremovedfromthereactor,exceptforsampling.Theexperimentalstudywasconductedintwophaseswithvariedﬂux.InphaseI,Intermittentpermeationwasstudiedatthreedifferentﬂuxof15,20and25L/m2hwithvariedsuctionpressurewhileinphaseII,airspargingwasinvestigatedatfourdifferentairﬂowratesof0,1,2and4L/hwithconstantﬂuxof25L/m2h.

TheaveragesupernatantTOCwas10.88mg/LwithfairlystableTOCremovalefﬁcienciesofover89%duringthewholeoperation.Finallythisstudyinﬂuencethatintermittentpermeationwasmoreeffectiveformembranefoulingcontrolcomparedwithairsparging.

ThecouplingofmembraneﬁltrationwithUASBrepresentedasanefﬁcienttreatmenttechnologyforrawmunicipalwastew-aterattheambienttemperature.Butlimitedstudiedareavailableonthissystemtherefore,detailedinvestigationsondemonstrationscale.

3.Discussion/summary

Thedecisionofapost-treatmentmethodfortheUASBreactortreatingsewageisachallengingtaskastoﬁndaproper,reliableandefﬁcientsystem,thatiseasyinoperationandmaintenance;technicallyfeasible,andeconomicallyviable(Chernicharo,2006).Variousphysico-chemicalandbiologicalprocessesarereportedintheliteratureandaresummarizedinTable3.Mostofthestud-iesreportedinthispaperhighlightedthetreatmentperformanceanddescribedtheiradvantages–disadvantagesintermsofspacerequirement.However,fewtreatmentsystemswereevaluatedonlifecyclecost,totalannualexpenditureandenergyconsumption.

Theoverlandﬂowsystemandﬁnalpolishingunitsormatura-tionponds,whichareclassiﬁedassettlingprocesses,arenowadayswidelyusedatfullscalelevelwithUASBreactor.ThesesystemsareabletoremoveremainingwellstabilizedsuspendedmatterintheefﬂuentofUASBreactor.InIndia,thesepondsarerunningatonedaydetentiontime;however,theefﬂuentisnotabletocomplywithdischargestandards.

MostoftheexistingfullscaleUASBbasedSTPsinIndiaareinte-gratedwithﬁnalpolishingunit/orponds;whileinBrazil,UASBarefollowedbyactivatedsludgeprocess(ASP),submergedaeratedﬁl-tersorstabilizationponds.TheSTPsbasedonUASB+pondsrarelyattaintheefﬂuentdischargestandardsofanyregulatoryagency.However,theUASBfollowedbyASPandotherhighrateaerobicmethodsattainstheefﬂuentwithlowresidualCOD,lowerthan50mg/L.Theproducedmethaneisnormallyburnedinﬂaresduetoimpropergascollectionfacilitiesandpooroperationandmain-tenance.Therefore,inordertoremovethecolloidalmatterandorganicmatter,DAFwasstudied.TheDAFsystemisabletoremovetheﬁnelydispersedorcolloidalorganicmatteruptothelevelrequiredtomeetthereusestandards.Fecalcoliformsarehowever,notremoved.Theefﬂuentcanbeusedtorecoverphosphatesandnitrogen.

Thepartialsulﬁdesoxidationtoelementalsulfurwasobservedfromtheapplicationofthesetechnologiesfortheanaerobicefﬂu-entscontaininglowsulﬁdes.However,thepost-treatmentofanaerobicefﬂuentscarriedoutbyhighratemicro-aerobicpro-cesses,wheresulﬁdeswereoxidizebacktosulfate,offersexcellentadvantagesatlowconcentrations.

Theotherpossibleoptionswouldbetointroducecertainmod-iﬁcationsinconﬁgurationoftheexistinganaerobicreactorsinordertoencouragetheoxygenlimitedenvironmentwhichfavoredtheformationofelementalsulfur,thereby,enablingitssepara-tionfromtheliquidphasebeforepost-treatmentunits.Bothways,biologicaloxidationofsulﬁdetoelementalsulfurinamicro-aerobicsystemcanbeasolutiontoremovesulﬁdefromtheliquidphasewiththeproductionofavaluablecompoundwhichcanbere-usedfortheproductionofsulphuricacidorinbioleach-ingprocesses.However,moreresearchontheapplicationofhighmicro-aerobicsystemsforcolloidalelementalsulfurremovalisnecessary.

Besidesbiologicalposttreatment,physic-chemicalprocessescanalsobeusedaspostandpre-treatmentforUASB.Twophysico-chemicalprocessesnamelychemicalcoagulation–ﬂocculationandCEPT(pre-treatment)andZeoliteColumn(posttreatment)systemwerecovered.Thesesystemscaneffectivelyremovetheﬁnelydis-persedorcolloidalorganicmatteruptothelevelrequiredtomeetthereusestandards.Theremovaloffecalcoliformsispoor.Themajordrawbacksoftheseprocessesarethehighdoseandcostofchemicalsusedandthelargesludgevolumegeneration.However,therecoveryofnutrientfromCEPTandZeoliteColumnsystemispossible.ThetreatedefﬂuentobtainedfromCEPT-Zeolitecolumnsystemcanbeusedforcropirrigationor/anddischargeinwaterbodies.

Fewmethodssuchasduckweedpondshavebeenreviewedinthispaperfortheremovalandrecoveryofnitrogenandphospho-rous.Besidesremoval/recoveryofnitrogenandphosphorous,theseprocessesalsoreducedtheorganicimpurities.Forahigherefﬂuentstandard,theUASBefﬂuentmaybetreatedinanintegratedDPandAPsystem,whichsatisﬁedthestandardsforunlimitedirrigationsetbyworldhealthorganization(WHO,1989)ifthelayoutoftheinte-gratedsystemisadjustedtoachievehighalgalremovalalongwithremovalofbacterialpathogens.CODandTSSconcentrationoftheﬁnalefﬂuentislowduetoalgaeremovalinthesecondduckweedstage.

Theperformanceofduckweedpondandconstructedwetlandsystemwasobservedtodependonthetemperature,hydraulicload,harvestingofplants,etc.Theefﬂuentofthesesystemshasnotbeenfoundtocomplywithdischargestandardsdespitetheirgoodnutrientremovalefﬁciencies.

Alsoanumberofaerobicposttreatmentsystems(bothsuspendedandattachedgrowthtype)wereevaluatedasthemethodswhichweremainlyresponsiblefortheremovalofresid-ualbiodegradablesolublematterincludingmicro-pollutantsandhazardouscompounds.Theactivatedsludgeprocessasaposttreatmentyieldsanefﬂuentwhichsatisﬁesdischargestandards;however,theprocessishighlyenergyintensive.

DHSandRBCaretheotheraerobicposttreatmentoptionsforthetreatmentofUASBefﬂuent.DHSandRBCreducetheBODandcoliformswellbelowtheefﬂuentdischargestandardsasreportedintheliterature.However,theseprocessesrequiregrowthmediasuchasspongeanddiscs,whichhaveveryhighcostandarenoteasilyavailable.ThecloggingofspongemediaisamajordrawbackofDHSanditscleaninghasnotbeingdiscussed.

Thesubmergedaeratedbio-ﬁlter(SABF)resultsinsigniﬁcantremovalofBODandSS.However,airoptimizationhasnotbeenperformed.Further,frequentbackwashingofSABF,whichiscarriedoutevery72hhascauses(i)sludgehandlingand(ii)greatlyaffectedthebioﬁlmgrownonthemedia.

StudiesoncontinuousaerationofUASBefﬂuentarestillinprogress.SuchatreatmentsystemhasbeenshowntoreduceBODoftheUASBefﬂuentby50%,andcanbeinstalledatexistingUASBbasedSTPspriortoﬁnalpolishingunit.

OnlylimitedstudyisavailableontheTFasaposttreat-mentoption.GenerallytheUASBreactorsareconstructedone-half

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undergroundand,ifTFiscoupledwithUASBreactorthecostofpumpingfortheapplicationofUASBefﬂuenttoTFwouldessentiallyincrease.SequenceofUASB-MBBRhasbeenstudiedonlaboratoryscaleandalthoughthecombinedsystemshowsgoodremovalefﬁcienciesoforganics,itwasadverselyaffectedbytem-peraturevariationforthenitrogencompoundsremoval.

TheefﬂuentpolishingisthethirdsteponUASBefﬂuenttreatment.Thetermpolishingmeansthecompleteremovalofpathogens,coloraswellasthehazardouscompoundsfromtheUASBefﬂuent.Areviewonvariousphysico-chemical,physicalandnaturalprocessescategorizedforpolishingtheefﬂuentofUASBhasbeenpresentedinthispaper.TheTSF-UViscapableofreducingorganicpollutantsandturbidityuptothelevelrequiredtomeetthereusestandards.Theslowsandﬁltrationsystemwasabletoreducethephysical,chemicalandmicrobiologicalpollutantsnotonlytothedesiredstandardsbutalsosatisﬁedwastewaterreusecriteriasetbyWHO(1989).However,therearefewdrawbackssuchasfrequentcloggingasﬁlterrunsonlyforshortperiod,i.e.7dayswithoutanynutrientremoval.Thecombinationofduckweedandpolishingpondsystemwasreportedtobeveryefﬁcient.Themajordisadvantagesarethelargepondarea,lowpathogensremovalandhighefﬂuentTSSconcentration.Anothercombinationisthepol-ishingpondandcoarserockﬁltersystem;however,theydonotprovideanefﬂuentwiththedesiredquality.

TheapplicationofmembraneﬁltrationmethodforpolishingUASBefﬂuentdeﬁnitelyproducestheefﬂuentqualityuptotherequireddischargestandardsbutitsuffersfromseveraldrawbackssuchashighcapitalcost,membranereplacementcost,frequentfoulingofmembrane,frequentcleaningandrequirementofsophis-ticatedautomation.

Amongallreviewedposttreatmentsystems,fewofthealter-nativeproduceﬁnalefﬂuentwithlowCOD,BODandTSS.Betweenallaerobicposttreatmentsystemspresented,theSBRfoundtobethemostcompactmethodandallowtheremovalofnutrientalongwithresidualCOD.ScantlyinformationisavailableintheliteratureoncouplingoftheSBRwithUASB.ThemajoradvantageofSBRoverotheraerobicsystemsisthesystemﬂexibilityforBODandnutrientremoval.

Generallylowcostsewagetreatmenttechnologiesarepreferredbydevelopingcountries.Therefore,itisnecessarytoevaluatethetreatmentsequencekeepinginviewoftotalinvestmentinclud-ingcapitalcost,operationandmaintenance(O&M)costandlandrequirement.AcomparisonhasthereforebeenmadeamongUASBreactorsanditsfewposttreatmentsystemswithconventionalASPsystem.ValuesobtainedonthebasisofthecriteriaofenergyrequirementandgenerationfromUASBreactor,i.e.energyauditofUASBreactorperMLD,aredescribedinthissectionandcom-parativereviewoflandrequirement,O&McostandcapitalcostaresummarizedinTable4.

ThebasisofenergyauditofaMLDUASBisgivenbelow:

•Negligibleenergyrequirement∼6kW-h/MLD(onlyforinitialpumping)(Tassou,1988).

•EnergyproductionintheformofBiogas(60–70%methane)50m3biogas/MLDsewagetreated(Arceivala,1998).

•Theelectricityproducedfrom1.0m3ofmethanegasgeneratedbyUASBis36,846kJatstandardconditionandapprox.7.0kW-hunderﬁeldconditions,since3600kJisapproximately1kW-h(Arceivala,1998;MetcalfandEddy,2003).

•Energysavingthroughreduceddieselconsumptionbymorethan70%byfeedingmethanegasintotheDual-FuelModeDieselEngine(Arceivala,1998).

ThebasisofenergyauditofaMLDaerobicposttreatmentsystemisgivenbelow:

•Energyrequirementofaerobicprocessasthesolewastewatertreatmentprocess,includinginitialpumpingisapproximately195kW-h/MLD(Tassou,1988).

Salientfeaturesofcomparativeenergyconsumptionaregivenasunder:

•Energyrequirementofposttreatmentaerobicsystemtreatingonly35%BOD(as65%BODremovaltakesplaceinanaerobicsys-tem)is195kW-h/MLD×0.35=68.25kW-h/MLD.

•HencetotalenergyconsumptionofintegratedUASB-aerobicpro-cessis(6+68.25)kW-h/MLD=74.25kW-h/MLDcomparedto195kW-h/MLDfortheaerobicprocessonly.

InTable4anefforthasbeenmadetosummarizeunitcost,effectonenvironmentandtheirﬁnalefﬂuentqualityinrelationtoadherencetoefﬂuentdischargingstandardsofvariousintegratedsystems.

Basedonexistingwasteandwastewatertreatmenttech-nologies,Lettinga(2008)suggested(i)aNaturalBiologicalMineralizationRoute(NBMR)followedbyphysico-chemicalmeth-odsforachievingthequalityoftreatedwastewaterforreuse/orintendedpurposesuchasforirrigationandindustrialreuseand,(ii)decentralizationofthesanitationandresourcerecoveryandreuse(DESAR),thatis,aconceptwhichincorporatesenvironmen-talprotection(EP)wherethewasteandwastewatertransportationiskeptatminimumlevelandwherepollutantsarebroughttoanacceptablevalueatthelocation.

3.1.Solutionsforsustainabilityandenvironmentalprotection

Theenvironmentalprotectionisamatterofeminentconcernforresearchers,policymakersandengineers.Drasticchangestowardsprotectionofenvironmentfrompollutionandtomaintainahighdiversityoflife,atlocalandgloballevels,andtopreventtheexhaustionofresourcesarenecessary.Sustainableenvironmentalprotectiontechnologiesmustbeneededforthesocietyinordertomakesustainablelifestyleandtoprotectenvironment.Althoughthesustainabilityapproachisnotnew,itisdifﬁculttounderstandandtoimplement.Quantiﬁcationofsustainabilityisvagueduetolackingofproperparameterswhichleadstoambiguouslythetargetsorproposedactionstakenbypoliticiansand/orpolicymak-ers.Forinstance,ifgovernmentimplementingextremelystringentstandardsforprotectingtheaquaticenvironmentfrompollution,manyquestionarises,likewhyasinglecountryorregionpursuingaparadisiacalnaturalenvironmentwhileatthesametimelittleifanymoneyortechnologyismadeavailabletocontributetothehighlyneededenvironmentalimprovementinlessprosperouscountries.Thewellevaluated,adequateandoverviewofthestateoftheartofthetechnologyforthepolicy/decisionmakersinpublicsanitation(PuSAn)wassummarizedinTable5andthesepotentialcombina-tionscanbeconsideredassustainablesolutionsifadoptedbasedonNBMR.

3.2.Selectionofsustainabletechnology

Thesuperiorityofsequentialanaerobic–aerobictreatmentsys-temsoverconventionalaerobicismoreprofoundwithincreaseinsewageconcentration.Incountriesoflimitedpercapitashareofwater,likeinAfrica,MiddleEastandIndiathetreatmentofconcen-tratedsewageviaconventionalaerobicsystemishighlyexpensive,especiallywithrespecttooperationalcosts.

BasedonthedataofTable3,theadvantagesofintroducingUASBreactoraheadofaerobicsystemisobvious,mainlyintermsofsludgeproductionandenergyconsumption.Inviewofthefactthataerationcostsincreaselinearlywithincreasingorganicloads,

Table4

SummaryofcapitalandannualO&McostandlandrequirementforvariousIntegratedUASBposttreatmentsystems(Satoetal.,2007).

ProcessTreatmentUnitcost

CapitalUnitrequirement

LandUnit(O&M)

AnnualUnitCountryReferenceRemarkscapacityUASB36,000

m3/d

44114/m3/d20US$/m3/d

IndiaTareetal.US$/m3/d

m2(2003)

US$1=Rs.48.27(2002/03)

UASB+pond20,000–400,000m3/d34.7–45.63US$/m/d1.70–1.98m2/m3//d––IndiaBinnieThamesWater(1996)UASB+pond

–

68.5–85.6

US$/m3/d

1.1–1.7

m2/m3//d––IndiaArceivala(1998)

US$1=Rs.32.43(1995)

UASB50,000PE17.8US$/PE0.12m2/PE

0.53

US$/PE

Egypt

Schellinkhout(1993)

CapitalcostincludeLandS$1=3.37LE(December1993)UASB+pond

50,000PE27.9US$/PE0.64m2/PE

0.53US$/EgyptSchellinkhout(1993)

CapitalcostincludePEland

S$1=3.37LE

(December1993)UASB+TF

50,000PE31.5US$/PE0.22m2/PE

0.71US$/PEEgyptSchellinkhout(1993)

CapitalcostincludelandS$1=3.37LE(December1993)

WSP30,0003m/d167US$/m3/d15.3m2/m3//d1.67US$/m3/dYemenArthur(1983)

WSP50,000

35.6

US$/PE

1.7

m2/PE

0.53

US$/PE

Egypt

Schellinkhout(1993)

CapitalcostincludelandS$1=3.37LE(December1993)ASP2150

186US$/m/d

9.5m/m/d

US$/m3

IndiaTareetal.(2003)

US$1=Rs.32.427(1995)

ASP

7000–650,000

313×PE−0.3

US$/PE

Sweden

BalmerandMattsson(1994)

U$1=8SEK(July1993)

ASP40,000–540,000PE212×PE−0.328US$/PEGreeceTsagarakisetal.(2003)

WSP3

20,000–400,000

m3/d

12.4–18.0US$/m/d12.5–14.0m/m/d–

–

IndiaBinnieThamesWater(1996)WSP25.7–34.3

US$/m3/d

5.6–15.6

m2/m3/d

India

Arceivala(1998)

US1$=Rs.32.427(1995)

ASP20,000–400,000

2m3/d

50.0–60.8US$/m3/d0.73–1.01m/m3/dIndiaBinnieThamesWater(1996)ASP102.8–119.9

US$/m3/d

1.1–1.4

m2/m3/d

India

Arceivala(1998)

US1$=Rs.32.427(1995)

ASP638×Q−0.219

US$/m3/d

ChinaLi(1987)

ASP334.3×Q−0.332m2/m/dChinaLietal.(1990)

ASP120,000–540,000PE4.05×PE−0.228m2/PEGreeceTsagarakisetal.(2003)ASP

10,000–500,000

3m/d

212×PE−0.328

m2/m3/d

Japan

JSWA(1999)

US$1=127.36JPyen(December2001)

Q=treatmentcapacity,PE=populationequivalent,capitalcostdoesnotincludelandcostunlessitmentionedinremark.

A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–12511247Table5

ComparisonofvariousoptionsofintegratedUASBpost-treatmentsystemsofdomesticsewage.

Integratedsystems

Economicaspects

EffectofRiskof

CompliancewithstringentTrackrecord

climaticenvironmentalefﬂuentstandards/reuseconditions

problems(odor,standards

insects,noise,aerosols)

ConstructionOperationLand

ProcessSludgeBOD&TSS

FecalcoliformsNitrogen

Phosphorus

cost

maintenancerequirementreliabilitygenerationcostUASB+CETP+++++++++++NIL√+√UASB+DAF

+++++√XX√+++++++NIL+UASB+coagulation++

+++

√XX√++

+++

NIL

NR++–ﬂocculationUASB+SSF++++++++++++√√NR+UASB+WSP

++++++++++++√

√NR√X+X+++UASB+constructed+

+++

++wetlands

UASB+duckweedponds++++++++++++X√X+XX++UASB+DHS+++++++++++√

X++XX+UASB+SBR+++√

++++++++++X√

+UASB+RBC+++++++++++XX+UASB+AFBR+++++++++++√XX+NR+UASB+SAB

+++++++++++√

XX+XXX+UASB+tricklingﬁlter+++++++++++++XNRNRNR+UASB+anaerobicﬁlters++++++++++XNRNRNR+UASB+EGSB

++++++++++++XXXX+UASB+overlandﬂow++

+++

+system

UASB+ASP(EA)+++√++++++++++NRNRNR+++UASB+CAS

NIL

+++=high;++=average;+=low;NR=notreported;

√=yes;X=no.

1248A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251A.A.Khanetal./Resources,ConservationandRecycling55 (2011) 1232–1251

1249

adoptingtheactivatedsludgesystemforpolishingofanaerobicefﬂuentsmaynotbethemostsustainableoptionforconcentratedsewage.However,otheraerobicsystems,suchasDHS,SBRandCFIDtypeSBRforUASBefﬂuentsposttreatmentreviewedinthispaperarepromisingoptionsforsewagemanagementatlowcost,lowlandrequirementandlowsludgeproduction.

Moreover,thepotentialofnutrientsrecoveryandpathogensremovalinanaerobicpost-treatmentforUASBefﬂuentsisconsid-erableandtheefﬂuentdischargestandardsestablishedbyvariousnationalandinternationalenvironmentalagenciescanbeachieved.

4.Conclusions

Basedonthesuitabilityfortheremovalofreducedcompounds,wellstabilizedsuspendedmatterorpoorlybiodegradablesolublematterandfortherecovery/removalofnutrients,theentireposttreatmentsystemsforUASBreactortreatingdomesticwastew-aterwereclassiﬁedintothreemaincategoriesnamelyprimary,secondaryandtertiarymethods,respectively.However,mostoftheseupcomingposttreatmentsystemslackinscaleupstudiesandimplementationonfullscale.Furtherstudiesarerequiredtoevaluatetheperformanceofthesesystemsondemonstrationandfull-scale.

Besidesfurtherresearch,theapplicationofthesecombinedsys-temstodevelopingcountriesnecessarilyneedtobeevaluateintermsofenergyconservationandtheireconomicalaspectstomeetthechallengesforfullscaledevelopment.Finally,basedontheover-allanalysisofvariousposttreatmentsystems,itwasconcludedthatthereisnoidealsystemapplicabletoallconditions.Eachsitu-ationmustbeanalyzedindividually,withtheconstantconcernofincorporatingthelocalspecialtiesinthestageofinvestigationanddecision.Onlythroughtheopeningofmultipleavenuesonecanreallyreachanefﬁcient,economicalandadequatesolution,notatthedesignstagebutthroughouttheoperationallifeofthetreat-mentplant.However,itcanbesaidthattheUASBsystemfollowedbyanaerobicsystemcanbetheidealconceptforfeasibleandsus-tainableenvironmentprotectioninadecentralizedsanitationwithresourcerecovery.

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