KineticHalfsaturationCoefficientConsiderations

Kinetic/Half-saturationCoefficientConsiderationsforPostDenitrificationMBBRMarijaPeric1*,BeverleyStinson1,DilliNeupane1,KathleenKharkar1,YaldaMokhayeri1,JohnCarr2,RoubenDerMinassian2,SudhirMurthy2,WalterBailey21AECOMWater,14504GreenviewDrive,Suite400,Laurel,MD,207082DCWASA,5000OverlookAvenueSW,WashingtonDC,20032*Towhomcorrespondenceshouldbeaddressed.Email:[e-mail]standtherobustnessofMBBRpostdenitrificationprocesstoachieveverylowlevelofNOx-N1mg/L,pilotwasimplementedattheBluePlainsAWTP.Itwasimportanttounderstandthelimitationsofthesubstrate,expressedasKs(half-saturationcoefficient)ontheoveralldenitrificationrates.Totaloffour100-gallonanoxicpilotreactorswereusedforthistesting.SDNRwasestimatedbymeasuringtherateofconsumptionofNOx-Novertime.Atanambienttemperatureof17°C,Ks,NOx-N(Ksfornitratelimitedtest)was2.5mg/Lonaverage,andKs,bsCOD(KsforbsCODlimitedtest)was6.5mg/L.Ks,NOx-Nremainedfairlyconstantduringthecoldtemperatureperiod(13.5°C)whileKs,bsCODconcentrationwasnotadefinitivefinding.In-situSDNRdatawereusedtodevelopamodelfortheMBBRsystem,basedonthedoubleMonodrelationship.Half-saturationcoefficientwasfoundtobeaveryimportantdesignconsiderationforfixedfilmpolishingapplication.KEYWORDS:PostDenitrification,Half-saturation,KineticLimitation,MovingBedBiofilmReactor(MBBR),LimitofTechnology(LOT),ENRINTRODUCTIONDCWaterandSewerAuthority’s(DCWASA)BluePlainsAdvancedWastewaterTreatmentPlant(AWTP)isa370MGDplantthatservesapopulationofmorethantwomillionpeopleintheWashingtonmetropolitanregion.TheexistingwastewatertreatmentprocessesattheBluePlainsAWTPconsistsofpreliminaryandprimarytreatment,secondaryhighrateactivatedsludgetreatment,nitrification/denitrificationactivatedsludgetreatment,effluentfiltration,chlorination/dechlorinationandpostaeration.DCWASAcurrentlyoperatesitsBluePlainsAWTPtomeetaneffluentTNlimitof7.5mg/LandTPlimitof0.18mg/L.Anewpermitisanticipatedthatwillrequireadditionaldenitrification.Theplantinvestigatedvariousalternativestomeetalimitoftechnology(LOT)futuretotalnitrogen(TN)goalof3-4mg/Lwhilesustainingthecurrentverystringenttotalphosphorous(TP)limitof0.18mg/L.WEFTEC2009Copyright©2009WaterEnvironmentFederation.AllRightsReserved.2359Figure1.ArialviewofBluePlainsAWTPOneofthealternativeprocessapproachesunderconsiderationinvolvesaninnovativeMovingBedBiofilmReactor(MBBR)technologyasadenitrificationpolishingstepdownstreamoftheexistingnitrification/denitrification(N/DN)activatedsludgesystem.TounderstandtheapplicabilityofMBBRtechnologyattheBluePlainsAWTPanditsabilitytoreliablyandcost-effectivelyperformtocomplywiththestringenteffluentTNrequirements,apilotplanwasundertakenbetweenDecember2007andJuly2008.MicrobialGrowthKineticsTheperformanceofbiologicalprocessesusedforwastewatertreatmentdependsonthedynamicsofsubstrateutilizationandmicrobialgrowth.Effectivedesignandoperationofsuchasystemrequiresanunderstandingofthebiologicalreactionsoccurringandanunderstandingofthebasicprinciplesgoverningthegrowthofmicroorganisms(Metcalf&Eddy,Inc.(2003)).Inbiofilmsystems,transportofsubstratesiscontrolledbydiffusion,boththroughthestagnantlayeronthetopofthebiofilmandontheinsideofthebiofilm.Hence,thepositivegradientforthediffusionhastobecreatedbythedifferenceintheconcentrationsofsubstrates.Kineticsofmicrobialgrowthgoverntheutilizationofthesubstrateandtheproductionofthebiomass,henceitwasimportanttounderstandthelimitationsofthesubstrate(usuallyexpressedasKs(half-saturationconstant))ontheoveralldenitrificationrates.Tounderstandprocessrobustnessandpossiblekineticlimitationstoachievetheselowlevels,pilotwasimplementedattheBluePlainsAWTPbetweenDecember2007andearlyJuly2008.ObjectiveThespecificdenitrificationrate(SDNR)dependsontheamountofbiomassavailablethatcandenitrifythenitrogenavailableinthepresenceofacarbonsource;inotherwords,itdependsontheconcentrationofNOx-NandsolubleCOD(sCOD).Tobetterunderstanddenitrification(removal)rates,in-situbatchtestswereconductedthroughoutthispiloting.Hence,theobjectiveofin-situSDNRtestswastoquantifythegrowthofhealthybiomassineachoftheanoxicMBBRreactorsandtoevaluatehalf-saturation(Ks)effectsunderlimitedNOx-NandsolubleCOD.WEFTEC2009Copyright©2009WaterEnvironmentFederation.AllRightsReserved.2360MBBRTechnologyTheMBBRtechnologyisarobust,attached-growth,biologicalwastewatertreatmentprocess.ItisarelativelynewtechnologyintheUnitedStates,appliedmainlytothenitrificationprocess,althoughthetechnologyitselfhasbeenprovenforbothnitrificationanddenitrificationinEuropeoverthepast20years.InanMBBRprocess,biomassgrowthisprimarilysupportedonthebiofilmcarrierandnotinsuspensionasinaconventionalactivatedsludgeprocess.Clarifiersandreturnactivatedsludgesystemsareoftennotrequired,whichmakesitverysimpleandcosteffectiveprocesstooperate.Thecarriersarekeptwithinthereactorbyanoutletsievearrangement.Thebiofilmisprotectedwithinplasticcarriers,whicharedesignedwithahighinternalsurfacearearelativetotheirvolume.Ingeneral,biofilmprocessestendtobemoreresilienttoprocesschangessuchaswastewatertemperature,flowsandloads,thansuspendedgrowth