DenitrificationStoichiometryandKineticsofMovi

DenitrificationStoichiometryandKineticsofMovingBedBiofilmReactorArbinaShrestha,RumanaRiffat,GeorgeWashingtonUniversity,WashingtonDC,USA;CharlesBott,VirginiaMilitaryInstitute,Lexington,Virginia,USA;ImreTakacs,EnviroSimAssociates,Bordeaux,France;BeverleyStinson,MarijaPeric,DilliNeupane,AECOM,USA;SudhirMurthy,DCWASA,Washington,DC,USAABSTRACTTheMovingBedBiofilmReactor(MBBR)technologywasdeterminedtobeapromisingandviableprocessfordenitrificationtoachievetotalnitrogenofbetween3-4mgN/Landnitrateconcentrationbelow1mgN/L.Apost-denitrifyingpilotMBBRsystemwassetupattheBluePlainsAWTP(AdvancedWastewaterTreatmentPlant)andabenchscalekineticevaluationwasconducted.Thisstudyisacompilationoftheex-situbenchevaluationsofthemediagrowninthepilotreactor.DenitrificationkineticsandstoichiometryofMBBRtechnologyusingmethanolasanexternalcarbonsourcewereinvestigated.Theaveragespecificdenitrificationrates(SDNR)weredeterminedbetween1.3-2.0gNOx-N/m2-day,andthestoichiometricC:Nratioswere4.6-5.8mgCOD/mgNOx-Nforatemperaturerangeof13-18oC.ExperimentalvalueswereusedtoobtainKsNOx-N(effectiveNOx-Nhalfsaturationconstant)fordifferenttemperaturesusingamodeldevelopedbasedontheMonodkineticexpressionandaspreadsheetinteractivesearchroutine.ValuesofeffectiveKsNOx-Nwereobservedbetween0.6and1.1mgN/Lforexperimentaltemperaturesrangingfrom12.5to20oC.KEYWORDSBiomassDensity,Denitrification,HalfSaturationConstant,MovingBedBiofilmReactor,SpecificDenitrificationRateINTRODUCTIONWastewatertreatmentfacilitiesdischargingintotheChesapeakeBayarefacingLimitofTechnology(LOT)standardstoreducetheirnitrogendischargetomorestringentlevels(3-4mg/LasTN).Accordingly,BluePlainsAWTPoperatedbyDCWASAwillhavetolimititsnitrogendischarge.Inthiscontext,DCWASAexploredanMBBR(MovingBedBiofilmReactor)processasapossiblemeansforaneffective,affordableandapplicabletechnologicalalternativetohelpmeetthecriteria.BluePlainsAWTPusuallyachievesloweffluenttotalnitrogenconcentration,i.e,approximately4to6mgN/L.Itbecomesdifficulttoattainthesedischargelevelsinwinter,whenmixedliquortemperaturesdecreaseto12oCandanoxicretentiontimeisreducedduetohigherflows(Doldetal.,2008).Eitheradaptationoftheexistingprocessordevelopmentofanewpost-denitrificationprocessisneededtoachievethisnewrequirement.153NutrientRemoval2009Copyright©2009WaterEnvironmentFederation.AllRightsReserved.Inanybiologicaltreatmentprocess,developmentofbiodegradationkineticsprovidesavaluabletoolthathelpsoptimizebiofilmreactordesignandultimatelyitsoperation.MBBRisanattachedgrowthbiologicaltreatmentprocesswherethebiofilmgrowsonasmallfreefloatingplasticmedia.Theplasticmediaisselectedbasedonthebiofilmsurfacearearequired.MBBRtechnologyutilizeswholereactorvolumeforbiomassgrowthsimilartoanactivatedsludgereactor.However,incontrasttoactivatedsludgeprocess,itdoesnotneedanysludgerecycle.Thereactormaybeusedforaerobic,anoxicoranaerobicprocesses(Ødegaard,2006).NitrogenremovalinMBBRplantsmaybeachievedbypredenitrification,postdenitrificationoracombinationofboth.TheMBBRpilotprocessatBluePlainsAWTPwasoperatedasapost-denitrificationprocessforadenitrificationpolishingstep.Inpostdenitrificationsystems,highdenitrificationratesmaybeobtainedbyadditionofadequateamountsofanexternalcarbonsource.Forthisresearch,methanolwasusedasanexternalcarbonsource.Methanolwasselectedmainlyfortworeasons,(i)DCWASAcurrentlyappliesmethanoltoitspost-nitrifiedeffluent,and(ii)itisthecheapestcarbonsourceintheUnitedStates(Mokhayerietal.,2008).Apilotscalepost-denitrificationMBBRwasevaluatedinthisstudy.Threedifferentparameters:specificdenitrificationrate(SDNR),effectiveKsNOx-Nhalfsaturationconstantandbiomassdensityweremeasuredoverarangeoftemperatures.TheprimaryobjectiveofthisresearchwastobetterunderstandthedenitrificationkineticsandquantifyspecificnitrogenremovalrateofheterotrophsusingmethanolasanexternalcarboninanMBBRsystem.EffectiveKsNOxforMBBRprocesswasalsodeterminedbygraphicallyfittingtheobserveddatatoanattachedgrowthmodelbasedonMonodtyperateequation.Finallycarbonyieldswereestimatedforthissystem.AnotherparameterinpostdenitrificationistheC:NratiowhichmaybeexpressedbytheamountofCOD(ChemicalOxygenDemand)addedasanexternalcarbonsourceandtheamountofNO3-Nenteringthedenitrificationreactor(Rustenetal.,1995b).Therefore,astoichiometricevaluationofC:NratioforMBBRprocesswasperformedusingCODandNOx-NvaluesoftheSDNRtest.METHODOLOGYSDNRBatchTestFortheSDNRtest,abenchscalereactorwassetupinatemperaturecontrolledincubator.Thereactorset-upisillustratedinFigure1below.154NutrientRemoval2009Copyright©2009WaterEnvironmentFederation.AllRightsReserved.Figure1.DiagramofMBBRBatchReactorApproximately2.1liters(30%offillvolume)ofmediaandaneffluenttomake7litersoftotalvolumewasobtainedfrompilotMBBRsthatwasoperatedatvarioustemperatures.EffluentwasusedfromexistingnitrifyingandpartiallydenitrifyingreactorsofBluePlainsAWTP.Incubatortemperaturewassetclosetothepilotinfluenttemperature.Assoonasthereactorwasplacedintheincubator,purifiednitrogengaswaspassedthroughtheliquortostripoffoxygen,maintainanoxicconditions,keepthemediainsuspensionandfo