DifferencesinAerationandAirScourEnergyRequi

DIFFERENCESINAERATIONANDAIRSCOURENERGYREQUIREMENTSBETWEENMEMBRANEBIOREACTORS,CONVENTIONALACTIVATEDSLUDGE(CAS),IFASANDMBBRFOLLOWINGPROCESSOPTIMIZATIONDipankarSen1,GeorgeCrawford2,DaveCommons31SantaClaraValleyWaterDistrict/Aquifas1290BryantAvenue,MountainView,CA940402CH2MHill3SanBernardinoMunicipalWaterDepartmentABSTRACTHollowfibermembranebioreactors(MBRs)wereevaluatedinfullscaletostudymethodsofreducingtheiraerationandairscourenergyrequirements.Thechangesweremodeledtodeterminemethodstooptimizetheaerationandairscourenergyconsumptionandcomparethetotaltotheaerationenergyrequirementsofconventionalactivatedsludge(CAS),IFASandMBBRsystems.TheresearchfoundthattheaerationandairscourenergyrequiredforanMBRcanbereducedtowithin33%aboveconventionalactivatedsludge(CAS)systemsoperatingwith50percentmorevolume(AerationOptimizedMode,AOM).Itcanbereducedfurthertowithin20%withautomatedDOandammonium-Nmonitoringandcontrolintheaerobiczoneandthemembranecell,oratfacilitieswhichcandischargeeffluentammonium-Nabove0.5mg/Lduringsomeweeks(AerationConstrainedMode,ACM).TheoptimizationtechniquesincludedreductionoftheDOsetpointinthemainreactorto1mg/LforAOMand0.5mg/LforACM;reductionintheairscourontimefrom100%to25%.Theairscourairflowratewascontrolledsuchthatitwasjustabove18mm/secduringtheontime.ThecombinationofthesetwochangesreducedtheDOlevelsinthemembranecellfromgreaterthan6mg/Ltolessthan4mg/L.TheenergyrequirementsforMBRswerethencomparedtoothercompactsolutionssuchasIFASandMBBRs.IntheMLEprocessconfigurationandACM,theMBRconsumed4to7%moreaerationandairscourenergythanIFASandMBBR,respectively.IntheEnhancedNutrientRemoval(ENR)mode,inwhichmethanolisaddedtothepost-anoxiccellstoincreasedenitrification,theMBRconsumedsimilaramountscomparedtoIFASandMBBRs,withallprocessesachieving4mg/LTotalNitrogen(TN).KEYWORDSMBR,Optimization,Energy,Modeling,AquifasINTRODUCTIONPastdatafrommembranebioreactors(MBRs)showthatenergyforaerationandmembranescourexceeds150%ofconventionalactivatedsludge(CAS)operatedforBiologicalNutrientRemoval(BNR).Insomeplants,thereportedvaluesforenergyconsumptionare200to300%ofCAS.Thetotalenergyconsumptionincludesairflowandmembranesuctionpumps.Thesevaluesreportedinthesestudiesarepriortooptimizationthathasrecentlybeenundertakenatafewfacilities.Theoptimizationindicatesthatthereareconsiderableopportunitiestoreducetheenergyrequirements.Theobjectivesofthisresearchareto(1)conducttheoptimizationofMBRsusingaprocess(plant)modelthathasbeencalibratedtofullscaleMBRsand(2)comparetheoptimizedsolutiontoCASandothercompacttechnologiessuchasIntegratedFixedFilmActivatedSludge(IFAS)andMovingBedBiofilmReactors(MBBRs).TheAquifasmodel(2007)wasusedtocombinethedifferent483MembraneTechnology2008Copyright2008WaterEnvironmentFederation.AllRightsReserved©optimizationtechniquesintheModifiedLudzackEttinger(MLE)andEnhancedNutrientRemoval(ENR)configurations.ThetwoconfigurationsareshowninFigures1to5.REVIEWOFENERGYREQUIREMENTSTheincreaseinenergybudgetinMBRscanbeattributedtothefollowingfactors:1.Reductioninalphavaluesinthemainaerationandinthemembranetanktank.ThesereductionsareafunctionoftheincreaseinoperatingMLSS.Typically,aMBRformediumsizedmunicipalfacilitiesoperatesat2to3timestheMLSSofaCAS.TheCASmaybedesignedwithaMLSSof3500mg/LinwinterwhiletheMBRisdesignedwith7000to8000mg/L.KrampeandKrauth(2003)presentedarelationshipofalphawithMLSS:α=e-0.08788XwhereX=MLSSing/L.Basedonthisrelationship,thealphadecreasesfrom0.74to0.5withtheincreaseinMLSSfrom3500mg/Lto8000mg/L.Thisisa33percentreductioninalpha.Corneletal(2003)presentedasimilarrelationship:α=e-0.046XwhereX=MLSSing/L.Basedonwhichalphadecreasesfrom0.85to0.69.Thisrepresentsa19percentreductioninalpha.2.Needtomaintainanairscourairflowinthemembranetankthatisinexcessoftheprocessdemand.TheprocessdemandistheoxygenrequiredtosatisfytherequirementsforCODandNremoval.Theairscourairflowisnecessarytocontrolthethicknessofbiomassthatbuildsupmembranes.Thereportedvaluesofairscourflowratesare60m3/m2/hor16to18mm/sec(Wagneret.al.,2006).Dimitriouet.al.(2006)applyasimilarconcepttomovetheliqudatvelocitiesof18mm/secandengineerthescour.3.IncreaseinmethanolrequirementsforpostanoxicdenitrificationintheENRconfiguration.Theadditionalmethanolisrequiredbecauseofsignificantlyhigherforwardflowthroughthepostanoxiccell.UnlikeaCAS,IFASorMBBR,wherethisforwardflowthroughthepostanoxiccellistypically1.5Q(sumtotalofinfluent,1Q,andRAS,0.5Q),inMBRs,thisforwardflowisthesumtotalofinfluent(1Q)andMBRrecycle(typically3to4Q).ForthesameoperatingDOintheaerobiccellupstrean,theDOloadenteringthepostanoxiccellis2.5to3timeshigher.Asmuchas50percentadditionalmethanolisrequiredtoconsumethisDO.484MembraneTechnology2008Copyright2008WaterEnvironmentFederation.AllRightsReserved©AnoxicAerobicRASPrimaryEffluent112233NitrateRecycle4455Figure1:ConventionalActivatedSludge(CAS)inMLEConfigurationAnoxicAerobicRASPrimaryEffluent112233NitrateRecycle4455Figure2:IntegratedFixedFilmActivatedSludge(IFAS)inMLEConfigurationThisfigureshowsmovingbedmedia(spongeorplastic)ScreenAnoxicAerobicPrimaryEffluent112233NitrateRecy