OvercomingHydraulicLimitationsoftheIntegrated

OvercomingHydraulicLimitationsoftheIntegratedFixed-filmActivatedSludge(IFAS)ProcessWilliamR.Leaf1,JoshuaP.Boltz2,JamesP.McQuarrie3,AdrienneMenniti4,GlenT.Daigger51CH2MHILL,Inc.,322EastFrontStreetSuite200,Boise,ID,83702USA,CorrespondingAuthor(E-mail:[e-mail])2CH2MHILL,Inc.,4350W.CypressStreet,Suite600,Tampa,FL,33607USA,(E-mail:[e-mail])3MetroWastewaterReclamationDistrict,6450YorkStreet,Denver,CO80229USA,(E-mail:[e-mail])4CH2MHILL,Inc.,2020SouthWest4thAvenue,3rdFloor,97201USA,(E-mail:[e-mail])5CH2MHILL,Inc.,9191S.JamaicaStreet,Englewood,CO,80112USA(E-mail:[e-mail])ABSTRACTIn2011morethan600movingbedbiofilmreactors(MBBRs)wereoperationalin50differentcountries.Approximately20systemsincorporatinganintegratedfixed-filmactivatedsludge(IFAS)processwereoperationalin2011,15ofwhichexistintheUnitedStatesofAmerica.SimilardesignstandardsareappliedtoMBBRandIFASprocessmechanicalcomponents:maximumplasticbiofilmcarriervolumetricfill,plasticbiofilmcarrierretentionscreens,airdiffusergrid(inaerobiczones),andmechanicalmixers(inanoxiczones).EachoftheseprocessmechanicalcomponentsinfluencesthehydraulicthroughputofMBBRandIFASsystems.TheauthorsareawareofIFASsystemsthathavebeensubjecttoahydraulicfailurethatresultedinplasticbiofilmcarrierloss.Hydraulicfailuresareanengineeringproblemthatcanbesolved.Whilehydraulicfailuresthatresultinplasticbiofilmcarrierlosshaveoccurredinlessthan1percentoftheknownexistingMBBRandIFASsystems,thefailureswerepublic.Potentialnegativepublicperceptionmayperpetuateutilityreluctancetoimplementthistechnology,whichhasnumerousbenefits.ThispaperpresentsexamplesofIFASprocessesthathavelostplasticbiofilmcarriersasaresultofhydraulicincidents.Inaddition,hydraulicfailuremechanismsthatresultinplasticbiofilmcarrierlossaredescribed.Finally,designfeaturesforovercominghydrauliclimitationsinherenttoIFASandMBBRprocessesaredescribed.Methodstoovercomethesehydrauliclimitationsinclude1)properselectionofmediageometry,2)suitablefillfractiongiventhehydraulicdesigncriterion,3)redirectingflowperpendiculartotraditionalbasinflowscheme,4)bypassing(low-pollutantload)wet-weatherflowsaroundtheIFASzone,5)splittinginternalmixed-liquorrecirculation(IMLR)flowwithaseriesofpumps,and6)processinstrumentationandcontroltooptimizebypassandIMLRoperations.KEYWORDS:Integratedfixed-filmactivatedsludge,movingbedbiofilmreactors,processmechanicaldesign,hydrauliclimitationsINTRODUCTIONIn2011morethan600movingbedbiofilmreactors(MBBRs)wereoperationalin50differentcountries.Approximately20systemsincorporatinganintegratedfixed-filmactivatedsludge(IFAS)processwereoperationalin2011,15ofwhichexistintheUnitedStatesofAmerica.SimilardesignstandardsareappliedtoMBBRandIFASprocessmechanicalcomponents.EachoftheseprocessmechanicalcomponentsinfluencesthehydraulicthroughputofMBBRand5236WEFTEC2011Copyright©2011WaterEnvironmentFederation.AllRightsReserved.IFASsystems.Theauthorsareawareofsix(6)IFASsystemsthathavebeensubjecttoahydraulicfailurethatresultedinplasticbiofilmcarrierloss.Hydraulicfailuresareanengineeringproblemthatcanbesolved.Whilehydraulicfailuresthatresultinplasticbiofilmcarrierlosshaveoccurredinlessthan1-percentoftheknownexistingMBBRandIFASsystems,thefailureswerepublic.Potentialnegativepublicperceptionmayperpetuateutilityreluctancetoimplementthistechnology.ThispaperpresentsexamplesofIFASprocessthathavelostplasticbiofilmcarriersduetohydraulicfailure.Inaddition,hydraulicfailuremechanisms(whichresultedinplasticbiofilmcarrierloss)aredescribed.Finally,designfeaturesforovercominghydrauliclimitationsinherenttotheIFAS(andMBBR)processaredescribed.MBBRandIFASProcessFlowDiagramandSystemFunctionMovingbedreactorscanbeoperatedasa2-(anoxic)or3-(aerobic)phasesystemswithbuoyantfree-movingplasticbiofilmcarriersthatrequireenergy(i.e.,mechanicalmixingoraeration)foruniformdistributionthroughoutthebulkphase(McQuarrieandBoltz2011).TheMBBR,whichisillustratedinFigure1,includesone,oraseriesof,submergedandcompletelymixedbiofilmreactor(s)followedbyaliquid-solidsseparationunit(e.g.,sedimentationbasin,dissolvedairflotation,orballastedflocculation).Figure1.MBBRprocessschematic.Figure2.IFASprocessschematic.Suspendedgrowthreactors,suchastheconventionalactivatedsludge(CAS)process,cultivatethegrowthofmicroorganismswhichbioflocculatetoformflocswhicharefreelysuspendedin5237WEFTEC2011Copyright©2011WaterEnvironmentFederation.AllRightsReserved.thebulkphase.Theflocculatedbacteria,orflocs,areresponsibleforbiologicaltransformationprocessesinCASsystems.Afterexitingthebioreactor,flocsareseparatedfromthebulkphasebysedimentationormembranes.Clarifier-coupledsuspendedgrowthreactorsrelyonreturnactivatedsludge,orunderflow,fromtheclarifiertoaccumulatethedesiredactivebiomassconcentrationinthebioreactor.Consequently,clarificationunitsmaybehydraulicallyorsolidslimited.TheMBBR,apurebiofilmreactor,retainsbacterialcellsinabiofilmattachedtofree-movingplasticbiofilmcarriers;therefore,biomassretentionintheMBBRisclarifierindependent.IFASprocesses,depictedinFigure2,combinesuspendedgrowtha