UnderstandingTheImportanceOfAerobicMixingBi

UNDERSTANDINGTHEIMPORTANCEOFAEROBICMIXING,BIOFILMTHICKNESSCONTROLANDMODELINGONTHESUCCESSORFAILUREOFIFASSYSTEMSFORBIOLOGICALNUTRIENTREMOVALDipankarSen*,CliffordW.Randall**,WilliamBrink***,GregFarren****,DickPehrson*****,WayneFlournoy*****,RhodesR.Copithorn****SantaClaraValleyWaterDistrict5750AlmadenExpressway,SanJose,CA95118,USA**DepartmentofCivilEngineering,VirginiaTech,Blacksburg,VA***Stearns&Wheler,Cazenovia,NY****AnneArundelCountyDPW,MD*****EntexInc,Raleigh,NCABSTRACTResearchwasconductedinIFASsystemstoidentifyproblemsthatarosewiththebiofilmwhentheintensityofmixingandaerationfallbeloworriseabovecertainthresholds.ThetypesofIFASmediaevaluatedincludefixedbedcordmedia(RinglaceandBioweb),movingbedsponge(spongemedia–LinporandCaptor;andplasticmedia–Kaldnes)andRBCmedia.Anaerationprocessdesignmodelandoperatingstrategieshavebeendevelopedtoaddresstheseproblems.ThispaperpresentstheIFASandMBBRdesignmethodologyandaerationprocessdesigncomponentoftheAquifasUnifiedModelforActivatedSludge,IFASandMBBRsystems.Thepaperreferencesspecificinstancesinfull-scaleIFASplantswhereproblemsoccurredbecauseofinsufficientorimpropermixingpatternsanddocumentshowtheywereresolved.Theproblemsobservedincludednuisancepredatordevelopmentandpluggingofmediaandscreens.TheIFASmodelwasupgradedtoincorporatecomputationofbiofilmthicknessandtheimpactoftheincreaseinbiofilmthicknessonthereductionineffectivesurfaceareaavailableonavarioustypesofmedia.Effectivesurfaceareaisthem2ofbiofilmsurfaceperm3oftankvolume.Themodelusestheeffectivesurfaceareaandtheamountofmediainthetanktocomputethesubstrate(COD,NH4NandOxidized-N)profiles.Additionalresearchisbeingconductedtoestablishcertainthresholdsformixingnecessaryaboveandbeyondtheaerationrequirementstomaintainathinbiofilm.ThepapershowshowtheprocessdesignmodelisappliedtoimprovethedesignofIFASandMBBRsystems.IninstanceswheretheaerationrequirementtosatisfytheDOset-pointissatisfiedbutthemixingisbelowthethresholdtomaintainathinbiofilm,themediafillvolumefractioninanaerobiccellcanbeincreasedandthefractionoftankvolumeoccupiedbytheaerobiccellcanbeincreased.Byincreasingthemediafillvolumefractioninasituationwherethebiofilminanaerobiccellistoothick,thetotalamountofbiofilmsurfaceareapresentintheaerobiccellisincreasedandthesolubleCODconcentrationisdecreased.Thisdecreasesthebiofilmthicknessobservedforthesameintensityofmixingandincreasestheeffectivesurfaceareaofbiofilm.Thisimprovestheperformanceofthesystem.Whenthemediafillvolumefractionisincreased,theairflowperunitofreactorvolumehastobeincreasedtosatisfytheadditionaloxygendemand.Thisincreaseinairflowhelpssatisfythemixingrequirementstomixthemediaandinducetherequisiteamountofbiofilmshear.Insomeinstanceswithfixedbedmedia,supplementalaerationsystemisinstalledbelowtheframestoincreasetheairflowduringdiurnalpeakloadhoursandthinoutthebiofilm.1098NutrientRemoval2007Copyright©2007WaterEnvironmentFederation.AllRightsReservedKEYWORDSIFAS,biofilm,mixing,modeling,AquifasINTRODUCTIONDatafromfullscaleIntegratedFixedFilmActivatedSludge(IFAS)plantsshowthatproblemsarisewithbiofilmwhentheintensityofmixingandaerationfallbelowcertainthresholds.TheseIFASplantsincorporatebiofilmsupportmediawithintheaerobicand,insomeinstances,theanoxiczonesoftheactivatedsludgesystem(Figure1).Thetypesofmediaappliedincludefixedbedcordmedia(RinglaceandBioweb),movingbedsponge(spongemedia–LinporandCaptor;andplasticmedia–Kaldnes)andRBCmedia.Atseveralplants,themediadevelopedathickerbiofilmwhentheintensityofmixingwasdecreased.Thisreducestheeffectivespecificsurfacearea(SSAe-m2ofbiofilmsurfaceperm3oftankvolume).ThereductioninSSAereducedtheorganicsandnutrientremovalachievedinthebiofilm.Atseveralplants,thethickerbiofilmledtoexcessivegrowthofnuisancepredatorsinthebiofilm(redworms).Whenthethicknesswasnotcontrolledorthebiofilmwasnotthinnedout,thewormsledtoareductioninnitrification.Inafewinstances,veryhighintensitiesofmixingresultinsparsecoverageofmediabythebiofilm.Theseobservationspointtotheneedforbetterphysicalandprocessmodels.ThemodelingiscomplicatedbecausethethicknessofbiofilmthatdevelopsatacertainmixingintensityandtheeffectivespecificsurfaceareaofbiofilmonacarrierparticlearedependentonCOD,NH4N,NOxNandDOconcentrationsineachcellofamulti-cellreactor.Therefore,theprocessmodeloftheIFASsystemhasto:1.Determinethesubstrate(COD,NH4NandNOxN)concentrationprofiles,2.Havetheabilitytocomputethebiofilmthicknessineachreactor(cell)withinanIFAStank,basedonthesubstrateconcentrationinthereactor,3.Incorporatefeaturesthatallowtheusertocorrecttheeffectivesurfaceareaforthebiofilmthicknessandrecomputethesubstrateprofile.Aswithactivatedsludgesystems,theprocessmodelhastocomputetheaerationrequirementsforeachaerobiccellfortheDOset-pointspecified.Theaerationrequirementshavetobecomparedagainstthethresholdsformixingandbiofilmshear.Thispaperdiscussestheresultsofresearchthathasestablishedsomeofthethresholds.Additionalresearchneedstobeconductedinconjunctionwithvariousmanufacturersofmediatocollectacompletedataset.Thepapershowshowthedesignercanaddressproblemsbyadjustingthesiz