EffectofTemperatureonBiofilmGrowthDynamicsa

1  EffectofTemperatureonBiofilmGrowthDynamicsandNitrificationKineticsinaFull-ScaleMBBRSystemCoreyBjornberg1*,Dr.WeiLin2,Dr.RobertZimmerman31HazenandSawyer,Cincinnati,OH45242USA2NorthDakotaStateUniversity,Fargo,ND58105USA.3CityofMoorhead,MN56561USA.*[e-mail],MinnesotaWastewaterTreatmentFacility(WWTF)operatesamovingbedbiofilmreactor(MBBR)tocomplywithseasonalammoniadischargelimitations.Sincebecomingoperationalin2003ithasshowntheabilitytoreachhighammoniaremovalefficiencyyearroundeventhoughitissubjecttotheextremetemperaturevariationsfoundintheupperMidwest.Monitoringdataforattachedbiomassrevealeddramaticchangesfromwintertosummermonthswithlargequantitiesofattachedbiomassinthecoldermonthsandsignificantlylessinthewarmermonths.Monitoringresultsindicatedthatattachedbiomass,biofilmthickness,andbiofilmmorphologychangesignificantlyfromwarmtocoldweathermonths.Attachedbiomassquantitieswerefoundtovaryupwardsoffourandhalftimeswhilebiofilmthicknesschangeduptothreetimesbetweenthewarmandcoldweathermonths.KEYORDS:Nitrification,MovingBedBiofilmReactor(MBBR),Biofilm,TemperatureINTRODUCTIONTheCityofMoorhead,Minnesotaoperatesawastewatertreatmentfacility(WWTF)withadesigncapacityof22,710m3/d(6mgd).Liquidtreatmentincludesbarscreens,aeratedgrittanks,flowequalization,primaryclarification,highpurityoxygenactivatedsludge,finalclarification,movingbedbiofilmreactor(MBBR)nitrification,polishingponds,andchlorination/dechlorinationbeforedischargingtotheRedRiveroftheNorth.In2003,theMoorheadWWTFbeganoperatingaseparatestageMBBRnitrificationprocesstocomplywiththeseasonaleffluentammonia-nitrogenlimitsbetweenthemonthsofJuneandSeptember.TheMBBRvolumeis2,970m3(104,880ft3)witha32%mediafill.SpecificationsfortheMBBRwerereportedbyZimmermanetal.(2005).PlasticmodulemediausedintheMBBRwasproducedbyHydroxyl-Pac(HydroxylSystems,Sidney,BritishColumbia,Canada).Thesecarriersaremadeofultravioletresistanthigh-densitypolyethylene,andareapproximately15mminlengthand21mmindiameter.Themediahasatotalspecificsurfaceareaof568m2/m3(176ft2/ft3)withaninternaloreffectivespecificsurfaceWEFTEC2009Copyright©2009WaterEnvironmentFederation.AllRightsReserved.44072  areaof388m2/m3(118ft2/ft3).Themediahasaspecificdensityof0.95g/cm3,allowingthecarrierstofloat,yettumbleandmixeasilywithinthereactor.SincebecomingoperationaltheMBBRhasbeensuccessfulincomplyingwithammonianitrogendischargelimitsduringtherequiredseasonalperiodaswellasduringtheperiodbetweenOctoberandMay,whenlimitsdonotapply.Theareaissubjecttoextremeambienttemperaturesthatcanvaryfromlowsbelow-33ºCinJanuarytohighsabove34ºCinJuly.ThisresultsinMBBRwatertemperaturefallingbelow9ºCinthewintermonthsandexceeding22ºCinthesummer.TheMBBRhasshowntheabilitytoachievehighremovalefficienciesnotonlyinthewarmsummermonths,butalsointhecoldwintermonths.Monthlyaverageinfluentandeffluentammonia-Nconcentrationsfor2007areshowninFigure1alongwithremovalefficiency.Removalefficiencyvariedbetween72%and99%butappearedtoberelatedcloselytoinfluentammonia-Nconcentrations,nottemperaturevariations. Figure1. AveragemonthlyinfluentandeffluentNH4+-Nandremovalefficiency.Watertemperaturevariedmorethan10ºCbetweenwinterandsummermonths,yetminimalapparentdecreaseinammonia-Nremovalefficiencywithtemperaturewasobserved.Similarresultsinotherattachedgrowthsystemshavebeenreported(Chenetal.,2006andSalvettietal.,2006).InJanuary2007,biomassmeasurementswithintheMBBRwereinitiatedtoevaluateseasonalvariationsintheamountofbiomassattachedtomedia.WhentheresultsofattachedbiomasswerecomparedtoMBBRtemperature(Figure2),analmostinverserelationshipwasexhibitedbetweenbiomassandoperatingtemperature.Agreaterquantityofattachedbiomasswasobservedduringwinterandspringmonthswhentemperatureswerelow,whileinsummerandearlyfall,whentemperatureswerehigh,thequantityofbiomassattachedtothemediawassignificantlylower.0%10%20%30%40%50%60%70%80%90%100%0510152025303540RemovalEffeciency,%NH4+-N,mg/LMBBRInfluentNH4-NMBBREffluentNH4-NNH4-NRemovalRateWEFTEC2009Copyright©2009WaterEnvironmentFederation.AllRightsReserved.44083   Figure2.ComparisonofbiomassandtemperaturevariationsintheMBBR.Itiswellacceptedthattemperaturehasasignificantimpactonbiologicalkinetics.Temperatureeffectsonsuspendedgrowthsystemshavebeensuccessfullyusedinsystemdesignandmodelsimulations,yetinattachedgrowthsystems,temperatureeffectispoorlyunderstood(Chenetal.,2006,Salvettietal,2006,ZhuandChen,2002,Wheaton,1994,OkeyandAlbertson,1989).Temperatureeffectsonbiofilmaredifficulttorecognizeassubstratediffusion,transport,andbiologicalactivityareallaffectedbytemperature.ItisalsowellacceptedthatDOsaturationistemperaturedependentsuchthathightemperatureproduceslowDOlevelsandastemperaturesdecreases,DOsaturationincreases.Dissolvedoxygenisbelievedtobeanunderlyingreasonfornoapparenttemperatureinfluenceonnitrificationrates(Ødegaardetal.,1994).Astemperaturesincrease,thediffusioncoefficientdrivingmasstransportintobiofilmalsoincreases.DOlevelsanddiffusionprocessesthatareheavilyinfluencedbytemperaturemakeitdifficulttodeterminehowbiofilmi