DesignAndOperationOfNutrientRemovalPlantsFo

DESIGNANDOPERATIONOFNUTRIENTREMOVALPLANTSFORVERYLOWEFFLUENTCONCENTRATIONSBjornRusten*andHallvardØdegaard***Aquateam-NorwegianWaterTechnologyCentre,P.O.Box6875Rodelokka,N-0504Oslo,Norway**NorwegianUniversityofScienceandTechnology(NTNU),Dep.ofHydraulicandEnvironmentalEngineering,S.P.Andersensvei5,N-7491Trondheim,Norway.ABSTRACTOutofatotalofsixplantswithnitrogenremovalinNorway,fourplantsusetheMBBR(MovingBedBiofilmReactor)process.InallfourcasestheMBBRprocesswaschosenbecauseitofferedaverycompacttreatmentsolutionandbecauseithadthelowestinvestmentcostsaswellasthelowesttotalannualcosts.AlltheMBBRplantsusethecombineddenitrificationprocess(pre-denitrification+post-denitrification)fornitrogenremoval,followedbychemicalprecipitationforphosphorusremoval.Thistypeofdesignoffersalotofoperationflexibilityandenablestheplantstoproduceveryloweffluentconcentrations.Externalcarbonsourcesusedforpost-denitrificationattheseplantsareethanol,methanolandmonopropyleneglycol.Theplantshavemaximumdesignflowsfrom1,125to7,200m3/h(7.1to45.6mgd)andatleasttwoparalleltrains.Allplantshaveprimaryclarifiers.Twooftheplantsusesedimentationforfinalsolidsseparation,whiletheothertwousedissolvedairflotation.ThepaperpresentstypicaldesigndatafornitrificationanddenitrificationinMBBRs,plusresultsfromguaranteetestswhereasingletrainisoperatedatdesignload,aswellasresultsfromnormaloperationoftheplants.Withinfluentconcentrationsabove50mgtotalN/L,theGardermoenWWTPhasdemonstratedeffluentconcentrations2mgtotalN/Land1mgtotalinorganicN/L.Fortheyear2005theLillehammerWWTPhadannualaverageeffluentconcentrationsof2.2mgBOD5/L,2.9mgtotalN/Land0.12mgtotalP/L.Yearsoffull-scaleexperiencefromMBBRplantsinNorway,usingcombinedpre-andpost-denitrificationpluspost-precipitation,hasdocumentedthatfinaleffluentconcentrationsbelow3mgtotalN/Land0.3mgtotalP/LcanbeachievedatlowwastewatertemperaturesandatacceptablecapitalandO&Mcosts.KEYWORDSBiofilm,carbonsource,chemicalprecipitation,denitrification,full-scale,MBBR,nitrification.INTRODUCTIONInNorwaychemicalprecipitationforremovalofphosphorusdowntoveryloweffluentconcentrations(0.3mgtotalP/L)hasbeenpracticedfordecades.Inthe1990’sarequirementforremovalofnitrogenwasimplementedforlargerwastewatertreatmentplants(WWTP)withdischargetosensitiveareas.Outofatotalofsixplantswithnitrogenremoval,oneplantusesthe1307NutrientRemoval2007Copyright©2007WaterEnvironmentFederation.AllRightsReservedactivatedsludgeprocessandtheotherfiveusebiofilmprocesses.FourofthefivebiofilmplantsusetheMBBR(MovingBedBiofilmReactor)process.ThreeoftheMBBRplantswereupgradedfrommechanical-chemicaltreatmenttomechanical-biological-chemicaltreatment,andthefourthplantwasbuiltfromscratch.TheMBBRprocesswaschosenbothbecauseitofferedaverycompacttreatmentsolutionandbecauseithadthelowestinvestmentcostsaswellasthelowesttotalannualcosts.Thepermitrequirementfortheseplantsis70%removalofnitrogenasanannualaverage.However,forthefirstfewyearstheplantswereperiodicallyoperatedwithveryloweffluentconcentrations,becausetheplantsupervisorswantedtoseehowwelltheplantscouldperform.Datafromprolongedperiodswithveryloweffluentconcentrationsarepresentedinthepaper,togetherwithdatafromprocessguaranteetestingwhereasingletrainwasoperatedatdesignloadconditions.ThemainobjectiveofthepaperistoshowthattheMBBRtechnologyhasdemonstrated(ineverydayfull-scaleoperation)thatveryloweffluentconcentrationsofbothtotalnitrogenandtotalphosphoruscanbeconsistentlyachievedatanacceptablecost.THEMOVINGBEDBIOFILMREACTOR(MBBR)PROCESSThemovingbedbiofilmreactorprocess(Eur.pat.no.0575314,USpat.no.5,458,779)wasdevelopedinNorwayinthelate80’iesandearly90’ties(Ødegaardetal.,1994;Ødegaardetal.,1999).TheideabehindthedevelopmentoftheMBBRprocesswastoadoptthebestfeaturesoftheactivatedsludgeprocessaswellasthoseofthebiofilterprocesswithoutincludingtheworst.Contrarytomostbiofilmreactors,theMBBRutilizesthewholetankvolumeforbiomassgrowth,asdoesalsotheactivatedsludgereactor.Contrarytotheactivatedsludgereactor,however,itdoesnotneedanysludgerecycle,asisalsothecaseinotherbiofilmreactors.Thisisachievedbyhavingthebiomassgrowoncarriersthatmovefreelyinthewatervolumeofthereactor,keptwithinthereactorvolumebyasievearrangementatthereactoroutlet.Sincenosludgerecirculationtakesplace,onlythesurplusbiomasshastobeseparated-aconsiderableadvantageovertheactivatedsludgeprocess.Thereactormaybeusedforaerobic,anoxicoranaerobicprocesses,asillustratedFigure1.Figure1-PrincipleoftheMBBR.a.Aerobicreactorb.Anoxicandanaerobicreactor1308NutrientRemoval2007Copyright©2007WaterEnvironmentFederation.AllRightsReservedInaerobicprocesses,thebiofilmcarriermovementiscausedbytheagitationsetupbytheair,whileinanoxicandanaerobicprocessesamixerkeepsthecarriersmoving.Fortheaerobicreactorsaspecialcoarsebubbleaerationsystemhasbeendeveloped.Thecarriersarekeptwithinthereactorbyanoutletsieve.Thismaybeverticallymounted,rectangularmeshsieves,butlatelythesieveismoreoftenshapedasacylindricalbarsieve,verticallyorhorizontallymounted.ProperdesignofaerationgridsandsievesisveryimportantforoptimumperformanceoftheMBBRpr