10YearsOfExperiencesOfAnMBBRProcessForPos

10YEARSOFEXPERIENCESOFANMBBRPROCESSFORPOST-DENITRIFICATIONK.Täljemark,H.Aspegren,C.Gruvberger,N.Hanner,U.NybergandB.AnderssonMalmöWaterSE-20580MalmöSwedenABSTRACTProblemswitheutrophicationandalgaebloomsinitiatedtheneedforenhancednutrientremovalatEuropeanwastewatertreatmentplantsintheendoftheeighties.WhenupgradingthetwoWWTPsinMalmö,sitespecificupgradingconceptshadtobeconsideredbecauseoftheexistingplantstructures.Asphosphorusremovalalreadywasimplemented,theupgradingwasfocusedonenhancednitrogenremoval.Thekeyelementintheupgradingprocedurewastheretrofitwithanappropriatepost-denitrificationprocess.Anewlydevelopedconceptwithamovingbedbiologicalreactorprocesswasfoundtobecost-effective.PilotplanttestswithanMBBRprocessforpost-denitrificationwereperformedfordemonstrationpurposeandtogetsomeideasforproperdesigncriteria.Thelaterexperiencesfromthefull-scaleplantoperationshowedgoodagreementwiththeselecteddesigncriteriafromthepilotplantdata.Thefutureeffluentstandardscouldbemetbytheupgradingoftheplants.Thepilotplanttestsaswellastheexperiencesfromfull-scaleoperationpointedouttheneedforacontrolsystemforexternalcarbondosage.Thecontrolactionwasbasedontheinstantaneousnitrateloadcalculatedfromon-linemeasurementsofnitrateandwastewaterflow.TheneedforanactivecontrolofthephosphorusconcentrationintheMBBRprocesswasalsostressedbytheexperiences.KEYWORDSMovingbedbiofilmreactor,postdenitrification,externalcarbon,phosphoruslimitationINTRODUCTIONManycoastalwatersinespeciallynorthernEuropehaveseriousproblemswithalgaebloomseverysummerandthesewatersareusuallyconsideredtobenitrogenlimited.Awayofdealingwiththeeutrophicationproblemistolimitnitrogenemissionsbybuildingorupgradingwastewatertreatmentplants(WWTPs)fornitrogenremoval.Thisprocessstartedinthelateeightiesandduringthefollowingyearsmanywastewatertreatmentfacilitieswereeitherbuiltorupgraded.InthelateeightiesthemostcommonprocessfornitrogenremovalwasbasedonanactivatedsludgeprocesswithalowF/Mratio.InSweden,where95%ofthepopulationisconnectedtowastewatertreatment,theexistingplantsweredesignedforBODandphosphorousremoval,asthemajorityofourinlandwatersarephosphoruslimited.Typically,thewastewatertreatmentplantsinSwedenwerehigh-rateactivatedsludgeplantsortricklingfilterplantsincombinationwithchemicalprecipitation.Itwasnotalwaysself-evidenthowtheseplantscouldbetransformedintolowF/Mratioactivatedsludgeunits.Therewasastrongincentivetodevisealternativeprocessschemesfornitrogenremovalthatcouldutilizeexistingfacilitiesinordertominimizetheoverallcost,investmentcostsaswellasrunningcosts,fortheupgrading.Obviously,ifexistingfacilitiesaretobeusedasite-specificupgradingconcepthastobeconsidered.Thisinturnmaytriggerthedevelopmentofnewornovelprocesses.InthecityofMalmöinsouthernSwedenitbecameevidentintheendoftheeightiesthatupgradingtheplantsforpost-denitrificationwithanovelprocessconceptnamelytheMBBRprocesscouldbeacost-effectivesolution.Inthispaper,theincentivesforchoosinganMBBRprocessandexperiencesfromthedevelopmentandfull-scaleoperationofthistypeofprocesswillbediscussedinmoredetail.PREREQUISITESFORUPGRADINGINMALMÖThecityofMalmöhadtwoWWTPs.TheSjölundaplantisdesignedfor550,000p.e.andtheKlagshamnplantisdesignedfor90,000p.e.Theactualloadsaresomewhatlowerthough.Thewastewaterismainlyofmunicipalorigin.TheSjölundaplanthasbeenupgradedthreetimesbefore.Theplantlayoutincludedprimarytreatmentwithpre-precipitation,biologicaltreatmentforBODremovalinanactivatedsludgeplantandatricklingfilterplantoperatedinparallelandtertiarytreatmentwithflotation.AkeyissueforupgradingtheSjölundaplantwastointegratetheexistingtricklingfiltersintheconcept.Extensivetestsmadeitplausiblethattheentirewastewaterflowcouldbenitrifiedintheexistingtricklingfiltersprovidedthatanactivatedsludgeprocessprecedesthefilters.Inordertoremovenitrogen,apost-denitrificationprocesshadtobeimplementeddown-streamthenitrifyingtricklingfilters.Theexistingflotationplantcouldstillbeusedfortertiarytreatmentandsuspendedsolidsremoval.Full-scaletestsattheKlagshamnplantwithdifferentupgradingconceptsshowedthattheavailableorganicmaterialintheprimaryeffluentwaslimitingthenitrogenremovalinapre-denitrificationsystem.Theeffluentnitrogenstandard,atleastatthepresentload,couldbereachedbyrunningtheactivatedsludgeplantasasingle-sludgeprocesswithnitrificationandpost-denitrificationwithadditionofmethanolorethanolasanexternalcarbonsource.Lookingatthedesignload,somedifficultiescouldbeforeseentoreachthestandardwithsecurity.Asatertiarytreatmentprocessforsuspendedsolidsremovalwasnecessarytobuildinordertoreachthephosphoruseffluentstandard,thepossibilitytointegrateapost-denitrificationprocessinthetertiaryprocesswasconsidered.Hereby,thenitrogenremovalcapacityoftheplantcouldbeincreasedsignificantly.Consequently,akeyelementintheupgradingofbothplantswaswiththeestablishmentofanappropriatepost-denitrificationprocess.PROCESSESFORPOST-DENITRIFICATIONFortheapplicationofpost-denitrificationprocesses,biofilmprocessesarepreferredsincetheylacktheinconvenienceasuspendedsinglesludgepost-denitrificationprocessinvolveswithd