Trackingparticlesizedistributionsinamovingbedbiofi

TrackingparticlesizedistributionsinamovingbedbiofilmmembranereactorfortreatmentofmunicipalwastewaterR.M.Åhl*,T.Leiknes*,H.Ødegaard**NTNU-NorwegianUniversityofScienceandTechnology,DepartmentofHydraulicandEnvironmentalEngineering,S.P.Andersensvei5,N-7491Trondheim,Norway.(E-mail:[e-mail],[e-mail],[e-mail])AbstractAstudyhasbeenconductedtoinvestigatetheeffectofloadingratesonmembranefoulinginamovingbedbiofilmmembranereactorprocessformunicipalwastewatertreatment,especiallyanalyzingthefateofsubmicroncolloidalparticlesandtheirinfluenceonmembranefouling.Twooperatingconditionsdefinedaslowandhighorganicloadingratesweretestedwherethedevelopmentandfateoftheparticulatematerialwascharacterizedanalyzingtheparticlesizedistributionsthroughouttheprocess.Analysisofthemembraneperformanceshowedhigherfoulingratesforthehigh-rateconditions.Thefractionofcolloidalsubmicronparticleswashigherinthemembranereactorindicatingthatfoulingbytheseparticleswasadominantcontributiontomembranefouling.KeywordsParticlesizedistribution,wastewater,membranebioreactors,fouling,INTRODUCTIONMembranebioreactors(MBR)arecommonlyunderstoodasthecombinationofmembranefiltrationandbiologicaltreatmentusingactivatedsludge(AS)wherethemembraneprimarilyservestoreplacetheclarifierinthewastewatertreatmentsystem(vanderRoestetal.,2002).Avarietyofprocessconfigurationsexistwherethemembraneisinstalledeitherinanexternalunitorimmersedintheaerationtankandwherethesystemsaredesignedtobeoperatedunderlow-pressurevacuum.OneofthebenefitsofAS-MBRscomparedtoconventionalactivatedsludgesystemsisthattheycanbeoperatedathighersuspendedbiomassconcentrationsresultinginlongsludgeretentiontimesevenatsmallreactorvolumes.However,ascommonwithallmembraneprocesses,theefficacyoftheMBRislargelyconstrainedbyfouling,i.e.theaccumulationofmaterialsonthesurfaceoforwithinthemembraneresultinginareductioninthemembranepermeability.FoulingisparticularlyaprobleminMBRssincetheprocessdealswithliquorshavinghighconcentrationsoftotalsolidsaswellasdissolvedcompoundssuchasextracellularpolymericsubstances(EPS).Foulingisthereforecausedbydifferentsubstancesandthemechanismsarerathercomplexandinterrelated.Certaintypesoffouling(reversible)canberemovedbybackwashing,i.e.cakeformationandloosedepositions,whileothersarepermanent(irreversible,foulingwhichisonlyrecoverablebychemicalcleaning).ThelatterisoftenattributedtoEPS(Nagaokaetal,1996,Changetal,2002).Asecondlimitation,clogging–whichreferstothefillingofthemembraneintersticeswithsolids–mustalsobesuppressedforsuccessfuloperation.Allthesephenomenalimittheflowofwaterthroughthemembranedemandingphysicaland/orchemicalmembranecleaningtorestorepermeability.BoththeconcentrationandthecharacteristicsofthesolidsarethereforeimportantparametersfordesigningMBRprocessesandforoptimizingfoulingcontrolandcleaningstrategies.AnalternativestrategytoreducetheeffectofmembranefoulingbyhighbiomassconcentrationsasinAS-MBRsystemsiscombiningmembraneswithbiofilmprocesses(BF-MBR).Conventionalbiofiltersarepronetocloggingathighloadsoforganicandparticulatematterresultinginfrequentbackwashing.Themovingbedbiofilmreactor(MBBR)isanalternativebiofilmreactorwhichtolerateshighparticulateandorganicloadingrates(Ødegaardetal.,2000).IntheKaldnesMBBRprocess,thebiofilmcarriersaresmallplasticcylinderswhicharesuspendedinthereactorbyaeration(Ødegaardetal.,1994).TheprocessisverycompactcomparedtoconventionalAStreatmentprocessandmaybeusedforlow-rateaswellashigh-ratesystems.ThesolidsconcentrationsintheeffluentfromaMBBRreactor(andinfluenttothemembranereactor)wouldtypicallybearound100-200mg/ldependingonwatercharacteristicsandloadingrate.Thisisdramaticallylowerthatinanactivatedsludge/membranereactorandifparticulatefoulingisanimportantfoulingmechanism,thereisapotentialwiththisprocesscombinationforefficientbiomassseparationbymembranefiltrationwhilemanagingparticulatefouling.Bycombiningacompactbiofilmprocesswithefficientparticleseparationstablehighqualityeffluentcanbeachieved.Membraneseparationafteramovingbedbiofilmreactoroffersthereforeoutstandingcontroloftheeffluent.Theobjectiveofthisstudyhasbeentoinvestigatetheeffectofloadingratesandmembranefoulingontheperformanceofamovingmedbiofilmmembranereactorforthetreatmentofmunicipalwastewater.Inparticular,potentialfoulingbytheparticulatematterhasbeenevaluatedbytrackingvariationsinparticlesizedistributionsthroughouttheprocess.Ahighandlowloadingrateonthebiofilmreactorwastestedwheretheperformanceofthemembraneseparationunitwasdefinedbyfoulingratesmeasuredforthegivenoperatingconditions.MATERIALANDMETHODSExperimentalsetupTheschematicofthetreatmentprocessandexperimentalconfigurationispresentedinFigure1.Municipalwastewaterfromacombinedsewersystemisfedtoapilotscalemovingbedbiofilmmembranereactor(MBB-M-R).Thewastewaterispretreatedusingasmallgravitysettlerandisthenpumpedfromanoverflowintothemovingbedbiofilmreactors(MBBR).Theoutletofthebioreactorsissubsequentlyledintothemembranereactortankfromwherepermeateisextractedthroughthemembraneundervacuum.Asmallvolumeoftheconce