TheDevelopmentofaBiofilmMembraneBioreactor

Desalination202(2006)135–143PresentedattheconferenceonWastewaterReclamationandReuseforSustainability(WWRS2005),November8–11,2005,Jeju,Korea.OrganizedbytheInternationalWaterAssociation(IWA)andtheGwangjuInstituteofScienceandTechnology(GIST).0011-9164/06/$–Seefrontmatter©2006ElsevierB.V.Allrightsreserved.doi:10.1016/j.desal.0000.00.000ThedevelopmentofabiofilmmembranebioreactorTorOveLeiknes*,HallvardØdegaardNTNU—NorwegianUniversityofScienceandTechnology,DepartmentofHydraulicandEnvironmentalEngineering,S.P.Andersensvei5,N-7491Trondheim,NorwayTel.+477359-4758;Fax[phone];email:[e-mail]31July2005;accepted23December2005AbstractMembranebioreactors(MBRs)arecommonlyunderstoodasthecombinationofmembranefiltrationandbiologicaltreatmentusingactivatedsludge.Developmentofabiofilm-MBRhasbeeninvestigatedcombiningamoving-bed-biofilmreactorwithasubmergedmembranebiomassseparationreactor.Treatmentefficiencieswerefoundtobehighwiththeproductionofaconsistenthigh-qualityeffluent,irrespectiveofloadingratesonthebioreactorormembranereactoroperatingmodes.Membraneperformance(fouling)isafunctionofthebiofilmreactoreffluentqualityandvarieswithloadingrates(HRT).Sustainableoperationwasfoundtocorrelatetothefateofthesubmicronparticlesizefractionthroughoutthetreatmentprocess.Keywords:Membranebioreactors;Biofilm;Membranefouling1.IntroductionMembranebioreactors(MBR)arecommonlyunderstoodasthecombinationofmembranefiltrationandbiologicaltreatmentusingactivatedsludge(AS)wherethemembraneprimarilyservestoreplacetheclarifierinthewastewatertreat-mentsystem[1,2].ThefirstgenerationofMBRs(late1970s,1980s)appliedtheuseofcross-flowoperatedmembranesinstalledinunitsoutsidetheAStankwithhighflowvelocitycirculationpumps.Adisadvantageofthecross-flowmembranesisthehighenergyrequiredtogeneratesufficientsludgevelocitiesacrossthemembranesurface,andthisprocessoptionwasthereforeconsiderednonviablefortreatingmunicipalwastewater.Thedevelopmentofsubmergedlowpressureconfigu-rationsinthelate1980s–1990s,byimmersingthemembranesintotheAStank,wasanimportantstepinmakingviablecommercialsolutionsfortheMBRprocess[3–5].Todayavarietyofprocessconfigurationsexistwherethemembraneisinstalledeitherinanexternalunitorimmersedintheaerationtankandwherethesystemsaredesignedtobeoperatedunderlow-pressurevac-uum.ComparedtoconventionalASsystems,*Correspondingauthor.136T.Leiknes,H.Ødegaard/Desalination202(2006)135–143severaladvantagesoftheAS-MBRshavebeenidentified[6]whichhavepromotedthedevelop-mentofcommercialAS-MBRoptions.Theseincludecompactunitswithsmallfootprints,completesolidsremoval,effluentdisinfection,operationathighersuspendedbiomassconcentra-tionsresultinginlongsludgeretentiontimes,lowsludgeproduction,andnoproblemswithsludgebulking.OneofthemajordrawbacksofAS-MBRsisfouling,whichiscommonforallmembranesystems,wheretheefficacyoftheprocessisconstrainedbytheaccumulationofmaterialsonthesurfaceoforwithinthemembraneresultinginareductioninthemembranepermeability.Membranefoulingiscausedbydifferentsub-stancesandthemechanismsarerathercomplexandinterrelated.Depositionofsolidsasacakelayer,poreplugging/cloggingbycolloidalparticles,adsorptionofsolublecompoundsandbiofoulingaresomeofthemainformsoffoulingthathavebeenidentified[7].FoulingisparticularlyaprobleminAS-MBRssincetheprocessdealswithliquorshavinghighconcentrationsoftotalsolidsaswellasdissolvedcompoundssuchasextracellularpolymericsubstances(EPS).Foul-ingisdefinedasreversible,i.e.canberemovedbybackwashingstrategies,orasirreversible,i.e.foulingwhichisonlyrecoverablebychemicalcleaning,wherethedominatingfoulingmecha-nismsubsequentlydeterminestheperformanceoftheprocess.OptimizingfoulingcontrolandcleaningstrategiesisthereforeanimportantaspectofdevelopinganddesigningMBRprocesses.AnalternativetotheAS-MBRiscombiningabiofilmreactorwithmembraneseparationofthesuspendedsolids(BF-MBR)whichmayreducetheeffectofmembranefoulingbyhighbiomassconcentrations[8,9].Althoughefficientinremovingsolubleorganicmatter,biofilmreac-torsdesignedastricklingfiltersorsubmergedfiltersusinggranularmediaarepronetocloggingwhenthewastewatercontainshighloadsofparticulatematter.Consequently,thereisalimittotheloadingratethatcanbeappliedtosuchprocesses,oftennecessitatingapretreatmentstepforparticleremovalpriortothebiofilmunit.Themoving-bed-biofilmreactor(MBBR)isanalternativeprocessdesignwhichutilizestheadvantagesofabiofilmreactorandwhichatthesametimecanhandlehighloadsofparticles.TheobjectiveofthisstudyhasbeentodevelopandinvestigatethepotentialsofaBF-MBRcom-biningtheMBBRwithmembraneseparation.2.MaterialsandmethodsAschematicoftheBF-MBRprocessconceptisshowninFig.1,wherethetreatmenttrainconsistsoftworeactors,theMBBRfollowedbyamembranereactorwithsubmergedmodulesandtheprocesscanthereforebedefinedasaFig.1.Schematicofbiofilm-MBRprocessconcept.T.Leiknes,H.Ødegaard/Desalination202(2006)135–143137biomassseparationmembranebioreactor.Bydividingthetworeactorsintoseparateentities,eachprocessstepcanbedesignedandoperatedatoptimalconditions.Forthepilotplantsinves-tigatedinthesestudies,municipa