TheMovingBedBiofilmReactor

Igarashi,T.,Watanabe,Y.,Asano,T.andTambo,N.:WaterEnvironmentalEngineeringandReuseofWater,HokkaidoPress1999,p.250-305TheMovingBedBiofilmReactorH.ØdegaardFacultyofCivilandEnvironmentalEngineering,NorwegianUniversityofScienceandTechnology,N-7034Trondheim,NorwayAbstractAnewbiofilmreactorforwastewatertreatment,themovingbedbiofilmreactor(MBBR),isdiscussed.Ageneraldescriptionofthereactorisgiven.Resultsfrominvestigationsofdifferentapplications(carbonaceousremoval,nitrificationandnitrogenremoval)whenusedformunicipalwastewatertreatment,arediscussed.Designvaluesaregivenanditisdemonstratedthatuseofthisreactorresultsinverycompacttreatmentplants.Keywords:Wastewater,biologicaltreatment,biofilmreactorINTRODUCTIONOverthelastdecadestherehasbeenagrowinginterestinbiofilmprocessesforwastewatertreatment.Thereareseveralreasonsforthefactthatbiofilmprocessesmoreandmoreoftenarebeingfavouredinsteadofactivatedsludgeprocesses,suchas:a.Thetreatmentplantrequireslessspaceb.Thefinaltreatmentresultislessdependentonbiomassseparationsincethebiomassconcentrationtobeseparatedisatleast10timeslowerc.Theattachedbiomassbecomesmorespecialised(higherconcentrationofrelevantorganisms)atagivenpointintheprocesstrain,becausethereisnosludgereturnTherearealreadymanydifferentbiofilmsystemsinuse,suchastricklingfilters,rotatingbiologicalcontactors(RBC),fixedmediasubmergedbiofilters,granularmediabiofilters,fluidisedbedreactorsetc.Theyhavealltheiradvantagesanddisadvantages.Thetricklingfilterisnotvolume-effective.MechanicalfailuresareoftenexperiencedwiththeRBC's.Itisdifficulttogetevendistributionoftheloadonthewholecarriersurfaceinfixedmediasubmergedbiofilters.Thegranularmediabiofiltershavetobeoperateddiscontinuouslybecauseoftheneedforbackwashingandthefluidisedbedreactorsshowhydraulicinstability.Forthesereasonsthemovingbedbiofilmprocess(Eur.pat.no.0575314,USpat.no.5,458,779)hasbeendevelopedinNorwayduringthelast10years(Ødegaardetal,1994,Ødegaardetal,1998a).Therearepresentlymorethan90treatmentplantsbasedonthisprocessinoperationorunderconstructionin17differentcountriesallovertheworld.Theyareusedformanydifferentpurposesformunicipalaswellasindustrialwastewatertreatment,likeorganicmatterremoval,nitrificationandnitrogenremoval.Inthispaperweshallfocusonthemunicipalapplications.2DESCRIPTIONOFTHEMOVINGBEDBIOFILMREACTOR(MBBR)Theideabehindthedevelopmentofthemovingbedbiofilmprocesswastoadoptthebestfromboththeactivatedsludgeprocessandthebiofilterprocesseswithoutincludingtheworst.Contrarytomostbiofilmreactors,themovingbedbiofilmreactorutilisesthewholetankvolumeforbiomassgrowth,asdoesalsotheactivatedsludgereactor.Contrarytotheactivatedsludgereactor,itdoesnotneedanysludgerecycle,asisalsothecaseinotherbiofilmreactors.Thisisachievedbyhavingthebiomassgrowoncarriersthatmovefreelyinthewatervolumeofthereactor,keptwithinthereactorvolumebyasievearrangementatthereactoroutlet.Sincenosludgerecirculationtakesplace,onlythesurplusbiomasshastobeseparated-aconsiderableadvantageovertheactivatedsludgeprocess.Thereactormaybeusedforbothaerobic,anoxicoranaerobicprocesses,seefigure1.a.Aerobicreactorb.Anoxicandanaerobicreactorc.Thebiofilmcarrier(K1)Fig.1.TheprincipleofthemovingbedbiofilmreactorandtheshapeofthebiofilmcarrierInaerobicprocesses,thebiofilmcarriermovementiscausedbytheagitationsetupbytheair,whileinanoxicandanaerobicprocessesamixer(normallyahorizontalshaftmountedbananamixer)keepsthecarriersmoving.Intheaerobicreactorsaspecialcoarsebubbleaerationsystemhasbeendeveloped.Thecarriersarekeptwithinthereactorbyanoutletsieve.Mostoftheplantsaredesignedwithverticallymounted,rectangularmeshsieves,butitissometimesshapedasacylindricalbarsieve,verticallyorhorizontallymounted.Thebiofilmcarrier(K1)ismadeofhighdensitypolyethylene(density0,95g/cm3)andshapedasasmallcylinderwithacrossontheinsideofthecylinderand“fins”ontheoutside(seefigure1c).Thecylinderhasalengthof7mm,andadiameterof10mm(notincludingfins).Latelyonehasintroducedalsoalargercarrier(K2)ofsimilarshape(lengthanddiameterabout15mm),intendedforuseinplantswithcoarseinletsieves.Oneoftheimportantadvantagesofthemovingbedbiofilmreactoristhatthefillingofcarrierinthereactormaybesubjecttopreferences.Thestandardfillingdegreeis67%,resultinginatotal,specificcarrierareaof465m2/m3.Sincethebiomassisgrowingprimarilyontheinsideofthecarrier,oneiscalculatingwithaneffectivespecificsurfaceareaof335m2/m3fortheK1carrierand235m2/m3forthelargerK2carrier,at67%filling.Inordertobeabletomovethecarriersuspensionfreelyitisrecommendedthatfillingdegreesshouldbebelow70%(correspondingto350m2/m3effectivespecificareaforK1).Onemay,however,useasmuchasneededbelowthis,whichisconvenient,especiallywhenupgradingplants–forinstancefromactivatedsludgetomovingbedreactors.3Therateexpressionnormallyusedinbiofilmprocessesisbasedonbiofilmcarrierarea(g/m2d).Becauseofsomeuncertaintywithrespecttohowmuchoftheavailablecarrierareathatisinfactcoveredbybiofilmandbecauseofeasyratecomparisonwithotherbiofilters,thevolumetricrates(g/m3reactorvolumed)havebeenusedearlierforthemovingbedreactor.Ithasbeendemonstrated,however