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Availableatficationinmoving-bedbiofilmreactorsRobertaSalvetti,AriannaAzzellino,RobertoCanziani,LucaBonomoD.I.I.A.R.EnvironmentalEngineeringDepartment,PolitecnicodiMilano,TechnicalUniversityofMilan,P.zaLeonardodaVinci,32-20133Milano,ItalyarticleinfoArticlehistory:Received31August2005Receivedinrevisedform4May2006Accepted9May2006Availableonline13July2006Keywords:NitrificationTemperaturePure-oxygenmoving-bedbiofilmreactor(PO-MBBR)AmmonialimitationOxygenlimitationMultivariatetechniquesABSTRACTTheeffectofwastewatertemperatureontherateofnitrificationwasstudiedintwopure-oxygenmoving-bedbiofilmreactors,fedonsecondaryeffluentfromamunicipalwaste-watertreatmentplant.ThefirstReactor(R1)wasoperatedunderammonia-limitingconditions,whilethesecondReactor(R2)wasoperatedunderoxygen-limitingconditions.Quitesurprisingly,theformershowedanegligibleinfluenceofthermalchangesonnitrificationrates,whilethelattershowedamuchhigherdependence.Inthispaper,atemperaturecoefficient‘‘y’’hasbeendefinedastheactual‘‘intrinsic’’biologicaltemperaturecoefficient,similartothecorrespondingcoefficientthatisusuallyadoptedforthedesignofactivated-sludgeprocesses.Inaddition,an‘‘apparent’’coefficientyahasbeenquantifiedindependently,whichwascalculatedaccordingtotheactualvaluesofnitrificationratesatdifferenttemperatures.Theactualbiologicaltemperaturecoefficient‘‘y’’,rangedbetween1.086and1.109(averagevalue1.098)underammonia-limitingconditions,whileunderoxygen-limitingconditionswasintherange1.023–1.081(averagevalue1.058).Theapparentvalueyawasneartounity(i.e.notemperatureeffect)underammonia-limitingconditions,whileonlyunderoxygen-limitingconditionsandatconstantdissolvedoxygenconcentration‘‘ya’’coincidedwith‘‘y’’.Anexplanationwasgiventhat,underoxygen-limitingconditions,thespecificbiomassactivity(i.e.theratioofnitrificationratetobiomassconcentration)wasstronglyinfluencedbythecombinedeffectsofoxygenpenetrationthroughthebiofilmandeffluenttemperature.&2006ElsevierLtd.Allrightsreserved.1.IntroductionMoving-bedbiofilmprocesseshaveprovedtobeveryreliablefortertiarynitrificationbecauseofthehighvolumetricloadingratesthatcanbeappliedandthelowsolidsbuild-upinthereactor.Totreatagivenvolumeofwastewater,thecapacityofamoving-bedbiofilmreactor(MBBR)canbesmallerthanrequiredforaconventionalactivated-sludgeprocessand,usually,thereisnoneedforatertiarysettlingtank.Comparedtofixed-bedbiofilmreactors(biofilters),MBBRshavemuchlowerheadlosses,filter-bedchannellingdoesnotoccur(i.e.allthebioreactorvolumeisused)andperiodicbackwashingisnotneeded.Moreover,existingconcretetankscanbeequippedandadaptedtoaMBBRconfigurationwithrelativelyminormodifications.MBBRsareusuallyfilledwithlow-density(slightlylessthan1.0gcm3)polyethyleneKMTsbiofilmcarriers.Onesuchcarrierconsistsofsmallcylindricalelements10mmindiameterand8mminheight,withsmalllongitudinalfinsthatprotrudeontheoutsidesurfaceandaninternalcrossmemberthatdivideseachelementintofourcircularsectors.Thevoidratioofthesupportmediaisashighas0.95withtheARTICLEINPRESS0043-1354/$-seefrontmatter&2006ElsevierLtd.Allrightsreserved.doi:10.1016/j.watres.2006.05.013Correspondingauthor.Tel.:+390223996431;fax:+390223996499.E-mailaddress:roberta.salvetti@polimi.it(R.Salvetti).WATERRESEARCH40(2006)2981–2993effectthat,inatankfilledwithwaterandcarriers,thevolumeofwateris95%.Thefillingratioisdefinedastheratiobetweenthevolumesoccupiedbythecarriers,consideredassolidcylinders,andthetotaltankvolume.Itsmaximumvalueforgoodmixingis0.7.Thetheoreticalspecificsurfaceareaofthesupportmediaisdefinedastheamountofsurfaceareaperunitvolumeofbiofilmcarrier,andforthemediadescribedis700m2m3(ØdegaardandRusten,1993).Sincethebiofilmgrowsintheprotectedinternalfacesofthemedia,theactualspecificsurfaceareacanbeassumedtobeabout500m2m3(Hemetal.,1994).Intheexperimentsdescribedinthispaper,theactualfillingratiowas0.5,sothatthesurfaceareaavailableforthebiofilmis250m2m3reactor.Theuseofpureoxygeninsteadofairenableshigherdissolvedoxygen(DO)concentrationstobemaintainedinthereactionvessel.Asaconsequence,greatertransferefficiencycanbeachievedandoxygencandiffusemoredeeplyintothebiofilm.Thisproduceshighernitrificationrates,and,conse-quently,smallerreactorvolumes.Ifpureoxygenisusedinthefirststage,theprocesscanbeconvenientlydividedintotwosequentialstages,withaera-tioninthesecondstage.ThefirststagecouldbeoperatedwithahighammoniaconcentrationandDOwouldbecometherate-limitingsubstrate.Sincezero-orderintrinsickineticscanbeassumedfornitrificationinbiofilms,zero-orderkineticscanalsobeassumedwithrespecttothenon-limitingsubstrate,i.e.ammonia.AsfarasDOisconcerned,ithasbeenfoundthatoxygencanbethereaction-limitingsubstrateiftheratioofoxygentoammoniaislowerthan2gO2(gNH4+-N)1(Hemetal.,1994)andthismayhappenevenwhenDOconcentrationishigh(5–10mgL1).Ifthesecondstagehadoperatedatlowammoniaconcen-tration,thenthereactionwouldhaveshiftedtoammonia-limitingconditions,whichwouldoccurwhentheoxygentoammoniaratioishigherthan5gO2(gNH4+-N)1.Ratiosof2–5gO2(gNH4+-N)1shouldbeavoidedsincet