Comparisonbetweenhybridmovingbedbiofilmreactoranda

Comparisonbetweenhybridmovingbedbiofilmreactorandactivatedsludgesystem:apilotplantexperimentDanieleDiTrapani,GiorgioMannina,MicheleTorregrossaandGaspareVivianiABSTRACTDanieleDiTrapaniGiorgioMannina(correspondingauthor)MicheleTorregrossaGaspareVivianiDipartimentodiIngegneriaIdraulicaedApplicazioniAmbientali,Universita`diPalermo,VialedelleScienze,90128Palermo,ItalyE-mail:[e-mail]parisonbetweenthetraditionalactivatedsludgesystem(AS)andahybridmovingbedbiofilmreactor(HMBBR).Inparticular,anexperimentalcampaignhasbeencarriedoutattheWWTPinPalermo(Italy),onapilotplantconsistingoftwopre-anoxicschemes.TheaeratedtankoftheHMBBRlinewasfilledwithsuspendedcarriers(AnoxKaldnesYK1),witha30%fillingratio.ThehydraulicloadoftheHMBBRlinewasincreaseduptotwotimestheASone.Further,inordertodistinguishtheadditionalcontributionoftheattachedbiomassfortheHMBBRsystem,inthetwolinestheMixedLiquorSuspendedSolids(MLSS)wasmaintainedasequalaspossible.Themonitoringperiodlastedthreemonthsduringwhichseveralparametersweremonitored.TheobtainedresultsshowedagoodtreatmentabilityoftheHMBBRsystem,referringtotheorganicmatterremovalaswellastotheammoniumremoval.Inparticular,inspiteoftheincreaseofthehydraulicloadfortheHMBBRline,thetwosystemsshowedasimilarperformanceintermsofbothorganicandnitrogenremoval.TheresultsdemonstratethehighertreatmentcapacityoftheHMBBRaddressingsuchsystemasaneffectivetechnologyfortheupgradingofoverloadedwastewatertreatmentplants.Keywords|biologicaltreatment,HMBBR,hybridbiofilmreactors,municipalwastewater,nitrification,organicandnitrogenloadINTRODUCTIONRecentlytherehasbeenanincreaseoftheurbanizationtogetherwiththeevenmorestricteffluentlimitsimposedattheoutletofmunicipalWastewaterTreatmentPlant(WWTP)bytheWaterFrameworkDirective(Chave2001).Inaddition,therehasbeenalsoanincreasinglylimitedareafortheconstructionofnewplants;thesefactorshaveoftenledtothenecessityofupgradingofexistingWWTP.ThesecondarytreatmentoftheWWTPisusuallyaccom-plishedbybiologicalprocessesthatcanbeclassifiedasbeingeithersuspendedorattachedgrowth.Theconventionalandmostlyusedsuspendedgrowthsystemisrepresentedbytheclassicalandwellknownactivatedsludgeprocess(AS).Indeed,thisprocesscanpresentsomeshortcomingswhenexposedtoincreasedhydraulicandorganicloads.ToincreasetheperformancesofanexistingASsystemitwouldbenecessarytoincreasetheamountofbiomassinsidetheaerobicreactor;however,insuchacase,themajorWWTPlimitationisrepresentedbythefollowinguniti.e.thesettlingtank.Inthelastyears,theideatocombinethetwodifferentprocesses(attachedandsuspendedbiomass)byaddingbiofilmcarriers,usuallyplasticcarriers,intotheaerationtankforbiofilmattachmentandgrowthhasbeenproposed.ThiskindofsystemisusuallyreferredasIFAS(IntegratedFixed-filmActivatedSludge)process(Randall&Sen1996;Sriwiriyarat&Randall2005;Sriwiriyaratetal.2005).Inthesesystemsthebiomassgrowsbothassuspendedflocsandasattachedbiofilm.Inthisway,itispossibletoobtainahigherdoi:10.2166/wst.2010.834891QIWAPublishing2010WaterScience&Technology—WST|61.4|2010biomassconcentrationintheaerobicreactor,butwithoutanysignificantloadincreasetothefinalclarifier.Therefore,theup-gradingofoverloadedexistingplants,nolongerabletomeettheeffluentlimits,canbeeasilyobtainablewithouttheconstructionofnewtanks.Furthermore,theincreaseoftheoverallsludgeageinthesystemleadstoafavourableenvironmentforthegrowthofnitrifyingbacteria(Randall&Sen1996).Asamatteroffact,severalstudieshavedemonstratedthatIFASprocesscanbeanalternativedesignforbiologicalnitrogenremovalandasacost-effectoptionforretrofittingWWTPstosustainnitrificationthroughoutthewinter(Sriwiriyarat&Randall2005).Inthelastyearsmanystudieshavebeencarriedoutonhybridsystems,withtheaimofinvestigatingtheprocessperformancesandalsotocomparedifferentcarriermedia,obtainingveryinterestingresultsandhighlightingtheeffectivenessofsuchsystemsbothforcarbonandnitrogenremoval(Morper&Wildmoser1990;Mu¨ller1998;Gebara1999;Hamoda&Al-Sharekh2000;Mu¨nchetal.2000;Germainetal.2007).Concerningthecarriermediathatisaddedforthegrowthoftheattachedbiomassitcanbefixedorfreelymovinginsidethereactor.Inthislattercase,whenthemediaisusedonitsown,theprocessisusuallycalledmovingbedbiofilmreactor(MBBR)(Ødegaard2006;Germainetal.2007).Morespecifically,intheMBBRprocessthebiofilmgrowsattachedonsmallcarrierelements,keptinconstantmotionthroughouttheentirevolumeofthereactor(Ødegaardetal.1994;Rustenetal.1995a,b;Ødegaard2006).Thecarriersarekeptwithinthereactorthroughasievearrangementatthereactoroutlet.ThetypicaladvantagesofMBBRsystemsisthelowheadloss,nofilterchannellingandnoneedofperiodicbackwashing(Pastorellietal.1999).Whenusedinahybridprocess,thesuspendedcarrierscanbekeptinthewholeorinapartoftheactivatedsludgevolume,dependingonthemainaimoftreatment.ThedifferencebetweenMBBRandIFASreliesonthepresenceofthereturnactivatedsludge(RAS).Morespecifically,whenevertheRASisreturnedtothetankwiththecarriersthesystemisreferredasIFASotherwiseisaddressedasMBBR.Nevertheless,recentlyinthecaseofmoveablecarriermediaIFASshavebeenaddressedasHMBBR(HybridMovingBedBiofilmReactors)process(Ma