BioresourceTechnologyjournalhomepage:www.elsevier.com/locate/biortechOzonationstrategiestoreducesludgeproductionofaseafoodindustryWWTP
J.L.Campos*,L.Otero,A.Franco,A.Mosquera-Corral,E.Roca
DepartmentofChemicalEngineering,SchoolofEngineering,UniversityofSantiagodeCompostela,LopeGómezdeMarzoas/n,E-15782SantiagodeCompostela,Spainarticleinfoabstract
Inthiswork,severalalternativesrelatedtotheapplicationofozoneindifferentstreamsofaseafoodindustryWWTPwereevaluatedtominimizetheproductionofwastesludge.TheWWTPwascomposedoftwocoagulation-flocculationunitsandabiologicalunitandgeneratedaroundof6550kg/dofsludge.Ozonewasappliedtosludgecomingfromflotationunits(110gTSS/L)atdosesupto0.03gO3/gTSSdur-ingbatchtests,nosolidssolubilizationbeingobserved.Ozonedosesrangingfrom0.007to0.02gO3/gTSSwerealsoappliedtotherawwastewaterinabubblecolumnreachinga6.8%ofTSSremovalforthehighestozonedose.Finally,theeffectofthepre-ozonation(0.05gO3/gTSS)ofwastewatercomingfromthefirstflotationunitwastestedintwoactivatedsludgesystemsduring70days.Ozonationcausedareductionoftheobservedyieldcoefficientofbiomassfrom0.14to0.07gTSS/gCODTremovedandaslightimprovementofCODremovalefficiencies.Onthebasisofthecapacityforozoneproductionavailableintheindustry,amaximumreductionofsludgegeneratedbytheWWTPof7.5%couldbeexpected.Ó2008ElsevierLtd.Allrightsreserved.Articlehistory:Received22May2008Receivedinrevisedform24July2008Accepted29July2008Availableonline1October2008Keywords:ActivatedsludgesystemExcess-sludgeOzoneWastewater1.IntroductionExcess-sludgetreatmentanddisposalsupposesbetween50and60%ofoperatingcostsofwastewatertreatmentplants(WWTPs).InEurope,theproductionofsludgereached10billionskgin2005andthisproductionwillincreaseduetothefactthatwaste-watertreatmentisexpandingbythebuildingofnewWWTPsandtheupgradingofexistingplantsbecauseofmorestringenteffluentcriteria(PaulandDebellefontaine,2007).MainalternativemethodsforsludgedisposalinEUarelandapplication(45%),landfill(38%)andincineration(17%)(Ginestet,2007).Landapplicationofsludgeisrestrictedtoavoidhealthrisksduetothepresenceofheavymetalsorpathogenswhilebuildingandoperatingcostsoflandfillareincreasingduetothelowerlandavailabilityandthestricterregulation.Incinerationmayalsobethefinaloptionforsludgedisposalbutthisprocessgeneratesashanddischargesheavymetalstotheatmosphere(BaloghandNollet,2008).Therefore,anaffectivestrategytoreducesludgedisposalcostscouldbetominimizeitsproductionwithintheWWTP.Technologiesforreducingsludgegeneratedfrombothprimaryandbiologicaltreatmentsaregainingattention(Ødegaard,2004;Benabdallahetal.,2007;LowandChase,1999;LiuandTay2001;Weietal.,2003).Anaerobicdigestionisthemostusedpro-cesstoreducesludgefromtheprimarysettlingtogetherwiththebiologicalexcess-sludge.However,enzymatichydrolysisofsolidsisthestepwhichlimitsthesolidsdegradationefficiency.Forthisreason,manyresearchershavefocusedtheireffortstoapplysev-eralpre-treatmentmethodstoimprovetherateofhydrolysis,thusenhancingsolidsremoval(Goeletal.,2003a;Bougrieretal.,2006;Sieversetal.,2004).Bougrieretal.(2006)studiedtheeffectsofultrasounds,ozonationandthermalpre-treatmentandfoundthatthermaltreatmentwasthemostefficientintermsofsolidssolubi-lizationandimprovementofbiogasproduction.Minimizationoftheexcess-sludgecomingfromthebiologicalreactormaybeachievedbyeitherchangesinoperationalcondi-tions(Extendedaeration,membranebioreactor,improvedaera-tion,chemicaluncoupler...)orbytreatmentoftherecycledactivatedsludge(Acidorbasehydrolysis,thermaltreatment,ozon-ation,mechanicaldisintegration...)(MahmoodandElliott,2006).Changesinoperationalconditionsaremainlyfocusedonpromot-ingdisappearanceofsuspendedorganicmatterbymechanismslikemaintenance,endogenousrespiration,anddecayofcellsorgrazingbyhigherorganismswhilethetreatmentofrecycledacti-vatedsludgeisfocusedonpromotinglysisofbiomassbyadverseenvironmentalconditions(pH,oxidantconditionsortemperature).Recentlyatechnicalandeconomicalevaluationofvariousroutestoallowwastewatertreatmentplantstoproducelesssludgeshowedthatozonationisthemostfeasibletechnologytoachievehighreductionpercentagesofsolids(Ginestet,2007).Differentstrategiesforozonetreatmentcanbeapplied(PaulandDebelle-fontaine,2007):(1)Suspendedsolidsmaybedirectlypumpedfromthebiologi-calunittowardsanozonationcontactorforpartialoxidationandthenpumpedbacktothebiologicaltankforfurtherbio-logicaldegradation.*Correspondingauthor.Tel.:+34981563100x16777;fax:+34981528050.E-mailaddress:joseluis.campos@usc.es(J.L.Campos).0960-8524/$-seefrontmatterÓ2008ElsevierLtd.Allrightsreserved.doi:10.1016/j.biortech.2008.07.0561070J.L.Camposetal./BioresourceTechnology100(2009)1069–1073(2)Ozonationcanbeusedtoimprovethebiodegradabilityofthesupernatantproducedbythesludgetreatmentlinebeforerecyclingittothebiologicalunit.(3)Ozonationcanbeappliedtothesludgeexitingfromtheset-tlerbeforerecyclingittothebiologicaltreatmentunitorbeforeanaerobicdigestion.(4)Suspendedsolidsmaybepumpedfromtheanaerobicdiges-tertowardsanozonationcontactorandthenrecyclingthemtothedigester.Goeletal.(2003b)observedanincreaseofsolidsremovaleffi-ciencyfrom25–35%to50–59%duringanaerobicdigestionwhenapre-ozonationof0.05gO3/gTSSwasapplied.Asimilarimprove-mentwasfoundwhenozonationwasusedaspost-treatmentofanaerobicdigestion(Goeletal.,2003c).However,whenadoseof0.05gO3/gTSSwasappliedtotherecycledactivatedsludgehighersolidsreductionswerereported(Sieversetal.,2004)andazerosludgeproductionsystemcouldevenbeachievedwithadoseof0.1gO3/gTSS(Songetal.,2003).Although,ozonationcausesadecreaseintheVSS/TSSratiooftrea-tedsludgenoaccumulationofinorganicsolidsintheaerationtankwasobserved(Sakaietal.,1997).Nevertheless,sludgeozonationmaycauseaslightincreaseoftotalorganiccarbonandsuspendedsol-idsinthefinaleffluent(Saktaywinetal.,2006;Sakaietal.,1997).MostworksfocusedonminimizingsludgebyozonationweredoneinmunicipalWWTPswhilelimitedinformationisavailableontheefficiencyofthistechnologyappliedtoWWTPstreatingindustrialwastewater.Forthisreason,thisworkwasfocusedonstudyingthefeasibil-itytoreduceexcess-sludgeproductiongeneratedinaseafoodindustryWWTP.ThisWWTPwascomposedoftwocoagulation-flocculationunitsandabiologicalunitandproducedaboutof6550kg/dofsludgewhileozonewasalreadygeneratedwithintheindustryfordisinfectionprocesses.Inafirststep,thedifferentstreamsoftheWWTPweremonitoredtodeterminetheircharac-teristicsandtoquantifytheamountofsludgegenerated.Later,theapplicationofozoneindifferentstreamsoftheWWTPwasevaluatedatlabscaleinordertodefinethebestoptiontobestud-iedatfullscale.2.Methods2.1.Batchozonationtests2.1.1.OzonationofsludgefromflotationunitsOzone(IberOzonoMod.St.8G)wasappliedtosludgecomingfromflotationunits(110gTSS/L)atdosesof0.001gO3/gTSS(pH5)and0.035gO3/gTSS(pH5and8).Experimentswerecarriedoutinglassbeakersof2Lwithmechanicalagitation(200rpm)tomaintaincompletemixing.Theairflowrateappliedwas0.6L/minwithanozoneinletconcentrationof16mgO3/Lair.(1Atm,20°C).Afinebubblediffuserwasemployedtopromotetransferofozonetotheliquidphase.Batchtestsweredoneintriplicate.2.1.2.OzonationofrawwastewaterOzonedosesof0.007,0.013and0.02gO3/gTSSwereappliedtorawwastewater(5.5gTSS/L)inabubblecolumnwithaworkingvolumeof1L.Airwithanozoneconcentrationof14mgO3/Lair(1Atm,20°C)wassuppliedatflowrateof0.6L/mintothecolumnthroughaporousplatelocatedatthebottom.Dosesweremodifiedbyvaryingthecontacttime.Eachdosewasassayedintriplicate.2.2.ContinuousoperationTwoactivatedsludgesystems,consistingofamixingbasinwithaworkingvolumeof2Lcoupledtoanexternalsettlerof1L,wereusedtotesttheeffectofozoneapplicationtostreamcomingfromthefirstflotationunitoftheWWTP.Acontrolreactor(R1)wasdi-rectlyfedwiththisstreamwhileasecondreactor(R2)wasfedwiththesamestreampreviouslyozonated(0.05gO3/gTSS)(Table1).Aperistalticpumpwasusedtofeedeachreactorandanairliftpumptorecyclesludgefromthesettlertotheaerationbasin.Bothactivatedsludgesystemswereoperatedatahydraulicretentiontimeof1dayandatroomtemperature(18–25°C).IntheaerationbasinpHvaluerangedfrom7.5to8.5anddissolvedoxygenconcentrationwashigherthan2mgO2/L.ThesystemswereinoculatedwithactivatedsludgecollectedfromtheaerobicreactoroperatinginamunicipalwastewatertreatmentplantplacedinBertamiráns(ACoruña,Spain).Wastesludgewasperiodicallywithdrawnfrombothsystemsinordertomaintainasimilarconcentrationofbiomassandagoodqualityoftheeffluentwithregardstosolidsconcentration.Anoverallmassbalanceappliedtothewholeoperationalperiodwascarriedouttodeterminethefateofsolids(Eq.(1)):InletSolids–OutletSolids–Solidsaccumulated–Solidspurged=Solidsdegraded(gTSS)FinfÁXTSSinfiÁDtiÀFefÁXTSSefiÁDtiÀDTSSreactorÁVreactorXiiÀTSSpurgeiÁVpurgei¼Solidsdegradedð1Þibeing:Finf,Fef:Influentandeffluentflowrate(L/d)TSSinfTSSef,TSSpurge:ConcentrationofTSSininfluent,effluentandpurgestreams(gTSS/L)Dti:Durationofperiodi(d)DTSSreactor:IncreaseofTSSinthesystemduringthewholeoper-ationalperiod(gTSS/L)Vreactor:Volumeofsystem(L)Vpurge:Volumepurgedfromthesystem(L).2.3.BiodegradabilitybatchtestsBiodegradabilityofbothinfluentandeffluentofthebiologicalreactoroftheWWTPwasstudiedinbatchtestsbymeasuringthevalueofsolubleCODalongtimeuntilachievingaconstantva-lue.Vialswithaworkingvolumeof500mLcontaining250mLofwastewaterand25mLofsludgecomingfromtheindustrialbio-logicalreactoroftheWWTP(finalbiomassconcentrationof2–2.5gVSS/L)wereused.Batchtestswerecarriedoutat20°CandpHadjustedto7.8withNaOH10N.Highconcentrationsofoxygenweremaintainedbyaeratingwithwetairtoavoidwaterloss.Eachtestwasdoneinquadruplicate.2.4.AnalyticalmethodsAmmonia,nitrate,nitrite,volatilesuspendedsolids(VSS),nonvolatilesuspendedsolids(NVSS),totalsuspendedsolids(TSS)andsludgevolumetricindex(SVI)weredeterminedaccordingtotheStandardMethods(APHA-AWWA-WPCF,1998).Concentra-tionsoftotalorganiccarbon(TOC)andinorganiccarbon(IC)wereTable1
Characteristicsoftheinfluentg/LR1InfluentR2InfluentTS4.98±0.364.55±0.32VS1.48±0.291.35±0.36TSS0.79±0.020.68±0.04VSS0.69±0.010.60±0.02CODT2.84±0.732.79±0.70CODs1.55±0.691.69±0.72J.L.Camposetal./BioresourceTechnology100(2009)1069–10731071measuredwithaShimadzuTOCanalyzer(TOC-5000).TotalandsolubleChemicalOxygenDemand(COD)wasdeterminedbyasemi-micromethod(Sotoetal.,1989).Dissolvedoxygenconcentrationinthereactorwasmeasuredusingaspecificelectrode(AqualiticOX-921)andanIngoldelec-trodeU-455connectedtoapH/mVsensorCrison506wasusedforpHmeasurement.Ozoneconcentrationinthegaswasobtainedusinganon-dispersiveUVspectrophotometer(Cecil,CE2021)at254nm.3.Resultsanddiscussion3.1.MonitoringoftheWWTPInstantaneoussampleswereweeklycollectedduring3monthstodeterminethecharacteristicsofthedifferentstreams.Oncecoarsesolidsareseparatedbyusingagrid,rawwastewateristrea-tedincoagulation/flocculationandflotationunit(A)where70%and90%oftotalCODandsolids,respectively,areremoved(Fig.1).Thestreamcomingfromtheseunitsistreatedbyanaero-bicsubmergedfixedbedreactorwithplasticcarrierswithaneffi-ciencyofonly50%withregardstosolubleCOD.Inordertodeterminewhetherthelowefficiencyobservedwasduetothepresenceofnon-biodegradablecompounds,biodegradabilitybatchtestswithinfluentandeffluentofthebiologicalreactorweredone.DuringtheseassaysareductionofsolubleCODofaround90%wasfoundforbothstreamswhichsuggeststhattheCODremovaleffi-ciencyofthebiologicalsystemmaynotbelimitedbythecharac-teristicsofwastewaterbutbyoperationalproblems.Asnocontrolstrategieswereappliedtoavoidbiofilmovergrowth,carri-erswerecompletelycloggedwhichsuggeststhatthelowefficiencyofsolubleCODremovalmightbeduetoalimitationofoxygentransfer.Theincreaseofsolidsconcentrationofwastewaterfrom0.83to1.23gTSS/Lduringthebiologicaltreatmentcouldbecausedbythecloggingofthesubmergedfixedbed.Sincethesur-faceofthecarrierwastotallycovered,biomassgrowsinsuspen-sionandcannotberetainedinthesystem.Takingintoaccountthataroundof1gCOD/Lisremoved,anincreaseof0.43gVSS/Lisexpected(YH:0.43gVSS/gCODremoved,(Wiesmann,1994)).Theeffluentofthebiologicalreactorisfinallytreatedbyasecondcoag-ulation/flocculationandflotationunit(B),itssolidscontentbeingreducedfrom1.23to0.29gTSS/L.TheaverageoverallefficiencyoftheWWTPwas84%and95%forCODTandTSS,respectively.Sludgeismainlygeneratedinthefirstcoagulation/flocculationandflotationunit(85%),theremainingbeingrelatedtosolidscom-ingfromthebiologicalunit.Sludgeischaracterizedbyahighcon-tentoforganiccompounds(95%)andawatercontentof89%.Finally,thewatercontentisreducedto76%byacentrifuge.3.2.OzonationofsludgecomingfromflotationunitsTakingintoaccounttheamountofozonegeneratedintheindustry,adoseof0.007gO3/gTSScouldbeappliedtosludgecomingfromflotationunits.FirstozonationassayswereperformedatpH=5.5andbyapplyinganozonedoseof0.001and0.035gO3/gTSS;neithersolidsnorCODreductionwasobserved.Asec-ondcampaignwasthenconductedatpH=8,atthehighestozonedoseonly;similarresultswereobtainedindicatingthat,withinthetestedrange,pHvariationsdidnotaffectsolidsremoval.OzonehasbeensuccessfullyusedtoreducesludgeproductionofWWTPs(Scheminskietal.,2000;Saktaywinetal.,2004;Goeletal.,2004).Inaerobicsystems,ozonationisgenerallycarriedoutinthereturnsludgelinewheresolidsconcentrationsare4–8gTSS/L(Ahnetal.,2002;Songetal.,2003).O3canbealsousedtoimprovesolidshydrolysisasapre-orpost-treatmentofanaer-obicdigestiontreatingstreamswithsolidscontentaround-CODT: 10809±3621 mg/LCODs: 2187±1207 mg/LTSS: 5.53±2.73g/LN Total: 125±40mg/LNH4+-N: 35±18mg/LCODT: 3070±1697mg/LCODs: 2123±937mg/LTSS: 0.83±0.42g/LN Total: 117±35mg/LNH4+-N: 35±18mg/LInfluent300 m3/dGridHomogenizationtankCoagulation-flocculationunitAFlotationunitVSS/TSS: 0.95±0.02Water content: 89±3%12820 kg/dCODT: 1800±918mg/LCODs: 1190±490mg/LTSS: 0.29±0.15 g/LN Total: 104±48mg/LNH4+-N: 34±10mg/LEffluentBiologicalreactorCODT: 2607±1658mg/LCODs: 1086±703mg/LTSS: 1.23±0.89 g/LN Total: 81±37mg/LNH4+-N: 24±8mg/LCoagulation-flocculationunitBFlotationunitVSS/TSS: 0.94±0.03Water content: 89±5%1470 kg/dCentrifugeWastewaterstreamSludgestreamHomogenizationtankVSS/TSS: 0.95±0.01Watercontent: 76±4%LimeSludge: 6550 kg/dFig.1.FlowsheetoftheWWTPandcharacteristicsofthedifferentstreams.1072J.L.Camposetal./BioresourceTechnology100(2009)1069–107325gTSS/L(Goeletal.,2003c).Inbothcasessolidsconcentrationsarequitelowerthan110gTSS/Lpresentinsludgecomingfromflo-tationunits.Totestwhetherthishighsolidsconcentrationlimitedtheefficiencyofozonationduetoalowgas-liquidtransferrate,as-sayswerecarriedoutwithsludgediluted1:20(5.5gTSS/L)atadoseof0.035gO3/gTSS.Areductionofsolidsconcentrationof17.1%wasobservedwhichwouldconfirmthereactionwaslimitedbythemasstransferrate.Asdilutionofsludgecomingfromflota-tionunitswasnotafeasibleoptionintermsofcostmanagement,ozonationofthisstreamwasruledout.3.3.OzonationofrawwastewaterAnotherpossibleoptiontoreducetheproductionofsludgewastohydrolyzesolidscontainedinrawwastewaterbyapplyingozoneinthehomogenizationtankoftheWWTP.Ozonedosesof0.0070.013and0.02gO3/gTSSweredirectlyappliedtothisstreaminabubblecolumnandsolidssolubilizationpercentagesof2,4.3and6.8%,respectively,wereobtained.TheseresultsaresimilartothoseobservedbyYeometal.(2002)duringbatchozon-ationtestsofsludgefrommunicipalWWTPsatthesamedose.Theseauthorsobservedthattherewaslineartendencybetweenthepercentageofsludgesolubilizedandtheozonedoseupto0.05gO3/gTSS,thenthispercentageasymptoticallyincreasedun-tilamaximumvalueof32.7%at0.5gO3/gTSSwasreached.Dur-ingtheseassaystotalCODconcentrationmaintainedpracticallyconstantwhichagreeswiththoseresultsobtainedbyBougrieretal.(2006)whofoundthatmineralizationphenomenononlyoc-curredforozonedoseshigherthan0.18gO3/gTSS.Inletandoutletozoneconcentrationsingasphaseweremea-suredalongthedifferentexperimentstoknowwhethermasstransferfromgastoliquidwasthelimitingstepoftheprocess.Ozonewasneverdetectedintheouletgas.Suchhighozonecon-sumptionefficiencywasalsofoundbyAhnetal.(2002).Only2%ofsolidswereremovedattheozonedoseavailableintheindustry(0.007gO3/gTSS),therefore,ozonationofrawwaste-watercouldbenotconsideredasuitablestrategyinordertore-ducethesludgeproduction.3.4.OzonationofwastewatercomingfromthefirstflotationunitToreduceexcessbiomassproductionofactivatedsludgesys-tems,ozoneisgenerallyappliedtoafractionoftherecycledacti-vatedsludge(YasuiandShibata,1994;Sakaietal.,1997).SuchstrategyisnotpossibleinthiscasebecausethebiologicalunitofthisWWTPisbasedonattachedbiomasstechnology.Apossiblealternativewouldbetheozonationofthestreamcomingfromthefirstflotationunit.Sincethisstreamcontained0.83gTSS/Land,takingintoaccounttheamountofozonegeneratedintheindustry,adoseof0.05gO3/gTSScouldbeapplied.Twoactivatedsludgesystemswereoperatedduring70days.Acontrolreactor(R1)wasfedwithwastewatercomingfromthefirstflotationunitandasecondreactor(R2)fedwiththesamewastewaterpreviouslyozonated(0.05gO3/gTSS).TheaverageefficiencyoftotalCODremovalwas75.6%forR1and85.5%forR2(Fig.2A).ThehigherefficiencyobservedforR2isre-latedtothelowercontentofsolidsofitseffluent(39mgTSS/L)comparedtothatofthecontrolunit(59mgTSS/L)(Fig.3AandB).R2showedalsoaslightlyhigherefficiencythanR1intermsofsolubleCODremoval:88.4%and90.4%forR1andR2,respec-tively(Fig.2B).Theobtainedefficienciesagreewiththoseex-pectedaccordingtothebiodegradabilitybatchtestsdoneforthiswastewater.Periodicwithdrawalsofsolidswerecarriedouttomaintainsimilarconcentrationsofbiomassinbothsystems(Fig.4).Noaccu-mulationofinorganicsolidswasobservedforR1andR2sincetheratioofVSStoTSSremainedconstantaround0.9duringwholeoperationalperiod.A4000CODT inf(mg/L)35003000250020001500100050000102030405060800BCODT ef(mg/L)CODS inf(mg/L)250020001500100050006006005004003002001000704003002001000102030405060070Time (d)Time (d)Fig.2.ConcentrationoftotalandsolubleCODduringtheoperationalperiod:InfluentR1(j),influentR2(N),effluentR1(h)andeffluentR2(4).ATSSinf(g/L)1.00.80.60.40.20.00.50.40.30.20.10.0BVSS inf(g/L)TSSef(g/L)1.000.800.600.400.200.000.50.30.20.10.0010203040506070010203040506070Time (d)Time (d)Fig.3.ConcentrationofTSSandVSSduringtheoperationalperiod:InfluentR1(j),influentR2(N),effluentR1(h)andeffluentR2(4).VSS ef(g/L)0.4CODS ef(mg/L)700500J.L.Camposetal./BioresourceTechnology100(2009)1069–10731073)8LS/S Tg6( noti4tranecnoc2 sdlioS0010203040506070Time (d)Fig.4.ConcentrationofTSSofR1(j)andR2(h).(ArrowsindicatepurgesofsolidsforR1(;)andR2(\")).Table2
FateofTSSduringthebiologicaltreatmentReactorWashed-out(%)Purged+Accumulated(%)Degraded(%)R18.843.248.0R26.123.570.4AmassbalanceappliedtooveralloperationperiodshowedanimportantincreaseofTSSdegradationwhenozonewasapplied(Table2).Theobservedyieldcoefficientwas0.14and0.07gTSS/gCODTremovedforR1andR2,respectively,whichindicatesthatareductionof50%insludgeproductionispossible.Thisreductionwashigherthanthoseof20–35%observedbyotherauthorswhichappliedadoseof0.05gO3/gTSStoreturnactivatedsludgeofamunicipalWWTP(Sieversetal.,2004;BöhlerandSiegrist2004;Dytczaketal.,2006).Nevertheless,Delerisetal.(2002)achievedareductionof70%atthesamedose.Basisontheseresults,areduc-tionofsludgegeneratedbytheWWTPof7.5%couldbeexpected.4.ConclusionsWhenozonewasappliedtosludgecomingfromtheflotationunits,nosolidsremovalwasobservedduetolimitationofozonetransferratecausedbythehighsludgesolidsconcentration.Ontheotherhand,averylowpercentageofsolidsremovalwasob-tainedwhenrawwastewaterwasozonatedatthedoseavailableintheindustry.Therefore,bothstrategiesofozonationarenotinterestingintermsofcosteffectiveness.Areductionof7.5%ofsludgegeneratedbytheWWTPcouldbeexpectedwhenozoneisappliedtowastewaterpriortothebiolog-icaltreatment.Moreover,theefficiencyofthebiologicalsystemimprovedwithregardstoCODT,CODSandTSSremoval.Theappli-cationofozonetothisstreamisafeasibleoptiontominimizesludgeproductionwhichwouldnotrequireanimportantcapitalcost.AcknowledgementsThisworkwasfundedbytheXuntadeGalicia(PGI-DIT05TAM028E).AuthorswanttothankMarOrge,MónicaDosilandMiriamVieitesfortheirsupportintheanalyticaltechniques.ReferencesAhn,K.H.,Park,K.Y.,Maeng,S.K.,Hwang,J.H.,Lee,J.W.,Song,K.G.,Choi,S.,2002.Ozonationofwastewatersludgeforreductionandrecycling.WaterSci.Technol.46(10),71–77.APHA,AWWA,WPCF,1998.StandardMethodsfortheExaminationofWaterandWastewater,20thed.,AmericanPublicHealthAssociation,Washington,DC.Balogh,S.J.,Nollet,Y.H.,2008.Mercurymassbalanceatawastewatertreatmentplantemployingsludgeincinerationwithoffgasmercurycontrol.Sci.TotalEnviron.389,125–131.Benabdallah,T.,Dosta,J.,Márquez-Serrano,R.,Mata-Álvarez,J.,2007.Effectofultrasoundpre-treatmentinmesophilicandthermophilicanaerobicdigestionwithemphasisonnaphthaleneandpyreneremoval.WaterRes.41,87–94.Böhler,M.,Siegrist,H.,2004.Partialozonationofactivatedsludgetoreduceexcesssludge,improvedenitrificationandcontrolscummingandbulking.WaterSci.Technol.49(10),41–49.Bougrier,C.,Albasi,C.,Delgenes,J.P.,Carrere,H.,2006.Effectofultrasonic,thermalandozonepre-treatmentsonwasteactivatedsludgesolubilisationandanaerobicbiodegradability.Chem.Eng.Process45(8),711–718.Deleris,S.,Geaugey,V.,Camacho,P.,Debellefontaine,H.,Paul,E.,2002.Minimizationofsludgeproductioninbiologicalprocesses:Analternativesolutionfortheproblemofsludgedisposal.WaterSci.Technol.46(10),63–70.Dytczak,M.A.,Londry,K.L.,Siegrist,H.,Oleszkiewicz,J.A.,2006.Ozonationreducessludgeproductionandimprovesdenitrification.WaterRes.41,543–550.Ginestet,P.,2007.ComparativeEvaluationofSludgeReductionRoutes.IWAPublishing,UK.Goel,R.,Tokutomi,H.,Yasui,H.,2003a.Anaerobicdigestionofexcessactivatedsludgewithozonepretreatment.WaterSci.Technol.47(12),207–214.Goel,R.,Tokutomi,H.,Yasui,H.,Noike,T.,2003b.Optimalprocessconfigurationforanaerobicdigestionwithozonation.WaterSci.Technol.48(4),85–96.Goel,R.,Tokutomi,H.,Yasui,H.,Shibayama,C.,2003c.High-performanceclosedloopanaerobicdigestionusingpre/postsludgeozonation.WaterSci.Technol.47(12),261–267.Goel,R.,Komatsu,K.,Yasui,H.,Harada,H.,2004.Processperformanceandchangeinsludgecharacteristicsduringanaerobicdigestionofsewagesludgewithozonation.WaterSci.Technol.49(10),105–113.Liu,Y.,Tay,J.H.,2001.Strategyforminimizationofexcesssludgeproductionfromtheactivatedsludgeprocess.Biotechnol.Adv.19(2),97–107.Low,E.W.,Chase,H.A.,1999.Reducingproductionofexcessbiomassduringwastewatertreatment.WaterRes.33(5),1119–1132.Mahmood,T.,Elliott,A.,2006.Areviewofsecondarysludgereductiontechnologiesforthepulpandpaperindustry.WaterRes.40,2093–2112.Ødegaard,H.,2004.Sludgeminimizationtechnologies–Anoverview.WaterSci.Technol.49(10),31–40.Paul,E.,Debellefontaine,H.,2007.Reductionofexcesssludgeproducedbybiologicaltreatmentprocesses:effectofozonationonbiomassandonsludge.OzoneSci.Eng.29,415–427.Sakai,Y.,Fukase,T.,Yasui,H.,Shibata,M.,1997.Anactivatedsludgeprocesswithoutexcesssludgeproduction.WaterSci.Technol.36(11),163–170.Saktaywin,W.,Tsuno,H.,Nagare,H.,Soyama,T.,Weerapakkaroon,J.,2004.Advancedsewagetreatmentprocesswithexcesssludgereductionandphosphorousrecovery.WaterRes.39,902–910.Saktaywin,W.,Tsuno,H.,Nagare,H.,Soyama,T.,2006.Operationofanewsewagetreatmentprocesswithtechnologiesofexcesssludgereductionandphosphorusrecovery.WaterSci.Technol.53(12),217–227.Scheminski,A.,Krull,R.,Hempel,D.C.,2000.Oxidativetreatmentofdigestedsewagesludgewithozone.WaterSci.Technol.42(9),151–158.Sievers,M.,Ried,A.,Koll,R.,2004.Sludgetreatmentbyozonation-Evaluationoffull-scaleresults.WaterSci.Technol.49(4),247–253.Song,K.G.,Choung,Y.K.,Ahn,K.H.,Cho,J.,Yun,H.,2003.Performanceofmembranebioreactorsystemwithsludgeozonation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