Thermalalkalinesolubilizationofwasteactivatedsludg

Thermal-alkalinesolubilizationofwasteactivatedsludgeasapre-treatmentstageforanaerobicdigestionA.G.Vlyssides*,P.K.KarlisLaboratoryofOrganicandChemicalTechnology,DepartmentofChemicalEngineering,NationalTechnicalUniversityofAthens,9HeroonPolytechniouStrasse,Zographou,15700Athens,GreeceReceived10February2003;receivedinrevisedform20May2003;accepted26May2003AbstractThisworkstudiedthehydrolysiskineticsandthesolubilizationofwasteactivatedsludgeunderamediumrangetemperature(50–90
C)andpHinthealkalineregion(8–11),asapretreatmentstageforanaerobicdigestion.Thehydrolysisrateforthesolubili-zationofvolatilesuspendedsolids(VSS)followedafirst-orderrate.Alinearpolynomialhydrolysismodelwasderivedfromtheexperimentalresultsleadingtoasatisfactorycorrelationbetweenthehydrolysisratecoefficient,pH,andtemperature.AtpH11andatemperatureof90
CtheconcentrationoftheVSSwas6.82%,theVSSreductionreached45%withintenhoursandatthesametimethesolubleCODwas70.000mg/landthetotalefficiencyformethaneproduction0.28lofCH4pergofVSSloading.2003ElsevierLtd.Allrightsreserved.Keywords:Wasteactivatedsludge;Thermal;Pretreatment;Alkaline;Lime;Solubilization;Hydrolysis1.IntroductionSludgehandlingrepresents30–40%ofthecapitalcostandabout50%oftheoperatingcostofmanywaste-watertreatmentfacilities.Abouttwo-thirdsofthesludgeinGreeksecondarytreatmentplantsiswasteactivatedsludge(WAS).TheWAShastobestabilizedsufficientlytoreduceitsorganiccontent,odorproblems,andpathogencontaminationbeforeultimatedisposal.Anaerobicdigestionisusedasacommonmethodforprimaryandsecondarysludge(PSandWAS)stabiliza-tionleadingtoanenergyrecoverybonusintheformofmethanegasproduction.Thedemandsforhigherefficiencyprocesses,havepromptedtheneedforpre-treatmentmethodsinordertoimprovesubstratesolu-bilizationanddigestibility.MorestudiesfocusonthermalorthermochemicalprocessasapretreatmentstageofWAS.Thesestudiesincludethermalpretreat-mentinthemoderatetemperaturerangeof60–100
C(Hiraoka,1984;LiandNoike,1997),inthemediumtemperaturerangeof100–175
C(Haugetal.,1978),andinahightemperaturerangeof175–225
C(Haug,1983).Also,thermochemicalpretreatmentisemployedatambienttemperatureandalkalineconditions(Linetal.,1997)oracidic(KarlssonandGoransson,1993)conditions.Themostimportantparameterforcontrollingan-aerobicdigestionistheconcentrationofbicarbonatealkalinitywhichhastobeashighaspossible.Thevol-atileacidsproducedduringdigestioncaninhibitthemethaneproductionduetothefactthatthepHcanbedecreasedtowardstheacidicregion.ThebicarbonatesareinequilibriumwiththesolubleCO2producingastrongbufferingatneutralpH.Carbondioxideisadi-gestionproduct,sothestrengthofthebufferingdependsonthepresenceofcations.Hence,inmanyanaerobicdigestioncases,itisnecessarytoaddappropriateamountsofalkaliintheformoflimeorsodiumorpotassiumhydroxideinordertokeepthepHintheneutralregion(MukherjeeandLevine,1992).Inaddi-tion,alkalinetreatmentoforganicmaterialwasreportedtoinduceswellingofparticulateorganic,makingthecellularsubstancesmoresusceptibletoenzymaticattack(BaccayandHashimoto,1984).Asaresultthebiode-gradabilityofthesolidphasemaybeenhancedsignifi-cantly.ThepretreatmentofWASatmoderatetemperature(50–90
C)inalkalineconditions,byaddinglimewas*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](A.G.Vlyssides).0960-8524/$-seefrontmatter2003ElsevierLtd.Allrightsreserved.doi:10.1016/S0960-8524(03)00176-7BioresourceTechnology91(2004)201–206examinedsystematicallyregardingtheireffectonWASsolubilizationandanaerobicbiodegradation.2.Methods2.1.MethodologyTheWASusedfortheexperimentalworkwasob-tainedfromthesecondarystageofasoftdrinkspro-ductionindustrywastewatertreatmentplant,operatedatasludgeretentiontimeof10days.About200lof5%WAScontentwastransferredtothelaboratory,filtered,dried,mixedandstoredinanairproofboxat4
Cim-mediately.Solidsuspensionsof10%(w/v)in250mlofwaterwerepreparedbyusingtheabovedescribedstoredrawmaterial.Thetotalvolatilesuspendedsolids(VSS)contentofthesludgeusedwas6.82%(w/v).Inordertostudythehydrolysisprocessoftheactivatedsludge,aseriesofexperimentswereperformedundervarioustemperatureandpHvalues.Atotalof20experimentalsetswerecarriedat50,60,70,80and90
CwithpHat8,9,10,11.About10batchreactorsof250-mlactivevolumeeach,wereusedineachsetofexperiments.Theappli-cationof10batchreactorsinsteadofonehadtodowiththeaccuracyoftheintermediateresults.Thebatchhy-drolysisprocesstookover10handbyterminatingeveryone-hourtheprocessofonereactor,moreaccuratere-sultswereobtained.SolubleCODandVSSweremea-suredforthetotalcontentofthereactorseveryhourbyterminatingonereactoratatime.Theanaerobicbiodegradabilityofthefinalsolubleproductswasevaluatedbyusingamodifiedbiochemicalmethanepotential(BMP)testasdescribedpreviouslybyWangandLatchaw(1990).AccordingtotheBMPtest,eachassaybottlewaspreparedbyplacing50mlofathermophilicmicrobialinoculumand50mloftestingsampleina125-mlserumbottle.Theseedinoculawereobtainedfromacontinuousflow5lthermophilicfer-mentor(55
C),operatedonasyntheticfeedcontaininganutrientsolution.Priortointroductionoftheseedinoculaandreactionsamples,theserumbottlesweregas