02Effectofairflowrateandturningfrequencyonbiodryin

Effectofair-flowrateandturningfrequencyonbio-dryingofdewateredsludgeLingZhao,Wei-MeiGu,Pin-JingHe*,Li-MingShaoStateKeyLaboratoryofPollutionControlandResourcesReuse,KeyLaboratoryofYangtzeRiverWaterEnvironment,CollegeofEnvironmentalScienceandEngineering,TongjiUniversity,1239SipingRoad,Shanghai200092,PRChinaarticleinfoArticlehistory:Received16January2010Receivedinrevisedform11May2010Accepted4July2010Availableonline13July2010Keywords:Bio-dryingDewateredsludgeAir-flowrateTurningfrequencyEnergyutilizationabstractSludgebio-dryingisanapproachforbiomassenergyutilization,inwhichsludgeisdriedbymeansoftheheatgeneratedbyaerobicdegradationofitsorganicsubstances.Thestudyaimedatinvestigatingtheinteractiveinfluenceofair-flowrateandturningfrequencyonwaterremovalandbiomassenergyutilization.Resultsshowedthatahigherair-flowrate(0.0909m3h
1kg
1)ledtolowertemperaturethandidthelowerone(0.0455m3h
1kg
1)by17.0%and13.7%underturningpertwodaysandfourdays.Withthehigherair-flowrateandlowerturningfrequency,temperaturecumulationwasalmostsimilartothatwiththelowerair-flowrateandhigherturningfrequency.Thedoubledair-flowrateimprovedthetotalwaterremovalratioby2.86%(19.5gkg
1initialwater)and11.5%(75.0gkg
1initialwater)withturningpertwodaysandfourdaysrespectively,indicatingthattherewasnoremarkableadvantageforwaterremovalwithhighair-flowrate,especiallywithhighturningfrequency.Theheatusedforevaporationwas60.6e72.6%ofthetotalheatconsumption(34,400e45,400kJ).Thehigherair-flowrateenhancedvolatilesolids(VS)degradationthusimprovingheatgenerationby1.95%(800kJ)and8.96%(3200kJ)withturningpertwodaysandfourdays.Withthehigherair-flowrate,heatconsumedbysensibleheatofinletairandheatutilizationefficiencyforevaporationwashigherthanthelowerone.Withthehigherturningfrequency,sensibleheatofmaterialsandheatconsumedbyturningwashigherthanlowerone.ª2010ElsevierLtd.Allrightsreserved.1.IntroductionNowadaysthecontinuousproductionoflargeamountofsewagesludgeisbecomingaworldwideenvironmentalproblem.Volumeandquantityreductionaswellasstabiliza-tionaretheprincipalmethodsfordewateredsludgetreat-ment.Thetypicalstabilizationapproach,composting,requiresarelativelylongresidencetime(30e50days),thusitbecomeslesspracticalforlargequantitiesofsludge(Navaee-Ardehetal.,2010).Inmanycountries,thelimitedlandresourcecoupledwithincreasingenergyproblemmadecombustionanattractivesludgetreatmentoption,especiallyinbigcities(Hongetal.,2009).Nowalmostallsludgeincin-erationplantsrequirepre-dryingofsludge.Thermaldryingisastrategyobtainingproductwithhighsolidcontentquickly,whileinmostcasesnotcost-effectiveandenvironmentallyfriendlyduetotheconsumptionofnon-renewableenergyresource.Also,theinvestmentandopera-tionalcostsarerelativelyhighresultingfromthecomplicatedapparatuses.Moreover,theproblemofplasticphaseofsludgeneedsavoidance.Afewcommercialthermaldryersadoptedheatpumpstotransformlow-qualityheatsourceintohigh-*Correspondingauthor.Tel./fax:þ862165986104.E-mailaddress:[e-mail](P.-J.He).Availableat(2010)6144e61520043-1354/$eseefrontmatterª2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.watres.2010.07.002qualityone,andtheenergyconsumptioncouldbereducedto468e2016kJkg
1evaporatedwater(Peregrinaetal.,2008).Basedonthis,todrysludgefromawatercontentof80e40%,theenergyconsumptionisestimatedas312e1344kJkg
1initialsludge.Bio-drying,analternativepretreatmentmethodtargetingincinerationhasbeendevelopedinrecentyears,whichaimsatremovingwaterfrombio-wasteswithhighwatercontent(Zhangetal.,2009).Derivedfromcomposting,themainmech-anismofbio-dryingisusingtheheatgeneratedduringtheaerobicdegradationoforganicsubstancestoevaporatewater(Freietal.,2004;Tre´mieretal.,2009;Velisetal.,2009).Ifthewatercontentofsludgedecreasesfrom80%to40%,about1667-kJenergyisrequiredforevaporating0.667kgofwaterfrom1-kginitialsludgecalculatedfromlatentheatofwaterevaporation(2500kJkg
1).Andafractionofthisenergyisexpectedfrombio-generatedheat.Thelowerheatingvalue(LHV)ofthedriedsludgewouldincreasefromabout0.5MJkg
1to6MJkg
1(Stastaetal.,2006).Moreover,bio-dryingisanticipatedtobeaneffectivevolumereductionprocesswhichwillbenefitshort-termstorageandtransportation,andreduceodoremissionduringtransportation.Throughaerationandturningofthematrix,masstransferconditionsareimprovedandwaterevaporationisenhanced.Itisaprocessinwhichwastedriesitselfbymeansofinherentchemicalenergy(Rulkens,2008).Bio-dryinghasmostlybeenstudiedformunicipalsolidwastes(MSW)(Zhangetal.,2008;Adanietal.,2002),whileafewstudieswerereportedforsewagesludgebio-drying.ComparedwithMSW,therearesomeproblemsinbio-dryingofdewateredsludge,i.e.,itcontainslessbiodegradableorganicsubstances,moremoisture,andtheporosityofthebiomassmatrixislower,whicharealldisadvantageoustoeffectiveaerationandenergyrecovery(Ragazzietal.,2007).Howtoeffectivelyandeconomicallyutilizethebio-gener-atedenergytoremovemorewaterinlesstimeunderoptimaloperationsisacrucialproblemofbio-drying(Choietal.,2001;Velisetal.,2009).Aerationandturningaretwoimportantoperationsw