环境工程英语原文对照翻译.docx
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环境工程英语原文对照翻译.docx
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环境工程英语原文对照翻译
ReducingtheHeavyMetalContentofSewageSludgebyAdvancedSludgeTreatmentMethods
ABSTRACT
Waste-activatedsludge(WAS)processesarekeytechnologiestotreatwastewater:
theireffluentscanmeetstringentdischargestandards,thusensuringaminimumresidualimpactontheaquaticenvironment.Thepresenceofheavymetalsintheexcesssludgeposes,however,seriousproblems,andconsiderablyhampersthefinaldisposalalternatives,especiallyintheagriculturaluse(soilimprovement/amendment).ThisarticlestudiestheeffectofthermalhydrolysisandFenton’speroxidationontheheavymetalcontentofthedewateredsludge.AcidthermalhydrolysisreducestheheavymetalcontentinthefiltercakeexceptforCu,Hg,andPb.AlkalinethermalhydrolysisreleasesCu,Pb,andCr.Fenton’speroxidationtransfersCd,Cu,andNifromthefiltercakeintothefiltrate.Landapplicationoftheresidualcakecanhencebereconsidered.
INTRODUCTION
WASTE-ACTIVATEDSLUDGE(WAS)processesarekeytechnologiestotreatwastewater:
theireffluentscanmeetstringentdischargestandards,thusensuringaminimumresidualimpactontheaquaticenvironment.Throughtheirmicrobialactivity,thesebiologicalprocessesproducehugeamountsofWAS,nowcommonlycalledbiosolids.ThisexcesssludgeisaninevitabledrawbackinherenttotheWASprocess.IncreasingamountsofWAShavetobedealtwithdueto
(1)thehigherwastewatercollectionrates,
(2)thereliabilityandefficiencyofwastewatertreatmentplants,and(3)thewidespreadapplicationofdenitrificationanddephosphatation.Thepresenceofheavymetalsintheexcesssludgeposes,however,seriousproblems,sincetheyareamongthemostimportantfactorsinassessingthefinaldisposalalternativesofthesludge,forexample,theywillaccumulateintheflyashintheeventofincineration,canbereleasedthroughleachinginlandfills,ormaypreventlandapplication.
Manycountrieshaveimposedlegalrestrictionsconcerningtheheavymetalcontent,whenusingthesludgeforlandapplication(e.g.,agriculturaluseasfertilizerorsoilamendment),sincehighconcentrationsinthesludgemayresultinheavymetalaccumulationincultivatedsoils.IntheEuropeanUnion,themaximumacceptableconcentrationsarefixedbytheDirective86/278/EECoftheEuropeanCommission(CommissionoftheEuropeanCommunities,1986).Mostcountriesapply,however,amorestringentlegislation.InFlanders,thestandardsaredescribedinVLAREA(FlemishGovernment,1997).BothEuropeanandFlemishlegalstandards,aswellastheaverageconcentrationsofheavymetalsinmunicipalsewagesludgeinFlanders(Aquafin,2004),arepresentedinTable1.SomemetalspresentintheWASdonotcomplywiththeselegalstandards,thusnecessitatingtheapplicationoflesssustainableandmoreexpensivedisposalroutes.
Heavymetalsinsludgecannotberemovedbycommonsludgetreatmentmethodssuchasaerobicoranaerobicdigestion.
Someadvancedsludgetreatmentprocesses(AST)wererecentlyconfirmedashavingthepotentialtoenhancethedewaterabilityofthesludge.Themostpromisingtechniquesincludeacidandalkalinethermalhydrolysis(NeyensandBaeyens,2003a;Neyensetal.,2003a,2003b)andtheperoxidationofthesludge(NeyensandBaeyens,2003b;2003c;Neyensetal.,2002).Inaddition,Dewiletal.(2005)demonstratedanincreaseofthethermalconductivityofperoxidatedsludge,henceimprovingitsdryingcharacteristics.Neyensetal.(2004)concludedthatthesetechniquesenhancethedewaterabilityofthesludgebydegradingtheextracellularpolymericsubstances(EPS)structures.SincealargepartoftheheavymetalsareboundtotheseEPS,theASTprocessesaresupposedtoalsoreleaseheavymetalsfromthesludgetothewaterphase,thusreducingtheheavymetalconcentrationintheresidualsludgecakeafterdewatering.Indoingso,thewaterphaseneedsatraditionaltreatmentformetalremoval(settling,flotation,etc.)priortobeingrecycledtotheplantinfluent.ThisarticlestudiestheeffectofthermalhydrolysisandFenton’speroxidationontheheavymetalcontentofthedewateredsludge.
EXPERIMENTALSETUPANDPROCEDURE
Sludgesused
Theexperimentsmakeuseofthickenedactivatedsludgesamples,takenfromthefull-scalewastewatertreatmentplant(WWTP)ofLeuven(locatedinFlanders,Belgium).
TheWWTPofLeuvenisahigh-loadWASplanthavingasludgeODS/DS(organicfractionofthetotalamountofdrysolids)fractionof67%.Noprimarysedimentationispresent.
Theactivatedsludgewascollectedandsettledinthelaboratoryforabout4h.Thesupernatantwasthereafterpouredoff.Theresultingdrysolidscontentwasabout3wt.%.
Hydrolysis
Hydrolysis(acidoralkaline)ofthesludgeimpliesatreatmentatmoderatetohightemperature.AcidthermalhydrolysisatlowpHrequiressulphuricacid(fromasolutioncontaining1.75kgH2SO4/L),whereasalkalinethermalhydrolysisoccursathighpH,obtainedbyaddingCa(OH)2(fromasuspensioncontaining50gCa(OH)2/L).Afterreaction,thesludgeisneutralizedbyaddingCa(OH)2orH2SO4for,respectively,acidoralkalinethermalhydrolysis.Ciba®ZETAG7878FS40polyelectrolyteisthereafteraddedbeforedewateringthesludge.
Theexperimentswerecarriedoutinareactor,constructedasapressurevesselwithanelectricallyheatedshell.Thetemperatureofthesludgeiskeptconstant.Samplesof10Lsludgewerebatchtreated.Table2summarizestheoperatingconditionsofthedifferenttreatmentmethods,previouslydeterminedtobeoptimalconditions(Neyensetal.,2003a,2003b).
Fenton’streatmentofthesludge
Neyensetal.(2002,2003c)showedthattheoptimumactivityfortheperoxidationisreachedunderthefollowingconditions:
(1)additionof25gH2O2kg-1DS,
(2)thepresenceof1.67gFe2+kg-1DS,(3)atpH3and(4)atambienttemperatureandpressure.
ThistreatmentwascarriedoutbyfirstadjustingthepHofthesludgeto3usingH2SO4·Fe2+(i.e.,FeSO4)wasaddedatthegivenconcentrationandH2O2wasthereafteraddedattherequiredamountfromasolutioncontaining390gH2O2/Lsolution.
Themixturewasstirredat200rpm.Theoxidationreleasesreactiongases(mostlyCO2,H2O,andsmallorganicmolecules),andthetimeofreactionwasconsideredasthetimeuntilthegasproductionwasstopped.Thistimewasabout60min.Afterreaction,thesludgemixturewasneutralizedwithCa(OH)2andpolyelectrolyte(PE)wasadded.ThePEusedwasCiba®ZETAG7878FS40.Thetestswereperformedinabatchreactorof10L.
Dewatering
Thesludgewasdewateredusingavacuum-assistedBuchnerfiltrationatavacuumpressureof0.5bar,assessedfora100-mLsampleduringasettimeof5min.Thefiltercakewasanalyzedforheavymetals.
Heavymetalanalysis
Cr,Cu,Ni,Zn,Cd,andPbweremeasuredbyICPOES.Hgwasdeterminedbycoldvaporatomicabsorptionspectroscopy(AAS).Priortotheanalysis,thesludgesamplesweresubmittedtoamicrowavedestructionmethod.Approximately0.2gwasintroducedinaTeflontube.TwomillilitersofHNO3(65wt.%)6mLofHCl(fuming),and2mLofHFwereadded.Thetubescontainingthemixturewereinsertedinamicrowaveoven,applyingapowerof400Wfor20min.Thereafter,22mLof4wt.%boricacidwasaddedtoneutralizetheremaining(hazardous)HF.
RESULTSANDDISCUSSION
Generalities
Heavymetalsinsewageareconcentratedinthesludgethroughtwotreatmentsteps(BrownandLester,1979):
(1)primarysettlingremovesaproportionofmetalswhichiseitherinsolubleoradsorbedontoparticles(OliverandCosgrove,1974),and
(2)additionalmetalremovaloccursintheWASprocessbytheadsorption(biosorption)ofdissolvedorfinelysuspendedmetalcompoundsontosludgeflocs(OliverandCosgrove,1974;YoshizakiandTomida,2000).Someauthorsreportthattheactivatedsludgeprocessismoreefficientinremovingmetalsthantheprimarysettling.
BiosorptionoftheheavymetalionsintheWASprocessseemstotakeplacemainlybytherapidionexchangereactionsoncellsurfaces(YoshizakiandTomida,2000).Thesurfacesofthemicro-organismsintheactivatedsludgearecoveredwithEPS.ThepotentialbindingsitesforheavymetalsontheEPSincludecarboxylates,amines,thiols,phosphates,andotherfunctionalgroups(Sekietal.,1998).EPShenceactasionexchangerswhichcombinecationicandanionicexchangefunctions,andthisactionseemstofacilitatethecombinationofsludgewithvariousmetalcationsandmetalcomplexanions.BydegradingtheEPS,itwasassumedpossibletoreleaseheavymetalsfromWAS.
ThetreatmentofthesludgewithASTmethodsaffectstheextractabilityoftheheavymetalspresentinthethickenedsludgebytwopossiblemechanisms:
(1)theEPSmechanismorbiologicalmechanism,and
(2)thephysicochemicalmechanism.
ThehighmolecularweightEPSoftheBioflocincludespolysaccharides,proteins,nucleicacids,lipids,andotherpolymericcompounds,andprovidemanyfunctionalgroupsthatmayactasbindingsitesforthemetals(ForsterandLewin,1972;Chengetal.,1975;BrownandLester,1979;Liuetal.,2001).TheASTmethodshavethepotentialtodegradetheEPS,thusdestroyingthebindingsites.Heavymetalsarethusreleased.TheprocessofdestroyingtheEPSbyASTmethodsisdescribedindetailbyNeyensetal.(2004).
Physicalandchemicalfactorsaffectingmetalremovalaretemperature,pH,durationofacidification,presenceofanoxidant,concentrationofcomplexingagents,WASflocsize,etc.
Heavymetalsoccurindifferentforms:
exchangeable,boundtocarbonates,boundtoironandmanganeseoxides,boundtoorganicmatter,andresidual(Naoumetal.,2001).Theexchangeablefractionislikelytobeaffectedbychangesinwaterionic
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