给排水专业英语Unit20

Unit20SludgeutilizationanddisposalRawsludgefromactivatedsludgetreatmentplantshasbeenapplieddirectlyontoagriculturallandparticularlyintheUnitedKingdom.Thispracticeisconsideredunsatisfactorybecauseofthepresenceofpathogensinthesludgeinhighnumbers.Therehasbeennothoroughstudy,however,whichhasshownthatthereisanincreaseintheriskofacquiringillnessesassociatedwithpathogensintherawsludgewhenproperhandlingprocedureandnon-entrytothelandfollowingapplicationisobserved.Reuseofcompostedsludgeasasoilconditionerinagricultureandhorticulturereturnscarbonnitrogen,phosphorusandelementsessentialforplantgrowthbacktothesoil.Lesschemicalfertilizersarerequiredandtheorganiccarbonhelpstoimprovesoilaeration,waterpercolationandrootgrowth.Thenitrogenandphosphorusarealsoreleasedgraduallyforplantuptakecomparedtothemoresolublechemicalfertilizers.Thepotentialofleachingofthenutrientstogroundorsurfacewaterbyrainfallrun-offismuchreduced.Pathogensandheavymetalscan,however,limitthereuseofsludge.Pathogensshouldbereducedtolevelsthatdonotposehealthhazardstoworkershandlingthesludge,potentialhealthhazardsfromthespreadingofhelmintheggsandfromhorticulturalproducecontaminatedbypathogens.Compostingofthesludgetoattainatemperatureof55℃fortwoweeksfollowedbywindrowmaturationproducescompostthatmeetstheseconditions.Stabilizedsludge,whichhasbeendewateredanddriedonsandbedstoattainalowmoisturecontent,canmeetthesameconditions.Heavymetalsandtoxicchemicalaredifficulttoremovefromsludge.Preventingthesechemicalsfromenteringthewastewaterorsludgebetheaimofwastewatermanagementforsludgeintendedforreuseinageicultureorhorticultur.Reusemaystillbepossibleforpurposessuchasminesiterehabilitation,highwaylandscapingorforlandfillcover.Sludgethathasbeenconditionedforreuseiscalled„biosolids‟.Conversionofsludge,whichisheavilycontaminatedbyheavymetalsortoxicchemicals,tooilistechnicallyfeasible(Enersludgeprocess).Afull-scalepantisoperatinginPerth,WesternAustralia(Bridleetal.,2000).Theconversionisbyapyrolysisprocess,heatingdriedsludgetoahightemperatureintheabsenceofoxygenorwithacontrolledamountofoxygen.Capitalandrunningcostsofanoilfromsludgeprocessarehigh.Thermaldepolymerizationuseshydrouspyrolysistoconvertreducedcomplexorganicstooil.Thepremacerated,grit-reducedsludgeisheatedto250℃andcompressedto40MPa.Thehydrogeninthewaterinsertsitselfbetweenchemicalbondsinnaturalpolymerssuchasfats,proteinsandcellulose.Theoxygenofthewatercombineswithcarbon,hydrogenandmetals.Theresultisoil,lightcombustiblegasessuchasmethane,propaneandbutane,waterwithsolublesalts,carbondioxide,andasmallresidueofinertinsolublematerialthatresemblespowderedrockandchar.Allorganismsandmanyorganictoxinsaredestroyed.Inorganicsaltssuchasnitratesandphosphatesremaininthewateraftertreatmentatsufficientlyhighlevelsthatfurthertreatmentisrequired.Theenergyfromdecompressingthematerialisrecovered,andtheprocessheatandpressureisusuallypoweredfromthelightcombustiblegases.Theoilisusuallytreatedfurthertomakearefinedusefullightgradeofoil,suchasno.2dieselandno.4heatingoil,andthensold.Thechoiceofawastewatersolidtreatmentmethoddependsontheamountofsolidsgeneratedandothersite-specificconditions.However,ingeneral,compostingismostoftenappliedtosmaller-scaleapplicationsfollowedbyaerobicdigestionandthenlastlyanaerobicdigestionforthelarger-scalemunicipalapplications.SludgedisposalWhenaliquidsludgeisproduced,furthertreatmentmayberequiredtomakeitsuitableforfinaldisposal.Typically,sludgesarethickened(dewatered)toreducethevolumestransportedoff-sitefordisposal.Processesforreducingwatercontentincludelagooningindryingbedstoproduceacakethatcanbeappliedtolandorincinerated;pressing,wheresludgeismechanicallyfiltered,oftenthroughclothscreenstoproduceafirmcake;andcentrifugationwherethesludgeisthickenedbycentrifugallyseparatingthesolidandliquid.Finalorultimatedisposalofsludge,whichcannotbereused,isbylandfillingorincineration.Sincesludgeforlandfillingusuallycontainsheavymetalsortoxicchemicals,liningofthelandfillwithclayorplasticlinermayberequiredtopreventcontaminationofgroundwater.Incinerationofsludgeisbyamultiplehearthfurnaceorfluidizedbedfurnace.Energyinputisrequiredtodrythesludgebeforecombustionisself-sustaining.Combustionfluegasesusuallyneedtreatmenttomeetairpollutioncontrolstandards.Investmentandoperatingcostsarehigh.Thereareconcernsaboutsludgeincinerationbecauseofairpollutantsintheemissions,alongwiththehighcostofsupplementalfuel,makingthisalessattractiveandlesscommonlyconstructedmeansofsludgetreatmentanddisposal.Thereisnoprocesswhichcompletelyeliminatestherequirementsfordisposalofbiosolids.InSouthAustralia,aftercentrifugation,thesludgeisthencompletelydriedbysunlight.Thenutrientrichbiosolidsarethenprovidedtofarmersfree-of-chargetouseasanaturalfertilizer.Thismethodhasreducedtheamountoflandfillgeneratedbytheprocesseachyear.