外文翻译中文译文和致谢Word格式文档下载.docx
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外文翻译中文译文和致谢Word格式文档下载.docx
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外文翻译中文译文和致谢@#@外文翻译@#@Coagulationandflocculation@#@14.1@#@Introduction@#@Coagulationandflocculationprovidethewatertreat-mentprocessbywhichfinelydividedsuspendedandcolloidalmatterinthewaterismadetoagglomerateandformflocs.Thisenablestheirremovalbysedi-mentation,dissolvedairflotationorfiltration.Colloidalparticles(colloids)aremidwayinsize1betweendissolvedsolidsandsuspendedmatter.Colloidsarekeptinsuspension(stabilised)byelectrostaticrepulsionandhydration.Electrostaticrepulsionoccursbecausecolloidsusuallyhaveasurfacechargeduetothepresenceofadoublelayerofionsaroundeachparticle.Thus,thecolloidhasanelectriccharge,mostlyanegativeone.Hydrationisthereactionofparticlesattheirsurfacewiththesurroundingwater.Theresultingparticle-wateragglomerateshaveaspecificgravity,whichdifferslittlefromthatofwateritself.@#@Thesubstancesthatfrequentlyaretoberemovedbycoagulationandflocculationarethosethatcauseturbidityandcolour.Surfacewatersintropicalcountriesoftenareturbidandcontaincolouringmaterial.Turbiditymayresultfromsoilerosion,algalgrowthoranimal/vegetabledebriscarriedbysurfacerun-off.Substancesleachedfromdecomposedorganicmatter,leaves,orsoilsuchaspeatmayimpartcolour.Bothturbidityandcolouraremostlypresentascolloidalparticles.@#@Theelectrostaticrepulsionbetweencolloidalparticleseffectivelycancelsouttheelectronicattractionforces(VanderWaals¡@#@¯@#@forces)thatwouldattachtheparticlestogether.Certainchemicals(calledcoagulatingagents,coagulants)havethecapacitytocompressthedoublelayerofionsaroundthecolloidalparticles.Theyreducetherangeoftheelectrostaticrepulsion,andthusenabletheparticlestoflocculate,i.e.toformflocs.Theseflocscangrowtoasufficientsizeandspecificweighttoallowtheirremovalbysettling,flotationorfiltration.@#@Generallywatertreatmentprocessesinvolvingtheuseofchemicalsarenotsosuitableforsmallcommunitywatersupplies.Theyshouldbeavoidedwheneverpossible.Chemicalcoagulationandflocculationshouldonlybeusedwhentheneededtreatmentresultcannotbeachievedwithanothertreatmentprocessusingnochemicals.Iftheturbidityandcolouroftherawwaterarenotmuchhigherthanispermissiblefordrinkingwater,itshouldbepossibletoavoidchemicalcoagulationinthetreatmentofthewater.Aprocesssuchasslowsandfiltrationormulti-stagefiltrationwouldservebothtoreducetheturbidityandcolourtoacceptablelevels,andtoimprovetheotherwaterqualitycharacteristics,inasingleunit.Aroughingfiltercanservetoreducetheturbidityloadontheslowsandfilter,ifnecessary.@#@.@#@14.2Coagulants@#@Alum(Al2(SO4)3.nH20)wheren=14,16,or18,dependsontheformofalumsupplied.Thismaybeinliquidsolution,brokencrystallinegranules2-5cmsize(kibbled)orcrystallineblocks.Itisbyfarthemostwidelyusedcoagulant.Ironsalts(e.g.ferricchloride(FeCl3),orferricsulphate(Fe2(SO4)3.9H2O)canbeusedaswellandinsomeinstanceshaveadvantagesoveralum.AsignificantadvantageofironsaltsoveraluminiumisthebroaderpH2rangeforgoodcoagulation.Thus,inthetreatmentofsoftcolouredwaterswherecolourremovalisbestobtainedatlowpHs,ironsaltsmaybepreferredascoagulants.Iron@#@saltsshouldalsobeconsideredforcoagulationathighpHs,sinceferrichydroxideishighlyinsolubleincontrasttoaluminiumsalts,whichformsolublealuminateionsathighpHs.SodiumaluminateismostlyusedforcoagulationatmediumpHs.Syntheticorganicpolyelectrolyteshavebecomeavailableascoagulantsbutaregenerallynoteconomicalforsmallwatersupplysystems,noraretheyreadilyavailable.@#@Theuseofextractsfromthecrushedseedsofcertaintropicaltreesisdescribedindetailinthenextsection,14.3.@#@Coagulantssuchassolublealuminiumandironsaltsreactwiththealkalinityofthewater,andhydrolyseinit.Forexample,alumreactstoformaluminium-hydroxidefloc,Al(OH)3,agelatinousprecipitate.Therequiredalkalinitymaybenaturallypresentinthewaterorithastobeaddedthroughdosageoflime,Ca(0H)2orsodiumcarbonate,Na2CO3(alsocalledsodaash).@#@Forgoodcoagulationtheoptimaldoseofcoagulantshouldbefedintothewaterandquicklyandproperlymixedwithit.Theoptimaldosewillvarydependinguponthenatureoftherawwateranditsoverallcomposition.Itisnotpossibletocomputetheoptimalcoagulantdoseforaparticularrawwater.Alaboratoryexperimentcalledthejartestisgenerallyusedfortheperiodicdeterminationoftheoptimaldose.@#@Thejartestmaybebrieflydescribedasfollows:
@#@Aseriesofsamplesofwaterareplacedonaspecialmultiplestirrerandthesamples,typically800mlin1-litrebeakers(orjars),aredosedwitharangeofcoagulant,e.g.10,2
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