1492Ch08

Manahan,StanleyE.WATERTREATMENTEnvironmentalChemistryBocaRaton:CRCPressLLC,20008WATERTREATMENT____________________________________________________8.1.WATERTREATMENTANDWATERUSEThetreatmentofwatermaybedividedintothreemajorcategories:•Purificationfordomesticuse•Treatmentforspecializedindustrialapplications•TreatmentofwastewatertomakeitacceptableforreleaseorreuseThetypeanddegreeoftreatmentarestronglydependentuponthesourceandintendeduseofthewater.Waterfordomesticusemustbethoroughlydisinfectedtoeliminatedisease-causingmicroorganisms,butmaycontainappreciablelevelsofdissolvedcalciumandmagnesium(hardness).Watertobeusedinboilersmaycontainbacteriabutmustbequitesofttopreventscaleformation.Wastewaterbeingdischargedintoalargerivermayrequirelessrigoroustreatmentthanwatertobereusedinanaridregion.Asworlddemandforlimitedwaterresourcesgrows,moresophisticatedandextensivemeanswillhavetobeemployedtotreatwater.Mostphysicalandchemicalprocessesusedtotreatwaterinvolvesimilarphenomena,regardlessoftheirapplicationtothethreemaincategoriesofwatertreatmentlistedabove.Therefore,afterintroductionstowatertreatmentformunicipaluse,industrialuse,anddisposal,eachmajorkindoftreatmentprocessisdiscussedasitappliestoalloftheseapplications.8.2.MUNICIPALWATERTREATMENTThemodernwatertreatmentplantisoftencalledupontoperformwonderswiththewaterfedtoit.Theclear,safe,eventastefulwaterthatcomesfromafaucetmayhavestartedasamurkyliquidpumpedfromapollutedriverladenwithmudandswarmingwithbacteria.Or,itssourcemayhavebeenwellwater,muchtoohardfordomesticuseandcontaininghighlevelsofstain-producingdissolvedironandman-©2000CRCPressLLCganese.Thewatertreatmentplantoperator’sjobistomakesurethatthewaterplantproductpresentsnohazardstotheconsumer.AschematicdiagramofatypicalmunicipalwatertreatmentplantisshowninFigure8.1.Thisparticularfacilitytreatswatercontainingexcessivehardnessandahighlevelofiron.Therawwatertakenfromwellsfirstgoestoanaerator.Contactofthewaterwithairremovesvolatilesolutessuchashydrogensulfide,carbondioxide,methane,andvolatileodoroussubstancessuchasmethanethiol(CH3SH)andbacterialmetabolites.Contactwithoxygenalsoaidsironremovalbyoxidizingsolubleiron(II)toinsolubleiron(III).TheadditionoflimeasCaOorCa(OH)2afteraerationraisesthepHandresultsintheformationofprecipitatescontainingthehardnessionsCa2+andMg2+.Theseprecipitatessettlefromthewaterinaprimarybasin.Muchofthesolidmaterialremainsinsuspensionandrequirestheadditionofcoagulants(suchasiron(III)andaluminumsulfates,whichformgelatinousmetalhydroxides)tosettlethecolloidalparticles.Activatedsilicaorsyntheticpoly-electrolytesmayalsobeaddedtostimulatecoagulationorflocculation.ThesettlingoccursinasecondarybasinaftertheadditionofcarbondioxidetolowerthepH.Sludgefromboththeprimaryandsecondarybasinsispumpedtoasludgelagoon.Thewaterisfinallychlorinated,filtered,andpumpedtothecitywatermains.AeratorLimePrimarybasinSecondarybasinFilterCleanwaterCO2Cl2SludgelagoonCoag-ulantFigure8.1.Schematicofamunicipalwatertreatmentplant.8.3.TREATMENTOFWATERFORINDUSTRIALUSEWateriswidelyusedinvariousprocessapplicationsinindustry.Othermajorindustrialusesareboilerfeedwaterandcoolingwater.Thekindanddegreeoftreat-mentofwaterintheseapplicationsdependsupontheenduse.Asexamples,coolingwatermayrequireonlyminimaltreatment,removalofcorrosivesubstancesandscale-formingsolutesisessentialforboilerfeedwater,andwaterusedinfoodprocessingmustbefreeofpathogensandtoxicsubstances.Impropertreatmentofwaterforindustrialusecancauseproblemssuchascorrosion,scaleformation,©2000CRCPressLLCreducedheattransferinheatexchangers,reducedwaterflow,andproductcontamination.Theseeffectsmaycausereducedequipmentperformanceorequipmentfailure,increasedenergycostsduetoinefficientheatutilizationorcooling,increasedcostsforpumpingwater,andproductdeterioration.Obviously,theeffectivetreatmentofwateratminimumcostforindustrialuseisaveryimportantareaofwatertreatment.Numerousfactorsmustbetakenintoconsiderationindesigningandoperatinganindustrialwatertreatmentfacility.Theseincludethefollowing:•Waterrequirement•Quantityandqualityofavailablewatersources•Sequentialuseofwater(successiveusesforapplicationsrequiringpro-gressivelylowerwaterquality)•Waterrecycle•DischargestandardsThevariousspecificprocessesemployedtotreatwaterforindustrialusearedis-cussedinlatersectionsofthischapter.Externaltreatment,usuallyappliedtotheplant’sentirewatersupply,usesprocessessuchasaeration,filtration,andclarifi-cationtoremovematerialthatmaycauseproblemsfromwater.Suchsubstancesincludesuspendedordissolvedsolids,hardness,anddissolvedgases.Followingthisbasictreatment,thewatermaybedividedintodifferentstreams,sometobeusedwithoutfurthertreatment,andtheresttobetreatedforspecificapplications.Internaltreatmentisdesignedtomodifythepropertiesofwaterforspecificapplications.Examplesofinternaltreatmentincludethefollowing:•Reactionofdissolvedoxygenwithhydrazineorsulfite•AdditionofchelatingagentstoreactwithdissolvedCa2+andpreventformationofcalciumdeposits•Additionofprecipitants,suchasphosphateusedforcalciumremoval•Treatmentwithdispersantstoinhibi