读书报告英文.docx
《读书报告英文.docx》由会员分享,可在线阅读,更多相关《读书报告英文.docx(11页珍藏版)》请在冰豆网上搜索。
读书报告英文
Waterdisinfection
Abstract
Waterdisinfectionisanecessaryprocessforthecontrolofthepathogenicmicro-organisms.Chlorination,chloramination,ozonation,andultravioletsystemarethemostcommonmethodsusedfordrinkingwaterandwastewatertreatmentHowever,theuseofchemicaldisinfectantsleadstotheformationofdisinfectionby-products(DBPs).,trihalomethanes(THMs),halogenicaceticacids,haloacetonitrils(HAN),halo-aldehydesandhaloketonsconsistmainlyofDBPs.DBPsprovideanunintendedhealthhazard.Theoverallpurposeofthispaperwastointroduceseveraldisinfectionmethods,somedisinfectionby-products(DBPs)anddifferenttechniquesfortheremovalofDBPs.
Keywords:
Disinfectionmethods;Disinfectionby-products;removingDBPs
1.Disinfectionmethods
Disinfectionisusuallythefinalstageinthewatertreatmentprocessinordertolimittheeffectsoforganicmaterial,suspendedsolids(SS),andothercontaminantslikepathogenicmicro-organisms.Theprimarymethodsusedforthedisinfectionofwaterinverysmall(25,500people)andsmall(501–3,300people)treatmentsystemsareozonation,ultravioletirradiation(UV),andchlorination[1].Thedisinfectionprocesshasbeenroutinelycarriedoutsincethedawnofthe20thcenturytoeradicateandinactivatethepathogensfromwaterusedfordrinkingpurpose.Disinfectantsinadditiontoremovingpathogensfromdrinkingwater,serveasoxidantsinwatertreatment.Theyarealsousedfor:
(1)removingtasteandcolor;
(2)oxidizingironandmanganese;
(3)improvingcoagulationandfiltrationefficiency;(4)preventingalgalgrowthinsedimentationbasinsandfilters;Chlorineanditscompoundsarethemostcommonlyuseddisinfectantsforwatertreatment.Chlorine’spopularityisnotonlyduetolowercost,butalsotoitshigheroxidizingpotential,whichprovidesaminimumlevelofchlorineresidualthroughoutthedistributionsystemandprotectsagainstmicrobialrecontamination.Thedisinfectionprocessisaffectedbydifferentphysicochemicalandbiologicalfactors.Thedisinfectionefficiency(Ct)isaproductofresidualdisinfectantandthecontacttimeofchlorineinthewater.Thisproductisusedasadesignparameterforthedisinfectionfacility.Disinfectantshavevaryingcapacitiestoinactivateorkillpathogens.Thetypesandnatureoforganismsaswellastheprocessconditions,includingtemperatureandpH,alsoaffectdisinfection.Generally,inactivationoforganisms’increaseswithincreasingCt..Foraspecificcontacttime,requiredchlorinedosesfordisinfectionareconsequentlyhigherinwinterthaninsummerconditions.However,inmostdrinkingwaterutilitiestheapplicationofadisinfectant(suchaschlorine)inadditionmaintainsadequateresidualstoavoidthereappearanceofmicroorganismsinthewaterdistributionsystem.Thedisinfectantresidualsdepleterapidlywhenthewatertemperatureishigh,whichexplainsthedifficultyofmaintainingminimumresiduallevelinthelargedistributionsystemsduringsummer.Also,microbialactivitywithindistributionsystemsishigherinwarmthanincoldwaters.Tomaintainanadequatelevelofresidualdisinfectantinthedistributionsystem,higherdisinfectantdosesareappliedduringthesummer.Generally,theconditionsaffectingthedisinfectionefficiencyandtherequirementstomaintaindisinfectantresidualsinthedistributionsystemssimultaneouslyaffecttheformationofDBPs.
Therearenumerousalternativedisinfectantsthathavebeenlesswidelyusedinsmallandverysmallwatertreatmentsystems,includingchlorinedioxide,potassiumpermanganate,chloraminesandperoxone(ozone/hydro-genperoxide[HP]).Aneffectivedisinfectantshouldbeableto:
(1)destroyalltypesofpathogensinwhatevernumberpresentinthewater
(2)destroythepathogenswithinthetimeavailablefordisinfection,functionproperlyregardlessofanyfluctuationsinthecompositionorconditionofthewater(3)functionwithinthetemperaturerangeofthewater(4)notcausethewatertobecometoxicorunpalatable(5)besafeandeasytohandle
(6)determineitsconcentrationinthewaterandprovideresidualprotectionagainstrecontamination.
1.1Chlorination
Chlorinehasbeensuccessfullyusedforthecontrolofwaterborneinfectiousdiseasesfornearlyacentury,andchlorinationisoneofthemosteffectivepublichealthmeasureseverundertaken.Chlorineisusedtodisinfectwaterineithergaseousform(Cl2),orashypochloritesalts.Allformsofchlorinereactwithwatertoproducehypochlorousacid(HOCl),whichrapidlydissociatestoformthehypochloriteionaccordingtothefollowing.InadditiontoHOClandthehypochloriteion(OCl+),chlorinemayalsobefoundintheformofmonochloramine(NH2Cl)anddichloramine(NHCl2).Thedominantformofchlorinedependsuponthecombinationofparameterssuchastemperature,pHandammoniaconcentrations【2】.AsthepHincreasestheconcentrationofthehypochloriteionrelativetohypochlorous(HOCl)acidincreases,whilethepresenceofammoniatendstoincreasetheconcentrationofmonochloramine.Knowledgeofthedominantformofchlorineinaparticulardisinfectionprocessisimportant.Withthedifferingformscomevaryingoxidizingstrengthsandthusbiocidalefficiencies.Thechlorinedisinfectionprocessoccursprimarilythroughoxidationofcellwallsleadingtocelllysis(bacterial)orinactivationoffunctionalsitesonthecellsurface.Hypochlorousacidisthemostpotentofthefourmainoxidizingforms.Inadditiontodifferencesinoxidizingstrengthsbetweenformsofchlorine,thedisinfectioneffectivenessvariesacrosstherangeofmicro-organisms.Protozoans,helminths,andvirusesarethemostresistant,followedbybacterialpathogens,witheachspeciesvaryinginresistance.Chlorineisveryeffectiveagainstentericbacteria,butlesseffectiveagainstotherbacterialspecies.
1.2Chloramination
Chloraminesareformedduringareactionbetweenchlorine(Cl2)andammonia(NH3).Chloraminesareamines,whichcontainatleastonechlorineatomthatisdirectlybondedtonitrogenatoms(N).Inorganicchloraminesareformedwhendissolvedchlorineandammoniareact.Duringthisreaction,threedifferentinorganicchloraminesareformed:
monochloramine(NH2Cl),dichloramine(NHCl2),andtrichloramine(NCl3).Therearemanysimilaritiesbetweenchlorineandchloramine.Themostimportantfactisthatbothofthemprovideeffectiveresidualdisinfectionwithminimalrisktopublichealth.Thedifferenceisthatmonochloramineis200timeslesseffectiveasadisinfectantthanchlorine.Ontheotherhand,chlorineformsmanyby-products,includingtrihalomethanes(THM)andhaloaceticacids(HAA),whereaschloramineformsasignificantlyloweramountofTHMsandHAAs,butalsoformsN-nitrosodimethylamine(NDMA)【3】.
1.3Chlorinedioxide
Chlorinedioxide(ClO2)isusedbothasadisinfectantandanoxidantinwatertreatments.Ithasseveraldistinctchemicaladvantages,whichcomplementthetraditionaluseofchlorineinwatertreatments[8].ChlorinedioxideishighlyeffectiveincontrollingwaterbornepathogenswhileminimizinghalogenatedDBPs.Also,abroad-spectrummicrobiocideisaseffectiveaschlorineagainstviruses,bacteria,andfungi,andmoreeffectivethanchlorinefortheinactivationoftheencystedparasitesGiardiaandCryptosporidium【4】.Furthermoreitisaneffectivecontrolstrategyfortaste,odor,color,andironandmanganeseremoval.Chlorinedioxidepresentsseveralotheradvantagesthanotherdisinfectants,whichcanbesummarizedasfollows:
(1)thebactericidalefficiencyisrelativelyunaffectedbypHvaluebetween4and10
(2)Chlorinedioxideisclearlysuperiortochlorineinthedestructionofspores,bacteria,viruses,andotherpathogenicmicro-organismsonanequalresidualbase
(3)therequiredcontacttimeforClO2islower
(4)chlorinedioxidehasbettersolubility
(5)nocorrosionassociatedwithhighchlorineconcentrations
(6)chlorinedioxidedoesnotreactwithammonia
(7)itdestroysTHMprecursorsandincreasescoagulation
(8)ClO2destroysphenolsandhasnodistinctsmell
(9)itisbetteratremovingironandmagnesiacompoundsthanchlorine,especiallycomplexbounds.
1.4Ozonation
Ozonehasbeenusedforwaterdisinfectionforabout80yearsinFrance,Germany,andotherEuropeancountries.Itisnowundergoingacriticalevaluationasapossiblealternativetochlorinewhenusedaloneorinconjunctionwithotherdisinfectionsystems.Ozoneisproducedwhenoxygen(O2)moleculesaredissociatedbyanenergysourceintooxygenatomsandsubsequentlycollidewithanoxygenmoleculetoformanunstablegas,ozone(O3).Disinfectionbyozonationisachievedusingtheformationoffreeradicalsasoxidizingagents.Themethodismoreeffectiveagainstvirusesandbacteriathanchlorination.Thelowsolubilityofozoneinwateristhemainfactorthatgreatlyreducesitsdisinfectioncapacity,andanyozoneresidualproducedrapidlydissipatesasaconsequenceofitsreactivenature.Theabsenceofalastingresidualmayalsobeseenasadisadvantageasthismayallowpossiblemicrobialre-growthandmakeitdifficulttomeasuretheefficiencyofthedisinfectionprocess.Ozoneisthemostefficientchemicaldisinfectantcurrentlyappliedindrinkingwatertreatment.Evenformicroorganismssuchasprotozoawhicharedifficulttoinactivatewithotherdisinfectants,ozoneprovidesadequateinactivationwithreasonabledosesandcontacttimes[5].
Themechanismsofdisinfectionusingozoneinclude,directoxidationofthecellwallwithleakageofcellularconstituentsoutsideofthecell,reactionswithradicalby-productsofozonedecomposition.Theeffectivenessofdisinfectiondependsonthesusce