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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