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3.1OzoneChemistry

Ozoneexistsasagasatroomtemperature.Thegasiscolorlesswithapungentodorreadilydetectableatconcentrationsaslowas0.02to0.05ppm（byvolume）,whichisbelowconcentrationsofhealthconcern.Ozonegasishighlycorrosiveandtoxic.

Ozoneisapowerfuloxidant,secondonlytothehydroxylfreeradical,amongchemicalstypicallyusedinwatertreatment.Therefore,itiscapableofoxidizingmanyorganicandinorganiccompoundsinwater.Thesereactionswithorganicandinorganiccompoundscauseanozonedemandinthewatertreated,whichshouldbesatisfiedduringwaterozonationpriortodevelopingameasurableresidual.

Ozoneissparinglysolubleinwater.At20oC,thesolubilityof100percentozoneisonly570mg/L（Kinman,1975）.Whileozoneismoresolublethanoxygen,chlorineis12timesmoresolublethanozone.Ozoneconcentrationsusedinwatertreatmentaretypicallybelow14percent,whichlimitsthemasstransferdrivingforceofgaseousozoneintothewater.Consequently,typicalconcentrationsofozonefoundduringwatertreatmentrangefrom<

0.1to1mg/L,althoughhigherconcentrationscanbeattainedunderoptimumconditions.

Basicchemistryresearch（Hoigné

andBader,1983aand1983b;

Glazeetal.,1987）hasshownthatozonedecomposesspontaneouslyduringwatertreatmentbyacomplexmechanismthatinvolvesthegenerationofhydroxylfreeradicals.Thehydroxylfreeradicalsareamongthemostreactiveoxidizingagentsinwater,withreactionratesontheorderof1010-1013M-1s-1,approachingthediffusioncontrolratesforsolutessuchasaromatichydrocarbons,unsaturatedcompounds,aliphaticalcohols,andformicacid（Hoigné

andBader,1976）.Ontheotherhand,thehalf-lifeofhydroxylfreeradicalsisontheorderofmicroseconds,thereforeconcentrationsofhydroxylfreeradicalscanneverreachlevelsabove10–12M（GlazeandKang,1988）.

AsshowninFigure3-1ozonecanreactbyeitherorbothmodesinaqueoussolution（Hoigné

andBader,1977）:

∙Directoxidationofcompoundsbymolecularozone（O3（aq））.

∙Oxidationofcompoundsbyhydroxylfreeradicalsproducedduringthedecompositionofozone.

Figure3-1.OxidationReactionsofCompounds（Substrate）During 

Ozonation 

of 

Water

Thetwooxidationpathwayscompeteforsubstrate（i.e.,compoundstooxidize）.Thedirectoxidationwithaqueousozoneisrelativelyslow（comparedtohydroxylfreeradicaloxidation）buttheconcentrationofaqueousozoneisrelativelyhigh.Ontheotherhand,thehydroxylradicalreactionisfast,buttheconcentrationofhydroxylradicalsundernormalozonationconditionsisrelativelysmall.Hoigné

andBader（1977）foundthat:

∙Underacidicconditions,thedirectoxidationwithmolecularozoneisofprimaryimportance;

and

∙Underconditionsfavoringhydroxylfreeradicalproduction,suchashighpH,exposuretoUV,oradditionofhydrogenperoxide,thehydroxyloxidationstartstodominate.

ThislattermechanismisusedinadvancedoxidationprocessessuchasdiscussedinChapter7,Peroxone,toincreasetheoxidationratesofsubstrates.

Thespontaneousdecompositionofozoneoccursthroughaseriesofsteps.Theexactmechanismandreactionsassociatedhavenotbeenestablished,butmechanisticmodelshavebeenproposed（Hoigné

Glaze,1987）.Itisbelievedthathydroxylradicalsformsasoneoftheintermediateproducts,andcanreactdirectlywithcompoundsinthewater.Thedecompositionofozoneinpurewaterproceedswithhydroxylfreeradicalsproducedasanintermediateproductofozonedecomposition,resultinginthenetproductionof1.5molehydroxylfreeradicalspermoleozone.

Inthepresenceofmanycompoundscommonlyencounteredinwatertreatment,ozonedecompositionformshydroxylfreeradicals.Ozonedemandsareassociatedwiththefollowing:

∙Reactionswithnaturalorganicmatter（NOM）inthewater.TheoxidationofNOMleadstotheformationofaldehydes,organicacids,andaldo-andketoacids（Singer,1992）.

∙Organicoxidationbyproducts.Organicoxidationbyproductsaregenerallymoreamenabletobiologicaldegradationandcanbemeasuredasassimilableorganiccarbon（AOC）orbiodegradabledissolvedorganiccarbon（BDOC）.

∙Syntheticorganiccompounds（SOCs）.SomeSOCscanbeoxidizedandmineralizedunderfavorableconditions.Toachievetotalmineralization,hydroxylradicaloxidationshouldusuallybethedominantpathway,suchasachievedinadvancedoxidationprocesses.

∙Oxidationofbromideion.Oxidationofbromideionleadstotheformationofhypobromousacid,hypobromiteion,bromateion,brominatedorganics,andbromamines（seeFigure3-2）.

∙Bicarbonateorcarbonateions,commonlymeasuredasalkalinity,willscavengethehydroxylradicalsandformcarbonateradicals（Staehelinetal.,1984;

GlazeandKang,1988）.Thesereactionsareofimportanceforadvancedoxidationprocesseswheretheradicaloxidationpathwayispredominant.

Source:

GuntenandHoigné

1996.

Figure3-2.ReactionofOzoneandBromideIonCanProduceBromateIonandBrominatedOrganics

3.2OzoneGeneration

3.2.1OzoneProduction

Becauseozoneisanunstablemolecule,itshouldbegeneratedatthepointofapplicationforuseinwatertreatment.Itisgenerallyformedby,combininganoxygenatomwithanoxygenmolecule（O2）:

3O22O3

Thisreactionisendothermicandrequiresaconsiderableinputofenergy.

Schö

nbein（Langlaiset<

biblio>

）firstdiscoveredsyntheticozonethroughtheelectrolysisofsulfuricacid.Ozonecanbeproducedseveralways,althoughonemethod,coronadischarge,predominatesintheozonegenerationindustry.Ozonecanalsobeproducedbyirradiatinganoxygen-containinggaswithultravioletlight,electrolyticreactionandotheremergingtechnologiesasdescribedbyRice（1996）.

Coronadischarge,alsoknownassilentelectricaldischarge,consistsofpassinganoxygen-containinggasthroughtwoelectrodesseparatedbyadielectricandadischargegap.Voltageisappliedtotheelectrodes,causinganelectronflowthroughacrossthedischargegap.Theseelectronsprovidetheenergytodisassociatetheoxygenmolecules,leadingtotheformationofozone.Figure3-3showsabasicozonegenerator.

Figure3-3.BasicOzoneGenerator

3.2.2SystemComponents

AsshowninFigure3-4,ozonewatertreatmentsystemshavefourbasiccomponents:

agasfeedsystem,anozonegenerator,anozonecontactor,andanoff-gasdestructionsystem.Thegasfeedsystemprovidesaclean,drysourceofoxygentothegenerator.Theozonecontactortransferstheozone-richgasintothewatertobetreated,andprovidescontacttimefordisinfection（orotherreactions）.Thefinalprocessstep,off-gasdestruction,isrequiredasozoneistoxicintheconcentrationspresentintheoff-gas.Someplantsincludeanoff-gasrecyclesystemthatreturnstheozone-richoff-gastothefirstcontactchambertoreducetheozonedemandinthesubsequentchambers.Somesystemsalsoincludeaquenchchambertoremoveozoneresidualinsolution.

Figure3-4.SimplifiedOzoneSystemSchematic

3.2.2.1GasFeedSystems

Ozonefeedsystemsareclassifiedasusingair,highpurityoxygenormixtureofthetwo.Highpurityoxygencanbepurchasedandstoredasaliquid（LOX）,oritcanbegeneratedon-sitethrougheitheracryogenicprocess,withvacuumswingadsorption（VSA）,orwithpressureswingadsorption（PSA）.Cryogenicgenerationofoxygenisacomplicatedprocessandisfeasibleonlyinlargesystems.Pressureswingadsorptionisaprocesswherebyaspecialmolecularsieveisusedunderpressuretoselectivelyremovenitrogen,carbondioxide,watervapor,andhydrocarbonsfromair,producinganoxygenrich（80–95percentO2）feedgas.Thecomponentsusedinpressureswingadsorptionsystemsaresimilartohighpressureairfeedsystemsinthatbothusepressureswingmolecularabsorptionequipment.Lowpressureairfeedsystemsuseaheatreactivateddesiccantdryer.

OxygenFeedSystems-Liquidoxygenfeedsystemsarerelativelysimple,consistingofastoragetankortanks,evaporatorstoconverttheliquidtoagas,filterstoremoveimpurities,andpressureregulatorstolimitthegaspressuretotheozonegenerators.

AirFeedSystems-Airfeedsystemsforozonegeneratorsarefairlycomplicatedastheairshouldbeproperlyconditionedtopreventdamagetothegenerator.Airshouldbecleananddry,withamaximumdewpointof-60º

C（-80º

F）andfreeofcontaminants.Airpreparationsystemstypicallyconsistofaircompressors,filters,dryers,andpressureregulators.Figure3-5isaschematicoflargescaleairpreparationsystem.

Particlesgreaterthan1mandoildropletsgreaterthan0.05mshouldberemovedbyfiltration（Langlaisetal.,1991）.