Encyclopedia of Physical Science and Technology - Environmental Science

290 Pages·2001·5.1 MB·English

by  Robert A. Meyers (Editor-in-Chief)

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Drinking Water Quality
and Treatment
Dan Askenaizer
MontgomeryWatsonEngineers
I. OverviewofDrinkingWaterSources
II. ProtectionofSourceWater
III. OverviewofBasicDrinkingWater
TreatmentProcesses
IV. Disinfection
V. DistributionSystem
VI. DisinfectionBy-Products
VII. AlternativestoChlorination
VIII. AdvancedTreatmentMethods
IX. NewEmergingIssuesforDisinfection
X. DrinkingWaterRegulations
GLOSSARY Filtration Filtration is the process of removing sus-
pendedsolidsfromwaterbypassingthewaterthrough
Disinfection Watersystemsadddisinfectantstodestroy apermeablefabricorporousbedofmaterial.Themost
microorganisms that can cause disease in humans. common filtration process employs a granular media
Primarymethodsofdisinfectionincludechlorination, (e.g.,sand,anthracitecoal).Filtrationisusuallyacom-
chloramines, chlorine dioxide, ozone, and ultraviolet binationofphysicalandchemicalprocesses.
light. Membrane filtration Membrane separation processes
Disinfectionbyproducts Sidereactionscanoccurinwa- usesemipermeablemembranestoseparateimpurities
terwhenchemicaloxidantssuchaschlorineandozone fromwater.Themembranesareselectivelypermeable
areusedtocontrolpotentiallypathogenicmicroorgan- towaterandcertainsolutes.Adrivingforceisusedto
isms.Thesereactionscanformlowlevelsofdisinfec- force the water to pass through the membrane, leav-
tionbyproducts,severalofwhichhavebeenregulated ingtheimpuritiesbehindasaconcentrate.Theamount
forpotentialadversehumanhealtheffects. and type of material removed depends upon the type
651
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652 DrinkingWaterQualityandTreatment
ofmembrane,thetypeandamountofthedrivingforce facet of providing safe drinking water. Water treatment
andthecharacteristicsofthewater. professionalsdealwithawiderangeofwaterqualitiesand
Ozone Ozoneisacolorlessgasthatisextremelyunsta- they have a growing array of treatment methods at their
ble and is a strong oxidizing agent that is capable or disposal.Duringthe1970sand1980stherewasagrow-
reacting with a wide variety of organic and inorganic inginterestandconcernwithgroundwatercontamination
solutesinwater.Effectivenessofozonedisinfectionis duetoorganicchemicalssuchassolventsandpesticides.
afunctionofthepH,temperatureofwaterandmethod In the early 1990s and in to the new millennium, there
forozoneapplication. is a growing awareness of the need to balance the risks
Ultravioletlight Ultravioletlightiselectromagneticen- of the need to disinfect the water to reduce the threat of
ergythatislocatedintheelectromagneticspectrumat diseasefrommicroorganismsagainstthepotentialhealth
wavelengthsbetweenthoseofX-raysandvisiblelight. risks from disinfection byproducts that are formed as a
UVlightthatiseffectiveisdestroyingmicrobialenti- resultsofaddingadisinfectant.Microorganismssuchas
tiesinlocatedinthe200-to310-nmrangeoftheen- GiardiaandCryptosporidiumpresentchallengestoregu-
ergyspectrum.MosttypicalapplicationsofUVatwa- latorsandwatertreatmentengineers.Thepurposeofthis
tertreatmentplantsapplyUVlightinthewavelength chapteristoprovideanoverviewofdrinkingwaterquality
rangeof250to270nm. andtreatmentmethods.
I. OVERVIEW OF DRINKING
WATER is the most abundant compound on the surface WATER SOURCES
oftheearth.Thephysicalandchemicalpropertiesofwa-
terareimportantissueswithregardtowatersupply,water Thevastmajorityoffreshwaterintheworldisprovidedby
quality,andwatertreatmentprocesses.Withtheincreasing precipitation resulting from the evaporation of seawater.
growthofurbanareasandactivitiesthatcanpossiblyin- This transfer of moisture from the sea to land and back
troducecontaminantsintodrinkingwatersources,source to the sea is referred to as the hydrologic cycle. Figure 1
water protection has become an increasingly important presentsadepictionofthehydrologiccycle.
FIGURE1 Hydrologiccycle.
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DrinkingWaterQualityandTreatment 653
Abouttwothirdsoftheprecipitationwhichreachesland B. Groundwater
surfaceisreturnedtotheatmospherebyevaporationfrom
Groundwater sources are beneath the land surface and
watersurfaces,soil,andvegetation,andthroughtranspira-
include springs and wells. As can be seen from the
tionbyplants.Theremainderoftheprecipitationreturns
hydrologiccycle,whenrainfallstotheground,somewa-
ultimately to the ocean through surface or underground
ter flows along the land to streams or lakes, some water
channels.
evaporatesintotheatmosphere,someistakenupbyplants,
The following section presents a brief overview of
andsomeseepsintotheground.Aswaterseepsintothe
threesourcesofwater:surfacewatersources,groundwa-
ground,itentersazonereferredtoastheunsaturatedzone
tersources,andtheuseofreclaimedwater.IntheUnited
orvadosezone.Watermovesthroughtheunsaturatedzone
States around 35% of the population served by commu-
intothesaturatedzone,wheretheinterconnectedopenings
nity water systems drink groundwater while nearly 65%
betweenrockparticlesarefilledwithwater.
ofthepopulationservedbycommunitywatersystemsre-
Groundwater quality is typically constant over time;
ceive water taken primarily from surface water sources.
however,changesinhydrogeologicalconditionscanlead
Increasingly,communitiesarelookingtoreclaimedwater
todifferencesinwaterqualityoverarelativelyshortdis-
tomeetaportionoftheirneeds.
tance.Thechemistryofthegroundwaterisinfluencedby
thecompositionoftheaquiferandbythechemicalandbi-
A. SurfaceWater ologicalprocessesthatoccuraswaterinfiltratesthrough
theaquifer.
Surface waters sources for drinking water include lakes,
rivers, canals, runoff, and impounding reservoirs. Wa-
C. ReclaimedWater
ter quality conditions in streams and rivers can change
dramatically over a short period of time. Surface water
An additional source of water that must be included in
sourcessuchasstreamsandriversaresusceptibletochem-
any discussion of sources is the use of reclaimed water
icalspillsandaccidentalreleases.
fornonpotableandpotableuses.Reclaimedwatersources
Therearethreestages(trophiclevels)inthelifecycle
include desalination of brackish water or seawater, and
ofabodyofwater.Theseareoligotrophic(lownutrients,
reuseorrecyclingofwastewaterthroughtheapplication
minimal microbiological activity), mesotrophic (moder-
of appropriate treatment technology. In some cities, re-
atenutrients,moderatemicrobiologicalactivity),andeu-
claimedwastewaterhasbeenusedtoirrigategolfcourses
trophic(highnutrients,highmicrobiologicalactivity).
andparks.Inaddition,severalcommunitieshaveseriously
Water quality in lakes and reservoirs can change
evaluatedandstudiedtheuseofreclaimedwastewaterto
throughouttheyearasthewatercanstratifyduringwarmer
augment the drinking water supply (e.g., adding highly
months. Thermal stratification can be a significant pro-
treatedwastewatermeetingdrinkingwaterstandardsun-
cessinmanylakes.Duringthewarmermonthsoftheyear
dertheSafeDrinkingWaterActtoareservoirthatisthe
the warmer water (and therefore less dense water) will
source for raw water for the community’s drinking wa-
stay near the surface of the water body while the cooler
tertreatmentplant).Thisdrivetowardutilizingagreater
and more dense water is trapped below. In the absence
amount of reclaimed water comes from growing urban
of strong winds there will be little mixing of the colder,
populations and constraints on the development of new
denserwaterbelowwiththewarmer,lessdensewaternear
water sources. Some public health authorities are reluc-
thesurface.Undercertainconditionswhereadequatenu-
tant to support or endorse the planned use of reclaimed
trientsarepresentthiscanleadtoadepletionofoxygenin
watertoaugmentadrinkingwatersupply.However,itis
thelowerpartsofthewaterbodyandcanthuscausewater
alreadythecasethattherearemanysurfacewatersources
qualityissuessuchastasteandodorproblemsandprob-
(riverandstreams)thataresubjecttosewagecontamina-
lemswithironandmanganese(whichwillhaveincreased
tionpriortotheiruseasapotabledrinkingwatersupply.
solubilityunderthereducingconditions).Astemperatures
In these instances, in effect, the cities are practicing un-
coolandthetemperatureofthesurfaceofthelakecools,
plannedindirectpotablereuse.
thistogetherwithwindactioncancausemixingthrough-
outthereservoir.
Depending upon nutrient, temperature, and carbonate II. PROTECTION OF SOURCE WATER
conditionstheupperregionsofalakeorreservoircanbe
susceptibletoalgalblooms(whichcancausechangesin Natural waters acquire their chemical characteristics by
sourcewaterturbidity,alkalinity,taste,odor,andpH)and dissolutionandbychemicalreactionswithsolids,liquids,
canmakeitdifficulttotreatthewaternearthesurfaceof andgaseswithwhichtheyhavecomeintocontactduring
thelake. the various parts of the hydrological cycle. An example
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654 DrinkingWaterQualityandTreatment
(cid:1)
TABLEI WaterQualityResultsforThreeDifferentSourcesa Saltwaterintrusion(increasedsalinity)
(cid:1)
(fromSnoeyink,Jenkins,1980) Wastewaterdischarges(bacterialcontamination,
Constituent Source1b Source2c Source3d depletionofdissolvedoxygenincreasedlevelsof
(mg/L) (reservoir) (riverwater) (groundwater) inorganicandorganiccontaminants)
(cid:1)
Industrialdischarges(accidentalorplanneddischarges
SiO2 9.5 1.2 10 ofchemicalcontaminants)
Fe(III) 0.07 0.02 0.09 (cid:1)
Hazardouswastefacilities(releaseoftoxic,reactive,
Ca2+ 4.0 36 92
corrosive,orflammablecontaminants)
Mg2+ 1.1 8.1 34 (cid:1)
Minedrainage(aciddischarges,increasesin
Na+ 2.6 6.5 8.2
sediments,turbidity,color)
K+ 0.6 1.2 1.4 (cid:1)
Agriculturalrunoff(pesticides,herbicides,fertilizers)
HCO− 18.3 119 339 (cid:1)
3 Livestock(microbialcontamination,increasederosion
SO2− 1.6 22 84
4 increasednitrates)
Cl− 2.0 13 9.6 (cid:1)
UrbanRunoff(petroleumproducts,metals,salts,silts,
−
NO 0.41 0.1 13
3 andsediments)
Totaldissolvedsolids 34 165 434 (cid:1)
Landdevelopment(increasederosionandsediment
TotalhardnessasCaCO3 14.6 123 369 loading,increasedhumanactivitiesthatcanrelease
contaminantstotheenvironment)
a(From Snoeyink, V. L., Jenkins, D. (1980). “Water Chemistry,” (cid:1)
JohnWileyandSons,NewYork.) Atmosphericdeposition(acidrain)
(cid:1)
b(Source1.PardeeReservoir.EastBayMunicipalUtilityDistrict, Recreationalactivities(swimming,boating,camping)
Oakland,CA,1976.)
c(Source2.NiagaraRiver,NiagaraFalls,NewYork.)
Activities to protect and enhance the quality of sur-
d(Source3.Groundwater,Dayton,OH.)
face water sources include conducting sanitary surveys,
programs to monitor source water quality and activities
ofthiswouldbeweatheringreactions,whicharecaused toprovidewatershedcontrol.Otheractivitiesthatcanbe
bytheinteractionofwaterandatmospherewiththecrust undertaken to protect source water include storm-water
of the earth. Table I presents examples of water quality for management,developmentandimplementationofemer-
threedifferentsources gencyresponseprocedures(tocontainandcleanupspills
Beginning in the early 1970s there was a growing in- topreventcontaminationofsourcewater).
terest in the presence of and increased understanding of A sanitary survey is an on-site review of the water
healtheffectsassociatedwithlowlevelsoforganiccom- source,facilities,equipment,operation,andmaintenance
poundsinwater.Organiccompoundsinwatercanoccur ofapublicwatersystemtoevaluatetheadequacyofthe
dueto(1)degradationofnaturallyoccurringorganicmate- source,facilities,equipment,operation,andmaintenance
rial(i.e.,leaves),(2)humanactivitiessuchashandlingand forproducinganddistributingdrinkingwater.
disposalofchemicals,and(3)chemicalsreactionsduring Monitoringprogramscanbeconductedofbothchem-
thetreatmentofwater(i.e.,theproductionofdisinfection icalandmicrobiologicalparametersatlocationsthrough-
byproducts). out a source of supply. A thorough monitoring program
Protecting sources of drinking water has become an can provide valuable information toward an understand-
increasingly important aspect of providing safe drinking ingandidentificationofpossiblechangesinsourcewater
water. Source water quality management is the first step quality.
towardensuringanadequatesupplyofsafedrinkingwater. Forgroundwatersources,thepotentialeffectivenessof
Potential sources of contamination or water quality agroundwatermanagementprogramdependsuponthede-
problemsinsourcewaterincludethefollowing: greetowhichpotentialcontaminationisaccuratelyidenti-
fiedandthepracticalityoftheresponse,remediation,and
(cid:1)
Climate(primarilyprecipitationcausingincreased protectionmeasuresthataredeveloped.Allresidentialand
levelsofsediment,turbidity,andothercontaminants) commercial development and industrial and agricultural
(cid:1)
Temperature(canaffectbiologicalactivity,oxygen activities within the well field zone of influence and up-
saturation,andmasstransfercoefficients) streamofthegeneraldirectionofgroundwaterflowshould
(cid:1)
Watershedcharacteristics(steepslopes,vegetative be investigated, and monitoring programs can be imple-
cover,wildlife) mentedtodetectandcontrolcontaminantsthatcouldbein-
(cid:1)
Geology(e.g.,mineralcontent) troducedintothegroundwater.Forsurfacewatersources,
(cid:1)
Presenceofnutrients(canstimulatemicrobiological sourceprotectioncaninvolvesuchactivitiesasstormwa-
growth) termanagementandcontrolsonactivitiesinawatershed
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DrinkingWaterQualityandTreatment 655
thatcouldimpactthequalityofthesourcewater,includ- nonionic.Anionicpolymersionizeinwatertoformnega-
ingactivitiessuchasfishing,boating,swimming,hunting, tivesitesalongthepolymermolecule.Cationicpolymers
andcamping. ionize to form positively charged sites, while nonionic
polymersexhibitonlyslightionization.
Flash or rapid mixing is an important part of coag-
III. OVERVIEW OF BASIC DRINKING ulation. The purpose of flash mixing is to quickly and
WATER TREATMENT PROCESSES uniformlydispersewatertreatmentchemicalsthroughout
the water. Effective flash mixing is especially important
Theamountandtypeoftreatmentappliedbyagivenpub- when using metal salt coagulants, since their hydrolysis
licwatersystemwillvarydependinguponthesourcetype occurswithinasecondandsubsequentadsorptiontocol-
andquality.Many,ifnotmost,groundwatersystemscan loidalparticlesisalmostimmediate.Rapidmixprocesses
provideadequatetreatmentthatinvolveslittleornotreat- cantypicallybeaccomplishedinjustafewminutes.
mentofthesource.Surfacewaters,however,areexposed
totheatmosphereandsurfacerunoffandaremorelikely B. Flocculation
tocontaincontaminants.Surfacewatersystems,therefore,
Flocculationisagentlemixingphasethatfollowstheini-
mustimplementagreaterleveloftreatmenttoprovidesafe
tialrapidmixstep.Duringtheflocculationstepthechemi-
andpotabledrinkingwater.
callytreatedwaterissentintoabasinwherethesuspended
Waterutilitiescanuseavarietyoftreatmentprocesses
particlescancollideandformheavierparticlescalledfloc.
together at a single treatment plant to remove contam-
Gentleagitationandappropriatedetentiontimesareused
inants, remove turbidity, and provide disinfection. The
toallowthisprocesstooccur.Typicaltimeforthefloccu-
most common physical processes used at public water
lationstepcouldbeontheorderof15–30min.Afterthe
systemswithsurfacewatersuppliesincludecoagulation,
flocculation step the water can then move into the sedi-
flocculation, sedimentation, and filtration. The follow-
mentationstep.
ing sections provide descriptions of physical and chem-
ical processes that can be used to treat drinking water.
In addition to a description of some basic water treat- C. Sedimentation
ment methods (coagulation, flocculation, sedimentation
Alsoknownasclarification,thepurposeofsedimentation
and filtration, slow sand filtration, lime-soda softening,
istoremoveamajorityofthesettleablesolidsbygravita-
granularactivatedcarbon),thefollowingsectionalsopro-
tionalsettling.Byremovingthemajorityofthesettleable
vides description of some typical water quality issues
solidsinthesedimentationstepthiswillmaximizedown-
thatcanbeaddressedthroughtreatment(ironandmanga-
streamunitprocessessuchasfiltration.Duringthesedi-
neseremoval,tasteandodorproblem,corrosioncontrol)
mentationstep,thevelocityofwaterisdecreasedsothat
and additional treatment methods (membranes and ion
suspendedmaterialcansettleoutofthewaterstreamby
exchange).
gravity. The key to effective sedimentation is proper co-
agulation and flocculation of suspended material in the
A. Coagulation raw water. Removal and disposal of the sludge from the
sedimentation basin are important parts of the treatment
Colloidalsuspendedparticlesinwaterhavelikeelectrical
process.
chargesthattendtokeeptheminsuspension.Coagulation
isdefinedasthedestabilizationofthechargeoncolloids
D. Filtration
and suspended solids, including bacteria and viruses, by
useofacoagulant.Themostcommonlyusedcoagulants Filtration is the process of removing suspended solids
aremetalsaltcoagulantssuchasaluminumsulfate,ferric fromwaterbypassingthewaterthroughapermeablefab-
chloride,andferricsulfate.Inwater,metalsaltsundergo ricorporousbedofmaterial.Themostcommonfiltration
hydrolysis.Theproductsofthishydrolysisreadilyadsorb process employs a granular media (e.g., sand, anthracite
to colloid particles and cause the destabilization of their coal).Filtrationisusuallyacombinationofphysicaland
electricalcharge.Animportantparameterindetermining chemicalprocesses.Mechanicalstrainingremovessome
the effectiveness of a given coagulant is the pH of the particlesbytrappingthembetweenthegrainsofthefilter
water. medium(suchassand).Adhesionisanequallyimportant
Synthetic polymers such as polydiallyl dimethyl am- processbywhichsuspendedparticlessticktothesurface
monium(PDADMA)arealsousedascoagulants.Organic offiltergrainsorpreviouslydepositedmaterial.Theaver-
polymerscanbeusedastheprimarycoagulantorasaco- agefiltrationrateintheUnitedStatesis5–6gal/min/ft2of
agulantaid.Polymersareclassifiedasanionic,cationic,or filterarea.Ataconventionaltreatmentplantthefiltersare
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656 DrinkingWaterQualityandTreatment
FIGURE2 Conventionaltreatmentprocess.
precededbycoagulation,flocculation,andsedimentation. fromwaterthroughacombinationandphysicalstraining
Atadirectfiltrationplant,thefiltersareprecededbycoag- andmicrobiologicalprocessesinaslowsandfilter.When
ulationandflocculationonly;withtheflocbeingremoved in operation, the surface of the filter bed is covered by
directly by the filters. Figure 2 presents a schematic for a thin layer of medium, known as the “schmutzdecke.”
aconventionaltreatmentplanttogetherwithexamplesof Thislayercontainsalargevarietyofmicroorganismsand
potentialapplicationpointsforchemicaladdition.Figure3 enablesthesefilterstoremovelargenumbersofbacteria.
presentstheschematicforadirectfiltrationplant. Slowsandfiltersdonotrequirehighlytrainedoperators,
have minimal power requirements, and can tolerate rea-
sonablehydraulicandsolidsshockloadings.Someofthe
1. SlowSandFilters
disadvantagesofslowsandfiltersincludethelargeamount
Slowsandfiltersareoperatedatverylow filtrationrates of land they require; the filters can be easily clogged by
without the use of coagulation. Slow sand filters are a excessiveamountsofalgae;theyarenotveryeffectiveat
simple, reliable and easy to operate system. The filtra- removing color; and intermittent operation of the filters
tion rate for slow sand filters is typically 50–100 times maydegradethequalityofthefiltereffluentbypromoting
slower than that of granular media filters. Therefore, a anaerobicconditionswithinthefilterbed.Thefiltersmust
muchlargerareaisneededforthefilterbedtoproducean beperiodicallycleanedbyscrapingoffathinlayerofsand
equivalent amount of water. Contaminants are removed fromthesurfaceofthefilterbed.
FIGURE3 Directfiltrationprocess.
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DrinkingWaterQualityandTreatment 657
TABLEII GeneralClassificationofWater heatingthecoalunderanaerobicconditions.Theproduct
Hardnessa ofthisprocessisthenactivatedbyexposuretoamixtureof
Levelofhardness(mg/L) Classification steamandairatatemperatureof1500◦F,whichoxidizes
thesurfaceofthecarbonporesandallowsthesurfaceto
0–75 Softwater
attractandholdorganiccompounds.Afterbeingusedata
75–150 Moderatelyhardwater
watertreatmentplant,thespentGACcanberegenerated
150–300 Hardwater
throughsteam,thermalregenerationandchemicalmeans.
>300 Veryhardwater
aHardnessissometimesexpressedasgrainspergal-
G. IronandManganeseRemoval
lon, where 1 grain of CaCO3/gallon is equivalent to
17.1mg/LasCaCO3. Thepresenceofironandmanganeseisdrinkingwaterhave
beenassociatedwithunpleasanttasteandodors,staining
of laundry and fixtures as well as causing the formation
E. Lime-SodaSoftening
of mineral deposits. Iron and manganese are commonly
The hardness of water is defined as the concentrations foundinsoilininsolubleforms.Whenwatercontainscar-
of divalent metallic cations in water and is expressed as bondioxide(orisanacidicwater)thentheferricironcan
mg/LofCaC0 . Table II presents a general classification bereducedtotheferrousform(whichissolubleinwater)
3
ofwaterhardness.Theprincipaldivalentmetalliccations andmanganeseisreducedtoaformthatisalsosolublein
thatcontributetothehardnessofawaterarecalciumand water.Treatmenttoremoveironandmanganeseincludes
magnesium,withcontributionsfromiron,manganese,and oxidation(aeration,chlorination,chlorinedioxide,potas-
strontium.Themainpurposeofwatersofteningistore- siumpermanganate,ozone)followedbyclarificationand
duce the levels of calcium and magnesium in the water filtration;ionexchange;stabilizationthroughuseofase-
toreducethehardnessofthewater.IntheUnitedStates questeringagent,andlimesoftening.
thereareover1000treatmentplantsweresofteningisprac-
ticed.Historically,softeningwasimportantduetothehigh
H. TasteandOdorControl
consumption of soap by hard water. With today’s syn-
theticdetergentsthisisnolongeramajorissue.However, Themostfrequentcausesoftasteandodorinadrinking
thereareotherbenefitstosofteningincludingremovalof wateraremetabolitesofalgae(mostcommonlyblue-green
heavymetals,metallicelementsandorganiccompounds; algae), actinomycetes (filamentous bacteria), and decay-
effective destruction of bacteria, viruses and algae; and ing vegetation. Other potential causes of taste and odor
improvementinboilerfeedwaterandcoolingwaters. issuesarehydrogensulfide,agriculturalrunoff,industrial
While there are many variations, the primary method chemicalspills,andsewagepollution.
ofsofteningawateristheadditionoflime(calciumhy- The most common odor-producing compounds are
droxide) and soda ash (sodium carbonate) to the water. geosminand2-methlyisoborneal(MIB)whichcanimpart
Thepurposeofaddingthesecompoundstoawateristo objectionableodoratverylowconcentrations.Thesecom-
change the hardness compounds such that they become poundsareresponsiblefortheearthy-mustyodorsinwater
insoluble and precipitate (e.g., calcium and magnesium andhavebeenisolatedfromactinomycetes(Actinomyces,
areconverted,respectively,tocalciumcarbonate(CaCO , Nocardia,Streptomyces)andfromblue-greenalgae(e.g.,
3
partiallysoluble)andmagnesiumhydroxide(Mg(OH) ). AnabaenaandOscillatoria).Controlmethodsfortasteand
2
Inadditiontochemicaltreatment,ionexchangeresinsand odorinclude:preventionatthesource(reservoirmixing,
membranescanbeusedtosoftenawater. aquaticplantcontrol,reservoirmanagement),removalof
a particular constituent at the treatment plant (aeration,
oxidation,adsorption),andcontrolwithinthedistribution
F. GranularActivatedCarbon
system (minimization of dead-ends, use of blow-off and
Granularactivatedcarbon(GAC)hasbeenusedasasub- cleanoutassemblies,distributionsystemflushing).
stituteforgranularfiltermediaandasanadditionalpro-
cess in conventional treatment plants for the removal of
I. CorrosionControl
organiccompoundsincludingcompoundsproducingtaste
and odors, pesticides and other synthetic organic com- All waters are corrosive to some degree. Corrosion can
pounds. GAC can be manufactured from a large variety reducethelifeofapipebyreducingwallthicknessuntil
of materials including wood, nuts, shells, coal, peat, or there are leaks, it can result in encrustations that reduce
petroleumresidues.GACusedinwatertreatmentplantsis the effective carrying capacity and can result in corro-
typicallymanufacturedfrombituminousorlignitecoalby sion by-products at the consumer’s tap that have public
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658 DrinkingWaterQualityandTreatment
healthimplications(i.e.,leadandcopper).Thetendency mentforcontrollingand/orreducingleadleachinginclude
ofawatertobecorrosivewilldependonitsphysicaland pHadjustment,carbonateadjustment,useofcorrosionin-
chemicalcharacteristicsaswellasonthenatureofthema- hibitors,calciumcarbonatedeposition,aswellaspainting,
terialitcomesintocontactwith.Themostcommontypes coating,andusageofcathodicprotectionsystems.
ofmaterialsusedindistributionsystemsincludecastiron,
ductileiron(castironcontainingasmallamountofalloy-
J. MembraneFiltrationTechnology
ingelementssuchasmagnesium),asbestos-cement,steel,
copper,galvanizediron,andplastics. Membraneseparationprocessesusesemipermeablemem-
The mechanisms of corrosion in a water distribution branestoseparateimpuritiesfromwater.Themembranes
systemaretypicallyacomplexandinterrelatedcombina- areselectivelypermeabletowaterandcertainsolutes.A
tionofphysical,chemical,andevenbiologicalprocesses. drivingforceisusedtoforcethewatertopassthroughthe
Thebasicprinciplesthataffectcorrosionofmaterialsin- membrane,leavingtheimpuritiesbehindasaconcentrate.
cludesolubility,describedbychemicalequilibriaamong Theamountandtypeofmaterialremoveddependsupon
materialsandconstituentsinthewater;andtherateofdis- thetypeofmembrane,thetypeandamountofthedriving
solution,whichisdescribedbychemicalandelectrochem- force,andthecharacteristicsofthewater.Importantissues
icalkinetics.Electrochemicalcorrosionoccurswheretwo involvedwiththeoperationofmembranesystemsinclude
differentmetalshaveanelectropotentialbetweenthemare membranefoulinganddisposaloftheconcentrate.
immersedinacommonbodyofwater.Allwaterscanact Therearetwoclassesofmembranetreatmentsystems.
as an electrolyte, but the degree to which they do so de- These include low-pressure membrane systems (such as
pendsonthedissolvedchemicalspresent.High-velocity microfiltration (MF) and ultrafiltration (UF)), and high-
waterflowcancausepittinganderosionofsurfacesdue pressuremembranesystems(suchasnanofiltration(NF)
to cavitation. Certain types of bacteria, sulfate-reducing andreverseosmosis(RO)).Low-pressuremembranesare
bacteria,andironbacteriacanalsocancauseinternalcor- operatedatpressuresrangingfrom10to30lb/in.2 (psi),
rosioninpipingmaterial.Otherfactorsthatcaninfluence whereas high-pressure membranes, including nanofiltra-
thecorrosivityofagivenwaterincludetheconcentration tionareoperatedatpressuresrangingfrom75to250psi.
ofdissolvedsaltsinthewater,levelofdissolvedgasesin Figure 4 presents a general description of various mem-
thewater,watertemperature,andstressandfatigue. branes types and their ability to remove impurities from
In terms of lead solubility, the most important water water.
qualityparametersarepH,alkalinity,dissolvedinorganic MFcanremoveparticlesthataregreaterthan0.5µm
carbonateandorthophosphatelevels.Ingeneral,lowpH indiameter.UFiscapableofremovingcolloids,bacteria,
levelshavebeenassociatedwithhigherleadlevelsatthe viruses,andhigh-molecular-weightorganiccompounds.
tap.SoftwatersthatarelowinpHandalkalinityareof- Some advantages of using low-pressure membranes
ten corrosive toward lead and other metals. Water treat- include small waste stream, limited chemical usage, a
FIGURE4 Membranesizeranges.
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DrinkingWaterQualityandTreatment 659
relativelysmallareaneededforequipment,goodpathogen movecarbonatehardness.Ontheotherhand,WACresins
reduction,andnodisinfectionby-productformation.Low- areeasiertoregeneratethanSACresinsanddonotresult
pressure membranes, however, are ineffective for the insodiumconcentrationincreaseasSACresinsdo.
removal of dissolved organic matter. Therefore, color- AnimportantissuefortheapplicationofIXtechnology
causing organic matter, taste-and-odor causing com- isthewastestreamproducedbytheprocess.Thevolume
pounds such as Geosmin and methylisoborneol (MIB), of the waste stream is not large on the order of 2 to 5%
andman-madechemicalscanpassthroughthemembranes of the water volume treated. However, the waste stream
intothetreatedwater. containsahighconcentrationofacid(HCl),base(NaOH),
NFmembraneshavebeenusedsuccessfullyforground- orsalt(NaCl).Thewastestreamalsocancontainahigh
water softening since they achieve greater than 90% re- concentrationofthecontaminantremovedfromthewater
jectionofdivalentionssuchascalciumandmagnesium. (e.g.,NO−,Pb2+,etc...).
3
NFmembranesarealsocapableofremovinggreaterthan
90%ofnaturalorganicmatterpresentinthewater.There-
IV. DISINFECTION
fore,theyarealsoexcellentcandidatesfortheremovalof
color,andalsoDBPprecursormaterial.
Disinfection is an important step in ensuring that water
ROmembraneshavelongbeenusedforthedesalination
is safe to drink. Water systems add disinfectants to de-
ofseawateraroundtheworld.Thesemembranescancon-
stroy microorganisms that can cause disease in humans.
sistently remove about 99% of the total dissolved solids
Primary methods of disinfection include chlorination,
(TDS) present in the water, including monovalent ions
chloramines,chlorinedioxide,ozone,andultravioletlight.
suchaschloride,bromide,andsodium.
Fromahistoricalperspective,theChick–Watsonmodel
hasbeenthepredominantmodelusedtodescribetheki-
K. Ion-ExchangeTechnology neticsofusingdisinfectantstoinactivatemicroorganisms.
Chick’slaw(1908)expressestherateofdestructionofmi-
Ion-Exchange(IX)technologyremovesundesirableions
croorganismsusingtherelationshipofafirst-orderchem-
from raw water and exchanges them for desirable ions.
icalreaction.Watson(1908)refinedtheequationtopro-
Thetwomostcommonapplicationsofionexchangeare
for water softening (Ca2+ and Mg2+ removal), either at duce an empirical relationship that reflected changes in
thedisinfectantconcentration.TheChick–Watsonmodel
the water treatment plant or as a point-of-entry (POE)
canbeexpressedasfollows:
treatmentprocess,andforindustrialapplications,suchas
theproductionoffullydemineralizedwater. logN/N =−kCnT,
o
ExamplesofionsthatcanberemovedusingIXinclude
where N = theinitialconcentrationofbacteria, N=the
nitrate,arsenic,selenium,barium,radium,lead,fluoride, o
concentrationofsurvivingbacteriaattimeT,k=ratecon-
andchromate.
stant(coefficientofspecificlethality),C=concentration
IntheIXprocesswaterpassesthrougharesinbedwhere
of the disinfectant, and n=concentration of the dilution
contaminantionspresentinthewaterareexchangedwith
(empiricallyderived).TheChick–Watsonmodelleadsto
ionsontheresinsurface,thusremovingthecontaminant
a correlation between the level of inactivation and the
ionsfromthewaterandconcentratingthemontheresin.
product of the concentration of the disinfectant (C) and
Theresinisfrequentlyregeneratedtoremovethecontam-
the contact time (T). The United States Environmental
inant from the resin surface and replenish the resin with
Protection Agency’s (USEPA) Surface Water Treatment
theoriginalexchangeion.Therearefourprimarytypesof
Rule includes tables that associate specific CT values
IXresins:
(mg*min/L)withagivenlevelofinactivationofGiardia
andviruses.
1. StrongAcidCationic(SAC)Resin
2. WeakAcidCationic(WAC)Resin
A. Chlorine
3. StrongBaseAnionic(SBA)Resin
4. WeakBaseAnionic(WBA)Resin Chlorination of potable water has been practiced in the
United States since 1903. Chlorine can be applied by
SAC and WAC resins are used to remove cations from drinking water treatment plants as chlorine gas, sodium
water(e.g.,Ca2+,Mg2+,Ra2+,Ba2+,Pb2+),whileSBA hypochlorite solutions, or as solid calcium hypochlorite.
and WBA resins are used to remove anions from water Free chlorine reacts rapidly with many substances in
(e.g., NO−, SO2−, ClO−, HAsO2−, SeO2−). During wa- the water, including microorganisms. The effectiveness
3 4 4 4 3
tersoftening,SACresinscanremovebothcarbonateand of chlorine to provide disinfection is affected by many
noncarbonatehardness,whereasWACresinscanonlyre- variablesincludingamountofoxidizablesubstancesinthe
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660 DrinkingWaterQualityandTreatment
water(thatexertademandonthechlorine),concentration 3. CalciumHypochlorite
of particulate matter, pH, temperature, contact time, and
Calcium hypochlorite is a white solid that contains 65%
thelevelofresidualchlorine.Theformationofdisinfec-
availablechlorineanddissolveseasilyinwater.Calcium
tionbyproductsincludingtrihalomethanes(THMs)isone
hypochlorite is very stable and can be stored for an ex-
of the major concerns with regard to the use of chlorine
tendedperiodoftime.Calciumhypochloriteisacorrosive
disinfection.
material with a strong odor. Reactions between calcium
hypochlorite and organic material can generate enough
1. ChlorineGas heat to cause a fire or explosion. It must be kept away
fromorganicmaterialssuchaswood,cloth,andpetroleum
Elementalchlorineisatoxic,yellow-greengasatnormal
products.Calciumhypochloritereadilyabsorbsmoisture,
pressures.Athighpressures,itisaliquid.Chlorinegasis
formingchlorinegas.
typically supplied as a liquid in high pressure cylinders.
Calcium hypochlorite can be dissolved in a mixing/
Chlorinegasisreleasedfromtheliquidchlorinecylinder
holdingtankandinjectedinthesamemannerassodium
byapressurereducingandflowcontrolvalveoperatingat
hypochlorite. Alternatively, where the pressure can be
pressureslessthanatmospheric.Thegasisledtoaninjec-
loweredtoatmospheric,suchasatastoragetank,tablets
torinthewatersupplypipewherehighlypressurizedwater
of calcium hypochlorite can be directly dissolved in the
ispassedthroughaventuriorificecreasingavacuumthat
freeflowingwater.
drawsthechlorineintothewaterstream.Adequatemixing
andcontacttimemustbeprovidedafterinjectiontoensure
complete disinfection of pathogens. Gaseous chlorine, V. DISTRIBUTION SYSTEM
whenaddedtowater,rapidlyhydrolyzestohypochlorous
acid(HOCl)andhydrochloricacid(HCl)asfollows:
Whilebeyondthescopeofthischapter,avitalcomponent
Cl +H O←−→− HOCl+H++Cl− towardensuringthedeliveryofsafedrinkingwateristhe
2 2
seriesoftransmissionmains,firehydrants,valves,pump
Hypochlorousacidissubjecttofurtherreactionincluding
stations,boosterchlorinationstations,storagereservoirs,
disinfection,reactionswithvariousorganicandinorganic
standpipes, and service lines that constitute the distribu-
compoundsordissociationtohydrogenandhypochlorite
tionsystem.Theproperdesign,constructionmaterial,and
ion(OCl−)asfollows:
maintainingtheintegrityofthedistributionsystemandthe
HOCl←−→− H++OCl− individualcomponentsareimportanttomaintainthemi-
crobiologicalsafetyofthedrinkingwater.
The relative concentrations of hypochlorous acid and
hypochloriteionaredependentonthepHandthetemper-
ature.Generally,hypochlorousacidisabetterdisinfecting VI. DISINFECTION BY-PRODUCTS
agentthanishypochloriteion.
In1974trihalomethanes(THMs),werefirstidentifiedin
finished drinking water. In 1975 the USEPA conducted
2. SodiumHypochloriteSolution
theNationalOrganicsReconnaissanceSurveyof80cities
Sodiumhypochloriteisavailableasasolutioninconcen- intheUnitedStatesandobservedthattheoccurrenceof
trationsof5to15%chlorine.Sodiumhypochloriteiseas- THMswaswidespreadinchlorinateddrinkingwaterand
ier to handle than chlorine gas or calcium hypochlorite. wasassociatedwiththepracticeofusingchlorinetodis-
It is, however, extremely corrosive and should be kept infect the water. Later studies demonstrated that THMs
away from equipment that could be damaged by corro- continuedtoforminthedistributionsystem.
sion. Hypochlorite solutions decompose and should not THMs are a class of organic compounds where three
bestoredformorethan1monthandmustbestoredina hydrogen atoms in the methane molecule have been re-
cool,dark,dryarea. placed with three halogen atoms (chlorine or bromine).
The sodium hypochlorite solution is diluted with wa- The four THMs identified were chloroform, bromodi-
terinamixing/holdingtank.Thedilutedsolutionisthen chloromethane,dibromochloromethane,andbromoform.
injectedbyachemicalpumpintothewatersupplypipe THMswereimportanttoregulatorsinitiallyassuspected
at a controlled rate. Adequate mixing and contact time human carcinogens. Recent information suggests that
mustbeprovided.Sodiumhypochloritecanbegenerated somedisinfectionby-products(DBPs)mayhaveadverse
onsitebyusingelectrolysisofsodiumchloridesolution. developmentalandreproductiveimpacts.
Hydrogen gas is given off as a by-product and must be The production of THMs can be shown simply as
safelydispersed. follows: