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Copyright J.N. Ramaswamy, Ph D, PE www.PDHSite.com
Design of Sewage Treatment Plants, Course #407
Presented by:
PDH Enterprises, LLC
PO Box 942
Morrisville, NC 27560
www.pdhsite.com
This course discusses design of sewage treatment plants and explains the procedures and standards
required for preliminary, primary, secondary (biological) and advanced treatment methods for domestic
sewage.
The course describes preliminary treatment units such as bar screen, comminutor, and grit chamber withillustrations and design particulars.
Primary sedimentation is explained with illustration and design features. Numerical example with
solution is provided for better understanding.
Secondary (biological) treatment is explained in considerable length. Variety of secondary treatment
process units such as trickling filter (all forms), activated sludge units (all modifications), and different
types of stabilization ponds are described. Design standards are furnished for each unit. Numerical
problems with solution are provided for easy understanding.
Disinfection theory is explained. Different types of disinfectant such as chlorine, ozone, and chlorine
dioxide are described with their design and procedures for use.
Various methods of treated effluent disposal such as disposal in water bodies, disposal on land,
recreational, and municipal reuse are described.
Sludge treatment steps such as thickening (mechanical and flotation), digestion (aerobic and anaerobic),
conditioning, and dewatering (drying bed, vacuum filter, centrifuge) are explained with illustrations.
Sludge disposal by land filling, lagooning, incineration, and ocean disposal is described.
To receive credit for this course, each student must pass an online quiz consisting of twenty-five (25)
questions. A passing score is 70% or better. Completion of this course and successfully passing the quiz
will qualify the student for four (4)hours of continuing education credit.
Course Author:
JN Ramaswamy, PhD, PE
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DESIGNOFSEWAGETREATMENTPLANTS
By
J.N.Ramaswamy,Ph.D.,P.E.
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TABLEOFCONTENTS
I.
Introduction
II. PreliminaryTreatment
III.
PrimaryTreatment
IV. SecondaryTreatment
V. AdvancedTreatment
VI. Disinfection
VII.
EffluentDisposal
VIII.SludgeTreatmentandDisposal
ListofFigures
II.1. Handcleanedandmechanicallycleanedracks
II.2. Brushcleanedscreen
II.3. Plan&crosssectionalviewofacomminuter
II.4. Crosssectionofsutro&proportionalflowweirIII.1. Rectangularsedimentationtank
III.2. Typicalcircularsedimentationtank
IV.1. Cutawayviewofatricklingfilter
IV.2. Highratetricklingfilterflowsheets
IV.3. Underdrainblocksfortricklingfilters
IV.4. Flowdiagramforconventionalactivatedsludgeprocess
IV.5. Flowdiagramforcompletemixactivatedsludgeprocess
IV.6. Flowdiagramforstepaerationactivatedsludgeprocess
IV.7. Flowsheetforcontactstabilizationtank
IV.8. FlowsheetforextendedaerationtankIV.9. Crosssectionofanactivatedsludgeaerationtankwithdiffusers
IV.10. Mechanicalaerators
IV.11. Flowsheetforoxidationditch
IV.12. Schematicofaeratedandaerobicanaerobiclagoon
VI.1. Chlorinationflowdiagram
VI.2. DistributionofHOCLandOCLatdifferentpHsandtemperatures
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VI.3. Residualchlorinecurve
VIII.1. Schematicofaconventionalsinglestagedigester
Viii.2. Schematicofaconventionaltwostagedigester
VIII.3. Crosssectionofastandardratedigester
VIII.4. Planandsectionofatypicalsludgedryingbed
Listoftables
II.1. Valuesof
IV.1. Operationalcharacteristicsoftricklingfilters
IV.2. Designparametersforactivatedsludgeprocesses
IV.3. Operationalcharacteristicsofactivatedsludgeprocesses
IV.4. Designparametersforstabilizationponds
V.1. Applicationdataforadvancedtreatmentprocesses
VIII.1.Sludgequantitiesproducedfromdifferenttreatmentprocesses
VIII.2.Solidsloadingrateformechanicalthickeners
VIII.3.Solidsloadingrateforflotationthickener
VIII.4.Arearequiredfordryingbeds
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I. INTRODUCTION
Sewagetreatmentplants,alsocalleddomesticwastewatertreatmentplants,aredesignedtoconverta
rawsewageintoanaccessiblefinaleffluent,andtodisposeofthesolidsremovedintheprocess. Itis
thereforerequiredtodeterminethecharacteristicsoftherawsewageandtherequiredcharacteristics
oftheeffluentortherequiredtreatment,beforeproceedingwiththedesignofthetreatmentplant. It
isgenerallynecessarytoobtaintheapprovalofaregulatorybodybeforeproceedingwithconstruction
ofanysewagetreatmentplant. Theregulationsoftheagencyusuallyestablishmanyofthebasicdesign
considerations.Manystateshaveestablishedclassificationsforvariousstreamswithintheirboundaries.
Theseclassificationsgenerallyestablishtreatmentstandardsoreffluentstandardswhichlimitthe
pollutionmaterialintheeffluent. Thetreatmentstandardortheeffluentstandardsareestablishedtakingintoaccounttheabilityofthereceivingwaterstoassimilatethewasteandtheusestowhichthe
receivingwatersareput.
Periodsofdesignfortreatmentplantsvary. Anormaldesignperiodwouldrequiretreatmentunitstobe
designedforpopulationandsewageflowsanticipatedsome15to20yearsaftercompletionof
construction. Unitsaredesignedtobereadilyexpandableasthepopulationincreases.
Waterconsumptionrecords,whereavailable,areagoodbasisfordeterminingdomesticflowrates.
About70to80%ofdomesticwaterconsumptionmaybeexpectedtoreachthesewer. Intheabsence
ofanybetterbasis,manyregulatoryagenciesacceptarateof100gallonspercapitaperday(gpcd). If
commercialsewageflowisquitesmallincommunities,thecommercialflowisincludedasdomestic
flow. Thedesignaverageflowrateistheaverageflowduringsomemaximumsignificantperiodsuchas
4,8,12,or16hr,dependingoncircumstances.
Determinationofimportantcharacteristicsofsewageisessentialtotheproperdesignoftreatment
works.Whereonlypopulationdataareavailable,acceptableequivalentsfordesignoftreatmentworks
are0.20lbofsuspendedsolids(SS)perdaypercapitaor250partspermillion(ppm)and0.17lbof
biochemicaloxygendemand(BOD)perdaypercapitaor200ppm. Sewagetreatmentprocessesmaybe
classifiedaspreliminary,primary,secondaryoradvanced(tertiary). Thepurposeofpreliminary
treatmentistoremovedeleteriousmaterialswhichwoulddamageequipment,interferewiththe
satisfactoryoperationofaprocessorequipment,orcauseobjectionableshorelineconditions. Primarytreatmentcanusuallybeexpectedtoremove50to60%suspendedsolidsand25to35%BOD.
Secondarytreatmentusingconventionalbiologicalprocessesmayremoveupto90%ofsuspended
solidsand75to90%BOD. Differentbiologicalprocessunitsaredeployedinsecondarytreatment.
Tertiaryoradvancedtreatmentmaybeexpectedtoremoveover95%ofbothBODandSSinadditionto
reducingsomeundesirablechemicals.
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Priortodisposingoftheeffluent,itissubjectedtodisinfectionbyinjectingchlorineorozoneintothe
effluentorpassingultravioletraysintotheeffluent.
Theeffluentdisposalmethodsinuseare:dischargetostreamsandrivers,landdisposaltoirrigate
certaincrops,deepwelldisposal,andsubmarineoutfallsextendingintotheocean.
Sludgeiscollectedandsubjectedtothefollowingtreatmentpriortodisposal:thickening(eithergravity
orflotation),digestion(aerobicoranaerobic),anddewateringusingsandbedsorequipmentsuchas
vacuumfilterorcentrifuge.
Dewateredsludgeisdisposedofonland,processedascompostandsoldtofarmers,depositedin
sanitarylandfill,orincinerated.
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II. PRELIMINARYTREATMENT
Preliminarytreatmentofsewageincludesscreening,grindingandgritremoval.
II.1ScreeningThefirstunitoperationencounteredinsewagetreatmentplantsisthefiltering
operationorscreening. Ascreenisadevicewithopenings,generallyofuniformsize,usedtoretain
coarsesewagesolids. Thescreeningelementmayconsistofparallelbars,rodsorwires,grating,wire
mesh,orperforatedplate,andtheopeningsmaybeofanyshape,generallycircularorrectangularslots.
Ascreencomposedofparallelbarsorrodsiscalledarackorabarscreen. Thematerialremovedbythe
screeningdevicesisknownasscreeningsorrakings.
Accordingtothemethodofcleaning,racksandscreensaredesignatedashandcleanedormechanically
cleaned. Accordingtothesizeofopenings,screensaredesignatedascoarse,orfine.
II.1.1RacksTheseareclassifiedundercoarsescreenandaremadeofbarsofsteelweldedintoa
framethatfitsacrossthechannelwithopeningbetweenbarsrangingfrom3to6in. Thesearemainly
usedinsewagetreatmentplantstoprotectpumps,valves,pipelines,andotherappurtenancesfrom
damageorcloggingbyragsandlargeobjects. Thebarsrunverticallyorataslopevarying30to800with
thehorizontal. Largeobjectsarecaughtontherack,carriedupbytravelingrakes,andscrapedand
collected. Theapproachvelocityofthesewageintherakingorscreeningchannelshallnotbebelowa
selfcleaningvalue(1.25ft/sec)orrisetoamagnitudeatwhichtherakingsorscreeningswillbe
dischargedfromthebarsorscreens(3.0ft/sec)orthelossofheadthroughtherackorscreenshallbe
suchasnottobackuptheflowtoplacetheentrantsewerunderpressure. FigureII.1showsahand
cleanedandamechanicallycleanedrack.
FigureII.1(a)Handcleanedrack (b)MechanicallycleanedRack
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Hydrauliclossthroughbarracksisafunctionofbarshapeandthevelocityheadoftheflowbetweenthe
bars. Velocitiesof2to4ft/secthroughtheopenareahavebeenusedsatisfactorily. Thefollowing
equationisusedtocalculatetheheadloss.
hL=(w/b)1.33hvsin..Eq.(II.1)
wherehL=headloss,ft
=abarshapefactor
w=maximumcrosssectionalwidthofbarsfacingdirectionofflow,ft
b=minimumclearspacingofbars,ft
hv=velocityheadofflowapproachingrack,ft
=angleofrackwithhorizontal
Theheadlosscalculatedusingtheaboveequationisapplicableonlyifthebarsareclean. Headloss
increaseswiththedegreeofclogging. Aminimumallowanceforheadlossthroughhandcleanedscreenis6in. Formechanicallycleanedscreensmanufacturersliteratureprovidestheallowanceforhead
loss. Valuesof forseveralshapesofbarsaregiveninTableII.1below.
TableII.1Valuesof
Bartype
Sharpedgedrectangular 2.42
Rectangularwithsemicircularupstreamface 1.83
Circular 1.79
Rectangularwithsemicircularupstreamanddownstreamfaces 1.67
____________________________________________________________________________________
II.1.2.FinescreenThesearemechanicallycleaneddevicesusingamediumofperforatedplate,woven
wirecloth,orcloselyplacedbarsthroughwhichthesewageflows. Theopeningsareusually3/16inor
less. Onevarietyoffinescreensusedisthedrumtype. Inthisscreenthefiltermediumisacylinder,
furnishedwithamechanicalmeansofrotation,andwithselfcleaningdevices. Thedrumis
approximately1/3to2/3submergedinthesewage. Theliquidpassesthroughthescreenandflowsout
atoneend. Thesolidswhichareremovedfromtheliquidareraisedabovetheliquidlevelasthedrum
rotatesandareremovedbybrushes,scrapers,and/orabackwash. Thebackwashmayutilizewater,air,orsteam.
Anothervarietyoffinescreenisthedisktypescreen. Thesescreensconsistofaroundflatplate
revolvingonanaxisinclined100to25
0fromthevertical. Thesewageflowsthroughthelowertwothirds
oftheplate. Astheplaterotates,theretainedsolidsarebroughtabovetheliquidwherebrushes
removethemfordisposal. Commonlyamotorisusedtoprovidetherotation. Headlossthroughfine
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screensmaybeobtainedfrommanufacturersratingtablesormaybecalculatedbymeansofthe
commonorificeformula:
hL=(1/2g)(Q/CA)2Eq.(II.2)
WhereC=coefficientofdischarge
Q=dischargethroughscreen,cfs
A=effectivesubmergedopenarea,ft2
g=accelerationduetogravity,ft/sec2
hL=headloss,ft
FigureII.2 showsabrushcleaneddiskscreenandabrushcleaneddrumscreen.
FigureII.2Brushcleaneddiskscreen (d)Brushcleaneddrumscreen
II.1.3.ComminutingdevicesAcomminutingdeviceisamechanicallycleanedscreenwhich
incorporatesacuttingmechanismthatcutstheretainedmaterialwithoutremovalfromthesewage
flow. Thistendstoreduceodors,flies,andunsightlinessoftenfoundaroundsewagescreeningshandledbyothermeans. Acomminutingdevicehasasubmergedrevolvingdrumwithopeningsvaryingfrom
to3/8in. Coarsematerialiscutbycuttingteethandshearbarsattherevolvingdrumwhichpasses
throughastationarycuttingcomb. Thecomminutedsolidsthenpass,withsewageliquor,outofthe
bottomopeningandbackintothedownstreamchannel. Thisrequiresaspecialvoluteshapedbasinto
giveproperhydraulicconditionsforsatisfactoryoperation. Thebasinshapemakesitsinstallationmore
expensive. Acomminutingdeviceisoftenusedinlocationswheretheremovalofscreeningswouldbe
difficultsuchasinaverydeeppit. FigureII.3showsplanandcrosssectionalviewsofacomminuter.
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FigureII.3Planandcrosssectionalviewsofacomminuter
II.1.4.DisposalofscreeningsLargevariationisreportedinthevolumeofscreeningsremovedper
milliongallonsofsewage. Thefactorsaffectingthequantityofscreeningsareasfollows:
1. Clearopeningbetweenbars
2. Percentageofcombinedsewersinthetributarysystem
3. Characterofindustrialwastetreated,and
4. Habitsoftributarypopulation
Incinerationhasbeenfoundtobeasatisfactorymeansofscreeningsdisposal.
Screeningsgrindershavebeenusedfordisposalofscreenings. Thematerialisreducedinsizeand
returnedtotherawsewage. Thegrindersarelocatednearthesourceofscreeningstobeprocessed.
Grindersusedarethehammermilltypeorthedisintegratortype. Acomminutingdeviceisnota
substituteforagrinder. Screeningsfromagrinderareusuallydisposedofasrawsludge.
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Anothermethodofdisposingofscreeningsisbyburial. Ifthismethodischosen,suitableandsufficient
areamustbeavailable.
II.1.5.GritremovalMinutepiecesofmineralmatterlikesand,andgravel,andmaterialsthatarenotof
mineraloriginlikecoffeegrounds,seeds,andsimilarmaterialconstitutegrit. Gritinsewagehastwo
characteristics:(1)Theyarenonputrescibleand(2)theyhavesubsidingvelocitiessubstantiallygreaterthanthoseoforganicputresciblesolids.
Gritchambersarelocateddownstreamofscreenchambers. Thepurposeofagritchamberisthree
fold:(1)theprotectionofmovingmechanicalequipmentfromabrasionandaccompanyingabnormal
wear,(2)thereductionofpipecloggingcausedbydepositionofgritparticlesorheavysludgeinpipes
andchannels,particularlyatchangesindirectionofconduits,and(3)reductionoffrequencyofdigester
andsettlingtankcleaningrequiredasaresultofexcessiveaccumulationofgritintheseunits.
Therearetwotypesofgritchambers: horizontalflowandaerated. Inthehorizontalflowtype,theflow
passesthroughthechamberinahorizontaldirection. Aconstantvelocityofflowthroughthegrit
chambermustbemaintainedat1ft/secforalldepthsofflowinordertopreventsettlingoforganic
solids. Thisisaccompaniedbymeansofprovidingasutroweiroraproportionalflowweir.
FigureII.4showscrosssectionofthetwoweirs.
FigureII.4Crosssectionof(a)sutroweir(b)proportionalflowweir
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Theaeratedtypeconsistsofaspiralflowaerationtank,thespiralvelocitybeingcontrolledbythe
dimensionsandthequantityofairsuppliedtotheunit. Thedetentionprovidedis3minutesatthe
maximumflowrate.
Thegritsolidsarerakedbyarotatingmechanismtoasumpatthesideofthetank,fromwhichtheyare
movedbyareciprocatingrakemechanism. Thequantitiesofgritvaryfromonelocationtoanotherdependingonthetypeofseweragesystem,thecharacteristicsofthedrainagearea,theconditionofthe
sewers,thefrequencyofstreetsanding,thetypeofindustrialwastes,thenumberofgarbagegrinders
served,andtheproximityanduseofsandybathingbeaches. Thereisawiderangeinthequantityof
gritvaryingfrom1/3ft3to24ft
3permilliongallonofsewagetreated. Becauseofthewidevariation,a
factorofsafetymustbeusedincalculationsconcerningtheactualstorage,handling,ordisposalofthe
grit.
Commonmethodofgritdisposalisasfill,coveredifnecessarytopreventobjectionableconditions. Grit
alsoisincineratedwithsludge. Incoastalcitiesgritandscreeningsarebargedtoseaanddumped.
Generallythegritmustbewashedbeforeremoval.
II.1.6.PretreatmentPretreatmentisusedtoremovematerialsuchasgreaseandscum,fromsewage
priortoprimarysedimentationtoimprovetreatability. Pretreatmentmayincludeskimming,grease
traps,preaerationandflotation.
Askimmingtankisachambersoarrangedthatfloatingmatterrisesandremainsonthesurfaceuntil
removedwhiletheliquidflowsoutcontinuouslythroughdeepoutlets. Thismaybeaccomplishedina
separatetankorcombinedwithprimarysedimentation. Theobjectistoseparatethelighterfloating
substancesfromsewage. Thematerialremovedincludesoil,grease,soap,piecesofcork,andvegetable
debrisandfruitskins.
Greasetrapsaresmallskimmingtanks. Theyaresituatedclosetothesourceofgrease,whichmaybe
anindustry,ahousesewer,orasmalltreatmentplant. Theinletissituatedjustbelowthesurfaceand
theoutletatthebottom. Detentiontimesof10to30minareused. Theymustbecleanedperiodically.
Preaerationofsewagepriortoprimarysedimentation,ifpracticed,isclassifiedaspretreatment. The
objectiveofpreaeratingsewageistoimprovetreatabilityandtocontrolodor. Detentiontimesofpre
aerationtanksrangefrom10to45min. Tankdepthsaregenerally15ftandairrequirementsrange
from0.1to0.4ft3/galofsewage.
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III. PRIMARYTREATMENT
Primarytreatmentconsistsofsettlingthesewageinasedimentationtank.Wheneveraliquid
containingsolidsinsuspensionisplacedinarelativelyquiescentstate,thosesolidshavingahigher
specificgravitythantheliquidwilltendtosettle,andthosewithlowerspecificgravitywilltendtorise.
Theseprinciplesareutilizedinthedesignofsedimentationtanks. Theobjectiveoftreatmentby
sedimentationistoreducethesuspendedsolidscontentbyremovingreadilysettleablesolidsand
floatingmaterial.
Efficientlydesignedandoperatedprimarysedimentationtanksshouldremovefrom50to65%ofSSand
25to40%ofBOD. Sedimentationtanksarenormallydesignedonthebasisofasurfaceloadingrateat
theaveragerateofflow,expressedasgallons/day/ft2ofhorizontalarea. Theeffectofsurfaceloading
rateanddetentiontimeonSSremovalvarieswidelydependingonthecharacterofthesewage,proportionofsettleablesolids,concentrationofsolids,andotherfactors.Whentheareaofthetankhas
beenestablished,thedetentionperiodinthetankisgovernedbywaterdepth.
Surfacesettlingratesnotfollowedbysecondarytreatmentshallnotexceed600gallonsperdayper
squarefoot(gpd/ft2)fordesignflowof1mgdorless. Higherratesmaybepermittedforlargerplants.
Normally,primarydetentiontanksaredesignedtoprovide90to150minofdetentionbasedonthe
averagerateofsewageflow.Weirloadingsshouldnotexceed10,000gallons/linearft/dayforplants
designedforaverageflowsof1MGDorless. Forplantsdesignedforhigherflows,theweirloadingrate
canbeincreaseduptoamaximumof15,000gallons/linearft/day.Weirrateshavebeenfoundtohave
lesseffectonefficienciesofremovalthanoverflowrates. Aminimumwaterdepthof7ftisrecommended.
III.1.Tanktype,sizeandshapeAlmostallsedimentationtanksaredesignedasrectangularorcircular
tankswithmechanicalcleaningmechanism. Theselectionoftheshapeisgovernedbythesizeofthe
installation,byrulesandregulationsofpermittingauthorities,bylocalsiteconditionsandtheestimate
ofcost. Twoormoretanksshouldbeprovidedinorderthattheprocessmayremaininoperationwhile
onetankisoutofserviceformaintenanceandrepairwork.
III.1.1.Rectangulartanks Thelengthofrectangulartanksisrestrictedto300ft. Tankwidthsmaynot
bemorethan80ft,butitshouldbedividedinto4bayssothatthecleaningmechanismcanbeinstalled
ina20footwidthbay. ArectangulartankisshowninFigureIII.1.
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FigureIII.1Rectangularsedimentationtank
Attachedtothechainatintervalsof10ft,are2inthickcrosspiecesofwood,orflights,6to8indeep,
extendingthefullwidthofthetankorbay. Linearconveyorspeedsof2to4ft/minarecommon. The
solidssettlinginthetankarescrapedtosludgehoppersinsmalltanksandtotransversetroughsinlarge
tanks. These,inturn,areequippedwithcollectingmechanisms(crosscollectors),ofthesametypeas
thelongitudinalcollectors,whichconveysolidstooneormoresludgehoppers. Screwconveyorsmay
alsobeusedinsteadofcrosscollectors.Wherecrosscollectorsarenotprovided,multiplehoppersmust
beinstalled. Ifacommonwithdrawallineisused,provisionismadetoisolateandcontrolthe
withdrawalfromeachhopperindividually. Itisdesirabletolocatethesludgepumpingfacilitiescloseto
thehoppers.
Rectangulartanksareusedwheregroundareaisatapremium. Theyarealsousedwheretankroofsor
coversarerequired.
Theinletarrangementisanimportantelementinthedesignofrectangulartanks. Influentchannels
mustbeprovidedacrosstheinletend.Withmultipleunits,theflowisdistributedtoeachunitas
uniformlyaspossibletoobtainmaximumefficiency. Oneeffectivemethodistheuseofdistribution
boxesorchambersaheadofthesedimentationunitswithgatesororificestoadjusttheflowbetween
theunits. Baffleboardsinfrontoftheinletsareusedtodistributesewageflowslaterallyandvertically
andtopreventshortcircuiting. Bafflesareinstalledapproximately2to3ftinfrontoftheinletsand
submerged18to24in.
Outletstructuresincludeeffluentchannelsandweirslocatedneartheeffluentendofthetank. Effluent
weirsareadjustableforlevelingandsufficientlylongtoavoidhighheadswhichresultinupdraft
currents. Thecrestisfrequentlyprovidedwith900Vnotchestoprovideuniformdistributionatlow
flows.
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Scumisusuallycollectedattheeffluentendofrectangulartanks. Thescumremovalmethodconsistsof
achainandflighttypeofcollectorthatcollectsthescumatonesideofthetankandscrapsitupashort
distancefordepositinscumhoppers,whenceitisusuallydisposedofwiththesludgeproducedatthe
plant.
III.1.2.CirculartanksThediameterofroundtanksvariesfrom10to180ftwithnosinglefactor
influencingtheselectionotherthanthesizeoftheplant. Thesidewalldepthvariesfrom7to14ft.
Floorsaredeepestatthecenterandsloperadiallyupwardstothetankwallsatarateof1inperft. The
slopefacilitatessludgewithdrawalanddrainageofthetank.
Inonetypeofcirculartanks,thesewageiscarriedtothecenterofthetankinapipesuspendedfroma
bridgeorencasedinconcretebeneaththetankfloor. Atthecenterofthetank,sewageentersacircular
welldesignedtodistributetheflowequallyinalldirections. Theremovalmechanismmoves
continuouslyataperipheralspeedof5to8ft/minandmayhavetwoorfourarmsequippedwith
scrapers. Thearmsalsosupportbladesforscumremoval. Inthesecondtype,asuspendedcircular
aluminumbaffleatashortdistancefromthetankwallformsanannularspaceintowhichthesewageis
distributedinatangentialdirection. Thesewageflowsspirallyaroundthetankandunderneaththe
baffle,theclarifiedliquidbeingskimmedoffoverweirsonbothsidesofacentrallylocatedweirtrough.
Greaseandscumareconfinedtothesurfaceoftheannularspace. Intervalsofpumpingthesludgevary
fromoncein30mintooncein12hoursdependinguponthevolumetobepumpedandtheplant
operatingschedules.
Thevolumeofsludgeproduceddependsupon:
1. Characteristicsoftherawsewage
2.
Periodofsedimentation
3. Conditionsofthedepositedsolids,and
4. Periodbetweensludgeremovaloperations.
ExampleIII.1showsatypicaldesignofaprimarysedimentationtank.
ExampleIII.1
Designaprimarysedimentationtankgiventhefollowingdata:Sewageflow=5mgd,surfaceoverflow
rate=600gpd/ft2,depthoftank=10ft,removalefficiency=60%,SSinrawsewage=200mg/L,specific
gravityofthesludge=1.03,andmoisturecontentofsludge=95%,weirloadingrate=15,000gpd/ft.Sludgeispumpedoutofthehoppers3timesadayfor30minutesdurationeachtime.
Solution
Surfacearea=5,000,000/600=8,333oruse8,340ft2
Totalvolume=10x8,340=83,400ft3
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Provide4rectangulartankseach120ftlongand20ftwidewhichgivesalengthtowidthratioof6:1and
atotalvolumeof96,000ft3.
Two75ftdiametercirculartankswithatotalvolumeof88,313ft3alsowillbesuitable.
Designflow/tank=5,000,000/4=1,250,000gpd
Weirlength/tank=1,250,000/15,000=83linearft
Weightofdrysolidsremoved/milliongallons=200x8.34x1x(60/100)=1,000lb
Volumeofsludge/milliongallonofsewage=1,000/{8.34x1.03x(5/100)}=2,330gallons
Volumeofsludge/5mgdofsewage=2,330x5=11,650gpd
Sludgevolumepumpedeachtime=11,650/3=3,883oruse3,890gallons
Adding10%forscum,volumetobepumped=3,890x1.1=4,668gallons
Pumpingrate=4,668/30=155.6oruse160gpm
Checkfordetentiontime
Usingrectangulartanks:Flow=5,000,000gallons/day=208,333gal/hr
Volumeoftank=96,000ft3=718,000gal
Detentiontime=718,080/208,333=3.45hrs
Usingcirculartanks:Flowasbefore=208,333gal/hr
Volumeoftank=660.581;Detentiontime=660,581/208333=3.17hr
Squaretanksarealsousedbuttheyarefewerinnumber. Thedesignfeaturesaresameasforcircular
tanks.
FigureIII.2showsatypicalcircularsedimentationtank.
FigureIII.2Typicalcircularsedimentationtank
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IV. SECONDARYTREATMENT
Secondarytreatmentofsewageinvolvesbiologicalprocessesthatconvertthefinelydividedand
dissolvedorganicmatterintoflocculentsettleablesolidsthatcanberemovedinsedimentationtanks.
Thecommonbiologicalprocessesare:
1.
Tricklingfilter
2.
Activatedsludge
3. Aeratedlagoons
4. Stabilizationponds
IV.1.TricklingfilterAtricklingfilter,consideredasattachedgrowthsystem,consistsofabedwith
highlypermeablemediatowhichmicroorganismsareattachedandthroughwhichsewageispercolated.
Thefiltermediausuallyconsistsof,rocks,varyinginsizefrom1to4in.indiameter. Thedepthofrock
varieswitheachparticulardesign,usuallyfrom3to8ft;anaveragedepthis6ft. Tricklingfilters
employingaplasticmediahavebeenbuiltwithdepthsof30to40ft. Thefilterbedisusuallycircular,
andthesewageisdistributedoverthetopofthebedbyarotarydistributor. Eachfilterhasanunder
drainsystemforcollectingthetreatedeffluentandanybiologicalsolidsthathavebecomedetached
fromthemedia. Theunderdrainsystemhastwofunctions:oneasacollectingunitfortheeffluentand
theotherasaporousstructurethroughaircancirculate. FigureIV.1showsacutawayviewofatrickling
filter.
FigureIV.1Cutawayviewofatricklingfilter
Thetricklingprocessdependsonbiochemicaloxidationofcomplexorganicmatterinthesewage. Soon
afterafilterisplacedinoperation,thesurfaceofthemediabecomescoatedwithzooglea,aviscous
jellylikesubstancecontainingbacteriaandotherbiota. Underfavorableconditionsthezoogleaabsorbs
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andutilizesuspended,colloidal,anddissolvedorganicmatterfromthesewagewhichpassesina
relativelythinfilmoveritssurface. Eventuallypopulationequilibriumisreached. Asbiotadie,they,
togetherwiththemoreorlesspartlydecomposedorganicmatter,aredischargedfromthefilter. This
dischargeistermedsloughing. Thesloughingmayoccurperiodicallyorcontinuously. Secondarysettling
isprovidedtoretainthesettleablesolidssloughedfromthefilter.
Tricklingfiltersareexpectedtoremove70to80%ofBOD. Theypredominateinsmallerplants. They
havetheabilitytorecoverfromshockloadsandtoprovidegoodperformancewithaminimumofskilled
technicalsupervision. Theyareclassifiedbyhydraulicororganicloadingashighrateandlowrate. The
hydraulicloadingisthetotalvolumeofliquid,includingrecirculation,perdaypersquareunitofthe
filterarea. Thegeneralpracticeistousemilliongallonsperacreperday(mgad). Organicloadingisthe
poundsof5day,200C,BODperdaypercubicunitofthefiltermedia. TheTenStateStandardshas
sponsoredpoundsperdayper1,000ft3. Therangeofloadingsencounteredandotheroperational
characteristicsforthehighrateandlowratefiltersareshowninTableIV.1
TableIV.1Operationalcharacteristicsofhighrateandlowratetricklingfilters
Alowratefilterisalsocalledastandardrateoraconventionalratefilterandisrelativelysimpledevice
andishighlydependable,producingaconsistenteffluentqualitywithvaryinginfluentstrength. Alarge
populationofnitrifyingbacteriaisprevalent. Headlossthroughthefiltermaybe5to10ft. Odorsarea
commonproblem,especiallyifthesewageisstaleorseptic. Nuisancecausingfilterflies(Psychoda)may
breedinthefiltersunlesscontrolmeasuresareemployed.
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Filtereffluentorfinaleffluentisrecirculatedinhighratefiltersresultinginhigherorganicloadings.
FlowdiagramsforvarioushighratetricklingfilterconfigurationsareshowninFigureIV.2.
FigureIV.2Highratetricklingfilterflowsheetswithvariousrecirculationpatterns(a) singlestagefilters(b)twostagefilters
Recirculationoffiltereffluentaroundthefilterresultsinthereturnofviableorganismsandimproves
treatmentefficiency. Recirculationalsoaidsinpreventingpondinginthefilterandinreducingthe
nuisanceduetoodorsandpsychodaflies.
EquationsareavailabletopredicttheBODremovalsintricklingfilters.Mostcommonlyusedequationis
byTheNationalResearchCouncil,anempiricalformula,whichisshownbelowforthefirststagefilter:
E1=1/{1+0.0085(W/VF)0.5..Eq.(IV.1)
whereE1=fractionalefficiencyofBODremovalforprocess,includingrecirculationandsedimentation
W=BODloadingtofilter,lb/day
V=volumeoffiltermedia,acreft.
F=recirculationfactor
Therecirculationfactoriscalculatedbymeansofthefollowingformula:
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F=(1+R)/{1+(R/10)}2 Eq.(IV.2)
whereR=recirculationratioQr/Q
Therecirculationfactorrepresentstheaveragenumberofpassesoftheinfluentorganicmatterthrough
thefilter
ExampleIV.1illustratestheuseoftheNRCformulasinthedesignoftricklingfilters.
ExampleIV.1
Atownisconsideringtheuseofatricklingfilterfortreatmentofitssewage. Atwostagefilteris
contemplated. Theinfluentflowis2mgdwithasettledBODcontentof200mg/liter. ThedesiredBOD
intheeffluentqualityis30mg/liter. Ifthefilterdepthsare6ftandtherecirculationratiois4:1,what
arethefilterdiametersassumingthefractionalBODremovalefficienciesarethesameinboththe
filters.
Solution
1. ComputeE1andE2,theBODremovalefficiencies
Overallefficiency=(20030)/200=85%
E1+E2(1E1)=0.85
E1=E2=0.615
2. Computetherecirculationfactor
F=(1+R)/{1+(R/10)}2=(1+4)/1.42=5/1.96=2.55
3. ComputetheBODloadingforthefirstfilter
W=200x8.34x2=3,334lb/day
4. Computethevolumeforthefirststagefilter
E1=1/{1+0.0085(W/VF)0.5}
0.615=1/{1+0.0085(3,334/2.55V)0.5}
V=0.1acreft
5. Computethediameterofthefirstfilter
Area=0.1/6=0.017acres=726ft2
Diameter={(726x4)/3.14}0..5=30ft
6.
ComputetheBODloadingforthesecondfilter
W=(IE1)W=0.385x3,334=1,284lb/day
E2=1/[1+{0.0085/(1E1)}x(W/VF)0.5]
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0.615=1/{1+[0.0085/(10.615)]x(1284/2.55V)o.5}
V=1.39acreft.,Area=1.39/6=0.23acresor10,020ft2;diameter=113ft.
IV.1.1.PhysicalfacilitiesFactorsthatmustbeconsideredinthedesignoftricklingfiltersinclude:
(1)thetypeanddosingcharacteristicofthedistributionsystem,(2)thetypeoffiltermediatobeused,
(3)theconfigurationoftheunderdrainagesystem,(4)provisionforadequateventilation,and(5)the
designoftheadequatesettlingtanks.
IV.1.1.1.DistributionsystemsThecommonarrangementofdistributionsystemistoprovidetwoor
moreofrotaryarms. Theyaremountedonapivotinthecenterofthefilterandrevolveinahorizontal
plane. Thearmsarehollowandcontainnozzlesthroughwhichthesewageisdischargedoverthefilter
bed. Thedistributorassemblyisdrivenbythedynamicreactionofthesewagedischargingfromthe
nozzlesorbyanelectricmotor. Thespeedofrevolutionnormallyis1revolutionin10minutesorless.
Clearanceof6to9inshouldbeallowedbetweenthebottomofthedistributorarmandthetopofthe
bed. Nozzlesarespacedunevenlysothatgreaterflowperunitoflengthisachievedattheperiphery
thanatthecenter. Theheadlossthroughthedistributorwillbeintherangeof2to5ft.
Dozingtanksprovidingintermittentoperationorrecirculationbypumpingmaybeemployedtoensure
thattheminimumflowwillbeadequatetorotatethedistributoranddischargethesewagefromall
nozzles.
Fixednozzledistributionsystemisalsoinuse. Itconsistsofaseriesofspraynozzleslocatedatthe
pointsofequilateraltrianglescoveringthefilterbed. Asystemofpipesplacedinthefilterdistributes
thesewageuniformlytothenozzles. Specialnozzleshavingaflatspraypatternareused.
IV.1.1.2.FiltermediaTheidealfiltermediashouldhavehighsurfaceareaperunitofvolume,should
belowincost,hasahighdurability,anddoesnotclogeasily. Themostsuitablematerialiscrushedrock
orgravelgradedtoauniformsizeof1to3in. Othermaterialssuchasslag,cinders,orhardcoalhavealsobeenused. Stoneslessthan1indiametermustbeavoidedastheydonotprovidesufficientpore
spacebetweenthestonesforfreeflowofsewageandsloughedsolids. Pluggingofthemediaand
pondinginsidethefilterwilloccur.
IV.1.1.3.UnderdrainsUnderdrainsarepartofthecollectioninatricklingfilter. Thecollectionsystem
consistsoffilterfloor,collectionchannel,andunderdrains. Theunderdrainsarespeciallydesigned
vitrifiedclayblockswithslottedtopsthatadmitthesewageandsupportthemedia. Theunderdrains
arelaiddirectlyonthefilterfloor,whichareslopedtothecollectionchannelata1to2percent
gradient. Underdrainsmaybeopenatbothendstofacilitateeasyinspectionandflushingintheevent
ofclogging. Theyalsoventilatethefloor,providingairformicroorganismsthatliveinthefilterslime.FigureIV.3providesanunderdrainsystem.
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FigureIV.3Underdrainblocksfortricklingfilters
IV.1.1.4.VentilationNaturalventilationoccursbygravitywithinthefilteranditisconsidered
adequateifthetricklingfilterisproperlydesigned,constructed,andoperated. Forcedventilationis
practicedatarateof1ft3perft
2offilterareaindeeporheavilyloadedfilters. Duringperiodsof
extremelycoldtemperaturestheairflowmustberestrictedto0.1ft3perft2inordertopreventfreezing
ofthefilter.
Filtersshouldbedesignedsuchthattheentiremediacanbefloodedwithsewageandthendrained
withoutcausinganyoverflows. Floodingisaneffectivemethodforflushingafiltertocorrectponding
andtocontrolfilterflylarvae.
IV.2.Activatedsludgeprocess Activatedsludgeisdefinedassludgeflocproducedinaraworsettled
sewagebythegrowthofzooglealbacteriaandotherorganismsinthepresenceofdissolvedoxygen,and
accumulatedinsufficientconcentrationbyreturningflocpreviouslyformed. Thisisconsideredasa
dispersegrowthsystem.
Activatedsludgeprocessisdefinedasabiologicalsewagetreatmentprocessinwhichamixtureof
sewageandactivatedsludgeisagitatedandaerated. Theactivatedsludgeissubsequentlyseparated
fromthetreatedsewage(mixedliquor)bysedimentation,andwastedorreturnedtotheprocessas
needed. Thetreatedsewageoverflowstheweirofthesettlingtankinwhichseparationfromthesludge
takesplace.
Activatedsludgeflocsarecomposedofasyntheticgelatinousmatrixinwhichfilamentousand
unicellularbacteriaareimbedded,andonwhichprotozoaandsomemetazoancrawlandfeed.
Activatedsludgediffersfromothersludgeinappearance,physicalcharacteristics,andbiological
composition. Goodactivatedsludgehasadistinctivemusty,earthyodorwhileincirculationinthe
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aerationbasin. Itisalightbrown,flocculantprecipitatethatsettlesrapidlyinitsmotherliquor,leaving
asupernatantliquidthatisclear,colorless,odorlessand,sparkling.
Theadvantagesofthisprocessareproducingaclear,sparklingand,nonputrescibleeffluent,freedom
fromoffensiveodorsduringoperation,removingmorethan90%ofBODandSS,relativelylow
installationcost,somecommercialvalueinthesludgeand,therequirementofhydraulicheadandsurfaceareafortheplantisless. Thedisadvantagesincludeuncertaintyconcerningtheresultstobe
expectedunderallconditions,sensitivitytochangesinthequalityoftheinfluent,highcostofoperation,
thenecessityforconstantskilledattendance,anddifficultyindewateringanddisposingofthelarge
volumeofsludgeproposed.
Theeffluentfromtheactivatedsludgeprocessisnormallyclear,odorless,sparkling,highindissolved
oxygen,andlowinBOD. Itcanbeexpected,ingeneral,thattheeffluentwillcontainfrom10to20mg/l
ofBODandSS.
Theconventionalactivatedsludgeprocesstogetherwiththesixmodificationsarelistedbelowandthey
aredescribedindetail: DesignparametersfortheseprocessesarefurnishedinTableIV.2
1. Conventionalactivatedsludgeprocess
2.
Completemixactivatedprocess
3. Taperedaerationactivatedsludgeprocess
4.
Stepaerationactivatedsludgeprocess
5. Modifiedaerationactivatedsludgeprocess
6.
Contactstabilizationactivatedsludgeprocess
7. Extendedaerationactivatedsludgeprocess
TableIV.2Designparametersforactivatedsludgeprocesses
BODloading
Process lbBOD/1000ft3 lbBOD/day/ Sludgeage Aerationperiod Returnsludge
perday lbMLSS days hours percent
____________________________________________________________________________________Conventional 2040 0.20.4 515 48 2550
Completemix 50120 0.20.6 515 35 25100
Taperedaeration 3040 0.20.5 515 67.5 30
Stepaeration 3050 0.20.5 515 57 50
Modifiedaeration 75100 1.55.0 0.20.5 1.53 515
Contactstabilization3050 0.20.5 515 69 100
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Extendedaeration 1030 0.61.5 510 2030 100
IV.2.1.ConventionalactivatedsludgeprocessThisistheearliestactivatedsludgesystem. Theflow
diagramforthisprocessisshowninFigureIV.4
FigureIV.4Flowdiagramplusoxygendemandandsupplyforconventionalactivated
sludgeprocess
Theaerationbasinisalongrectangulartankwithairdiffusersononesideofthetankbottomtoprovide
aerationandmixing. Settledsewageandreturnactivatedsludgeentertheheadofthetank,getaerated
forabout6hoursandflowdownitslengthinaspiralflowpattern. Constantaerationisprovidedby
diffusedairormechanicalmeans.Duringthisperiod,adsorption,flocculation,andoxidationofthe
organicmattertakeplace. Themixedliquorissettledintheactivatedsludgesettlingtank,andsludgeis
returnedatarateofapproximately25to50percentoftheinfluentflowrate. Theaboveprocessis
illustratedinExampleIV.2.
ExampleIV.2
Data: Volumeofaerationtank=120,000ft3or0.898mg
Settledsewageflow=3.67mgd
Returnsludgeflow=1.27mgd
Wastesludgeflow=18,900gpdor0.0189mgd
MLSSinaerationtank=2,350mg/l
SSinwastesludge=11,000mg/l
InfluentsewageBOD=128mg/l
EffluentBOD=22mg/l
EffluentSS=26mg/l
Usingtheabovedatacalculatetheloadingandoperationalparameters.
Solution
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BODload=3.67x128x8.34=3,920lb/day
MLSSinaerationtank=0.898x2,350x8.34=17,600lb
BODloading=3,920/120=32.7lb/day/1000ft3
BODloading=3,920/17,600=0.22lb/day/lbofMLSS
Sludgeage=(2,350x0.898)/(26x3.67+11,000x0.0189)=7days
Aerationperiod=(0.898x24)/3.67=5.9hr
Returnsludgerate=(1.27x100)/3.67=35%
BODremoval={(12822)x100}/128=83%
Sludgeproduction=(0.0189x11,000x8.34)/3,920=0.44lbSSwasted/lbBODapplied
IV.2.2.CompletemixactivatedprocessProcessflowdiagramforthisprocessisshowninFigureIV.5
FigureIV.5Flowdiagramplusoxygendemandandsupplyforcompletemixactivatedsludgeprocess
Thesettledsewageinfluentandthereturnsludgeflowareintroducedatseveralpointsintheaeration
tankfromacentralchannel. Themixedliquorisaeratedasitpassesfromthecentralchanneltothe
effluentchannelsatbothsidesoftheaerationtank. Theaerationtankeffluentiscollectedandsettled
intheactivatedsludgesettlingtank. Theorganicloadontheaerationtankandtheoxygendemandare
uniformfromoneendtotheother. Asthemixedliquorpassesacrosstheaerationtankfromthe
influentportstotheeffluentchannel,itiscompletelymixedbydiffusedormechanicalaeration.
IV.2.3.TaperedaerationactivatedsludgeprocessTheobjectiveoftaperedaerationistomatchthe
quantityofairsuppliedtothedemandexertedbythemicroorganisms,astheliquortraversesthe
aerationtank. Thusonlythearrangementofthediffusersandtheamountofairconsumedareaffected
inthisprocess. Attheinletoftheaerationtankwherefreshsettledsewageandreturnactivatedsludge
firstcomeincontact,theoxygendemandisveryhigh. Thediffusersarespacedclosetogetherto
achieveahighoxygenationrateandthussatisfythedemand. Asthemixedliquortraversesthetank,
synthesisofnewcellsoccurs,increasingthenumberofmicroorganismsanddecreasingthe
concentrationofavailablefood. Thisresultsinalowerfood/microorganism(U)ratioandaloweringof
theoxygendemand. Thespacingofdiffusersisincreasedtowardthetankoutlet,toreducethe
oxygenationrate. Thisresultsintwoadvantages:loweringofaerationcostandavoidanceofover
aerationcreatinginhibitionofgrowthofnitrifyingorganisms.
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IV.2.4.StepaerationactivatedsludgeprocessInthisprocess,thesettledsewageisintroducedat
severalpointsintheaerationtanktoequalizetheUratio,thusloweringthepeakoxygendemand. A
typicalflowsheetforthisprocessisshowninFigureIV.6
FigureIV.6Flowdiagramplusoxygendemandandsupplyforstepaerationactivatedsludgeprocess
Theaerationtankissubdividedintofourormoreparallelchannelsthroughtheuseofbaffles. Each
channelcomprisesaseparatestep,andtheseveralstepsarelinkedtogetherinseries. Returnactivated
sludgeentersthefirststepoftheaerationtankalongwithaportionofthesettledsewage. Thepipingissoarrangedthatanincrementofsewageisintroducedintotheaerationtankateachstep. Flexibilityof
operationisanadvantageinthisprocess. Otheradvantagesare:higherBODloadingsper1,000ft3of
aerationtankvolume,solubleorganicsremovalinashortperiod,andbetterutilizationoftheoxygen
supplied. ExampleIV.3showsthedesignofatreatmentplantusingtheaboveprocess.
ExampleIV.3
Data:
Settledsewageflow=7.40mgd(989,000ft3/day)
BODcontent=7,900lbs
DesignmaximumBODloading=40lbs/1000ft3/day
Designminimumaerationperiod=6hr
Numberofaerationtanksrequired=4
MinimumoperatingMLSS=2,000mg/l
Numberoffinalcircularclarifiers=4
Determine(1)thedimensionsoftheaerationtanks,and(2)thedimensionsoftheclarifiers.
Solution
VolumeoftankbasedonBODloading=7,900/(40/1000)=198,000ft3
Volumeoftankbasedonaerationperiod=(7,400,000x6)/(24x7.48)=247,000ft
3
Usethehighervalueof247,000ft3
NowBODloading=7,900/247=31lb/1000ft3/day
Assumeeachaerationtanktohaveawidthof24ftandliquiddepthof13ft
Lengthofeachtank=247,000/(4x13x24)=198ft
Sizeofeachaerationtank=198ftx24ftx13ft
Assumeoverflowrateof800gpd/ft2forclarifiers
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Surfaceareaofeachclarifier=7,400,000/4x800=2310ft2anddiameter=54ft
Detentiontime=(2,310x13x24)/(989,400/4)=2.7hr
IV.2.5.ModifiedaerationactivatedsludgeprocessTheflowdiagramforthisprocessissimilartothat
ofconventionalprocessexceptthatthisprocessusesshorteraerationtimes,usually1.5to3hours,and
ahighfoodtomicroorganismratio. TheMLSSconcentrationisrelativelylow,whereastheorganicloadingishigh. BODremovalisintherangeof60to75percent. Thesludgehaspoorsettling
characteristicsandtheeffluentcontainshighsuspendedsolids.
IV.2.6.ContactstabilizationactivatedsludgeprocessFlowsheetforthisprocessisshownin
FigureIV.7.
FigureIV.7Flowsheetforcontactstabilizationtank
Thisprocesscontainstwoaerationtanks;oneforaeratingthemixtureofsettledsewageandreturn
sludgeforaperiodof30to90mincalledthecontacttankandtheotherisaseparateaerationtankto
aeratethereturnsludgefromthefinalclarifierfor3to6hourscalledstabilizationtank. BODremoval
occursbyadsorptioninthecontacttankandbyabsorptioninstabilizationtank. Aportionofthereturn
sludgeiswastedpriortorecycletomaintainaconstantmixedliquorvolatilesuspendedsolids(MLVSS)
concentration. Theaerationtankvolumerequirementsareapproximately50%ofconventional
process. Byconvertinganexistingconventionalplantintoacontactstabilizationplantwithminor
modificationtopiping,theplantcapacitycanbeevendoubledwithalittleadditionalcost.Thisprocess
isexcellentfortreatingsewagenotcontainingindustrialwastes. ExampleIV.4showsthedesignofa
contactstabilizationplant.
ExampleIV.4
Acitywithapopulationof2,000personshasbuiltacontactstabilizationplantfortreatingitssewage
withthefollowingdata:
Volumeofaerationtank=2,500ft3
Volumeofreaerationtank=5,000ft3
Volumeofaerobicdigester=4,500ft3
Volumeofsedimentationtank=3,660ft3andsurfacearea=300ft
2
CalculatetheBODloading,aerationperiods,anddetentiontimes.
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Solution
Hydraulicload=2,000x100gal/person/day=200,000gal/day
BODload=2,000x0.2lb/person/day=400lb/day
BODloadingonaerationtanks=400/(2,500+5,000)=0.05lb/ft3/day
Aerationperiodinaerationtank=(2,500x7.48x24)/200,000=2.25hrAerationperiodinstabilizationtank=(5,000x7.48x24)/200,000=4.5hr
Detentiontimeinsedimentationtankwith1005recirculation=(3,660x24x7.48)/(2x200,000)
=1.64hr
IV.2.7.ExtendedaerationactivatedsludgeprocessFlowsheetforthisprocessisshowninFigureIV.8
FigureIV.8Flowsheetforextendedaerationtank
Thisprocessoperatesintheendogenousphaseofthegrowthcurve,whichnecessitatesaloworganic
loadingandlongaerationtimeof24hrorgreater. Henceitisapplicabletosmalltreatmentplantless
than1mgdcapacity. Theprocessisstableandcanacceptvariableloading. Finalsettlingtanksare
designedforalongdetentiontimeandalowoverflowratevaryingfrom200to600gpd/ft2. The
processisextensivelyusedforprefabricatedpackageplants. Primarysedimentationisomittedand
separatesludgewastingisgenerallynotprovided.
IV.2.8.AerationdevicesTherearetwomethodsofprovidingaeration,oneisdispersingdiffusedair
andtheotherisusingmechanicalmeans. Indiffusedaeration,bubbleairdiffusersareusedandthey
aresetatadepthof8ftormoretoprovideadequateoxygentransferanddeepmixing. Thediffusers
aremadeofhallowporousstainlesssteeltubes12ftinlengthorhallowporousdisksabout6in.in
diameter. Theindividualdiffusersareattachedalongasubmergedairheaderabout10ftinlength
attachedtoanairsupplyhangerpipewhichisdesignedwithrotatingjoints. Fromdataobtainedfrom
existingplants,theaveragepowerconsumptionisfoundtobe0.563kwhrperlbofBODremoved.
FigureIV.9showsacrosssectionofanaerationtankwithfinebubblediffusersystem.
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FigureIV.9Crosssectionofanactivatedsludgeaerationtankwithdiffuser
Mechanicalaeratorsareofverticaldrafttubetype. Flowthroughthedrafttubeisinducedbyamotor
drivenpropeller,coneorotherrotarydevice. Theseaeratorsaredesignedforinstallationin14to30ft2,
hexagonal,orsquaretanks8to18ftdeep. Fromdataobtainedfromexistingplants,theaveragepower
consumptionisfoundtobe0.446kwhrperlbofBODremoved. FigureIV.10showsthreevarietiesofmechanicalaerators.
FigureIV.10Mechanicalaerators(a)surfaceaerator,(b)simplexcone,(c)turbineaerator
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IV.2.9.OperationalcharacteristicsanddesignparametersTheoperationalcharacteristicsforthe
differentactivatedsludgeprocessesareshowninTableIV.3
TableIV.3Operationalcharacteristicsofactivatedsludgeprocesses
IV.2.10.OperationaldifficultiesTwomostcommonoperatingproblemsinactivatedsludgeplantsare
risingsludgeandbulkingsludge. Occasionally,denitrificationofgoodsettlingsludgetakesplaceina
sedimentationtankafterrelativelyashortsettlingperiod. Thenitritesandnitratesinthesewageare
convertedtonitrogengasmuchofwhichistrappedinthesludgemass. Ifenoughgasisformed,the
sludgemassbecomesbuoyantandrisesorfloatstothesurface. Risingsludgeproblemcanbe
overcomeby(1)increasingtherateofreturnactivatedsludge,(2)decreasingtherateofflowof
aerationliquor,(3)increasingthespeedofsludgecollectingmechanisminthesedimentationtanks,and
(4)decreasingthemeancellresidencetimebyincreasingthesludgewastingrate.
Bulkedsludgehaspoorsettlingcharacteristicsandpoorcompactability. Twotypesofsludgebulking
havebeenidentified. OneiscausedbythegrowthoffilamentousmicroorganismssuchasSphaerotilus
andtheotheriscausedbyboundwaterinwhichthebacterialcellscomposingtheflocswellthroughthe
additionofwatertotheextentthattheirdensityisreducedandtheywillnotsettle. Bulkingofsludgeis
causedbyfluctuationsinflowandstrength,ph,temperature,nutrientcontent,airsupplycapacity,
sedimentationtankdesign,returnsludgepumpingcapacitylimitation,shortcircuitingorpoormixing,
lowdissolvedoxygenintheaerationtank,andoverloadingtheaerationtanks. Tocontrolthebulking,at
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least2mg/ldissolvedoxygenmustbemaintainedintheaerationtank,food/microorganismratiomust
bemaintainedfrom0.2to0.4perday
Anotherproblemencounteredinsewagetreatmentplantsisfoamformationduetothepresenceof
soap,detergentsandothersurfactants. Largequantitiesoffoammaybeproducedduringstartupof
theprocess,whentheMLSSarelow,orwheneverhighconcentrationsofsurfactantsarepresentinthesewage. Thefoamingactionproducesafroththatcontainssludgesolids,grease,andmicroorganisms.
Thefroth,besidesbeingunsightly,isahazardtoworkmen,becauseitisslipperyevenaftercollapse. To
controlfoamformation,screenedeffluentorclearwaterissprayedthroughnozzlesmountedalongthe
topoftheedgeoftheaerationtankcontinuouslyorintermittentlybyaclockcontrolledprocess.
Anotherapproachistoaddasmallquantityofantifoamingchemicalattheinletoftheaerationtankor
intothespraywater.
IV.3.OxidationditchThisisanextendedaerationprocessinaclosedloopreactorandisgoodforsmall
communities. AflowsheetforatypicaloxidationditchisshowninFigureIV.11
FigureIV.11Flowsheetforanoxidationditch
Itconsistsofanelongatedovalchannelabout3ftdeepwithverticalwallsandacenterdividingwall.
Horizontalbrushrotorsareplacedacrosstheditchtoprovideaerationandcirculation. Thescreened
sewageenterstheditch,isaeratedbytherotors,andcirculatesatabout1to2ft/sec. Theoperation
canbeeitherintermittentorcontinuous.
IV.4.StabilizationpondsAstabilizationpond,alsocalledanoxidationpond,isarelativelyshallow
bodyofwatercontainedinanearthenbasinofcontrolledshape. Pondsarepopularwithsmall
communities. Stabilizationpondsareclassifiedasaerobic,aerobicanaerobic,andanaerobic.
IV.1.AerobicpondsAerobicstabilizationpondscontainaerobicbacteriaandalgaeinsuspension.
Aerobicconditionsprevailthroughoutthedepth. Therearetwotypesofaerobicponds. Inthefirsttype
thedepthislimitedto6to18in.inordertoprovidemaximumproductionofalgae. Inthesecond
variety,thedepthmaybeupto5ftsothatmaximumoxygencanbeproduced. TheBODremovalisup
to95percent. Aerobicpondsareusedprimarilyforthetreatmentofsolubleorganicwastesand
effluentsfromwastewaterplants. Stabilizationpondsmaybeemployedinparallelorseries
arrangementtoachievespecialobjectives. Parallelunitsprovidebetterdistributionofsettledsolids.
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Recirculationofpondeffluenthasbeenusedeffectivelytoimprovetheperformanceofpondsystemsin
series. Thepondisdesignedonthebasisof100personsperacreperdaywithadetentionperiodof200
days.
IV.2.AerobicAnaerobicpondsThreezonesexistintheseponds:(1)asurfacezonewhereaerobic
bacteriaandalgaeexistinasymbioticrelationship,(2)ananaerobicbottomzoneinwhichaccumulatedsolidsareactivelydecomposedbyanaerobicbacteria,and(3)anintermediatezonethatispartlyaerobic
andpartlyanaerobic,inwhichthedecompositionoforganicwasteiscarriedoutbyfacultativebacteria.
Becauseofthis,thesepondsarealsoreferredtoasfacultativeponds. TheSSinthewastewaterare
allowedtosettletothebottomandalgaepresenceisnotarequirement. Thesepondsalsocanbe
operatedinseriesorparallel. Thedesignparametersaresameasforaerobicponds. Incoldclimates
duringthewintermonths,aportionoftheincomingBODisstoredintheaccumulatedsludge. Asthe
temperatureincreaseinspringandsummer,theaccumulatedisanaerobicallyconverted,andthe
oxygendemandofacidsandgasesproducedmayexceedtheoxygenresourcesoftheaerobicsurface
layerofthepond. InsituationswhereBODstoragewillbeaproblem,surfaceaeratorsare
recommended. Theaeratorsshouldhaveacapacityadequatetosatisfyfrom175to275%ofincomingBOD.
IV.3.AnaerobicpondsThesepondsareanaerobicthroughouttheirdepth. Tomaintainanaerobic
conditions,pondsareconstructedwithdepthsupto20ft. Stabilizationisbroughtaboutbya
combinationofprecipitationandtheanaerobicconversionoforganicwastestoCO2,CH4,andother
gaseousproducts,organicacids,andcelltissues. BODremovalisupto70%.
IV.4.AeratedlagoonsAnaeratedlagoonisabasininwhichsewageistreatedonaflowthroughbasis.
Oxygenissuppliedbymeansofsurfaceaeratorsordiffusedaerationunits. Dependingontheamountof
mixing,lagoonsareclassifiedasaerobicoraerobicanaerobic. Dependinguponthedetentiontime,the
effluentwillcontainabout1/3tothevalueoftheincomingBODintheformofcelltissues. Beforethe
effluentisdischarged,solidsmustberemovedbysettling. Asettlingtankisanormalcomponentofthis
system,
Inthecaseofaaerobicanaerobiclagoon,thecontentsofthebasinarenotcompletelymixed,anda
largeportionoftheincomingsolidsandthebiologicalsolidsproducedfromwasteconversionsettlesto
thebottomofthelagoon. Asthesolidsbegintobuildup,aportionundergoesanaerobic
decomposition. Themeancellresidencetimevariesfromabout3to6days. Theamountofoxygen
requiredvariesfrom0.7to1.4timestheamountofBODremoved. Iceformationmaybeaproblemin
somepartofthecountryinwintermonths. Theproblemcausedbyiceformationcanbeminimizedby
increasingthedepthofthelagoon. Ifthedepthisincreasedbeyond12ft,drafttubeaeratorsmustbe
used.
FigureIV.12showsschematicofaeratedlagoonandaerobicanaerobiclagoons.
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FigureIV.12Schematicof(a)anaeratedlagoon
(b) anaerobicanaerobiclagoon
DesignparametersforthedifferentformsofthestabilizationpondarefurnishedinTableIV.4.
TableIV.4Designparametersforstabilizationponds
IV.5.DesignofphysicalfacilitiesThefollowingmustbeconsideredwhiledesigningthephysical
facilities:(1)locationofinfluentlines,(2)outletstructuredesign,(3)dikeconstruction,(4)liquiddepth,
(5)treatmentoflagoonbottom,and(6)controlofsurfacerunoff.
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Forsmallponds,acenterinletispreferred. Forponds10acresormore,theinletcanbeinstalled400ft
fromthedike. Forlargeaerobicanaerobicponds,multipleinletsaredesirabletodistributethe
settleablesolidsoveralargerarea.
Theoutletstructure(s)shouldpermitloweringthewaterlevelataratelessthan1ft/week. Itshouldbe
largeenoughtoprovideeasyaccessformaintenance. Provisionforcompletedrainageofthepondisdesirable. Overflowstructuresmustbeprovided.
Dikesmustbeconstructedsuchthatseepageisprevented. Compactioncanbedonebytheuseof
conventionalequipment. Vegetationmustberemoved,andtheareauponwhichtheembankmentisto
beplacedshouldbescarified. Thedikemustbewideenoughtoaccommodatemowingmachinesand
othermaintenanceequipment. Awidthof8ftisadequate. Forouterslopesa3horizontalto1vertical
issatisfactory. Forinnerslopes1verticalto3to4horizontalissatisfactory. Afreeboardof3ftabove
themaximumwaterlevelisadequate. Liquiddepthsupto5ftwillhavesomeadvantage. Provisionof
largerdepthsisnecessaryforlargerponds.
Thebottomofpondsmustbemadeasflataspossible. Thebottomshouldbewellcompactedtoavoid
excessiveseepage.
Pondsshouldnotreceivesignificantamountofsurfacerunoff. Ifnecessary,provisionmustbemadeto
divertthesurfacewateraroundthepond.
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V. ADVANCEDTREATMENT
Manyofthesubstancesfoundinsewagearenotaffectedbyconventionaltreatmentoperationsand
processes. Thesesubstancesrangefromsimpleionssuchascalcium,potassium,sulfate,nitrate,and
phosphatetocomplexsyntheticorganiccompounds. Itisanticipatedtreatmentrequirementswillbe
morestringentthusrequiringadvancetreatmentfacilities. Becauseoftheirimportanceinpromoting
aquaticgrowths,compoundscontainingnitrogenandphosphorousreceiveconsiderableattention.
Unitoperationandprocessesadoptedinadvancedtreatmentareclassifiedas:
(1)Physical,
(2)Chemical,and
(3)Biological.
Selectionofaparticularunitprocessdependsupon:
(1)Theusetobemadeofthetreatedeffluent,
(2)Thenatureofthewastewater,
(3)Thecompatibilityofthevariousoperationsandprocesses,
(4)Availablemeansfordisposingoftheultimatecontaminants,and
(5)Theeconomicfeasibilityofthevariouscombinations.
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TableV.1showstheunitprocesses,theirapplicationandremovalofthesubstances.
TableV.1Applicationdataforadvancedtreatmentprocesses
V.1.PhysicalunitoperationsOfthemanyphysicaloperationsthathavebeenusedinadvanced
treatment,removalofammoniaandnitrogenshouldbegivenconsiderableattention.
V.1.1.AirstrippingofammoniaAirstrippingofammoniaisamodificationoftheaerationprocess
usedfortheremovalofgasesdissolvedinwater. Ammoniumionsinwastewaterexistinequilibrium
withammonia,asshowninthefollowingequation:
NH3+H2O NH4++OH
Eq.(V.1)
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AsthepHisincreasedto7,theequilibriumisshiftedtotheleftandtheammoniumionisconvertedto
ammonia,whichmayberemovedasagasbyagitatingthewastewaterinthepresenceofair.
V.1.2.FiltrationFiltrationcanbeusedtopreparethewastewaterforsubsequenttreatmentprocesses
orfordirectreuseashighlyclarifiedwater. Itmaybedirectlyappliedtothesecondarytreatmentplant
effluentorfollowingcoagulationsedimentationprocess. Theobjectiveoffiltrationistoproduceaneffluentthatconsistentlymeetstheestablishedtreatmentcriteriaatminimumcost. Thefollowing
typesoffiltersareinuse:
Dualormixedmediafilters Inrecentyears,sandfiltershavebeenreplaced,inmany
casesbydualormixedmediafilters. Thesefiltersconsistofdifferentdensitymediaofvaryingsize
inanattempttoapproximatereversegradation. Twoofthemostcommonlyappliedschemesin
mixedmediafiltrationare:
(1) Dualmedia,composedofacoarseanthracitecoalapproximately12indeep;
(2) Mixedmediaconfigurationwhichutilizescoal,silicasand,andgarnetsand.
Bothofthesefiltersattempttocreateamoreidealfiltrationmechanismbyprovidingmediainwhich
thelargestparticlesareonthetopandthesmallestparticlesonthebottom.
GranularmediafiltersThesefiltersmaybeusedwithorwithoutpretreatment(bycoagulationand
sedimentation)forremovalofsolids.
V.1.3.OtheroperationsThefollowingoperationsarealsopracticedasappropriate:
a. DistillationThisisaunitoperationinwhichthecomponentsofaliquidsolutionareseparated
byvaporizationandcondensation. Volatilecontaminantssuchasammoniagasandlowmolecularweightorganicacidscanberemovedbythisprocess. Amongthevariousdistillation
processes,multistageflashevaporation,multipleeffectevaporation,andvaporcompression
distillationappearmostfeasible.
b. Flotation Flotationisusedforremovaloffinelydividedcolloidalandsuspendedmatterin
treatedsewage. Itsuseisincreasingespeciallyinconjunctionwiththeuseofpolymers.
c.
FoamfractionationThisoperationinvolvestheseparationofcolloidalandsuspendedmaterial
byflotationanddissolvedorganicsbyadsorption.
d. FreezingThisisanoperationofphysicalseparationsimilartodistillation.Wastewateris
sprayedintoachamberoperatedundervacuum. Aportionofthewastewaterevaporatesand
thecoolingeffectproducescontaminantfreeicecrystalsintheremainingliquid. Theiceisthenremovedandmeltedbyusingtheheatofcondensationofthevaporsfromtheevaporation
stage.
e. ReverseosmosisThisisaprocessinwhichwaterisseparatedfromdissolvedsaltsinsolution
byfilteringthroughasemipermeablemembraneatapressuregreaterthantheosmotic
pressurecausedbythedissolvedsalts.
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f. SorptionThisisaprocessdevelopedtoremovevariousformsofphosphate. Activated
aluminaisusedbypassingastreamofwaterthroughthesorptioncolumn. Regenerationofthe
activatedaluminaforreuseisaccomplishedbyusingsmallamountsofcausticandnitricacids.
V.2.ChemicalunitprocessesAvarietyofadvancedchemicalunitprocesseshavebeenappliedtothe
treatmentofsewage. Somehavebeenusedtobothtreatedanduntreatedsewagewhilesomeothers
havebeenusedtotreatedeffluents. Thefollowingarethemostcommonunitprocesses.
a. CarbonadsorptionFollowingbiologicaltreatment,adsorptionhasbeenaccomplishedinfixed
andexpandedbedcolumnsofgranularcarbonandintanksusingpowderedcarbon.
b. ChemicalprecipitationThisprocessisusedforprecipitationofphosphorusbyadding
coagulantssuchasalum,lime,orironsalts,polyelectrolytes,andmetalions. Chemical
precipitationmaybecarriedoutinprimaryoractivatedsludgesettlingtanksorasaseparate
operation.
c. IonexchangeThisisaunitprocessinwhichionsofagivenspeciesaredisplacedfroman
insolubleexchangematerialbyionsofadifferentspeciesinsolution. Ionexchangeoperations
areeitherbatchorcontinuous. Exchangematerialisplacedinapackedcolumnorbedand,
watertobetreatedispassedthroughit.
V.2.1.OtherprocessesOtherchemicaltreatmentprocessesusedincludeelectrodialysis,oxidation,
andreduction.
a.
Electrodialysis Ioniccomponentsofasolutionareseparatedthroughtheuseofsemi
permeableionselectivemembranesinthisprocess. Applicationofanelectricalpotential
betweenthetwoelectrodescausesanelectriccurrenttopassthroughthesolution,whichin
turn,causesamigrationofanionstowardsthepositiveelectrodeandcationstowardthe
negativeelectrode.
b.
OxidationChemicaloxidationcanbeusedtoremoveammonia,toreducetheconcentrationof
residualorganics,andtoreducethebacterialandviralcontentofwastewaters. Chlorineor
hypochloritecanbeaddedtoremoveammoniabyformingmonochloramineanddichloramine
asintermediateproductsandnitrogengasandhydrochloricacidasendproducts.
c.
ReductionNitratemaybereducedelectrolyticallyandbytheuseofstrongreducingagents.
Whenreducingagentsareused,thereactionusuallymustbecatalyzed. Otherreducingagents
havebeentried. Theuseofthechemicaldependsonitsavailabilityatlowcostandshouldnot
produceanytoxiccompounds.
V.3.BiologicalunitprocessesThecommonbiologicalprocessunitsemployedare(a)bacterial
assimilationand(b)nitrificationdenitrification. Theseprocesseshavebeenusedprincipallyforthe
removalofnitrogeninvariousformsandindirectlyfortheremovalofphosphorus.
a. BacterialassimilationForcellsproduction,nitrogenandphosphorusarerequired. About
0.13lbofnitrogenand0.0026lbofphosphorusarerequiredforeachlbofcellsproduced. If
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thefoodsourceisproperlyselectedandadjusted,itshouldbepossibletoconvertallsoluble
formsofnitrogenandphosphorusintoorganicformscontainedinbacterialcells.
b. NitrificationdenitrificationThisprocessseemstobethemostpromisingoneforremoval
ofnitrogen. Ifthewastewatercontainsnitrogenintheformofammonia,firsttheammonia
isaerobicallyconvertedtonitratenitrogen(nitrification)andsubsequentlythenitratesare
convertedanaerobicallyintonitrogengas(denitrification). Aplugflowmixedreactor
wouldbeusedfornitrificationanddenitrification.Meancellresidencetimeisacontrol
factoranditvariesfrom2to4days.
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VI. DISINFECTION
Disinfectionoftreatmentplanteffluentinvolvesspecializedtreatmentforthedestructionofharmful
(pathogenic)andotherwiseobjectionableorganisms. Disinfectionhasbeenpracticedfordestructionof
pathogenicorganisms,moreparticularly,bacteriaofintestinalorigin. Thesurvivaltimeofpathogenic
organismsdependsupontemperature,pH,oxygenandnutrientsupply,dilution,competitionwithother
organisms,resistancetotoxicinfluences,abilitytoformspores,andothers. Disinfectiondoesnot
necessarilyimplysterilization(completedestruction)ofalllivingorganisms. Elementalchlorineis
commonlyemployedinmunicipaltreatmentapplications.Wastewaterdisinfectionisalsopracticedby
theapplicationofheat,irradiationbyultravioletrays,andoxidantssuchashalogens,andozoneetc.
Chlorineisshippedinliquidform,inpressurizedsteelcylindersranginginsizefrom100lbto1ton. One
volumeofchlorineliquidyields450volumesofchlorinevapor. Themoistgasiscorrosiveandsoall
pipinganddosingequipmentmustbenonmetalorresistanttocorrosion.
Chlorinegasisdrawnfromthepressurizedcylinderthroughasolutionfeederwhichcontrolstherateof
application. Theinjector,inasolutionfeedchlorinator,dissolvesthegasintothefeedwater. The
concentratedsolutionisthenappliedtotheprocesswater. SeeFigureVI.1forachlorinationflow
diagram.
FigureVI.1Chlorinationflowdiagram
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VI.1.Chlorine dioxideasadisinfectantChlorinedioxidemaybeproducedfromsodiumchloriteand
acid;fromsodiumchloriteandgaseouschlorine,orfromsodiumhypochlorite. Afterproduction,
chlorinedioxideisfedthroughPVCpipeusingadiaphragmpump. Safetyfeaturessuchaschlorinegas
detectors,floordrains,andemergencygasmasksshouldbeavailableatthegenerationandapplication
site. Themajoradvantageofchlorinedioxideisinitsuseasaresidualdisinfectant. Itdoesnotproduce
measurablequantitiesofbyproductssuchastrihalomethanes,becauseitdoesnotreactwithmany
chlorinedemandingsubstances. Otheradvantagesofchlorinedioxideincludealgaedestruction;iron
andmanganeseremoval,andresidualandgeneraldisinfectionproperties.
VI.2.OzoneasadisinfectantOzoneisastrongoxidizinggasthatreactswithmostorganicandmany
inorganicmolecules. Itismorereactivethanchlorine. Itdoesnotreactwitheffluenttoproduce
disinfectingspeciesbutdecomposestoproduceoxygenandhydroxylfreeradicals. Thehalflifeof
ozoneisapproximately10to30minandshorterifpHisabove8andhenceitmustbegeneratedatsite.
Ozoneisrarelyappliedsolelyfordisinfectionbecauseofthehighcostrelativetochlorine. Inmostcases
itsapplicationisforinactivationofmicroorganisms. Ozonedoesnotproduceanyhealthrelatedby
products. Theozonationsystemconsistsoffourpartsasfollows:
1. Agaspreparationsystem.
2.
Anelectricpowersupply.
3. Ozonegeneratingequipment.
4. Contactingequipment.
TwosystemsareavailableforozoneproductiontheOttosystemandWelsbacksystem. Ozonemust
beproducedatthetreatmentplant. Pipesleadingfromtheozonatorareusuallystainlesssteel. Ozone
isintroducedintotheeffluentbyinjectionthroughafilterheadatthebaseofacolumncontactororby
jettingintoanimpelleratthebaseofacontactcolumnorbydiffusionthroughvariousmediasuchas
ceramicandstainlesssteeldiffusers. Typically,thecolumnprovides5to10minofcontacttime
betweentheozoneandtheeffluent.
VI.3.ChemistryofchlorinationChlorineisusedintheformoffreechlorineorashypochlorite. In
eitherformitactsasapotentoxidizingagentandoftendissipatesitselfinsidereactionssorapidlythat
littledisinfectionisaccomplisheduntilamountsinexcessofthechlorinedemandhavebeenadded.
Reactionswithwater Chlorinecombineswithwatertoformhypochlorousandhydrochloricacidsas
showinthefollowingequation:
Cl2+H2O HOCl+H++Cl
(VI.1)
HypochlorousisaweakacidandpoorlydissociatesatpHlevelsbelow6. IndilutesolutionandatpH
levelsabove4,theequilibriumshownaboveisdisplacedgreatlytotherightandverylittleCl2existsas
suchinsolution. Hypochloritesareusedlargelyintheformofcalciumhypochlorites.Whensuch
compoundsaredissolvedinwater,theyionizetoyieldhypochloriteionasshownbelow:
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Ca(OCl)2+H2O Ca2++H2O+2OCl
(VI.2)
Thisionestablishesequilibriumwithhydrogenionsinaccordancewiththefollowingequation:
OCl+H+ HOCl..(VI.3)
TheamountsofOClionandHOClinthesolutiondependuponthepHasshowninFigureVI.2below.
FigureVI.2DistributionofHOClandOClatdifferentpHs&temperatures
ReactionswithAmmonia Ammoniumionsexistinequilibriumwithammoniaandhydrogenions. The
ammoniareactswithchlorineorhypochlorousacidtoformmonochloramines,dichloramines,and
trichloraminesdependingupontherelativeamountofeachandtosomeextentonthepHasfollows:
NH3+HOCl NH2Cl + H2O (monochloramine)(VI.4)
NH3+2HOCl NHCl2+2H2O (dichloramine)(VI.5)
NH3+3HOCl NCl3+3H2O (trichloramine)(VI.6)
Themono anddichloramineshavesignificantdisinfectingpowerandare,therefore,ofinterestinthe
measurementofchlorineresiduals.
Chlorinecombineswithawidevarietyofmaterials,particularlyreducingagents.Manyofthereactions
areveryrapid,whileothersaremuchsmaller. Thesesidereactionscomplicatetheuseofchlorinefor
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disinfectingpurposes. Theirdemandforchlorinemustbesatisfiedbeforechlorinebecomesavailableto
accomplishdisinfection. Thereactionbetweenhydrogensulfideandchlorine,asshownbelow,
illustratesthetypeofreactionthatoccurswithreducingagents.
H2S+4Cl2+4H2 H2SO4+8HCl(VI.7)
Fe2+,Mn
2+,andNO2areexamplesofotherinorganicreducingagentspresentineffluents. Afeworganic
reducingagentsmaybepresent,buttheirconcentrationsareverylow. Organiccompoundsthat
possessunsaturatedlinkageswillalsoneedchlorineandincreasethechlorinedemand.
Cl Cl
C=C +Cl2 CC .(VI.8)
H H H H
ChlorineAmmoniareactionsThereactionsofchlorinewithammoniaareofgreatsignificancein
disinfection.Whenchlorineisaddedtoeffluentcontainingnaturaloraddedammonia,theammonium
reactswithHOCltoformvariouschloramineswhich,likeHOCl,retainstheoxidizingpowerofthe
chlorine. Thereactionsbetweenchlorineandammoniaareshownbelow:
NH3+HOCl NH2Cl+H2O (monochloramine).(VI.9)
NH2Cl+HOCl NHCl2+H2O (dichloramine).(VI.10)
NHCl2+HOCl NCl3+H2O (trichloramineornitrogentrichloride).(VI.11)
Thedistributionofreactionproductsisgovernedbytheratesofformationofmonochloramineand
dichloramine,whicharedependentonpH,temperature,time,andinitialCl2:NH3ratio. Ingeneralhigh
Cl2:NH3ratios,lowtemperatures,andlowpHlevelsfavordichloramineformation. ItisevidentsomedichloraminecanbeanticipatedatpHlevelsbelow7. AtpHlevelsbelow7.5somenitrogentrichloride
canbeexpected. Dependingonthefreeammoniaandorganicnitrogencontent,theleveloffree
residualchlorinationapplied,contacttime,andpH,nitrogentrichloridecanposeaconsiderable
problemwhichmaybedisposedbyvariousmeans.
ChlorineResiduals: Timeofcontactandconcentrationofthedisinfectingagentareextremelyimportant
indisinfection.Whereotherfactorsremainingconstant,thedisinfectingactionmayberepresentedby
Kill=Cxt
WhereC=concentrationofthedisinfectingagent
t=timeofcontactKill=disinfectingeffect
Withlongcontacttimes,alowconcentrationofdisinfectantsuffices,whereasshortcontacttimes
requirehighconcentrationtoaccomplishequivalentkills.
Ithasbecomecommonpracticetorefertochlorine,hypochlorousacid,andhypochloriteionasfree
chlorineresidualsandchlorominesarecalledcombinedchlorineresiduals. Thereactionratebetween
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ammoniaandhypochlorousacidismostrapidatpH8.3andincreasesrapidlyasthepHisdecreasedor
increased. Forthisreason,itiscommontofindfreechlorineandcombinedchlorineresidualscoexisting
aftercontactperiodsof10,15,oreven60min.
Withmoleratiosofchlorinetoammoniaupto1:1,bothmonochloroamineanddichloroamineare
formed,therelativeamountsofeachbeingafunctionofthepH. Furtherincreasesinthemoleratioof
chlorinetoammoniaresultinformationofsometrichloramineandoxidationofpartoftheammoniato
nitrogengas. Thesereactionsareessentiallycompletewhen2molesofchlorinehavebeenaddedfor
eachmoleofammonianitrogenoriginallypresentinthewater. Chloraminesresidualsusuallyreacha
maximumwhen1moleofchlorinehasbeenaddedforeachmoleofammoniaandthendeclinetoa
minimumvalueofchlorinetoammoniaratioof2:1. Furtheradditionsofchlorineproducefreechlorine
residuals. Chlorinationtotheextentthatalltheammoniaisconvertedtotrichloramineoroxidizedto
freenitrogenorothergasesisreferredtoasbreakpointchlorinationbecauseofthepeculiarcharacter
ofthechlorineresidualcurve,asillustratedinFigureVI.3
FigureVI.3Residualchlorinecurve
Theoretically,itshouldrequire3molesofchlorineforthecompleteconversionof1moleofammoniato
nitrogentrichloride(trichloramine). Thefactthat2molesofchlorinearerequiredtoreachthebreakpointindicatesthatsomeunusualreactionsoccur. Nitrousoxide,nitrogen,andnitrogentrichloride
havebeenidentifiedamongthegaseousproductsofthebreakpointreaction. Thepresenceofnitrous
oxidecouldbeaccountedforbythefollowingreaction:
NH2Cl+NHCl2+HOCl N2O+4HCl..(V.12)
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Thetotalchlorinerequiredforformationofmonochloroamine,dichloramine,andthehypochlorousacid
forthefinaloxidationstepcorrespondsto2molesforeachmoleofammonia. Thiswouldindicatethat
nitrousoxideisthemajorendproductwhenammoniaisoxidizedbychlorineindilutesolutions.
VI.4.DesigncriteriaThedesigncriteria,asrecommendedbytheRecommendedStandardforSewage
works,
Great
Lakes
Upper
Mississippi
River
Board
of
State
Public
Health
&
Environmental
Managers(Ten
StateStandards),aregivenbelow:
1. Fornormaldomesticsewage,thefollowingmaybeusedasaguideinsizingchlorination
facilities.
Tricklingplanteffluent10mg/l
Activatedplanteffluent..8mg/l
Tertiaryfiltrationeffluent..6mg/l
Nitrifiedeffluent..6mg/l
2.
Standbyequipmentofsufficientcapacityshouldbeavailabletoreplacethelargestunitduring
shutdowns.
3. Anamplesupplyofwatershallbeavailableforoperatingthechlorinator.
4.
Theuseof1toncontainersshouldbeconsideredwheretheaveragechlorineconsumptionis
over150lbs.
5. Scalesforweighingcylindersshallbeprovidedatallplantsusingchlorinegas.
6. Abottleof56%ammoniumhydroxidesolutionshallbeavailablefordetectingchlorineleaks.
7.
Pipingsystemsshouldbeassimpleaspossible.
8. Agastightroomshallseparatethechlorinationequipmentfromanyotherportionofthe
building.
9.
Acleargas,gastight,windowshallbeinstalledinanexteriordoororinteriorwallofthe
chlorinatorroom.
10.
Thetemperatureoftheroomwherethechlorinationequipmentisinstalledmustbekeptat
least600Fandforcedmechanicalventilationshallbeinstalled. Switchesforfansandlightsshall
beoutsidetheroom.
11. Respiratoryairpacequipmentshallbeavailableandmustbestoredataconvenientlocation.
12.
Thechlorinecontacttankshouldbeconstructedsoastoreduceshortcircuitingtheflow.
13.Thedisinfectantshallbepositivelymixedasrapidlyaspossible,withthecompletemixbeing
effectedin3secondsandaminimumcontacttimeof15minutesprovidedatpeakhourlyflow
14.Facilitiesshallbeincludedforsamplingthedisinfectedeffluentaftercontactandequipment
shallbeprovidedtomeasurethechlorineresidual. Equipmentshallalsobeprovidedfor
measuringfecalcoliformusingacceptedtestprocedures.
15.Solutionfeedvacuumtypechlorinatorsaregenerallypreferredforlargeinstallations.
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VII.EFFLUENTDISPOSAL
Ultimatedisposalofwastewatereffluentswillbebydilutioninreceivingwaters,bydischargeon
landordesertareas,andbyevaporationintotheatmosphereaswellasseepageintotheground.
Disposalbydilutioninlargerbodiesofwater,suchaslakes,rivers,estuaries,oroceansisbyfarthe
mostcommonmethod. Theassimilativecapacityorselfpurificationcapacityofwaterbodiesmust
bedeterminedpriortodischargingtheeffluentintothem. Receivingwaterstandardsandeffluent
standardsareestablishedbyregulatoryagencies. Oneofthestandardsofreceivingbodyofwateris
tomaintainaminimumof5.0mg/lofdissolvedoxygen.
VII.1.DisposalbydilutionSmalllakesandreservoirsarecompletelymixed. Astreamisaliving
thingcapableofabsorbingsomepollutionbecauseoftheirabilitytopurifythemselvesthroughthe
actionoflivingorganisms. Thesourcesofoxygenreplenishmentinariverarereaerationfromthe
atmosphereandphotosynthesisofaquaticplantsandalgae. Inmostrivers,itisassumedthattheeffluentisevenlydistributedoverthecrosssectionoftheriver. InriveranalysistheStreeterPhelps
equationismostcommonlyused.
Thezonewheretherivermeetstheseaiscalledanestuary. Theebbandflowoftidesmaycause
significantlateralmixinginthereachesoftheriversneartheestuary. Estuarinewatersarevertically
stratified. Inmanyestuarinechannels,tidalactionmerelyincreasestheamountanddispersionof
thewastealongthelengthofthechannel.
Oceandisposalistypicallyaccomplishedbysubmarineoutfallsthatconsistofalongsectionofpipe
totransporttheeffluentsomedistancefromshore. Attheendoftheoutfall,theeffluentis
releasedinasimplestreamorjettedthroughamanifoldormultipleportdiffuser. Thedesignofan
outfallshouldmeetapplicablereceivingwaterstandards. Bacterial,floatablematerial,nutrient,and
toxicityrequirementswillgovernthedesignandlocationofmostoutfalls.
VII.2.DisposalonlandEffluentdisposalonlandincludesagriculturaluse,recreationaluse,ground
waterrecharge,spraying,andcontainment(ponding). Sprayingonirrigableland,woodedareas,
andhillsideshasbeenused. Theamountofeffluentdisposaldependsontheclimaticconditions,
theinfiltrationcapacityofthesoil,thetypesofgrassorcropsgrown,andthequalitystandards
imposedwhererunoffisallowed.
VII.3.Direct
&
Indirect
reuse
Theamountofeffluentthatcanbereusedisaffectedbytheavailabilityandcostoffreshwater,transportationandtreatmentcost,waterqualitystandards,and
thereclamationpotentialoftheeffluent. Itcanbeusedascoolingwaterinindustries. Agricultural
useofeffluentispracticeddependinguponthecrops. Fieldcropsthatarenormallyconsumedina
rawstatecannotbeirrigatedwiththeeffluent.
VII.4.RecreationaluseRecreationaluseincludesgolfcourseirrigationandparkwatering,
establishmentofpondsforboatingandrecreation,andmaintenanceoffishandwildlifeponds.
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VII.5MunicipaluseReclaimedwatercanheusedforlawnirrigationinadditiontousingforcar
washing,drivewaywashing,toiletflushing,clotheswashingetc. Toaccomplishthis,thereshould
beadualmunicipalwatersystem,onewithfreshwaterforcookinganddrinkingpurposesandthe
otherwiththereclaimedwaterforallusesotherthandrinkingandcoking. Intheconstructionof
thetwosystemscareshouldbetakentoseethatthereisnochanceofcrossconnectionbetween
thetwosystems.
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VIII. SLUDGETREATMENTANDDISPOSAL
Forproperdesignofsludgetreatmentanddisposalfacilities,sources,quantities,andcharacteristics
ofthesludgemustbeknown. Dataonquantitiesofsludgeproducedfromvariousprocessesand
operationsarepresentedinTableVIII.1
TableVIII.1Sludgequantitiesproducedfromdifferenttreatmentprocesses
Thevolumeofsludgedependsmainlyonitswatercontentandslightlyonthesolidmatter. The
characteristicsofsludgevarydependingonitsorigin,theamountofagingthathastakenplace,and
thetypeofprocessingtowhichithasbeensubjected. Sludgefromprimarysedimentationtankis
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usuallygreyandslimyandhasanoffensiveodor. Sludgefromchemicalprecipitationtanksisusually
blackandhasobjectionableodor. Activatedsludgehasabrownflocculentappearance. Thissludge,
wheningoodcondition,hasaninoffensivecharacteristicodor. Tricklingfilterhumusisbrownish,
flocculent,andrelativelyinoffensive. Digestedsludgeisdarkbrowntoblackandcontainsalarge
amountofgas. Itisnotoffensiveandhasodorlikethatofhottar,burntrubber,orsealingwax.
Sludgetreatmentincludesthefollowingtreatmentprocesses:thickening,digestion,conditioning,
anddewatering.
VIII.1.ThickeningWasteactivatedsludgeormixtureofprimaryandwasteactivatedsludgeare
subjectedtothickening. Theaimofthickeningisvolumereduction.Ifasludgeisthickenedfrom
1to4percentsolids,thevolumewillbereducedto25percentoftheoriginalvolume.Mechanical
(gravity)anddissolvedairflotationthickenersarecommonlyusedtothickensludge.
VIII.1.2.MechanicalthickenerDiluterawprimaryorwasteactivatedsludgeisfedintothe
thickeningtankcontinuously. Thickeningtankissimilartoacircularclarifier. FigureVIII.1shows
schematicofamechanicalthickener.
FigureVIII.1Schematicofamechanicalthickener
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Theresultingcontinuoussupernatantflowisreturnedtotheprimarysettlingtank. Thethickened
sludgecollectedatthebottomofthetankispumpedtothedigesters. Thethickenersaredesigned
onthebasisofhydraulicsurfaceloadingandsolidsloading. Typicalsurfaceloadingratesare400to
800gpd/ft2
. SolidsloadingsareshowninTableVIII.2
TableVIII.2Solidsloadingrateformechanicalthickeners
Aeratedmixedliquororfinaleffluentmustbeaddedtomaintainaerobicconditions.
VIII.1.3.FlotationthickenerTheseareusednormallywithwasteactivatedsludge. Itwillproducea
sludgewithapproximately4percentsolids. ThesolidsloadingratesaregiveninTableVIII.3
TableVIII.3Solidsloadingrateforflotationthickener
VIII.2.Digestion Digestionisclassifiedasanaerobicandaerobic. Althoughanaerobicdigestionhas
beenpracticedforoveracentury,aerobicprocesshasbeengrowinginpopularityforuse.
VIII.2.1.AnaerobicdigestionAnaerobicdigestionisclassifiedasconventionalorstandardrateand
highrate. Conventionaldigestioniscarriedouteitherasasinglestageortwostageprocess. See
FiguresVIII.2andVIII.3forschematics.
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FigureVIII.2Schematicofaconventionaldigesterinsinglestageprocess
FigureVIII.3Schematicoftwostagedigestionprocess
Thesludgeisnormallyheatedbymeansofcoilslocatedwithinthetankoranexternalheat
exchanger. Insinglestage,thefunctionsofdigestion,thickening,andsupernatantformationare
carriedoutsimultaneously. Acrosssectionofatypicalstandardratedigesterisshownin
FigureVIII.4.
Duetothestratificationandthelackofmixing,thevolumeofastandardratesinglestagedigesteris
notmorethan50percentutilized. Recognizingtheselimitations,mostconventionaldigestersare
operatedastwostagedigesters.
Inthetwostageprocess,thefirsttankisusedfordigestion. Itisheatedandequippedwithmixing
facilities. Thesecondtankisusedforstorageandconcentrationofdigestedsludgeandfor
formationofclearsupernatant. Tanksmayhavefixedrooforfloatingcovers. Tanksareusually
circularandthediametervariesfrom20to115ft.Waterdepthshouldbeminimum25ftatthe
center.
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FigureVIII.4 Crosssectionofastandardratedigester
Withtheexceptionofhigherloadingratesandimprovedmixing,therearenotmanydifferences
betweenahighratedigesterandthefirsttankinaconventionaltwostagedigester. Sludgeshould
bepumpedcontinuously. Theincomingsludgedisplacesdigestedsludgetoaholdingtank. Typical
volumesofdigestergas(methane)producedinanaerobicdigestionrangefrom8to12ft3/lbof
volatilesolidsadded. Gasproductionvariesfrom0.6to0.8ft3/capitainprimaryplantstreating
normaldomesticsewage. Insecondarytreatmentplantsthisisincreasedtoabout1.0ft3/capita.
Heatingvalueofdigestergasisapproximately600Btu/ft3
VIII.2.2.AerobicdigestionAerobicdigestersareusedtotreatonlywasteactivatedsludge,
mixturesofwasteactivatedsludgeortricklingfiltersludgeandprimarysludge. Advantagesof
aerobicdigestionare:(1)lowerBODconcentrationsinsupernatantliquor,(2)productionofan
odorless,humuslike,biologicallystableendproduct,(3)productionofsludgewithgooddewatering
characteristics,(4)recoveryofbasicfertilizervalues,(5)feweroperationalproblems,and(6)lower
capitalcost. Thedisadvantagesare(1)higherpowercost,and(2)theusefulbyproduct,methane
gas,isnotrecovered.
Aerobicdigestionissimilartoactivatedsludgeprocess. Asthesupplyofavailablesubstrate(food)is
depleted,themicroorganismswillbegintoconsumetheirownprotoplasmtoobtainenergyforcell
maintenance.Whenthisoccursthemicroorganismsaresaidtobeintheendogenousphaseorauto
oxidationphase.
Factorsthatmustbeconsideredindesigningaerobicdigestersincludehydraulicresidentialtime,
processloadingcriteria,oxygenrequirements,energyrequirementsformixing,environmental
conditions,andprocessoperation. Hydraulicresidencetimevariesfrom10to12days. Volatile
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solidsremovalrangesfrom45to75percent. Solidsloadingrangesfrom0.1to0.2ft3/day. Oxygen
requirementforcompleteoxidationofBODvariesfrom1.7to1.9lb/lbofcelltissuedestroyed. If
mechanicalaeratorsareusedformixing,horsepowerrequiredis0.5to1.0hp/1,000ft3volumeof
thetank. Inairmixing,airrequirementisbetween20and30ft3/min/1,000ft3oftankvolume. The
systemmayperformpoorlyifthetemperatureandpHfallbelow200Cand5.5respectively. ThepH
shouldbecheckedperiodicallyandnecessaryadjustmentmadeifnecessary.
VIII.3.ConditioningConditioningisperformedforthepurposeofimprovingitsdewatering
characteristics. Additionofchemicalsandheattreatmentarethemethodsmostcommonlyused.
Elutriation,aphysicalwashingoperation,isemployedtoreducethechemicalrequirement. The
chemicaldosagerequiredisdeterminedinthelaboratorybyfilterleaftest. Commonchemicals
usedareCaoandFeCl2.
VIII.4.DewateringMethodsusedfordewateringsludgeincludespreadingondryingbeds,vacuum
filtration,andcentrifugation. Thechoiceamongthesemethodsdependsonthecharacteristicsof
thesludge,themethodoffinaldisposal,theavailabilityofland,andtheeconomicsinvolved.
VIII.4.1.DryingbedsSludgeisplacedonthebedsin8to12inlayerandallowedtodry. After
dryingthesludgeisremovedanddisposedinalandfill,orgroundforuseasafertilizer. Atypical
sludgedryingbedisshowninFigureVIII.4.
Thedryingareaispartitionedintoindividualbeds,approximately20ftwideand20to100ftlong.
Theinteriorpartitionsconsistoftwoorthreecreosotedplanks,oneontopoftheother,toaheight
of15to18instretchingbetween