Design of Sewage Treatment Plants Course

7/26/2019 Design of Sewage Treatment Plants Course

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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

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