宝典提纲版英语城市水务工程

Figure13.1.Schematicshowingurbansurfacewatersource,watertreatmentpriortourbanuse,andsomesourcesofnon-pointurbandrainageandrunoffanditsimpacts.

treatment

pipesleaking

pollutionofgroundwater

pollutionofwaterways

wasteofscarceresource

2.1.WaterDemand

Asecurewatersupplyisofvitalimportanceforthehealthofthepopulationandfortheeconomy.Drinkingwaterdemanddependson:

affectthequalityofthewaterinthosereceivingwaterbodies.ThefateandtransportofthesepollutantsinthesewaterbodiescanbepredictedbyusingwaterqualitymodelssimilartothosediscussedinChapter12.

Thischapterbrieflydescribestheseurbanwatersystemcomponentsandreviewssomeofthegeneralassumptionsincorporatedintooptimizationandsimu-lationmodelsusedtoplanurbanwatersystems.Thefocusofurbanwatersystemsmodellingismainlyonthepredictionandmanagementofquantityandqual-ityofflowsandpressureheadsinwaterdistributionnetworks,wastewaterflowsingravitysewernetworks,andonthe

thenumberofinhabitantswithaccesstodrinkingwater

meteorologicalandclimatologicalconditionsthepriceofdrinkingwater

theavailabilityofdrinkingwater

anenvironmentalpolicythataimsatmoderateuseofdrinkingwater.

designefficienciesofwaterandwastewaterplants.Othermodelscanbeusedforthe

treatment

Table13.1showsanoverviewofthetotalannualwaterdemandinvariouscountries.Thetotalwaterdemandissub-dividedintodomesticuseandagriculturalandindus-trialwateruse.

Drinkingwaterdemandfordomesticuseshowsadailyandseasonalvariation.Thereisnogeneralformulaforpredictingdrinkingwaterdemand.Drinkingwatersup-plierstendtomakepredictionsonthebasisoftheirownexperienceandhistoricalinformationaboutwaterdemandintheirregion.

real-timeoperationofvarioussystems.

components

of

urban

2.DrinkingWater

Drinkingwaterissuesincludedemandestimation,watertreatment,anddistribution.

E020809n

Table13.1.Annualper-capitawaterdemandinvariouscountriesintheworld.Source:1)OECDdatacompendium2002and2)WorldResourcesInstitute.

Egypt

920

55

792

74

1993

China

439

22

338

78

1993

source:

OECDdatacompendium2002

source:

WorldResourcesInstitute

2.2.WaterTreatment

AsshowninFigure13.2,oneofthefirststepsinmostwatertreatmentplantsinvolvespassingrawwaterthroughcoarsefilterstoremovesticks,leavesandotherlargesolidobjects.Sandandgritsettleoutofthewaterduringthisstage.Nextachemicalsuchasalumisaddedtotherawwatertofacilitatecoagulation.Asthewaterisstirred,thealumcausestheformationofstickyglobsofsmallparticlesmadeupofbacteria,siltandotherimpurities.Oncetheseglobsofmatterareformed,thewaterisroutedtoaseriesofsettlingtankswheretheglobs,orfloc,sinktothebottom.Thissettlingprocessiscalledflocculation.

Afterflocculation,thewaterispumpedslowlyacrossanotherlargesettlingbasin.Inthissedimentationorclarificationprocess,muchoftheremainingflocandsolidmaterialaccumulatesatthebottomofthebasin.Theclarifiedwateristhenpassedthroughlayersofsand,coalandothergranularmaterialtoremovemicroorganisms–includingviruses,bacteriaandprotozoasuchasCryptosporidium–andanyremainingflocandsilt.Thisstageofpurificationmimicsthenaturalfiltrationofwaterasitmovesthroughtheground.

Thefilteredwateristhentreatedwithchemicaldisinfectantstokillanyorganismsthatremainafterthefiltrationprocess.Aneffectivedisinfectantischlorine,butitsusemaycausepotentiallydangeroussubstancessuchascarcinogenictrihalomethanes.

Alternativestochlorineincludeozoneoxidation(Figure13.2).Unlikechlorine,ozonedoesnotstayinthewaterafteritleavesthetreatmentplant,soitoffers

Beforewaterisusedforhumanconsumption,itsharmfulimpuritiesneedtoberemoved.Communitiesthatdonothaveadequatewatertreatmentfacilities,acommonproblemindevelopingregions,oftenhavehighincidencesofdiseaseandmortalityduetodrinkingcontaminatedwater.Arangeofsyndromes,includingacutedehydratingdiarrhoea(cholera),prolongedfebrileillnesswithabdominalsymp-toms(typhoidfever),acutebloodydiarrhoea(dysentery)andchronicdiarrhoea(Brainerddiarrhoea).Numeroushealthorganizationspointtothefactthatcontaminatedwaterleadstoover3billionepisodesofdiarrhoeaandanestimated2milliondeaths,mostlyamongchildren,eachyear.

ContaminantsinnaturalwatersuppliescanalsoincludemicroorganismssuchasCryptosporidiumandGiardialam-bliaaswellasinorganicandorganiccancer-causingchemicals(suchascompoundscontainingarsenic,chromium,copper,leadandmercury)andradioactivematerial(suchasradiumanduranium).Herbicidesandpesticidesreducethesuitabil-ityofriverwaterasasourceofdrinkingwater.Recently,tracesofhormonalsubstancesandmedicinesdetectedinriverwateraregeneratingmoreandmoreconcern.

Toremoveimpuritiesandpathogens,atypicalmunicipalwaterpurificationsysteminvolvesasequenceofprocesses,fromphysicalremovalofimpuritiestochemicaltreatment.Physicalandchemicalremovalprocessesincludeinitialandfinalfiltering,coagulation,flocculation,sedimentationanddisinfection,asillustratedintheschematicofFigure13.2.

E040712b

India

588 29 18 541 1990

Namibia

185 52 126 6 1990

country demand domestic agriculture industrial year

m3/capita m3/capita m3/capita m3/capita

Germany

490 67 2 389 1999

USA 1870 213 752 828 1990

Mexico

800 101 662 38 1999

screening

rapidmixing

flocculation

filtration

chlorination

unfiltered

water

Figure13.2.Typicalprocessesinwatertreatmentplants.

chlorine

chemicals

anthracite

coal

gravel

filteredwater

pump

reclaimedbackwashwater

backwash

water

oxygen

ozone

backwashwaterreclamationpond

ozonation

Figure13.3.A6-milliongallonperdaywatertreatmentplantatSanLuisObispo,locatedabouthalfwaybetweenLosAngelesandSanFranciscoonthecentralcoastofCalifornia.

noprotectionfrombacteriathatmightbeinthestoragetanksandwaterpipesofthewaterdistributionsystem.Watercanalsobetreatedwithultravioletlighttokillmicroorganisms,butthishasthesamelimitationasoxidation:itisineffectiveoutsideofthetreatmentplant.

Figure13.3isanaerialviewofawatertreatmentplantservingapopulationofabout50,000.

Sometimescalciumcarbonateisremovedfromdrinkingwaterinordertopreventitfromaccumulatingindrinkingwaterpipesandwashingmachines.

Inaridcoastalareasdesalinatedbrackishorsalinewaterisanimportantsourceofwaterforhigh-valueuses.

The

costofdesalinationisstill

high,but

decreasing

steadily.Thetwomostcommonmethodsofdesalinationaredistillationandreverseosmosis.Distillationrequiresmoreenergy,whileosmosissystemsneedfrequentmaintenanceofthemembranes.

2.3.WaterDistribution

Waterdistributionsystemsincludepumpingstations,distributionstorageanddistributionpiping.Thehydraulicperformanceofeachcomponentdependsupontheperformanceoftheothers.Ofinteresttodesignersare

E020809p

chlorine

boththeflowsandtheirpressures.Leakageofdrinkingwaterfromthedistributionsystemisaconcerninmanyolddrinkingwatersystems.

Theenergyatanypointwithinanetworkofpipesisoftenrepresentedinthreeparts:thepressurehead,p/γ,theelevationhead,Z,andthevelocityhead,V2/2g.(Amorepreciserepresentationincludesakineticenergycorrectionfactor,butthatfactorissmallandcanbeignored.)Foropen-channelflows,theelevationheadisthedistancefromsomedatumtothetopofthewatersurface.Forpressure-pipeflow,theelevationheadisthedistancefromsomedatumtothecentreofthepipe.Theparameterpisthepressure,forexampleNewtonspercubicmetre(N/m3),γisthespecificweight(N/m2)ofwater,Zistheelevationabovesomebaseelevation(m),Visthevelocity(m/s),andgisthegravitationalacceleration(9.81m/s2).

Energycanbeaddedtothesystemsuchasbyapump,orlostby,forexample,friction.Thesechangesinenergyarereferredtoasheadgainsandlosses.Balancingtheenergyacrossanytwositesiandjinthesystemrequires

columnwouldriseinapiezometer(atuberisingfromthepipe).Whenplottedinprofilealongthelengthoftheconveyancesection,thisisoftenreferredtoasthehydraulicgradeline,orHGL.ThehydraulicgradelinesforopenchannelsandpressurepipesareillustratedinFigures13.4and13.5.

Theenergygradeisthesumofthehydraulicgradeandthevelocityhead.ThisistheheighttowhichacolumnofwaterwouldriseinaPitottube,butalsoaccountsforfluidvelocity.Whenplottedinprofile,asinFigure13.5,thisisoftenreferredtoastheenergygradeline,orEGL.Atalakeorreservoir,wherethevelocityisessentiallyzero,theEGLisequaltotheHGL.

Specificenergy,E,isthesumofthedepthofflowandthevelocityhead,V2/2g.Foropen-channelflow,thedepthofflow,y,istheelevationheadminusthechannelbottomelevation.Foragivendischarge,thespecificenergyissolelyafunctionofchanneldepth.Theremaybemorethanonedepthwiththesamespecificenergy.Inonecasetheflowissubcritical(relativelyhigherdepths,lowervelocities)andintheothercasetheflowiscritical(relativelylowerdepthsandhighervelocities).Whetherornottheflowisaboveorbelowthecriticaldepth(thedepththatminimizesthespecificenergy)willdependinpartonthechannelslope.

Frictionisthemaincauseofheadloss.Therearemanyequationsthatapproximatefrictionlossassociatedwithfluidflowthroughagivensectionofchannelorpipe.TheseincludeManning’sorStrickler’sequation,which

thatthetotalheads,includinganyheadgainslossesHL(m)areequal.

HG

and

[p/γ

Z V2/2g

HG]sitei

[p/γ

Z

V2/2g

HL]sitej

(13.1)

Thehydraulicgradeisthesumofthepressureheadandelevationhead(p/γZ).Foropen-channelflow,thehydraulicgradeisthewatersurfaceslope,sincethepressureheadatitssurfaceis0.Forapressurepipe,

iscommonlyusedfororKutter’sequation,

open-channelflow,andChezy’s

Hazen–Williamsequation,and

thehydraulichead

is

the

height

to

which

a

water

V2/2g

HL

headloss

EGL

1

V2/2g

Figure13.4.Theenergycomponentsalonganopenchannel.

velocityhead

watersurface

2

HGL

channelbottom

Z1

Z2

elevationdatum

E020809q

V2/2g

HL

EGL

1

Figure13.5.Theenergycomponentsalongapressurepipe.

V22/2g

HGL

p1/y

p2/y

Z1

Z2

elevationdatum

TheenergybalancebetweentwoendsofachannelsegmentisdefinedinEquation13.5.Foropen-channelflowthepressureheadsare0.Thus,forachannelcon-tainingwaterflowingfromsiteitositej:

Darcy–Weisbachor

Colebrook–White

equations,

which

areusedforpressure-pipeflow.Theyalldefineflowveloc-ity,V(m/s),asanempiricalfunctionofaflowresistancefactor,C,thehydraulicradius(cross-sectionalareadivided

bywettedperimeter),R

slope,S HL/Length.

(m),andthefrictionorenergy

[Z

V2/2g]sitei

[Z

V2/2g

HL]sitej

(13.5)

TheheadlossHLisassumedtobeprimarilyduetofriction.Thefrictionlossiscomputedonthebasisoftheaveragerateoffrictionlossalongthesegmentandthelengthofthesegment.Thisisthedifferenceintheenergy

gradelineelevationsbetweensitesiandj;

V

kCRxSy

(13.2)

Thetermsk,xandyofEquation13.2areparameters.Theroughnessoftheflowchannelusuallydeterminestheflowresistanceorroughnessfactor,C.ThevalueofCmayalsobeafunctionofthechannelshape,depthandfluidvelocity.ValuesofCfordifferenttypesofpipesare

HL

(EGL1

EGL2)

[Z

V2/2g]sitei—[Z

V2/2g]sitej

(13.6)

listedinhydraulicstextsorhandbooksMays,2000,2005).

(e.g.

Chin,

2000;

Thefrictionlossperunitdistancealongthechannelistheaverageofthefrictionslopesatthetwoendsdividedbythechannellength.Thisdefinestheenergygradeline,EGL.

2.3.1.OpenChannelNetworks

Foropen-channelflow,Manning’sorStrickler’sequation

is

commonlyusedtopredicttheaveragevelocity,V(m/s),andtheflow,Q(m3/s),associatedwithagivencross-sectionalarea,A(m2).ThevelocitydependsonthehydraulicradiusR(m)andtheslopeSofthechannelaswellasafrictionfactorn.

2.3.2.PressurePipeNetworks

TheHasen–Williamsequationiscommonlyusedtopredicttheflowsorvelocitiesinpressurepipes.Flowsandvelocitiesareagaindependentontheslope,S,the

hydraulicradius,R(m),(whichequalshalfradius,r)andthecross-sectionalarea,A(m2).

thepipe

V

Q

(R2/3S1/2)/n

AV

(13.3)

(13.4)

factorsncanbefoundin

V

0.849CR0.63S0.54

(13.7)

(13.8)

Thevaluesofvariousfriction

AV

πr2V

tablesinhydraulicstextsandhandbooks.

Q

E020809r

TheheadlossalongalengthL(m)ofpipeofdiameterD

(m)containingaflowofQ(m3/s)isdefinedas

LetQijbetheflowfromsiteitositejandHibethe

headatsitei.Continuityofflowinthisnetworkrequires:

KQ1.85

HL

(13.9)

0.5

0.1

0.25

0.15

(13.14)

(13.15)

(13.16)

(13.17)

QDA

QDA

QABQDC

QDC

QAC

QCBQAC

QCA

QBCQCA

QAB

whereKisthepipecoefficientdefinedbyEquation13.10.

K

[10.66L]/[C1.85D4.87]

(13.10)

QBC

QCB

AnotherpipeflowequationforheadlossistheDarcy–Weisbachequationbasedonafrictionfactorf:

Continuityofheadsateachnoderequires:

HDHDHAHC

H

HCHAHBHA

H

22*(Q1.85)

(13.18)

(13.19)

(13.20)

(13.21)

(13.22)

2

HL fLV/D2g

(13.11)

DC

11*(Q1.85)

DA

ThefrictionfactorisdependentontheReynoldsnumberandthepiperoughnessanddiameter.

Giventheseequations,itispossibletocomputethedistributionofflowsandheadsthroughoutanetworkofopenchannelsorpressurepipes.Thetwoconditionsarethecontinuityofflowsateachnode,andthecontinuityofheadlossesinloopsforeachtimeperiodt.

Ateachnodei:

22*(Q1.85)

AB

25*((QCA

QAC)1.85)

)1.85)

11*((Q

Q

C

B

CB

BC

SolvingtheseEquations13.14to13.22simultaneously

forthe5-flowand4-headvariablesyieldstheflows

Qij

fromnodesitonodesjandheadsHiatnodesilistedinTable13.2.IncreasingHDwillincreasetheotherheadsaccordingly.

ThesolutionshowninTable13.2assumesnoeleva-tionheads,nostoragecapacityandnominorlosses.Lossesareusuallyexpressedasalinearfunctionofthevelocityhead,duetohydraulicstructures(suchasvalves,

Storageit

Storage

(13.12)

Qin

Qout

it

it

i,t1

Ineachsectionbetweennodesiandj:

HLit

HLjt

HLijt

(13.13)

wheretheheadlossbetweennodesiandjisHLijt.

TocomputetheflowsandheadlossesateachnodeinFigure13.6requirestwosetsofequations,oneforcontinuityofflows,andtheothercontinuityofheadlosses.Inthisexample,thedirectionofflowintwolinks,fromAtoC,andfromBtoC,areassumedunknownand

E020903k

QDC

=

0.21

henceeachvariables.

isrepresentedbytwo

non-negativeflow

QCA

=

0.00

QCB

=

0.13

Q=0.1

Q=0.25

A

K=22

B

K=11

K=25

K=11

HB

=

0.00

HD

=

1.52

D

K=22

C

Q=0.5

E020809s

Q=0.15

Figure13.6.Anexampleofapipenetwork,showingthevaluesofKforpredictingheadlossesfromEquation13.10.

Table13.2.FlowsandheadsofthenetworkshowninFigure13.6.

HC =0.26

HA =0.43

QBC=0.00

QAB=0.12

QAC=0.07

QDA=0.29

restrictionsormeters)ateachnode.ThissolutionsuggeststhatthepipesectionbetweennodesAandCmaynotbeeconomical,atleastfortheseflowconditions.Otherflowconditionsmayproveotherwise.Buteveniftheydonot,thispipesectionincreasesthereliabilityofthesystem,andreliabilityisanimportantconsiderationinwatersupplydistributionnetworks.

useofsatellitetreatment,suchasre-chlorinationatstoragetanks

targetedpipecleaningandreplacement.

Computermodelsthatsimulatethehydraulicandwaterqualityprocessesinwaterdistributionnetworksmustberunlongenoughforthesystemtoreachequilibriumconditions,i.e.conditionsnotinfluencedbyinitialboundaryassumptions.Equilibriumconditionswithinpipesarereachedrelativelyquicklycomparedtothoseinstoragetanks.

2.3.3.WaterQuality

ManyofthewaterqualitymodelsdiscussedinChapter12can be used to predict water quality constituentconcentrationsinopenchannelsandinpressurepipes.Itisusuallyassumedthatthereiscompletemixing,forexampleatjunctionsorinshortsegmentsofpipe.Reactionsamongconstituentscanoccuraswatertravelsthroughthesystematpredictedvelocities.Waterresidenttimes(theagesofwaters)inthevariouspartsofthenet-workareimportantvariablesforwaterqualityprediction,asconstituentdecay,transformationandgrowthprocessestakeplaceovertime.Computermodelstypicallyusenumericalmethodstofindthehydraulicflowandheadrelationshipsaswellastheresultingwaterqualityconcentrations.Mostnumericalmodelsassumecombinationsofplugflow(advection)alongpipesectionsandcompletemixingwithinsegmentsofeachpipesectionattheendofeachsimulationtimestep.SomemodelsalsouseLagrangianapproachesfortrackingparticlesofconstituentswithinanetwork.Thesemethodsarediscussedinmoredetailin

Chapter12.

Computerprograms(e.g.EPANET)existthatcanperformsimulationsoftheflows,headsandwaterqualitybehaviourwithinpressurizednetworksofpipes,pipejunctions,pumps,valvesandstoragetanksorreservoirs.Theseprogramsaredesignedtopredictthemovementandfateofwaterconstituentswithindistributionsystems.Theycanbeusedformanydifferentkindsofapplicationindistributionsystemsdesign,hydraulicmodelcalibra-tion,chlorineresidualanalysisandconsumerexposureassessment.Theycanalsobeusedtocompareandevaluatetheperformanceofalternativemanagement

3.Wastewater

Wastewaterissuesincludeitsproduction,itscollectionanditstreatmentpriortodisposal.

3.1.WastewaterProduction

Wastewatertreatmentplantinfluentisusuallyamixtureofwastewaterfromhouseholdsandindustries,urbanrunoffandinfiltratinggroundwater.Thecharacterizationoftheinfluent,bothindryweathersituationsandduringrainyweather,isofimportanceforthedesignandoperationofthetreatmentfacilities.Ingeneral,wastewatertreatmentplantscanhandlepuredomesticwastewaterbetterthandilutedinfluentwithlowconcentrationsofpollutants.Thedischargeofurbanrunofftothewastewatertreatmentplantdilutesthewastewater,thusaffectingthetreatmentefficiency.Theamountofinfiltratinggroundwatercanalsobesignificantinareaswitholdsewagesystems.

3.2.SewerNetworks

Sewerflowsandtheirpollutantconcentrationsvarythroughoutatypicalday,atypicalweek,andoverthesea-sonsofayear.Flowconditionscanrangefromfreesurfacetosurchargedflow,fromsteadytounsteadyflow,andfromuniformtograduallyorrapidlyvaryingnon-uniformflow.Urbandrainageditchesnormallyhaveuniformcrosssectionsalongtheirlengthsanduniformgradients.

Becausethedimensionsofthecrosssectionsaretypicallyoneortwoordersofmagnitudelessthanthelengthsoftheconduit,unsteadyfree-surfaceflowcanbemodelled

usingone-dimensionalflowequations.

Whenmodellingthehydraulicsofflowitisimportanttodistinguishbetweenthespeedofpropagationofthe

strategiesforimprovingwaterqualitythroughoutsystem.Thesecaninclude:

alteringthesourceswithinmultiplesourcesystems

alteringpumpingandtankfilling/emptyingschedules

a

3.3.WastewaterTreatment

kinematicwavedisturbanceandthespeedofthebulkofthewater.Ingeneralthewavetravelsfasterthanthewaterparticles.Thusifwaterisinjectedwithatracer,thetracerlagsbehindthewave.Thespeedofthewavedisturbancedependsonthedepth,widthandvelocityoftheflow.

Floodattenuation(orsubsidence)isthedecreaseinthepeakofthewaveasitpropagatesdownstream.Gravitytendstoflatten,orspreadout,thewavealongthechannel.Themagnitudeoftheattenuationofafloodwavedependsonthepeakdischarge,thecurvatureofthewaveprofileatthepeak,andthewidthofflow.Flowscanbedistorted(changedinshape)bytheparticularchannelcharacteristics.

Additionalfeaturesofconcerntohydraulicmodellersaretheentranceandexitlossestotheconduit.Typically,ateachendoftheconduitisanaccess-hole.Thesearestoragechambersthatprovideaccesstotheconduitsupstreamanddownstream.Access-holesinducesomeadditionalheadloss.

Access-holesusuallycauseamajorpartoftheheadlossesinsewagesystems.Anaccess-holelossrepresentsacombinationoftheexpansionandcontractionlosses.Forpressureflow,theheadloss,HL,duetocontractioncanbewrittenasafunctionofthedownstreamvelocity,VD,andtheupstreamanddownstreamflowcross-sectionalareasAUandAD:

Thewastewatergeneratedbyresidences,businessesandindustriesinacommunityconsistslargelyofwater.Itoftencontainslessthan10%dissolvedandsuspendedsolidmaterial.Itscloudinessiscausedbysuspendedparticleswhoseconcentrationsinuntreatedsewagerangefrom100to350mg/l.Onemeasureofthestrengthofthewastewaterisitsbiochemicaloxygendemand,orBOD5.BOD5istheamountofdissolvedoxygenaquatic

microorganismsmetabolizethe

will require in

fivedaysastheyin

organicmaterialthe

wastewater.

aBOD5concentration

Untreatedsewagetypicallyhas

rangingfrom100mg/lto300mg/l.

Pathogensordisease-causingorganismsarealsopres-entinsewage.Coliformbacteriaareusedasanindicatorofdisease-causingorganisms.Sewagealsocontainsnutrients(suchasammoniaandphosphorus),mineralsandmetals.Ammoniacanrangefrom12to50mg/landphosphoruscanrangefrom6to20mg/linuntreatedsewage.

AsillustratedinFigures13.7and13.8,wastewatertreatmentisamulti-stageprocess.Thegoalistoreduceorremoveorganicmatter,solids,nutrients,disease-causingorganismsandotherpollutantsfromwastewaterbeforeitisreleasedintoabodyofwaterorontotheland,orisreused.Thefirststageoftreatmentiscalledpreliminarytreatment.

Preliminarytreatmentremovessolidmaterials(sticks,rags,largeparticles,sand,gravel,toys,money,oranythingpeopleflushdowntoilets).Devicessuchasbarscreensandgritchambersareusedtofilterthewastewaterasitentersatreatmentplant,anditthenpassesontowhatiscalledprimarytreatment.

Clarifiersandseptictanksaregenerallyusedtoprovideprimarytreatment,whichseparatessuspendedsolidsandgreasesfromwastewater.Thewastewaterisheldinatankforseveralhours,allowingtheparticlestosettletothebottomandthegreasestofloattothetop.Thesolidsthataredrawnoffthebottomandskimmedoffthetopreceivefurthertreatmentassludge.Theclarifiedwastewaterflowsontothenext,secondarystageofwastewatertreatment.

Thissecondarystagetypicallyinvolvesabiologicaltreatmentprocessdesignedtoremovedissolvedorganicmatterfromwastewater.Sewagemicroorganismscultivated

HL K(V2/2g)[1

(A/A)]2

(13.23)

D

D U

ThecoefficientKvariesbetween0.5forcontractionandabout0.1forawell-designedcontraction.

suddengradual

Animportantparameterofagivenopen-channelconduitisitscapacity:theflowthatitcantakewithoutsurchargingorflooding.Assumingnormaldepthflowwherethehydraulicgradientisparalleltothebedoftheconduit,eachconduithasanupperlimittotheflowthatitcanaccept.

Pressurizedflowismuchmorecomplexthanfree-surfaceflow.Inmarkedcontrasttothepropagationspeedofdisturbancesunderfree-surfaceflowconditions,thepropagationofdisturbancesunderpressurizedflowina1mcircularconduit100mlongcanbelessthanasecond.Someconduitscanhavethestablesituationoffree-surfaceflowupstreamandpressurizedflowdownstream.

primarytreatment

streamortertiarytreatment

activatedcarbonabsorption

sedimentationtank

ammoniastripping

chlorination

Figure13.7.Atypicalwastewatertreatmentplantshowingthesequenceofprocessesforremovingimpurities.

secondarytreatmnt

sludge

removal

gritchamber

tricklingfilter

clarifier

precipitation

landfill

soilconditioner,fertilizer,landfill

released

Fixed-filmsystemsgrowmicroorganismson

sub-

stratessuchasrocks,sandorplastic,overwhichthewastewaterispoured.Asorganicmatterandnutrientsareabsorbedfromthewastewater,thefilmofmicro-organismsgrowsandthickens.Tricklingfilters,rotatingbiologicalcontactorsandsandfiltersareexamplesoffixed-filmsystems.

Suspended-filmsystemsstirandsuspendmicroorgan-ismsinwastewater.Asthemicroorganismsabsorborganicmatterandnutrientsfromthewastewater,theygrowinsizeandnumber.Afterthemicroorganismshavebeensuspendedinthewastewaterforseveralhours,theyaresettledoutassludge.Someofthesludgeispumpedbackintotheincomingwastewatertoprovide‘seed’microorganisms.Theremainderissentontoasludgetreatmentprocess.Activatedsludge,extendedaeration,oxidationditchandsequentialbatchreactorsystemsareallexamplesofsuspended-filmsystems.

Lagoons,whereused,areshallowbasinsthatholdthewastewaterforseveralmonthstoallowforthenaturaldegradationofsewage.Thesesystemstakeadvantageofnaturalaerationandmicroorganismsinthewastewatertorenovatesewage.

Figure13.8.WastewatertreatmentplantinSoest,theNetherlands(WaterschapValleienEem).

andaddedtothewastewaterabsorborganicmatterfromsewageastheirfoodsupply.Threeapproachesarecom-monlyusedtoaccomplishsecondarytreatment:fixed-film,suspended-filmandlagoonsystems.

E020809t

dryingbeds

filter

digester

thickener

denitrification

clarifier

aerationtank

rawsewage

separationofsolidsatstructures

outfalls.

Thesecomponentsorprocessesarebrieflydiscussedinthefollowingsub-sections.

Advancedtreatmentisnecessaryinsomesystemstoremovenutrientsfromwastewater.Chemicalsaresome-timesaddedduringthetreatmentprocesstohelpremovephosphorusornitrogen.Someexamplesofnutrientremovalsystemsarecoagulantadditionforphosphorusremovalandairstrippingforammoniaremoval.

Finaltreatmentfocusesonremovalofdisease-causingorganismsfromwastewater.Treatedwastewatercanbedisinfectedbyaddingchlorineorbyexposingittosuffi-cientultravioletlight.Highlevelsofchlorinemaybeharmfultoaquaticlifeinreceivingstreams,sotreatmentsystemsoftenaddachlorine