The Design of Small Bore Sewer Systems小口径管道系统的设计 31

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TheDesignofSmallBoreSewerSystems小口径管道系统的设计3

Iwr-JIFignre10.ExternalviewofsanitationblockinNewBussa,showingexposedconnectorsewerandjunctionboxes.-33-iIwIAIc,lJIIll-a-=--interceptortankwithgravityflow-Q-O-interceptortankwithindividualliftstationFigure11.SiteplanoftheMt.Andrewsmallboresewersystem.9fw.1c150Lengthofsewer(m)Figure12.Profileofthe50mmdiameterinflectivegradientsmallboresewersystematMtAndrew,Alabama-35-Thesystemwasdesignedtotestthreelow-costseptictankeffluentcollectionsystems:(a)asmallboresewerwithcontinuousbutnotuniformslope;(b)forcemainconnectionstoasmallboresewerwithcontinuousslope;and(c)asmallboresewerwithvariablegradient.Finaltreatmentanddisposalwereprovidedbyastabilizationpond.Thecollectionlineswerelaidoutinaconventionalmanner(Figure11).LineBanditscontinuation,LineD,both75mmdiameterPVCpipe,havenearlycontinuousslopebutprecisegradecontrolwasnotmaintained.Thegradewascontrolledonlytotheextentthatadequatefallexistsfromtheseptictankstothecollectionlineandthencetothepond.Ofthe18homesservedbythesetwolines,8requiredliftpumpstoenterthecollectionmain.LineCwasconstructedwithcontinuousslopetoitsjunctionwithLineD,butwithoutprecisegradecontrol;itisa50mmPVCpipeservingthreehomes.LineA,a295mlong50mmPVCpipewith10connections,wasconstructedwithanaveragefallof2%butataconstantdepthratherthantograde.Asaresult,therearefiveinflectionsbetweenpositiveandnegativegradients(Figure12).Nomanholesorcleanoutswereinstalledinanyofthelines.4.14Modifiedseptictanksinstalledupstreamofeachconnectionservedasinterceptortanks.Eachhastwocompartments;thefirstisa1920litresettlingtank,andthesecondisaliquidstoragecompartmentof195litrecapacity.Thetwocompartmentsarehydraulicallyconnectedwithsix50mmdiameterPVClamellasettlingtubessetat60’fromthehorizontalandwiththeiropening600mmabovethebottomofthefirstcompartment.Thetubesareintendedtoremoveanysolidspassingthroughthesettlingcompartmentandtopreventscumfromenteringthestoragecompartment.Theoutletfromthestoragecompartmentisa50mmPVCverticalstandpipewithitsopening300mmabovethefloor.Thisallowsforsurgestorageabovetheoutletandaventforthedrain.4.15Theresidentsinstalledthesewersthemselveswithtechnicalassistance.Thecostofinstallationin1975wasaboutUS$6.50perlinearmetre.Thissystemhasrequiredverylittlemaintenanceduringitseightyearsofoperation.Pumpingoftheinterceptortanks,theonlymaintenanceperformedonthesystem,hasbeenmorefrequentthanoriginallyanticipatedduetothesmallsettlingcompartmentvolume.Inspectionandsamplingofthetanksandlineswereperformedafter18monthsofoperation.Samplesofeffluentsfromtheinterceptortanksindicatedthatthesolidsremovalisnobetterandissometimesworsethanconventionalseptictanks,butthatsludge,scumandlargesuspendedsolidsareremovedeffectively.Toinspectthelines,sectionsofthe50mminflectivegradientsewerwereremoved.Onlyathingreyresiduewasfoundcoatingtheinsidewallsofthepipe.NOaccumulationsofheavysolidswereobserved.(b)Uestboro,Wisconsinfi/4.16Thissystemservesasmallruralcommunityofapproximately200persons.Ithas85houseconnections,andsoisamuchlargersystemthan21/R...T.Otis,AnAlternativePublicWastewaterFacilityforaSmallRural-Corunity,SmallScaleWasteManagementProject,UniversityofWisconsin,Madison,Wisconsin,1978.-36-...............................................vII....l.....I..................................t--.--smallboresewerwithmanholeFigure13.SiteplanoftheWestborosmallboreseweragesystems.-37-thatatMt.Andrew.Thesmallboresewers,designedfollowingtheSouthAustralianguidelines(para.4.61,areusedtocollecteffluentfrominterceptortanksfordirectdischargeintoacommonsubsurfacesoilabsorptionfieldwithoutadditionaltreatment(Figure13)22/.Thesewersare100mmdiameterPVCpipeslaidataminimumgradientof0.67%.Curvilinearalignmentsinboththehorizontalandverticalplanesbetweenadjacentmanholeswereallowed.Manholeswereinstalledattheupstreamendofeachline,atalljunctionsandatintervalsnotexceeding185m(spacingsgreaterthanthe122mtypicallyrequired’forconventionalsewerswerepermittedbecauseoftheavailabilityofhydraulicjettingequipmentabletoreachmorethan60m).ThefewhomeswithbasemPEtdrainsbelowtheinvertelevationofthecollectorsewerswereprovidedwithsmallresidentialliftstations.Inaddition,threecommunityliftstationswerenecessary.4.17Eachhouseholdhasaninterceptortank.Existingseptictankswereusediftheywerefoundtobeingoodcondition;iftheywerenot,newprefabricated3,785litre(1,000USgallon)single-compartmenttankswereinstalledregardlessofhomesize.Thetanks,whetherneworexisting,andallotherappurtenances,suchastheresidentialliftstations,werepurchasedbytheWestboroSanitaryDistrict,whichretainedowershipandresponsibilityfortheirmaintenance.Thecostsofpowertoruntheresidentialliftstationsarepaidbythosehouseholdsrequiringthem.4.18ConstructioncostsoftheWestborosystemweresignificantlygreaterthantheMt.Andrewsystem,becauseitisamoreconservativedesignandself-helplaborcouldnotbeused.Inparticular,problemsinvolvingfrostdamagetosewerlinesinpastwintersledtospecificationsrequiringthatthesmallboresewersbeburiedmuchdeeperthanwouldberequiredsimplyonhydraulicgrounds,significantlyincreasingcost;thiswouldnotbeanissueinthegreatmajorityofdevelopingcountries.ThetotalcostwasUS$245,635in1977,representingacostofUS$2,890perconnectionandUSS42.60perlinearmetre.4.19Thesmallboresewershaveperformedwelloverthesixyearssincethesystembecameoperational.Theonlyregularmaintenanceperformedhasbeenpumpingoftheinterceptortanks.Thetanksaredesludgedonastaggeredthree-yearcycle,exceptforthoselocatedatthelargercommercialestablishmentswhicharedesludgedannually.Nosewercleaninghasbeennecessary.Observationofthesewersindicatesthattheyrarelyflowmorethanone-eighthtoone-quarterfull.Solidsaccumulationinthesewersisprimarilyduetoslimegrowthswhichcommonlysloughfromthepipewall.However,minorproblemshaveoccurredwiththesystem.Infiltrationandinflowhavebecomeasevereproblemduringheavyrainsorsnowmelt.Thesourceshavenotbeenpositivelyidentifiedbuttheindividualinterceptortanksaresuspected.Coversoftwoofthe75.newtanksinstalledhavecollapsedbecauseofinsufficientreinforcement.Thereisapossibilitythatothersarecrackedandsoallowgroundw;:sztoseepin.Also,themanholeshavebeenfoundtobeasourceofgrit,stonesandotherdebris,aswellasofclearwaterinflow;fortunately,these22/Fordesigncriteriaforsoilabsorptionfieldsystems,seeU.S.-EnvironmentalProtectionAgency,ProcessDesignManualforLandTreatmentofWunicipalUater,October,1981.Table1:UnitCostsforSmallBoreSewers.ConventionalSewerandWaterMainInstallationfromSelectedProjecfsintheUnitedStatesofAmerica\INSTALLEDPIPECOSTPERMETRE(1982$/M)DEPTHOFCOLLECTORINSTALLATIONUnitedStatesIIWATERMAINO-2M2-4M4-6~INSTALLATIONCommunityPipeDia.Per-PipeDia.Per-PipeDia.Per-PipeDiametercentcentcent100mm200mmSavings100mm200mmSavings100mm200mmSavings100mm150mm200mnJestBoro,$24.90--$29.90$37.408,:-----20%8,:-----WisconsinJ3adgek,g$15.30$37.2040%---,---SouthDakotaSouthCorning,$38.50$46.8018%$52.90$75.6031%----.--NewYorklewCastle,$28.80$50.4043%$36.00$54.0033%---$25.20$27.00$45.OCVirginiaGardner,$13.90--$48.70$86.9044%$69.60$86.9020%---NewYorkIwWI-39-materialshavebeenretainedlargelywithinthemanholeswheretheycanberemovedeasily.Partoftheproblemwiththemanholesisdamagetothecoversinthecourseofsnow-clearingoperationsusingheavyequipment;thisgainwouldnotariseinmostdevelopingcountryapplications.Corrosionisbecomingsevereinthethreein-lineliftstationswhereferrousmaterialsdereused.Pumprails,chainsandsupportsforthefloatswitcheswillsoonrequirereplacementwithnon-corrosivematerials.Odorshavenotbeenaproblem.V.COSTS5.1Comparedtoconventionalsewerage,smallboresewerscanbesignificantlylesscostlytoconstructandoperate,andyetprovideasimilarlevelofservice.Withprimarysedimentationprovidedupstreamofeachconnectiontoremovetroublesomesolidsbeforethewastesenterthecollectors,blockagesrarelyoccur.Inaddition,flowsareattenuated,markedlyreducingpeaktoaverageflowratios.Asaresult,design,constructionandmaintenancestandardsestablishedforconventionalsewerscanberelaxed.Forexample,withoutthesolidsload,smallboresewersneednotbedesignedforself-cleansing.Pipegradientscanbereducedandsectionsdepressedbelowthehydraulicgradeline.Thisreducesthedepthofexcavationnecessary,butmoreimportantlyallowsgreaterconstructiontolerances,reducingthelevelofskillrequiredbythecontractorandtheextentofsitesupervisionbytheclientagency.Also,accessformechanicalcleaningequipmentisnotnecessarysincehydraulicflushing,ifneeded,issufficienttokeepthedrainsfree-flowing.Therefore,curvilinearalignmentispermittedandmanholescanbereplacedbysimplecleanouts.Materialcostsarealsoreducedbecauseattenuatedpeakflowsallowsmallerminirmmpermissiblepipediametersandsmallerliftstations;moreover,pumpingequipment,whichhandlesliquidsandnotsolids,canbesimpler.5.2InAustralia,effluentdrainsarerequiredtobelaidinthesamewayasconventionalsewerswithuniformgradientssoastoachieveaminimumof0.5m/setflowvelocity.Despitethisrequirement,constructioncostsarereportedtobe25to35%lessthanconventionalsewers*Thecostsofinterceptortanksanddrainconnectionsarebornebytheusers.Iftheprivatelybornecostsweretobeincludedinthetotalcosts,however,fewsavings,ifany,wouldresult.PracticeintheUnitedStatesofAmericaissimilartothatinAustralia.Uniformgradientshavebeenrequiredbutinmanycasesminimumflowvelocitieshavebeenreducedto0.3m/set.Reportedsavingsinconstructioncostsoverconventionalsewersrangefrom0%to50%.Inprojectswherethereportedsavingswerelow,thehighercostswereattributedtoahighpercentageofinterceptortankreplacement.onlyrecentlyhaveprojectsbeenconstructedintheUnitedStateswhereinflectivegradientswereallowed.Costsavingsintheseprojectsareexpectedtobegreater,butas-constructeddataarenotyetavailable.5.3StatesZ3/.Table1presentsunitcostsfromseveralprojectsintheUnited-Thesecostsarepresentedasdollars(1982)permetrefor23/Otis,R.J.,-SmallDiameterGravitySevers:AnAlternativeWastevaterCollectionMethodforUnseveredCorunities.U.S.EnvironmentalProtectionAgency,Cincinnati,Ohio,1985.-40-Table2:ShareofTotalProjectCostbyComponentfrom10UniformGradeSmallBoreSewerprojectsintheUnitedStatesofAmericaCOMPONENTPERCENTOFTOTALPROJECTCOSTInplacepipeInterceptorTanksLiftStationsServiceConnectionsStreetRepairStreetR.R.CrossingsResidentialLiftStationsForceMainSiteRestorationManholesCleanoutsMiscellaneous29%17%13%13%7%6%3%3%3%3%1%2%100%-41-installingcollectormainslaidwithauniformgradientatvariousdepths.Comparisonsbetween100mmpipeand200mmpipelaidatthesamedepthandgradientshow20%to44%savings.Totalprojectssavingswouldbegreaterbecausewithflatterorinflectivegradients,smallboresewerswouldavoiddeeperplacement.Iflaidsimilarlytowatermains,costsfromtheprojectinNewCastle’,(Virginia)suggestthesavingswouldexceed50%overconventionalsewers.5.4Table2summarizestheconstructioncostsbycomponentofthe10UnitedStatesuniformgradesmallboresewersreviewed.Thecostsareexpressedasapercentageofthetotalconstructioncosts.Thisshowsthatinterceptortanksandserviceconnectionsaccountforapproximately30%ofthetotalconstructioncosts.In-placepipecostsalsoaccountforapproximately30%ofthecosts.Thus,evengreatersavingswouldresultthanthoseexperiencedintheUnitedStatesincaseswhereinterceptortanksalreadyexistandwhereinflectivegradientsystemsareinstalled.5.5Fewoperationandmaintenancecostsdataareavailable.InAustralia,wherethereisnearly25yearsofexperience,maintenancehasbeenshowntoberequiredinfrequently,simpletoperformandrelativelyinexpensivewhencomparedtoconventionalsewerage24/.Maintenancepersonnelneednotbehighlyskilledtoperformthegeneralmaintenanceduties.Reasonsforthelowercostsarethatthesystemneedslittlewatertofunctionbecausetheinterceptortanksremovesolidswhichmaycauseblockages,thecollectorscanbelaidatshallowdepths,makinganynecessaryrepairsquickandsimpletoperform,andthepre-treatmentprovidedbytheinterceptortankscanreducefinaltreatmentcosts.Themostsignificantmaintenanceoperationistheregulardesludgingoftheinterceptortanks.Frequencyofdesludgingwilldependontanksizeanduserhabits,varyingbetween1to10years.5.6Indevelopingcountries,themosteconomicalapplicationforsmallboresewersislikelytobefortheupgradingofareaswhereexistinginstallations(septictanksorpour-flushlatrineswithleachpits)arefailingorarenotfunctioningproperlybecauseofincreasedwateruseorurbanexpansion,renderinginadequatetheareaavailableforseptictankdrainfieldsoron-sitedisposalofpour-flusheffluentsandsullage.Inthesecircumstances,theseptictankscanbeusedasinterceptortankstoreducethecostsofinstallingthesmallboresewers.Upgradingexistingsanitationfacilitiesinthismannerisfarmoreeconomicalthanconstructingconventionalsewers,notonlybecauseofthereducedconstructioncostsbutalsobecausesmallboresewersarenotdependentonagoodandreliablewatersupplytofunctionproperly.E/EnvironmentalProtectionAuthority.ComparisonofSeverageandComnEffluentDrainageforCountryTownships-ReportNo.65/79EastMelbourne,Australia,1979.r-43-INTERCBPTOItTAt?KVOLDHECALCULATIONSANNEX1Page11.Interceptortanksareusuallydesignedasseptictanks,andarecentreviewofseptictankdesignhashighlightedthewidevarietyofrecommendationsmadeinnationalcodesofpractice:/.InthisAnnexarationaldesignprocedureforinterceptortanksispresented,basedontheBrazilianseptictankcode?/.2.InterceptortanksaredesignedtoprovidespaceforfourseparF’lefunctions:(a)solidsinterception;(b)digestionofsettledsolids;(clstorageofdigestedsolids;and(d)storageofscum.Thesefunctionsandtheirtankvolumerequirementsarediscussedinturn.Solidsinterception3.Theinterceptedsolidscomprisetwofractions:thoseinthetankinfluentandthosewhichriseupfromthesludgelayersthroughflotationbythegasesproducedtherein.Theoreticallytemperatureaffectstherateofsedimentationbychangingtheviscosityoftheliquidphase,butinpracticeitsinfluenceissmallandusuallyignored.Howevertemperaturemarkedlyaffectstherateofanaerobicdigestionandhencegasproduction;thuswithincreasingtemperaturesmoresolidswillrisethroughflotation.Hydraulicsurgeswhichoccurasaresultofhighpeakflowsoverashortperiodoftimealsocausesomeofthesettledsolidstoberesuspended.Itisbecauseofthesetwofactors-solidsflotationandhydraulicresuspension-thatretentiontimesinseptictanksarelongerthanthosenormallyemployedinrawsewagesedimentationtanks.Oftenaminimummeanhydraulicretentiontimeofoneday4.8used(ratherthansixhoursatdryweatherflowinsedimentationtanks),butthereisaformulawhichrelatestheretentiontimetothecontrib‘.ingpopulationandthepercapitawastewaterflow?/:L/J.Pickford,TheDesignofSepticTanksandAqua-Privies,OverseasBuildingNoteNo.187,BuildingResearchEstablishment,Garston,England,1980.IConstn@oeImt+la$iodeFossasSepticaseDisposi@odosEfluentesFimais,NormaBrasileiraRegistrada7229,Associa$~~BrasileiradeNormasTecnicas,RiodeJaneiro,1982.IIC.M.FairandJ.C.Geyer,WaterSupplyandWasteEngineering,pa901,JohnWiley,NewYork,1954.TheformulaquotedhasbeenchangedfromUSgallonstolitres.TAG/TN/14-44-th=1.5-0.3log(Pq)ANNEX1Page2(1)whereth=minimummeanhydraulicretentiontime,daysP=contributingpopulation4=wastewaterflow,ledThisequationindicatesthattherequiredminimumretentiontimedecreasesasthepopulationservedorpercapitawastewaterflowincreases;itthusmakesinherentallowanceforthefactthatthemagnitudeofpeakflowsdecreaseswithincreasingflowrates.However,theminimumrecommendedretentionissixhours.4.Thevolumerequiredforsedimentation(Vh,m3)isthereforegivenbytheequation:vh=lo-(pq)th(2)Solidsdigestionandstorage5.Althoughthedigestionofinterceptedsolidsandthestorageofdigestedsolidsaretwoseparatefunctionsofaninterceptortank,itisnecessarytoconsiderthemtogethersincecurrentlyavailablefielddatadonotdistinguishbetweenthem4/.Theliteratureonseptictanksisfullofreferencestosludgeaccumtionrates;reportedvaluesvaryfrom25litrespercapitaperannum(lea)togreaterthan100lea.Areasonableestimateofsludgeaccumulation,basedontheaveragesludgeaccumulationin205Americanseptictanks,is0.19led?/,equivalenttoanannualstoragerequirementof70litrespercapita.Thusthecombinedsludgedigestionandstoragevolume‘(Vs,m3isgivenby:VS=70x1O-3PN(3)WhereNequalsdesiredintervalbetweensucessivedesludgingoperations,years.StorageofSCM6.Scumstoragerequirementsareoftenimplicitlyincludedinthesolidsstoragevolume,butthisisnotstrictlycorrect.Scumresultsfromfatsandgreaseinsullageandfromtoiletpaper.Astudyof268septictanksintheUSAindicatedthatthesubmergedscumvolume(i.e.,thevolumeYSolidsdigestionisaverytemperature-dependentprocess,andsoclearlylessvolumeisrequiredfordigestionathighertemperatures.However,morefieldresultsarerequiredbeforeareliabledesignrelationshipcanbeformulatedbetweenthedigestionrateininterceptortanksandtemperature.51SiR.Weibelandothers,StudiesonHouseholdSevageDisposalSystems,PartI,PublicHealthService,EnvironmentalHealthCenter,CincinnatiOhio,1949.TAG/TN/14-45-ANNEX1Page3belowtheinvertoftheoutletpipe)rarelyexceeds0.7cubicmetrest/.Thusthemaximumsubmergedscumdepthcd,,,m)isafunctionofthetanksurfacearea(A,m2);dss=0.7/A(4)Clearspacedepth7.Theclearspacedepth,whichistheminimumacceptabledepthofthesolidssettlingzonejustpriortodesludging,comprisesthesubmergedscumcleardepthandthesludgecleardepth.Thesubmergedscumcleardepthisthedistancebetweentheundersideofthescumlayerandthebottomoftheoutlettee,andshouldbeatleast75mml/.Thesludgecleardepthisthedistancebetweenthetopofthesludgelayerandthebottomoftheoutlettee;itsminimumvalue(d,,,m)isrelatedtothetanksurfaceareaasfollows8/:dsc=0.82-0.26A(5)subjecttoaminimumvalueof0.3mz/.Thustheminimumtotalclearspacecalculatedas(0.075+d,,)mustbecomparedwiththedepthrequiredforsedimentation(=Vh/A),andthegreaterdepthchosen.Designeurqle8.Designar:interceptortanktopretreatthewastewaterfromahouseholdof8personswho!ireduce70ledofwastewater;thetankistobedesludgedeverythreeyears-/.Thesolutionisasfollows:(a)Calculatetheminimummeanhydraulicretentiontimeforsettleablesolidssedimentationfromequation[l]:th-1.5-0.3log(Pq)-1.5-0.3log(8x70)=0.68day!Y7/!v91-’T.W.Bendixen,R.E.Thomas,A.A.McMahanandJ.B.Coulter,EffectofFoodWasteGrhdersonSepticTanks,PublicHealthService,RobertA.TaftSanitaryEngineeringCenter,Cincinnati,Ohio,1961.J.A.CotteralandD.M.Norris,SepticTankSystems,JournalofthaSanitaryEngineeringDivision,AmericanSocietyofCivilEngineers,(959SA4),715,1969.R.Laak,Multi-chamberseptictanks,JournaloftheEnviromentalEngineeringDivision,AmericanSocietyofCivilEngineers,106(EE3),539,1980.Inpractice,ofcourse,designerswouldseektooptimizethecombinationoftankvolumeanddesludgingfrequency,soastoproducealeast-costsolution.Thusseveraldesludgingintervalswouldnormallybeconsidered.TAG/TN/14-46-ANNEX1Page4Thusthevolumerequiredforsedimentationisgivenbyequation[2]as:vh-10-3(Pq)th*lo-3(8x70)0.68-0.39m3(b)Calculatethesolidsdigestionandstoragevolumefromequation131:VS=70x10-3PN=70x10S3(8x3)=1.68m3Cc)Assumeacross-sectionalarea(A)of3m2.Then:(I)themaximumdepthofsludge(V,/A)is(1.68/3),=0.56m;(ii)themaximumsubmergedscumdepthcd,,)isgivenbyequation141as(0.7/3),=0.23m;(iii)theminimumsludgeclearspaceisgivenbyequation[5]asIC.82-(0.26x311,=0.04m;thisislessthantheminimumof0.3mwhichisthereforeadopted;(iv)thetotalclearspacedepthistherefore(0.075+0.3)m,-0.375m;thisisgreaterthan(Vh/A),=(0.38/3)-0.13m,sothetotalclearspaceisthecontrollingfactorinthedesign.(d)Thetotaleffectivedepthisthereforethesumofthesludgedepth(0.56m),theclearspacedepth(0.375)andthemaximumsubmergedscumdepth(0.23m),i.e.1165mm.(4Thussuitableoverallinternaldimensionsofthetankwouldbe1mx3Px1.5n,asshowninFigure1.1.iL-lSCUMArCLEARSPACESLUDGEI560iI150013000I11Figure1.1.Dimensionsofdesignexampleinterceptortank.TAG/TN/14-47-ANNEXIIPage1SNALXBORESEWERDESIGNBXLVPLES1.Asinglesmallboreseweristobeconstructedtoserve20lots,ofwhichonly10havebeenbuilttodate(Figure11.1).Eachexistinghousehasamultipletapin-houselevelofwatersupplyservice.Provisionmustbemadeforthefutureconnectionof10lotslocatedabovethepresentupstreamterminusofthesewer.Thedesign(peak)flowis0.025litre/secondperconnection.2.Solution.Firstly,individualsectionsofthesewerareselectedforhydraulicanalysisonthebasisofisolatingsectionswithrelativelyuniformgradientsorflows.Inthiscaseninesectionswerechosen,asshowninFigure11.1.3.ThehydrauliccalculationsarepresentedinTableII.1anddescribed,columnbycolumn,below:Column1:StationnumberNumbersrelatingtothecommencementofeachsewersectionandstarting..fromthedownstreamterminusofthesewereColumn2:StationelevationsTheelevationinmetres,ofeachstationaboveadatum(whichinthiscaseistheelevationofStation1).Column3:DistanceThedistance,inmetres,ofeachstationfromStation1.Column4:ElevationdifferenceoversectionThedifference,inmetres,betweentheelevations(column2)ofadjacentstations.Column5:LengthofsectionThedifference,inmetres,betweenthestationdistances(column3)ofadjacentstations.Column6:AverageslopeofsectionColumn4dividedbycolumn5givestheaverageslopeofthesectioninm/m.c01ullm7:NumberofconnectionsservedThenumberoflotsconnectedupstreamofthedownstreamstationofthesection.0501001t260Lengthofsewer(ti)FigureII.1.Sewerprofilefordesignexamplesusedbranch1g20lotsePm=~extensionserving10lotsTAG/TN/14-49-ANNEXIIPage3Column8:DesignflowColumn7multipliedby0.025(thedesignflowinlitre/secondperlot)givestheflowinthesectioninlitres/second.Column9:PipediameterThepipediameter,inmm,selectedbythedesignerforeachsection.Initialchoicesmayproveinadequateandthepipesizemayhavetobeincreased(seecolumn10).Column10:FlowatfullpipeThecapacityoftheselectedpipediameteriscalculatedfortheslopecomputedinCO~WUI9.Manning’sequationwasusedinthisexamplewitharoughnessfactor(n)of0.013forplasticpipe.Theflowatfullpipemustbegreaterthanthedesignflow(column8).Ifitisnot,alargersizepipemustbeusedortheslopeincreased.4,Thecriticalsectionsinsmallboresewerdesignsare:(a)thosethatarecontinuouslyflooded;and(b)thoselaidlevel.Inthisexample,threesectionsofthesewerwillremainfullatalltimes:thesectionsbetweenstations3and4;6and7;and9and10.Thesesectionsmustbecarefullyanalysedhydraulicallytoensurethattheydonotbecomeexcessivelysurchargedduringpeakflowperiodsandbackupintoanyconnections.Tocheckthis,themaximumelevationtowhichthehydraulicgradientrisesmustbedetermined.Theelevationdifferencebetweenthiselevationandthedownstreamoutletfromthefloodedsectionestablishesthemaximumpermissibleslopeofthehydraulicgradientoverthesurchargedsection.Intheaboveexample,forthesectionbetweenstations3and4,Manning’sequationisusedtocalculatethehydraulicgradientforthe50mmpipeflowingfull:thisis0.0035m/mandthisisequivalenttothehydraulicgradelinerising(0.0035x24,thelatterfigurebeingthesectionlength),i.e.0.08mabovetheupstreamstation.Theinvertoftheoutletofanyinterceptortankdischargingintotheseweralongthissectionmustbeabovethehydraulicgradelineinordertoavoidbackflowintothetankduringperiodsofpeakflow.Ifthisisnotpossible,thenalargerpipediametermstbechosen,oranindividualliftstationprovided,orthesewerelevationatthedownstreamstationlowered;thechoicebetweentheseoptionsisessentiallyeconomic,although(especiallyindevelopingcountries)theconsequenceofpumpfailureintheliftstationmustb