dlyll41.1\ra.+s.Lll,t.-Jrb sldl frldll iJlj,r:rtelr}}Jlf el+l t r$r f.,! / tF.JlJt5tll ir.rl+Jl( 4*llJrr-,atl eJ)Design of Bar Raeks(Bar Sereons)6ifsh C-f :l+JtLlt rlslt4JJIl 4--J-..;lll. s : r-il--.i,rlfelt-lyl...ilrFl ,.aJll d-.. CtuY r+ C*:l ard a311rl dtfr u. sJl.....ii...J tJ|-lt sJt ...dlrJt u.r. (,jl,...u.bJl sSJ sil i.,lrtlll Cr.lllsJl... Otsrttlbrcl.jll .j/+.. dtsr'tl+ /il r-l O. .,Jl...:r'-- -tl Ll-Jtl !lrL. sll((^\l d1r.J lr'"rr'",.ttlD)..... Jtrt . rU {'i. r.'i..| (rill. elltll (Pl sjl,.. sfiil tU'lt 1rr. eLJ &r+ll s.ll.... ilrdl d.l! !-J sfr.ll Jrl, {r|| Jrttl sJl.....rti5tt3 g.rt .ltg, tl,lilb Cebill,'i. el& 1;ill sll,.... r.I d,l$l "ll((.-tl))c.zrcy' iltj At rllr,... ,rFh"sP c!N,,. dlt fuFll sjn rtttsrsl .rsll &Ji.Jl rrll sjl... .'{hlt g-tit ,-Stl sll. \riirJ15ill Sliill e'is.ll..... &ltt Lb sJ Ji, &iJ J (!a'' Ji (,ll.....tui sl .il dll. larLlcr.Jrtdelr! O. sjl((+r))..,. l& sl 43 e3ll rLilll !r..i)l .,J.....|.r11 & di..l f Cill tlr.ll rrJ sll.... p1.dil J d'F dJ.i. O5rtI sJl(s trlJ,-rD)'.lrr ul6.;.1 o"tt...{{+J OlJbJ iiuJl L'..t sll... c/P rer dI lri sjt(,r-.+rj))- ltafrIlnrz-/Jdit-.-3e --J.(\-,--]-_=-;$_l ,fJS^,.' -,;sl:+ 6rsrl gl Yl ,r:*^'^1V&LiYl !5i$L-l 'Jl.... ltLYl rar^ J.\!'4il:Jrlsildr J 4^ii\Jcl^AL i^$ d.,k++-9'r3 cal:L::11. ,'''-rll rs: !+6rc1-,"^lt3 Jdl + -l :.o Ja ,JS,Jl-lList of contentsSubleapage no,Part oneprestresslI.t:'11I't1IIIII1-l:1iII't'l'tIChapter one : Introduction1-1- Introduction1-2- ObjectivesChapterTVo21- Introduction2-2-Coarse Screens2-3:X'ineScreens2-4: APPLICABILITY2-5: ADVAIIITAGF,S ANDDTSADVAIITAGES2-6:DESIGN CRITERIA2-7: RemovalEfficiency2.8: OPERATION AND MAINTENANCE3-1:General&2:DesignpopulationI23471011t2t4t7ChapterThree: DESIGN pOpUtATtON & Volume ofwastewater18183-3:Estimating futureservice demand34: Volume of wastewater15: Population equivalents96: Capacityfactor3-7: Hydraulic Capacity3-7-l: X'lowDefi nitions and Identification3-7-2: Hydraulic Capacity for Wastewater Facilities toSERVESExisting Collection Systems3-7-3: Hydraulic Capacifyfor Wastewater FacilitiestoserveNew Collection Systems.Chapter FourDesign Criteria of Bar Screens4-1:Bar ScreensChapter5:Designof Bar Racks19202424252625,,,|285-1: calculation ofdesignflow5-2:Assumption5-3: Calculations of flow conditions in thethescreen3434incomingpipe to365-4:Design of rack(screen ) chamber 385-4-1: Compute bar spacingand thediurensions ofthc bar38rack chambcr.....'.':5-4-2: Compute the actual depthof flow and velocity in theRack chamber at peak design flow5-4-3: Compute thevelocity through theclear openingsOf the barrack5-4-4: Compute headloss through the barrack5-4-5: Compute the depth of flow and velocity in the rackchamber below the rack dr,u,5-4-6: Compute the headloss through the rack at 5}o/oClogging5-5:Bottom Slopeof thechannel Below the Rack5-5-1: Compute the critical depth of flow5-5-2: Calculate theelevation ofthe channel bottomnear free fall in the wet well5-6: Hydraulic profile through the bar Rackassumechannel floor is 0.005-7: Quantity of screenings5-8: Disposal ofscreenings the screenings will bedisposed of by sanitary and fillings5-9: DesignDetaly404l4l424244444444454646IIIIII'1IIFigures:ng. -l) Diagrrm of trashrackucedfor trertmentof CSOIFig .(2-2)RomagrM ICOMRTNGIIMcchanicalscreen(VERTICAL) for CSOfloatablescontroleX'ig. 3-1Infiltration allowances_Fig . (3-a Rrtioof peak hourly flow to designaverageflowFigure 4-l.Schematic of HeavyDuty MechanicalyCleanedBarScreenFigure 4-2 Estimate of ScreeningcollectedonBar Screensn& ($1 ): Sewerand BarscreenlayoutIiC. (5-2) :Hydraulicelemenbof circularpipeFig (5-3 ) I Detaib of rackchanber.la, binA c)Fig (54) hydraulicprofile through the Bar Rack{tg. (S-O The relationship betweenof openingof barsandt[s sgrrolingsfig (5{ ) deign detailsof the bar rackpsgs6922262831363740454547'tII1II'11I1II1IIIIIIrlI'1IlIItIIl1!i1IITables:Table (2-l) Design Parametersfor statioScreensTabIe(2-2) Design Parametersfordrum screensand rotaryscreenTable 3-1. CapacityfactorTable 3-2. Per capitasewageflowsTable 3-3.SewagecharacteristicsTable 4-l.Effrciencies of bar spacingTable (5 - l) : designfactors for ManuallyandMechanicallycleaned bar racks.pgst415l819t929350l|llTEn ffi!1-l- Introduction:The first 'nit found at fteatmentplants is usedto removetrash and coarsesolids (wood, cloth,paper,plastics, garbage, etc.).Screeningdevicesmayconsist of parallel bars, gratings, wire mesh, or perforated slots. Theparallel bar or rodconfiguration is calleda trash rack or barscreenand willhave openings(space between bars)of s7r" or more.The 1990 edition of"Recommended Standardsfor wastewater Facilitiesof the GreatLakes-upperMississippi RiverBoardof state public Health andEnvironmentalManagers"(commonly referred to as "Ten States' Standards")lists theseascoarse screens. This publication recommends openings for manually cleanedbarracksbe from1" to 13/4"andplaced on a slopeof30to 45 degrees fromthehorizontal. It also recommends approach velocities be at least 1.25 frs toprevent settlingand not more than3.0 fps to prevent forcing material*,roughthe openings.The term "screen"is usedfor screeningdevicesconsisting of perforatedplates, wedgewire elements,andwire clothandthe openingswill be lessthat %". "TenStates' Standards"refersto screenswith openingsofapproximately l/16 inchas fine screensthatcan be usedin lieu ofprimary sedimentation. commonly usedscreensinclude the hydrosieve orinclined fxedscreenandthe rotarydrum screen.A primary concemofscreeningdevicesis the potential headlossthrough them, which increaseswith thedegreeof clogging.If they arenot self cleaning;it is imperativethat the operatorclean the'mseveraltimes eachdaytLr prevent a darrmingefrect thatresultsin grit behg de1';r-rsitedupsrrealnfromtheunit. wherefinesclccnsareused, consideration shouldbe givento the prior removal offloatableoils andgreascs, rvhichcan clog theopening-r.l-2- Obiectives:l- Determination of peak designwet weatherflow, max.designdryweather flow and average designdryweatherflow.2- Calculation of flow conditionsin the incomingconduit.3- Designof rack (screen) chamber.4- Calculation ofbottomslopeofthe channelbelowthe rack.5- Hydraulicprofile throughthe bar rack.6- Calculation ofthe quantity of screenings.7- Disposalof screening.8- Providea designdetails.Il|filT[nTr0Chanter Two2-1- Introduction:ln 1994, the U.S. Environmental ProtectionAgency(EPA) recognizedthe importanceof controlling solidand floatablematerials underthe "nineminimumcontrols"described in the CombinedSewerOverflow (CSO)Control Policy. CSOscancontainhigh levels of floatablematerials,suspendedsolids, biochemicaloxygen demand (BOD),oils and grease,toxic pollutants,and pathogenicmicroorganisms.Floatables are often the mostnoticeableand problematic CSO pollutant. They create aesthetic problemsandboating hazards,threatenwildlife,foul recreationalareas, andcause beachclosures. Thereare numerousmethodsavailablefor floatablescontrol.includingbaffles, catch basin modifications,netting systems,containmentbooms,skimmingprocesses, andscreening and trashrack devices.Thesetechnologiesare summarizedin EPA's CSOTechnologyFactSheetentitled"FloatablesContol'(EPA 832-F-99-008).This fact sheetfocuseson screensandtrash racksfor CSO floatables control.Screens areconsideredan effectiveandeconomically efficient methodofremovingsolidsand floatables from CSOs.CSO screensare typicallyconstructedof steel parallelbars or wires,wire mesh (wedgewire),grating,or perforatedplate; somescreens,however,are constructedof milledbronzeor copperplates.In general,theopenings arecircular or rectangularslots, varyingin size from0.25to 15.24centimeter (0.1 to 6 inch) spacings.The amountand sizeof the solidsand floatablc'sremovedis dependent onthe type of screenand thc sizeof thescreenopenings.Solids areretnovedfiom theflow by twobasictreatmentrneclranisms;. Direct straining of all particleslarger thanthe screenopenings.Filteringof smaller particlesby strainingt'low tirrouph the matof3solids already deposited on thescreen.Generally therearetwo typesofbar screens- coarseandfine. Bothare usedatCSOcontol facilities, with eachdifferenttype providinga differentievelofremoval efficiency.While thereis no industry standardfor classifringscreensbased on aperture sizecoarsebarscreensgenerallyhave0.04to 0.08meter (1.5 to 3.0 inch)clear spacingbetweenbars and fine screens generallyhaveroundedor slottedopeningsof0.3 to 1.3 centimeters (0.1to 0.5 inch)clear soace.2-2-CoarseScreensCourse screens are constructedof parallel verticalbarsand areoftenreferredto as bar racks or bar screens.In CSO controland treatmentfacilities, coarsescreens areusually the first unit of equipment in the system.Thesescreensareusuallyset at 0 to 30 degreesfromvertical and are cleanedby an electricallyor hydraulicallydrivenrakemechanismthat removes thematerial entrainedon thescreen on a continuousor periodic basis. Therearethreetypesofbar screensusedat CSOcontrol facilities:trashracks;manuallycleanedscreens;andmechanicallyclenned screens.- Trash racksTrash racks (alsoknownas trash grates) are intendedto removeonly verylarge objectsfromtheflow stream.Trash racks are generallyprovidedat theintersectionof the combinedsewerand tire.sanitary interceptorto preventmajorblockagesin the interceptoror to protectpurlpingequipment.Sinceboth dry anJwetweather flowspassthroughthis type of screeningdevice,dailyclcaningis usually required.l'rash racks lvpically have 0.04to 0 08meter (1.5 to 3.0iuch) clear spacingbetwecn bars. Figure1 is a diagrarnof atvpicaltrasir rack.-Manually cleanedbar screensManually cleaned bar screens havea 2.54to 5.08 centimeter (1.0 to 2.0inches)clem spacingbetweenbars.The barsare set 30 to 45 degrees fromthe vertical and the screeningsare manuallyrakedontoa perforatedplate fordrainagepriorto disposal.-Mechanicallycleaned bar screensMechanicallycleanedbarscreenshavea 0.64to 2.54 centimeter (0.25 to1.0 inch)clear spacing betweenbars, The barsareset0 to 30 degreesfromthevertical.Electricallydrivenrakemechanismswill either continuouslyorperiodicallyremovematerialentrainedon thebarscreen itself. Thet}reecommontypes of mechanically cleaned screens are: ( 1) chaindriven, (2)climber type rake,and (3) catenary.Chain drivenmechanical raking systems consistof a series of bar rakesconnectedto chainson eachside ofthe barrack. Duringthe cleaningcycle,the rakestravel in a continuouscircuitfiomthebottomto the top of the barrack,removingmaterialsretainedon the barsand discharging themat the topof the rack.A disadvantage of chain-drivensystemsis thatthe lower bearingsand sprocketsare submergedin the flow and are susceptible to blockageand damagefrom gpit and other materials.Acceleratedchainwear andconosioncanalso be a problem.Clirnber-rypesystemsemploya singlerake mechanismmountedon a geardrivenrackand pinionsystem. The gear driveturnscogu'heelsthat movealong a pin rack mormtedon eachside of the bar rack. During the cleaningcycle,the rake rnechanisrntraverlsup andclownthe bar rack to removematerialsretainedon the bars. Screeningsare typicallydischargedfromthebarsat thetop of therack. This typeof barscreen has no submergedbearingsor sprockets and is, therefore,lesssuscepibleto blockages,damage andcorrosionthan chaindriven rurits.Catenarysystems also employ chain-drivenrake mechanisms, but allsprockets,bearings,and shafts are located abovethe flow level in thescreeninssAd oY&toaadonfuto.$f$ h na.c bfnC||fg hraEbr.toDqrlal bdrrir!lrih $idotl3SCDoS elr I it.aN0!6tlrircNb!r3gfidmoMisdnry orr{ntrilgxt lroll uoa!h 6oldtarlflCllot,tlriiro$arcdD6nG||$olhYglSctgotra{dOorFoooooo Ooooo000coocooaooO oooooc OSource:Metcalf and Eddy,1991.FIGIIRE(2-l)DIAGRAMOFTRASHRACKUSEDFORTREATMENTOF CSOSChannel. This in tum reducesthe potential for damageand corrosion andfacilitatesroutine maintenance. Duringthecleaningcycle,therakestravel ina continuouscircuit from thebottomto the top of thebarrackto removematerialsretainedon the bars. Screenings are typically dischargedfromthebars at the top oftherack. Thecleaningrakeis heldagainstthe barsby theweight of its chains,allowingthe rake to bepulledover largeobjcctsthatareIodged in the barsand that might otlerw'ise jam therakemechanism.2-3: Fine ScreensFine screens at CSO facilitiestypicallyfollow coanebar screeningequipmentand provide the next levelof physicaltreatment in removing thesmallersolidparticles from the wastestream,Bothfixed (static) and rotaqrscreenshavebeen used in CSO treatnentfacilities.Fixedfine screens are typicallyprovidedwithhorizontalor roundedslottedopeningsof 0.02 to 1.27centimeters (0.010 to 0.5 inches).The screens areusually constructedof stainless steel in a concaveconfiguration,at a slopeofapproximately 30 degrees.Flowis dischargedacrossthe top ofthescreen.The flow then passesthroughtheslotted openings and solids are retainedon the screen surface. Solidsare dischargedfrom the screensurfacebygravity and by washingonto a conveyerbeltor othercollectingsystem.Rotary fine screensinclude extemally and intemallyfed screens.Externallyfed screensallowwastewaterto flow over the top of the drummechanismand throughthe screens whilecollectingsolids on the screensurface.As thescreen rotates,a systemof cleaning brushesor sprayedwater removesdebrisfrom the drum. Internally fed systems dischargewastewater in thecenterof the drum,allowingwater to passthrough the screeninto adischargechannel,whilesolidsare removedfrom the screensurfacebycleaningbrushes or a waterspray.Screened n.raterialis usually washedfromthe screen with a highpressuresprayinto a discharge trough. Screendiameterscan rangefrom0.5 to 2 meters (1.6to 6.6 feet),whilethe lengthscan vary from 2 to 6 meters (6.6to 19.7feet). There arethreemodes ofoperationwhich include:Low Florv-no drumrnovement.IntennediateFlow-drum movesa shortclistanceaudstopswithbrushcomingon as head lossrises.HighFlows- continuousoperationwhere thedrumrotatesat I rpm andbrush at 10rpm.In responseto the needfor solids and floatables controlduring stormevents,proprietary screenproducts,such as the ROMAGTM screen(Figwe2-2), have beendesignedfor wet weatherapplications. TheROI,v{AGrMscreen partitions the flow, sendingscreenedflow to the CSOdischargepoint, while keepingsolidsand floatablesin the flow directedtowards the sanitarysewer.The ROMAGru screenworks as follows:excessflow enters thescreeningchamber,flowsovera spill weir andproceedsthroughthescreen into a channelwhichdischarges flow to a receivingwaterbody.Floatables trapped by thescreenmove laterallyalong the faceof the screenvia combVseparatorstothe transverse end section ofthe pipe wherethey can be directed to thesanitarysewer line for ultimateremoval at the wastewater treatmentplant.Screenblinding is prevented by a hydraulically-driven rakeassembly.7'Source: Pisano, 1995.FIGURE(2-2) ROMAGTM'COMBING'MECHANICAL SCREEN(VERTICAL) FORCSOFLOATABLES CONTROLboth sides to facilitateinspectionsand maintenance.The screenconsistsofhorizontalbarswith 4 mm (0. 16inches)openingsthat are mountedon a weirin the collectionsystem.Screensrangefrom2 to 9 meters (6.6-29.5feet)inlengthand330-1200mm (13-47.2 nches)in height. Unitscan be stackedtocreate a customizedmeshopeningfor a specifieddesign flow at a particularlocation. The nominalvelocity through thebar openingsis approximately1.5meters per second(4.9 feetper second).The hydraulicallydrivenmechanicalcombsusedto cleanthe screen movelaterallyalong the front face of the screen when activatedby a levelcontrol,which detectsrisingwater.Asthe screensurfaceis cleaned, capturedmaterialis transportedforwardto the endsectionfor storage andsubsequentremoval.The hydrauliccombingunit is located outside the screen andconsists ofanoil tank, pump and conholvalves.The ROMAGTMscreenma1, be clesigned for a variety of flow scenarios.Water maypassthrough thescreenhorizontally(RSWtype), as shown inIrigure (2-2); overthe top ofthe screen(RSO typc)or up fromunder rhesoeen(RS[D type.This unit hasproven useful in remote settingsand iscapableof handling flows from300-6100L/sec (6-140MOD).While screening is widelyusedto conholsolids and floatablesat theheadworksof wastewater treatrnent plants, screeningfor solidsat remotelocations,such as at CSO or stormwater overflow points,is less common'However, Sometypes of screensare effectivefor remotesolidsandfloatablescontrol due to their largeaperture size andself-cleaningability.As a result,mechanically-cleaned bar screens have proven to be a relativelysimpleandinexpensive means of removingfloatablesand visible solids' They aretypicallythe screenof choicein many CSOtreatmentfacilities,and are widelyusedor implementedat a large number of cSo facilitiesacrossthecountryandabroad.Therehas been lesssuccessin removingfine solids from storm waterandCSO overflows. However, proprietarymethods,suchas the RomagrMscreen,haveaddressed this issue.Morethan250RomagrMscreens havebeeninstalled in Europe since 1990.Recently,severalRomagrM screenshavebeen installedin the U.S. The first was installedin Rahway, NJ , n 1997 'In addition, Deerfield, Illinois has had successutilizing rotating finescreensat their overflow facilities.Theirfinescreenshave 1.02millimeter(0.04inch) openings that removeall large solids and floatables. The screenedwastewater is dischargedinside the screenand conveyed to a chlorine contacttank for disinfection prior to clischarge to the receiving stream' Thescreenings are conveyeclby internalconveyor s to a discl.rargc chuteforstorageand eYentualretu l to the POTW at the end of the over'flow event.Theentireoperationis autonatic {West et al., 1990)'10Sincescreeningis a physicaltreatrnentprocess, it will remove onlythoseobjectsthat arelarger thanthe screen openings.Screeningsystemsareveryeffectivein removing floatableandvisible solids,but do not remove asignificantamountof suspended solids. In caseswherewater qualityevaluationsindicatethe need for removal of suspendedsolidsor oxygendemandingmaterials,additional treatment processesdownstreamfromthescreeningunits would be required.Becausescreensat CSO control facilities removedebris, rags,and otherfloatablesthat wouldotherwise be dischargedinto a receivingstream, theyare vitalin preserving water quality andaesthetics.UnscreenedmaterialinCSOscan become a nuisance if thefloatables,andother solidsend up inreceivingwaters. Theycan create navigationalhazards,attractnuisancevectors,and retain bacteria and otherpollutants.Properlyscreenedandremoved materials in CSSs preventmaterialsfromsettling outin the system,thuspreventingpotential back upsand possibleoverflows elsewhere.Thescreenings and debristhatare removedfromthescreens are typicallynothazardous and can be disposedof in a licensedlandfillor incinerated.Negativeenvironmentalimpacts can occur fromimproperdisposalof screenedmaterials,such as by stockpiling in areasadjacent to receivingwatersor in areas wherethey may be seen by thepublic.2-6:DESIGNCRITEII.IAHydlauliclosses through bar screens are a functiouol approaclivelocity andtbe velocify through the bais. 'Ine headloss tlrougha clean bar screencatrbe estirnatedu:;ingthefollowingcquatiorr:11h1-( 1/0.7) * (1v2 -v27tzg1where :h1: headloss, ft (m)0.7 = an empiricaldischargecoefficientto account to turbulenceandeddy losses.V : velocityofflowthrough the openingsofthe bar racks, ff/s (m/s)v : approachvelocityin upstreamchannel,ff/s (m/s)g = accelerationdue to gravity, ff/J (nvs'?)Headloss increasesas the bar screen becomesclogged, or blinded.Forcoarsescreens, the approachvelocity should be at least0.38meters persecond (1.25 feet per second)to minimizedeposition, whilethe velocitythroughthe bars shouldbe less than 0'91 meters per second (3 feet persecond)to prevent entrainedsolids from beingforcedthroughthe bars'Instrumentation provided withmechanically-cleaned screensis configured tosenda signalto the cieaningmechanismso theheadloss across thescreenislimited to 6 inches.The following generalfactors should be considered in the design andoperation ofcoarseand hne screens:. Grit classifiersare effective in separating, washing,and dewateringgrit, sand, finds,and silt from an effluent flow normally downstreamform the screens.Coarsescreens with moving partsout of theflow stream are preferableto coarsescreenswith submerged parts.12Fine screensusing steel wiremeshor perforatedpanelsarevery proneto clogging fromftbrous materialsand arenot easilycleaned. Plasticmeshpanelshaveprovento be effective,are resistantto clogging andareeasilycleaned with water sprays.Pumping or conveyinglarge amountsof large and smallsolids typicallyremovedby screeningsystems hasprovento be very difficultand a majormaintenance problem.Screw conveyorsand compactor typescrewshavebeen shown to be effectivein handlingsolids,especiallythoseremovedbyfine screens.Designparametersfor differenttypesof screensaregiven onTables(2-1)ard(2- 2).Additional desienissuesto consider include:Grit will tendto accumulateupslreamanddownstreamof screens.Provisionsmustbe madefor easyaccessto such areas and alternativemethods of gritremoval, including vacuumsystems, high pressurewater cannonsor spraysystems.Backwaterfroma storage/sedimentation tank effluent weir can createquiescentsettlingconditionsin the barscreen channel.Therefore, a meansofflushingor backwashingthe screeningschannelshouldbe provided'A redundant or back-upbar screen should beprovidedso thatpeak flow to thefacility canbe maintainedrvithone unit out of service.Providing stopgroovesor slideTABLE(2-l) DESIGNPARAMETERS FOR STATIC SCREENSHydraulicloading, gaVmin/ftof widthInclineof screens, degreesfromvertical*Slotspace, Pmautomaticcontrolsone* Bauer}lydrasieves rNthave 3-stageslopeson eachscreen: 25",35", 45"Note: eal/mir/ft X 0.207 : l/m/s100-18035250- 1600None13.gates in the channelallows the userto isolatethe screen from the flowfor maintenance.. Guards,railings,and gratings shouldbe provided in the areaaroundthescreeningequipmentto ensure operatorsafety.Electricalfittings andequipment associatedwiththe screeningequipmentmustconform to theexposurerating for the space in which the equipmentis located.2-7: Removal EfficiencvRemoval efficiencyis a function of barscreen;spacingandfloatablesolidscharacteristics,Removalefficiencyincreases as the size and concentrationof the solidsincreasesand the spacingdimensiondecreases. Screeningstypically containing10-20percent dry solids will typically ; a bulkdensityranging from 640 to I 100 gramsper cubicmeter(40to 70 poundsper cfoot).Typical floatableremovalratesfor se screensrange from 3.5 to 84litersper 1000 c meters(0.469 to 1 1.2 cubicfeet perMG).The quantityofscreeningscan varygreatlyand, in general,depends on thefollowingfactors:. Configurationof the drainagesystem.. Time of year.. lntervalbetweenstorms.. Intensityof thestorm.TABLE (2-2)DESIGN PARAMETERSI'OR DRUMSCREENSAND ROTARYSCREENScreenspacing, prnScreenmaterialDrum speed,r/rnin100-420stainlesssteelor plastic74-167105 recommendedstainless steel or14il.ar., -a160-7020-506-24plastic30-65))t4-1670-15075-900.02-2.550SpeedrangeRecommendedspeedPeripheralspeed,ft/sSubmergenceof drum,o/oFlux density, gaVfflminOf submergenceHydraulic efficiency,% of inflowHead loss,in.BackwashVolume, %oofinflow0.5-330-50Pressure,Ib/in2Note: ga7 ftzlminx2,44= m3ft/mtin. X2.54 : cmft X 0.305 : cm;Ib/in.2X 0.0703= kg/cm2. Velocityof theflow through thescreens.. Screenaperture.Studies havefound averageCSO screenings loadsvaryingfromapproximately3.7xlO-e-8.23x10-8cubic meters per liter (0.5 to 1l cubicfeet per million gallons), with peaking factorsbasedon hourlyflowsrangingfrom2: I to greater than20: I .Fieldstudiesperformedin Canadaand Europehaverevealedthe followingfl oatable removal effi ciencies:. Samplings taken at differentCSOoutfalls in Montreal, Canadashowed that up to 80 percent of floatablematerial can beretained by properly designedbar screenswith 6.35 millimeters(0.25 inch) bar spacing.A year-long study was conducted in Germanyto determinetheefficiencyof an externallyfed rotaryscreen in controllingdownstreamfloatable pollution. The screen,which wasactivated by high flows, received42 percent of the CSOdischarge,with no visiblesolids reported afterfrequentinspectionsof river banks.A pilot studyin GreatBritaintesteda 4 mm ROMAGTMbarspaced"weir mount"storm overflowscreen. The averagesolidsloadingbefore the screen was 2369grams perminute,whilethe solids concentrationafterthe screen was 3.5 gramsper minute,exhibiting a 98.5 percent deflectionrate. In a similarstudy, on 11 differentoccasionsduringa 12 week period,averagemass reduction of floatablesand solidsmaterialgreater than 6 millimeters(0.24 inches)was98.5 percent.2-8: OPERATION ANDMAINTENANCEInstrumentation and control of screenstypicallyincludessomecombinationof thefollowino:. Manualstadstop.. AutomaticstarUstop on timer.. Automatic starUstooon differentialhead.toActivation of mechanicallycleanedscreensis triggered by remotesensingof flow into the screeningschannel,or the water level in thescreening channel.As screensare subjectto blinding fromgrease and the "first flush" ina CSOevent,the screenshould be keptcleanto minimizeheadless.Due to the intermittent nature of CSOsit is important for thescreening units spray system to be workingproperly to prevent solidsfrom drying and stickingto the screens,thus increasing headlosses.Fine screens can be cleaned with highpressure water,steam, orcleaning agents to maintain performance. Screening systemsshouldbe regularlyinspected to ensurethat chains and rollermechanismsare lubricated and functioning. The trunnions associated with finescreens are the least reliablecomponent due to the abusive forcesthey receive. The manufacturer's operation and maintenancemanualshould be consulted for the maintenance requirements andschedules.0lltrTilTlilIIIIIIDESIGNPOPULATION & Votumeofwastewater3-l: GeneralTherequired treatment is determinedby the influentcharacteristics,the effluentrequirements, and the treatment processesthat produce anacceptableeffluenlInfluent characteristicsaredeterminedby laboratory testingofsamples from the waste streamor froma similar waste strearn, or arepredicted on the basisof standardwastesfreams. Efluent qualityrequirementsare setby Federal, interstate, State, and local regulatoryagencies.Treaftnentprocesses are selected accordingto influent-effluent constraints andtechnicaland economic considerations.Treatment capacity is basedon the designpopulation, which is theprojected population obtained by analysis.Thedesigl populationis determinedby addingthe total resident and l/3 the non-residentpopulations andmultiplying by theappropriate capacityfactorwhich allowsfor variations inthe usingpopulation. Theresidentpopulation is determinedby adding thefollowins:Table3-1. Capacity factors.t'. ffective PopulationCaprcityFactorunder 5,0001.50iqqq___ rqqgq___ __20,0001.50llilOir{ro,11'-._0q_0_--50fr0ll1.10L-o_:lLa)rl1SThe nature of the activitiesof the personnel at military installationarea very important tactorin determining per capitawaste loadsbecausedifferent activitieshave differentwateruses.Table 3-2 illusnatesthis fact in termsof gallonspercapitaperday (grcd); table 3-3 showshowwasteloadingsvarybetweenresident andnonresident personnel.The valuesshownin table3-3, for thatportion of the contributingpopulationservedby garbagegrinders,will be increasedby 30 percentfor biochemicaloxygendemandvalues,100 percent forsuspendedsolids, and40 percent for oil andgrease. Contributingcompatible industrialor commercialflowsmust be evaluated for wasteloading on acase-by+asebasis.Table 3-2. Per capita sewageflows.NOTE: Add 30 gallonsper 8-hour shift per capitafor non-resident and civilian personnel.Table 3-3. Sewagecharacteristics.Type of Unit For Resident Personnel (gpcd)Permanent Field TrainineHospitalunits 300-600100All other unite 100 35ItemRegded_teriqll-elIb/capitafor 24 hrsNon-residentPersonnelIb/capitafor 8-hrshiftS Solids 0.?00.10lliochcrnicaIOxygcnDt'marrd 0.200.10Oil & Grease 0 0a)0.05l934: Volume of wastewater.a. Variationsin wastewaterflow. The rates of sewageflow atmilitary installationsvarywidelythroughoutthe day. The designofprocess elementsin a sewagetreatrnentplantis based on the averagedaily flow. Transmission elements,such as conduits, siphonsanddistributormechanisms, will be designedon the basisof an expectedpeak flow rate of three timesthe average rate. Clarifierswill bedesignedfor a peak hourlyflow rate (i.e.,1.75 times theaveragedailyrate). Considerationof the minimum rateof flow is necessaryin thedesignof certain elements,suchasgrit chambers,rneasuring devicesanddosing equrpment;for thispurpose, 40 percent of the averageflow raiewill be used.b, Average dailywastewaterflow.Theaverage dailywastewater flowto be usedin the design of newtreatmentplantswill be computedbymultiplying the designpopulation by the per capita rates of flowdeterminedfrom table 3-2, and thenadjustingfor suchfactorsasindustrial wastewater flow,stormwaterinflowand infiltration.Whereshift personnel areengaged, the flow will be computedfor theshift whenmost of the people areworking.A usefulcheckon sewagevolumeswould be to comparewaterconsumption to the sewage estimate(neglecting infiltration,which will be considered subsequently).About 60 to 80 percent of the consumed waterwill reappear as sewage,theother 20-40 percentbeinglostto irigation,fire-fighting,wash-down,and pointsofuse not connectedto the sewer.(l) Goodpractice requires exclusionof stormwater from the sanitarysewersystemto tlie maximumpracticalexlent.lntiltratioir must al:ro bekept to a minimum.Both rnust be carefltlly analyzed andthe mostrealisticpracticalquantitythatcan be usedin designmust be assigned20to these flows,Leakageof stormwater into sewer lines oftenoccursthrough manholecovers or collars, but thisusually is no more than20 ta 70 gallons per minute if manholeshavebeen constructed andmaintainedproperly.However;leakageintothe sewer mainsandlateralsthroughpipe joints ard olderbrick manholeswithincreasein groundwaterlevels canresultin large infiltration.The amorntof water thatactuallypercolatesinto thegroundwater table maybe negligibleif an area isoccupiedby properlyguttered buildingsand paved areas,or ifthe subsoilisrich in impervious clay. ln othersandyareas,up to 30 percentof rainfallmayquickly percolateandthen lift groundwaterlevels.lnfiltrationrateshavebeenmeasuredin submerged sewerpipe.Relativelynew pipewithtightjointsstill displayedinfihationsat arotnd1,000 gallons per daypermile, whileolder pipesleakedto over40,000 gallons per daypermile.Sewersbuilt firstusuallyfollowedthe contour of water coursesand areoften submergedwhilemorerecentsewersare not only tighter, but areusuallybuilt at higher elevationsas the systemhasbeenexpanded. Indesigningnew treatment facilities, allowforinfiltrationas givenin TM5-814-1/A,FM88-11,volume 1, exceptas modified by thisdesignmanual. Utilize existingflow records,sewerflow surveys,andexaminethe correlationbetweenrecordedflows and rainfalldata toimprovethe infiltrationestimate.The economicfeasibility ofimprovingthe collectionsystemto reduce the rateof infihationshouldbeconsidered.(2) Anothermethodforcalculatingtheinfiltrationcomponent of total flowisto multiply thentiles of a given pipe sizeandconditionby the diameterin inches and to sum theinch-miles. 'lhe sums of inch-miles of pipeestimatedaccordingto conditionsare thenmultiplied by factors betwecrt250 and500 to obtain gallori'da)'. lf ffitrationis knowr.rto be negligible atmanholes,thenan infiltrationallowanccmay be caictrlated basedupon?.1areaserved and figwe 3-1. Curve A should be used for worstconditionswhenpipes are old and joints are composedof jute orcement.CurveB applies to old pipes withhot or coldasphaltic joints orfornew pipesknownto have poorjoints. CurveC is usedfornew sewerswheregroundwater does not coverinverts andwhen joints and manholesaremodemandquitetight.Of course,field tess maybeconducted to morecloselvestimateinfi ltration.Figure3-1 Infiltrationallowances(3)Averagewastewaterflowis usuallyexpressed in million gallonsperday,but willbe calculatedintheappropriateunitsfor desigrroftheunitprocess mder consideration.c. Contributingpopulations. ln calculating contributingpopulations,use3.6personsper familyresiden-tialurit. In hospitals,cornt the number ofbeds, plusthe number of hospital staffeatingthreemeals at the hospital,plusthe number of shift employees havingone mealthere. Thistotal is thennmber of resident personnelto be usedin the designcalculations.Individuals will be countedonlyonce, eithel at home or at work.f-irecapacityfactor still applies ir.rcidculatingdesign populatiors.d. Industrialflon'.Industialwastewilterflows will be minimal at most22militaryinstallations. Whenindustialflows axe present,however,actualmeasurementis the bestway to ascertain flowrates. Modes of occurrence(continuous or intermittent)and period of discharge must also be known.Typicalindustrialdischargesincludewastewatersfrom the following:-wastewatertreatnentplant itself;-maintenancefacilities;-vehiclewash areas;-weaponscleaningbuildings;-boilerblowdowns;-swimmingpoolbackwashwater;-watertreatrnentplantbackwash;-coolingtowerblowdown;-tue fightingfacility,-photographiclaboratory;-medical or dentallaboratories.e. Stormwater flow.Including stormwaterflows is impotant in treatrnent plantdesign either whencombined sewersystemsare sen'edor whensignificantinflow entersthesewersystem.Combinedsewer systems will not be permittedin new military installations.Separatesewers are requiredandonlysanitaryflows are to be routedthroughtreatmentplants.For existingplantsthatare servedby combinedsewer systems,capacitieswill be determinedbypeak wet-weatherflowdetermined fromplant flow records.In the absenceofadequaterecords,hydrauliccapacitiesof for.r timesthedry-weatherflowwillbe used in the design. Q{eference to eistingsystemsis applicableto Armyfacilities only.)2),Suspendedsolids and organic loadingcan be interpretedas populationequivalentswhenpopulationdataconstitutethemain basis of design.Typicalpopulation equivalentsapplicableto militaryfacilitieswere givenin table 3-3.Theseequivalentvaluescan also be usedto convertnon-domesticwasteloadsinto population designvalues. Theeffectsof garbagegrindingwill beincorporated into population-equivalent values whenapplicable.Thewastestreamto be heatedat existingmilitary installationsshould,whenfeasible, becharacterized;this actualdata shouldbeusedin the desisn.3{:Capacitvfactor.A capaclty factor(CF)taken from table 3-1 is usedto make allowancesfor population variation,changes in sewage characteristics,andunusualpeakflows. Thedesign population is derivedby multipllng the actual authorizedmilitaryandcivilianpersorurel population(calledtheeffectivepopulation) by theappropriatecapacity factor. Where additions are proposed, the adequacy ofeach elementof the plantwill be checkedwithoutapplyingthe capacityfactor.Whentreatmentunits are determinedto be deficient,then capacityfactorsshould be usedto calculatetheplant capacityrequired after expansion.However, theuse of an unnecessarilyhighCF may so dilutewaste as toadversely effect somebiologicalprocesses. If the area servedby a plant willnot, accordingto the bestcurrentinformation,be expandedin the future,thecapacltyfactor will not be usedin designingheatnent componentsin facilitiesservingthat area" The followingequationmaybe wed to estirnatetotal flowto thesewageplant where domestic,industrial and storm waterflows areanticipated.21x-a+bWhere(eq.3-1)x = Totalflow to sewageplanta : Flow frompopulation(effectivepopulation x 100gpcd x capacityfactorb: Infiltration+ industialwastewater+ stormwater (4 x dry_weatherflow)3-7: HvdraulicCapacitv37-1:Flow Delinitions and ldentificationThe following flowsfor thedesign year shallbe identifiedand usedas a basisfor design for sewers, lift stations,wastewatertreatmentplants, treatmentunits,andotherwastewater handlingfacilities.whereany of the termsdefined in this Sectionare used in thesedesignstandards,the definitioncontained in thisSectionapplies.a. Design Average FlowThe designaverageflowis theaverageofthedaily volumesto be receivedfor acontinuous 12month period expressed as a volume perunit time. However,thedesignaverage flow for facilitieshavingcriticalseasonal high h1'draulicloadingperiods(e.g., recreational areas, campuses,industriarfacilities) shall bebasedon thedailyaverageflowduring theseasonalperiod.a. DesignMaximumDayFlowThe design maximumdayflow is the largestvolumeof flow to be receivedduringa continuous 24 hour period expressedasa volume perunit time.C. Design Peak Hourly FlowThe designpeak hourly flow is the largestvolume of flow to be receiveddwing a one hourperiod expressedasavolume per writtime.d. DesignPeakInstantaneous FlowThe designpeak instantaneous 1'low is theinshntaneousmaximumflowrate tobe received.3-?-2:IlvdraulicCapacitvfor WastewaterFacilities to serveExistinsCollectionSrntemsa. Projectionsshallbe made from actualflow datato the extent possible.IIea.cot.,lo^v vlttoo pr.(rjnog lrog 9 19911ygb. 'Ihe probabledegreeofaccuracyofdataand projectionsshallbeevaluated. 'lhisreliabilifyesrimationshouldincludean evaluationof the accuncyof existingdata as well as an evaluationof thereliabilitl,of estilnates of flow reductionEul