epa water treatment manual preliminary

7/22/2019 EPA Water Treatment Manual Preliminary

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ENVIRONMENTAL PROTECTIONAGENCY

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WASTEWATERTREATMENT MANUALSPRELIMINARYTREATMENT

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WASTE WATERTREATMENTMANUALSPRELIMINARYTREATMENT

Environmental Protection AgencyArdcavan, Wexford, Ireland.Telephone: 353-53-47120 Fax: 353-53-47119


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Environmental Protection Agency 1995

Partsof hispublication maybe reproduced without urtherpermission, provided thesource isacknowledged.

WASTEWATER TREATMENTMANUALS

PRELIMINARY TREATMENT

Publishedby the Environmental Protection Agency, Ireland.

The Agency personnel involved in thepreparation andproduction of his manualwere Ms. AnneButler, Mr. GerryCarty, Dr. MattCrowe, Dr. Paddy Flanagan andMs. MarionLambert (wordprocessing).

ISBN 1-899965-22-XPrice IRL15.OO 12/95/1000


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CONTENTS 1

LIST OF FIGURES 5LIST OF TABLES 6

PREFACE 7

ACKNOWLEDGEMENTS 8

ABBREVIATIONS 91. PRETREATMENT OFWASTEWATER 11

1.1 PRELIMINARY TREATMENT 111.2 NATURE OFWASTEWATER1.3 STORMWATER IN SEWAGE1.4 TYPICAL SEWAGE LOADS 13

1.4.1 HYDRAULIC LOADING 131.4.2 ORGANIC LOADING 131.4.3 LOAD VARIATION 13

1.5 OVERVIEW OF WASTEWATER TREATMENT 141.6 PRELIMINARY TREATMENT PROCESSES- OVERVIEW 141.7 ROLE OFPLANT OPERATOR 16

2. HYDRAULIC DESIGN, STORM OVERFLOWS ANDFLOW BALANCING 192.1 TREATMENT PLANT FLOWS 19

2.1.1 FLOW TOTREATMENT 192.1.2 FLOW CONTROLSANDOVERFLOWS 19

2.2 COMBINED SEWER OVERFLOW - DISCHARGE CRITERIA 242.2.1 STORM WATER OVERFLOW SETTING 24

2.3 STORM OVERFLOW STRUCTURES 252.4 OPERATIONS ANDMAINTENANCE OF OVERFLOWS 252.5 STORM FLOW BALANCING 25

3. SCREENING 333.1 DEFINITION 333.2PURPOSE OFSCREENING 33


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2 PRELIMINARYTREATMENT

3.3 SOURCES OF SCREENINGS 333.4 CHARACTERISTICS OF SCREENINGS 34

3.5 QUANTITY OF SCREENINGS 343.6TYPES OFSCREEN 343.7 MANUALBARSCREENS 353.8 COARSE SCREENS 35

3.8.1 TRASH RACKS 353.8.2 ROTATINGBAR NTERCEPTORS(R.B.I.) 373.9 MEDIUM SCREENS 37

3.9.1 CURVEDBARSCREENS 383.9.2VERTICAL AND INCLINED SCREENS 383.10 FINE SCREENS 39

3.10.1 INCLINEDBARSCREENS 393.10.2BAND SCREENS 393.10.3DRUM SCREENS 393.10.4ROTOMAT, SCREEZER.CONTRA-SHEAR 393.10.5 DISCREEN 423.10.6DISPOSABLEBAGS 43

3.11 SCREEN DESIGN 433.11.1 SELECTION 433.11.2STANDARDS 443.l1.3DESIGN 44

3.12 SCREENINGSDEWATERING 453.12.1 HYDRAULIC PRESS 463.12.2SCREW COMPACTORS 463.12.3WASHER DEWATERERS 473.12.4CENTRIFUGE 47

3.13 SCREENINGS DISPOSAL 473.14DISINTEGRATION 48

3.14.1 COMMINUTORS 483.14.2MACERATORS 483.14.3MUNCHERS 483.15 MAINTENANCE 493.16 COMMON OPERATING PROBLEMS 49

4. GRITREMOVAL 514.1 DEFINITION 514.2SOURCES . 514.3QUANTITIES OF GRIT 514.4 PROBLEMS 514.5SETTLEMENTTHEORY 524.6CONSTANT VELOCITY GRIT CHANNELS 53

4.6.1 PARABOLIC CHANNEL 53


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CONTENTS 3

4.6.2 CHANNEL WITH SUTROWEIRCONTROL 544.6.3 LENGTHOFCONSTANT VELOCITY CHANNEL 544.7 DETRITUS TANK . 554.8VORTEX GRITSEPARATORS 554.9AERATED GRIT TRAPS 554.10CROSS-FLOW DETRITER 554.11 GRITDEWATERING/WASHING 564.12 DISPOSAL OFGRIT 574.13 MAINTENANCE 574.14 COMMON PROBLEMS 58

5. OILS,GREASES & FATS 595.1 DEFINITION 595.2SOURCES 595.3NATURE ANDEFFECTS 595.4REMOVAL TECHNOLOGIES 605.4.1 PHYSICAL REMOVAL METhODS- GENERAL 605.4.2 BIOLOGICALTREATMENT 645.4.3CHEMICAL REMOVAL METHODS 655.4.4 DISPOSALOF OIL, FAT AND GREASESLUDGES. 665.5 OPERATIONAL PROBLEMS 66

6. FLOWMEASUREMENT 676.1 PRINCIPLES OF FLOW MEASUREMENT 676.1.1 INTRODUCTION 676.1.2BASIC PRINCIPLESOFFLOW MEASUREMENT 676.1.3UNITS OFMEASUREMENT 686.1.4STANDARDS 686.1.5 LOCATIONSOFFLOW MEASUREMENT DEVICES 686.1.6 VARIATION ANDACCURACY 706.2 MEASUREMENT DEVICES ANDSTRUCTURES 706.2.1TYPESOFMEASUREMENT DEVICE 706.2.2 STANDINGWAVEFLUME 716.2.3 PARSHALL FLUME 716.2.4PALMER BOWLUS FLUME 716.2.5 WEIRS-RECTANGULAR AND VEENOTCH 716.2.6VENTURI METER 746.2.7FLOW NOZZLE AND ORIFICE PLATE METERS 746.2.8 ELECTROMAGNETICFLOWMETER 746.2.9ULTRASONIC FLOWMETER 776.2.10 OTHER DEVELOPMENTS (ELECTROMAGNETIC ANDULTRASONIC) 776.3SELECTION OFAPPROPRIATE FLOW MEASUREMENT SYSTEM 776.4 ACCURACY OF FLOW MEASUREMENT 78


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PRELIMINARY TREATMENT

6.5 CALIBRATION ANDMAINTENANCE 786.5.1 INTRODUCTION 786.5.2 OPEN CONDUITFLOWMETERS 816.5.3CLOSED CONDUIT FLOWMETERS 81

7. CONTROLOF NUISANCE 837.1 INTRODUCTION 837.2 SOURCES OFNUISANCE 83

7.2.1 ODOURS 837.2.2 ODOUR CONCENTRATIONS 847.2.3 NOISE 847.2.4VISUALAPPEARANCE 85

7.3 CONTROL TECHNOLOGIES/ PERFORMANCE 857.3.1 ODOUR NUISANCE CONTROL 867.3.2NOISE NUISANCE CONTROL 887.3.3 VISUALNUISANCE CONTROL 90

7.4 GASCONTROLS 90REFERENCES 93GLOSSARY 95APPENDIX A: SCHEMATIC DRAWING OF PRETREATMENT WORKS 101USERCOMMENT FORM 103RECENT ENVIRONMENTAL PROTECTION AGENCYPUBLICATIONS 105


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CONTENTS 5

Figure 1.1:Wastewater Treatment Process 15Figure1.2:TypicalWastewater Preliminary TreatmentProcesses 17Figure2.1:HighSideWeirOverflow 20Figure2.2: VortexRegulatorThrottle Device 21Figure2.3. Throttle PipeDesign 22Figure2.4: TypicalHighSide Weir Criteria 27Figure2.5:Stilling PondOverflowCriteria 28Figure2.6: Vortex Chamber in CircularShaft 29Figure2.7:On-lineandOff-line Storage - 32Figure3.1:QuantitiesofScreenings Collectedrom Mechanically Cleaned BarRacks 35Figure3.2:RotatingBar Interceptor 37Figure3.3: CurvedBarScreen 38Figure3.4: Inclined Mechanic ilyRakedBarScreen 40Figure3.5:StepScreens 40Figure3.6: Screezer 41Figure3.7: Drum Screen- Rotamat Type 41Figure3.8: Discreen 42Figure3.9: Hydraulic ScreeningsPress Detail 46Figure3.10: SolidWasteScreenings - Top Feed Press 47Figure3.11: In-lineDisintegrator Muncher) 49Figure4.1: Cross-section ofParabolicConstant-VelocityDetritusChannel 53Figure4.2:ProportionalFlow Plate Weir(Sutro Weir) 54Figure4.3:Helical Flow Pattern in anAeratedGritChamber 56Figure4.4: Grit WasherandClassifer 57Figure5.1: Glass-ReinforcedPlastic (G.R.P.) GreaseTrap 61Figure5.2: GreaseSeparator 62Figure5.3: AeratedSkimmingTank 63Figure6.1:Flow-time graph 67Figure6.2: FlowMeasurement Locations 69Figure6.3: TypicalDiurnalFlowPattern inDry Weather 70Figure6.4: FlowMeasurement Devices 75Figure7.1: WetOxidation about Ozone 87Figure7.2:Section ThroughPeatBedBiofilter 88


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6 PRELIMINARY TREATMENT

Table2.1: Flow ControlDevices atStormwater Oveiflows - Options 23Table2.2:Recommended Storm OverflowStructures 26Table3.1: TypicalScreen Applications 36Table3.2: Screen Selection 43Table3.3: Screen Design Factors 44Table3.4: OperationalProblems atScreens 50Table4.1: Settling VelocityofGrit 52Table4.2: Typical Design Data 56Table4.3: Problems atGritPlants 58Table 5.1. OperationalProblems withGreaseTable 6. I:TypesofFlowMeasurement Devices and theirOperation 72Table 6.2:Application ofFlowMeasurement inWastewater Treatment 73Table 6.3:Typical Criteria Used in theSelection ofFlowMetering Devices (Ref 1). 79Table 6.4:Evaluation ofVarious TypesofFlowMetering in WasrewaterPreliminaryTreatment 80Table 6.5:Typical Flow Metering AccuracyTable 7.1: TypicalRangesofOdourandSulphurCompound Concentrations inGas Emissions atPretreatmentWorks 84


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PREFACE 7

The EnvironmentalProtectionAgencywas established in 1993 to licence, regulateand controlactivities forthe purposes ofenvironmentalprotection. In Section 60 of the Environmental Protection Agency Act, 1992,it is stated that "theAgency may, and shall ifso directedby the Minister, specify andpublish criteria andprocedures, which in the opinion of the Agency are reasonable and desirable for the purposes ofenvironmental protection, in relationto the management, maintenance, supervision, operationoruse ofallorspecified classesofplant, sewers ordrainage pipesvested n orcontrolledorusedbya sanitaryauthorityfor he disposal ofany sewageorother effluentto any waters". Criteria andproceduresnrelation othetreatment anddisposal of wastewater arebeing published by the Agency inanumberofmanualsunder thegeneral heading: 'WastewaterTreatment Manuals'. Where criteria and procedures are published by theAgency, a sanitary authority shall, in theperformance of tsfunctions, haveregard o hem.This manual onPreliminary Treatment setsoutthegeneral principles and practiceswhich shouldbefollowedby those involved in the treatment of wastewater. It provides criteria and procedures for the propermanagement, maintenance, supervision, operation and useof theprocesses and equipment required in thepreliminary treatment of wastewater. The Agency hopes that it will provide practical guidanceto thoseinvolved in plantoperation, use, management, maintenance and supervision. Furthermanuals areplannedfor secondary and tertiary treatment of wastewater. Wherereference is madein the document toproprietaryequipment, this is intended as indicating equipment type and is not to be interpreted as endorsing orexcluding anyparticular manufacturer orsystem.The Agency welcomes any suggestions which users of the manual wish to make. Theseshouldbe returnedto theEnvironmental Management and Planning Divisionat the Agency headquarters on theenclosed UserComment Form.


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The Agency wishesto acknowledge those whocontributed to and reviewed this manual. The draftmanualwas prepared undercontract to theAgency by M.C. O'Sullivan& Co. Ltd. A review panelwas establishedby theAgency to assist inthe finalisation ofthe manual and we acknowledge below hosepersonswho tookthe time to offervaluable information, advice and in many casescomments andconstructive criticism on thedraftmanual. Wegratefully acknowledge the assistance offered by thefollowing persons:

M. Beirne, Environmental HealthOfficers AssociationProfessor T. Casey, University College, Dublin.,R. Dunne, Dept.of heEnvironmentJ. Fenwick, DublinCorporationP. Fullam, DublinCorporationJ. O'Flynn,Waterford CountyCouncil representing the County andCityEngineers Association)P. Ridge, Galway County Council

The Agency alsowishes to acknowledge the assistance of Engineering Inspectors of the Department of theEnvironment and theSanitaryServices sub-committee of the Regional Laboratory, Kilkenny, both ofwhomcommented onthedraftmanual.


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DOE Departmentof heEnvironmentWRc WaterResearch Centre, U.K.EPA Environmental ProtectionAgencyBOD Biochemical Oxygen DemandCOD Chemical Oxygen DemandDWF DryWeather FlowCCTV Closed CircuitTelevisionEEC European EconomicCommunityRBC Rotating Biological ContactorUV Ultra Violetp.e. Population EquivalentPLC ProgrammableLogicControllerHMSO HerMajesty'sStationary OfficeS. . Statutary InstrumentHMIP HerMajesty'sInspectorate ofPollutionRB RotatingBar Interceptorrpm revolutions perminute

ABBREVIATIONS 9


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PRETREATMENT OFWASTEWATER 11

1.1 PRELIMINARY TREATMENTSection 60 of the Environmental ProtectionAgencyAct, 1992 permits the agency (EPA) tospecify and publish criteria and procedures,which in the opinion of the Agency arereasonable and desirable for the purposes ofenvironmental protection, in relation to themanagement, maintenance, supervision, operationoruseofall or specified classesofplantvested inor controlled or used by a sanitaryauthority forthe reatment or disposal ofsewageor effluent toany waters. This document is prepared inaccordance with the foregoing, in respect ofwastewater preliminary treatment. Itsobjective, therefore, is to provide criteria andprocedures to properly manage, maintain,supervise, operate or use the processes andequipment required in the preliminary treatmentofwastewater.In interpreting these requirements,it is consideredappropriate that criteriabe evolvedin relation othefollowing topics:

management:criteria for theestablishment ofpreliminary treatment including siting,design,process and equipment selection andorganisational management to meetperformance objectives;maintenance: criteria for servicing andupkeep of equipment, approach channels andacconmodation works, renewal ofconsumable items, servicingand repair;supervision: superintendence of the works,maintenance of detailedperformance recordsand monitoring to check compliance withservice objectives, avoidance of nuisance andassessmentofoperating costs;operation or use: criteria for optimisedoperational performance and efficiency ofplant including disposal of by-products in anenvironmentally safemanner, minimisation ofnuisance from odours,flies, aerosols or othersocial impacts eitherat the plant or disposalsite.

These criteria are considered from the point ofview of the purpose, functioning and load

conditions which apply.Therefore, the criteria orselection ofprocess and equipment are reviewed,together withthe issueswhich arisein day to dayoperation.In considering these issues, regardmustbehad tothe variations in wastewater flow and othercharacteristics.Foul flows are subject tovariationwith daytime peaks and night-time low flows.The effects of rainfall can increase flowssubstantiallyand result ingreater oadsofgritandscreenings brought into the sewers throughgullies. Preliminary treatment processes must,therefore, cater for a range of flow conditionsbetween minimum andmaximum.1.2 NATURE OFWASTEWATERMunicipal foulsewageis derived from domestic,commercial and industrial wastestreams togetherwith stormwater run-off. Apart from faecalmatter, sewage contains a varietyof suspendedand floating debris including grit and other inertsolids washed in from pavement and roofsurfaces, paper,plastics, ags and otherdebris.Other constituents of sewage are derived fromprocess water from industry or commercialundertakings. These cangiverise to thefollowingconstituents:

slaughter house and butcher wastes caninclude animal hair, bone fragments, bloodandoffal;creameries result in milk and milk fat wasteswhich constitute a high carbohydrate loadwhich can result in operational problems inactivated sludge plants;

food processing and catering establishmentwastes can include grease, heated effluentsandorganic solids withahighbiological load;filling stations, garages and other servicecentrescan resultin dischargeofused oil andotherhydrocarbon products in sewage;industries involved in metal plating,computers and related areas can haveelevatedlevels of heavymetals which can be toxic tocritical organism species in activated sludge.


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Theymay also restrict thedisposal options forthesludge; andin general. many industries use detergents,dyes and solvents which may give rise tooperationalproblems, foaming, high nutrientloadings and other problems affecting thetreatment process and final effluent.

These characteristics of sewage must be takeninto account in the design, management andoperation of sewage treatment works. Themonitoring of incoming sewage should besufficient to identify the characteristics whichwould affect the operation and perfonnanceefficiency of the plant. In relation to pretreatmentprocesses, significant issuesinclude:

the amount of floating and suspended matterwill influence the amount of screenings andgrit to be removed, the nature of thesematerials and the potential for odour nuisanceanddisposal difficulties associated with hem;similarly, grease, oils and fat in the sewagestream will require removal if the levelsconstitute a problem for the downstreamtreatment process; andhigh organic loading, for example milk orbloodwastes, require more stringent standardsin relation to stormwater overflows to protectreceiving waters.

In accordance with the Urban WastewaterDirective, and arising from the application of the"Polluter Pays" principle, there is increasedemphasis on treatment of ndustrial wastewater atsource. Thepractical application ofSchedule 4 ofthe Directive will be that manyindustriallcominercial facilities will be obliged toinstall/upgrade their wastewater treatmentfacilities beforedischarge to themunicipal sewerto "ensure that the operation of a wastewatertreatment plantand the treatmentofsludge arenor mpeded".Where pretreatment of industrial wastewaters isdiscussed, it is not intended to advocate suchtreatment atthemunicipal plant.On the contrary,appropriate pre-treatment should be providedprior to discharge to the local authority sewer.Nevertheless, the municipal treatment plantoperator should be aware of the nature of sucheffluent streams and their implications for hisworks, in the event of treatment failures at the

source plantdue to break-down oroverloading. Inparticular ircumstances, the Local Authority mayprovide for treatment of industrial effluents onbehalfof the industry, by agreement.1.3 STORMWATER IN SEWAGEAll sewerage systems receive some level ofstormwater inflow. The three types of sewernetwork are asfollows:

combined systems: the traditional systemwhere all foul and storm lows discharge to acommon pipe network. In this system,sewage flows can increase dramaticallyfollowing rainfall with peak flows of up to30 timestheaverage flow(DWF);partiallyseparate: used during the 1960'sand early 1970's, the partially separatesystem involved draining the storm run-offfrom the backs of houses and the rearfootpaths to the foul system with a separatestorm drainage network in the roads to takeroadand frontofhouse run-off; andseparate system: in this system. a dedicatedfoul sewer is provided for foul flows only,withall stormrun-offdirected o the separatestorm sewers. In all systems, a degree ofmisconnection occurs and some storm run-off inevitably discharges to the foul system.Typically, 5-10m2 perhouse is connected tothe foul sewer, even in nominally separatesystems, producing peak flows of 4-5 timesDWF.

Increased storm flows can havea flushing effectuponthe sewerage system bringing a quantity ofstale sewageand debris to the reatment works inthe early periodof a storm. This is known as the"first foul flush" and can give rise to very strongsewage with very high loading on the treatmentplant and a substantially increased level of gritand other debris resulting from the flushing ofgulley pots and the resuspension of bed sedimentin sewers. One effectis an increased level ofgritand screenings content.As the storm continues, the strength of thesewage reduces significantly and can result in arelatively dilutesewage inflow for longerstorms,typically during winter rainfall. It follows thatoverflows to receiving waters should beprevented, as far as practicable, during the firstfoul flushstage.


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PRETREATMENTOFWASTEWATER13

The major effect of rainfall, therefore, isincreased flow to the works which, if allowedpass to the treatment process, will result inhydraulic overloading. In activated sludge plants,it will cause flushing out of the activated sludgebiomass giving rise to plant upset and possiblefailure, if a large percentage of the biomass iswashed out. Carryover of biomass to thesedimentation tanks will also have an immediatenegative impact on effluent quality. Loss ofbiomass in the process reactorcan lead to long-term plant failure until such time as the biomassgrows back. Excess flows might also result inflooding of the worksdepending onpipecapacitybetween thedifferent elements.For this reason, it is necessary to restrict theforward flow to the treatment plant. This requiresoverflowing of excess storm flows. In order toprevent pollution of the receiving streams fromthe effects of these overflows, arrangements arcrequired to limit the frequency and volume ofoverflow spill bythe useofstorage and topreventcarryover of solids by effective useofbaffles andscreens. This aspect is discussed in detail inChapter2.It is particularly mportant that overflows shouldnot occur during theperiodof first flush. If thescreenings removal equipment s underdesignedor inadequately maintained, "blinding" of thescreens mayoccur fromtheextra screenings loadcarried down the sewer, resulting in prematureoverflows. Suchoverflows could havepotentiallyvery serious pollution consequences due to veryhigh concentrations of BOD, ammonia and thepotential for hydrogen sulphide which isextremely toxic oaquatic life.In this document, flows to the treatment plantareexpressed in terms of multiples of dry weatherflow (DWF). This is the total volume of sewageduring a day which follows7 days without rainand may also be described as an average dailyflow indry weather.1.4 TYPICALSEWAGE LOADS1.4.1 HYDRAULIC LOADINGAs already stated, the flow or sewage to theworks is usually expressed in terms of dryweather low(DWF).Peak flowis thendescribedas a multiple of dry weather flow (e.g. 3 timesDWF). Dry weatherflow will vary according tothenatureof the contributing catchment and willinclude the following elements:

domestic sewage which is typically in therange 180-200 1/head. This isbased on normalper capita water consumption of 150 1/day,plus some leakage and sewer infiltration.Frequently, the domestic flow is taken toinclude normal commercial discharges frompremises such as public houses, restaurantsand similar establishments and a total figureof the order of 225 l/hd per day may beappropriate based on major flow surveyscarried out in Ireland. Actualflows shouldbeverified in each case,however;industrial effluent flows are normally thesubject of effluent licence conditions whichinclude a requirement for metering.Accordingly, these flows can be metered,providing detailsof average and peakvalues.Flows from institutions such as hospitals,schools or from hotels can be estimated fromresident population or from directmeasurement. Such site measurements arealwaysnecessary priorto the design stage ofanew wastewaterreatment plant;andinfiltration results from leakage into thesewers and will tend to be ata maximum inwinter time when the groundwater table is atits highest. Infiltration is a function of thecondition of the sewer system and can besignificant in older systems involvingmasonryculvertsor in newer systems wherepoor construction practices are employedresulting in leaks at joints and at manholes.The quantity of infiltration can only beestablished from flow measurement,particularly base night flow measurement.CCTVsurveys can also assist in dentificationof nfiltration.

1.4.2 ORGANIC LOADINGUrban wastewaters characterised in the UrbanWastewater Directive (91/27 1/EEC) as having 60g of BOD per population equivalent (p.e.) perday. Field investigations may give somewhatlowervalues, inpractise.This valuemaybe usedto estimate thepollution oad from thedomesticpopulation of a catchment. Surveys are usuallyrequired to establish the load fromcommercial orinstitutional development.


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1.4.3 LOAD VARIATIONA further complication arises from seasonalvariations. For example, tourism can result in amajor increase in foul flows in seaside townsresulting in markedly increased flows to thetreatment plant. This can present serioustreatment difficulties and may requiremobilisation of standby equipment to meet theseasonal peak flows. Therefore, peak conditionsmust be catered for inthe design and operation ofaplant.Wherepermanentmeasurement facilities are notavailable, inflows shouldbe monitored over thefull cycle, by fitting of a suitable flowmeasurement device. This can be combined withflow proportional sampling to establish theorganic loadof the plant.1.5 OVERVIEW OF WASTEWATERTREATMENTWastewater treatment processes can involvephysical, chemical and biological processesdepending on the requiredeffluent standards, thenature of the wastewater and the scale of theworks (Fig. 1.1). Among the processes whichmay arise in wastewater treatment are thefollowing:

preliminary processes: physical processesahead of the treatment stage as described inSection 1.6:primary sedimentation:nowadays generallyconfined to larger plants of at least 5000population equivalent (p.c.);biological treatment: in fixed or suspendedmedia reactors using biofiltration, activatedsludge or extended aeration or variants onthese. Other biological processes includerotating biological contactors (RBC) and theuse ofconstructedwetlands forfull treatmentor final polishing of wastewater.NitnficationlDenitification may be providedfor to reduce thenitrateconcentrations wheretheeffluent sdischarged toa sensitive marineenvironment;chemical treatment: may be used to adjustthe parameters of wastewater prior tobiological treatment (e.g. pH adjustment,reduction in heavy metals or nutrientadjustment). It may also be used in

conjunction with biological treatment forphosphate removal:final sedimentation: used to separate thesludge solids from the final effluent.Typically, this is carried out in radial flowsedimentation tanks, though horizontal flowrectangular anksare used in olderworks;tertiary systems: sand or microfiltrationsystems may be employed to enhance thequality of final effluents, where necessary.Other ertiary treatment processesmay includedisinfection using UV radiation or ozonetreatment; andsludge treatment: as discussed below.

Further handling and treatment processes aregenerally employed to deal with the surplussludge generated within the treatment works.These processes may include the followingelements:sludge draw-off, flow balancing andpumping;gravity thickening of sludge in circular tanksusually assisted by a rotating picket fenceand scraper mechanism. Mechanicalthickening is also an option. increasinglyusedinEurope;stabilisation treatment ofsludge may includeaerobic oranaerobic digestion treatment;

is achievedbybelt press or

inter-stage transfer of sludge is achievedusing positive displacement pumps, screwand beltconveyors.

1.6 PRELIMINARY TREATMENTPROCESSES -OVERVIEWThe purposeofpreliminary treatment is to ensurta satisfactory qualityof final effluent and finalsludge product and to protect the treatmentprocess from malfunction associated withaccumulation ofscreenings, debris, inorganic grit,excessive scum formation or loss of efficiencyassociated with grease or oil films or fataccumulations.

volume reduction of sludgemechanical dewatering incentrifuge system; and


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ReceivingWater

PRETREATMENT OFWASTEWATER 15

FIGu11.1: WASTEWATERTREATMENT PROCESSWastewater preliminary treatment processesessentially comprise physical processes requiredto ensure that the treatment plant can catersatisfactorily for the "pass-through" flows.Theirsatisfactory operation enables theplant toproducethe required final qualityofeffluentanda treatedsludge suitable for recovery or for the specifieddisposal objectives (e.g., disposal to agriculturalland).

The principal preliminary treatment processesemployed at a wastewater treatment works,therefore, maybedescribed asfollows (Fig.1.2):stormoverflows: involve an in-line controldevice toregulatethe maximum forward flowto reatment with facilities for accommodationof excess flows using either on-line storage,off-line storage oroverflow spillpipe;

PrefreatmentWorks

Settled StormWaterBypass BiologicalTreatment

TertiaryTreatment(ifrequired)

Sludge toRecovery\Disposal


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screening: ma include coarse and finescreening, usually mechanically operated. tointercept floating and suspended debris withancillary equipment to remove the screenings.flush organic matter hack to the sewage flowand compact the final screenings residue fordisposal offsite:grit removal: to intercept and separate outinorganic grit prticles including grit washingand storage facilities. Removal of gritprevents its downstream accumulation inprocess units and the potential for excessivewear in pumps, sludge dewatering plant andother machinery:oil, grease and fat: facilities for flotation andremoval byskimming ofoil, grease and fat arenecessary where these are significantconstituents of the wastewater inflow. This isdesirable to prevent blockages and scumformation and the accumulation of fat onconveyors and other elements of the worksresulting in reducedefficiency and excessivemaintenance requirement. Fatlgrease removalis best achieved on the contaminated streamrather than on the total flow, ifpracticable. forefficient performance and correct selection ofthe plant equired:flow measurement: required to quantify thehydraulic loadto reatment normally includingfacilities for proportional sampling foranalysis of organic. nutrient or otherparameters. It is alsohighly desirable to assistin controlof sludge flows and the addition ofchemicals:odour nuisance: odours arise at thepretreatment works primarily associated withthe removal of material from the sewagestreamand the storage of residues. This givesrise to the production of noxious gases.Treatment of odourmay require containmentand extraction of malodorous air fortreatment. Treatment processes can includechemical treatment using ozone, dry or wetscrubbers and adsorption filters. Biologicaltreatment of odours can be achieved usingpeat or compost beds. Odour maskingchemicals have occasionally been used as ashort-term strategy fordealingwith odours:monitoring and control system: the plantmonitoring and control facilitiesmay providefor monitoring of such parameters as inletflows, equipment status. wastewater level.

O\erflow duration and frequcnc\ . combinedith control of return flows from stormbalancing storage: andhazard zoning:wherepretreatment processesare enclosed in buildings. it is necessary tomonitor for hydrogen sulphide. methane,hydrocarbon concentration and oxygendeficiency.and togiveconsideration to hazardzoning of the various compartments andequipment contained therein.

In the following chapters. pretreatment processesareconsidered from thepointofview of purpose,design criteria, control and supervision, types ofequipment and specification. nature and disposalof residues.1.7 ROLEOF PLANTOPERATORIndividual sectionsof this manual will outline herole of the plant operator in the management,maintenance and general operation of eachprocess. In general. duties can he summarised asfollows:

record keeping: requires recording of allsignificant operational information includingweather conditions. changes in quantity orcharacteristics of the wastewater. details ofscreenings. grit or other residues indicatingtime/date, method of removal, estimatedvolume and end disposal option:routine maintenance: in accordance withdetailed plant manuals, recording ofmaintenance schedules, breakdowns andrepairs carried out on the plant:maintaining theworks naclean condition:by regular washing down of screens. walls.storage areas. etc. Controlling rodents,scavengers and similar pests within plants:prevention of solids accumulation: inchannels or tanks including flushing out ofsediment accumulations in the balancingstorage following astormevent:maintaining safe working conditions:including maintenance of gas monitoringequipment and alarms, maintaining equipmentguardsand following safe working practices.This would involve implementing anoccupational Health & Safety plandevisedforthe scheme by the authority responsible for it:and


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PRETREATMENTOF WASTEWATER17

effluent quality: ensuring that the worksproduces a final effluent treated to therequired standard.To summarise, the plant operator is responsiblefor carrying out regularand routine maintenanceto ensure continued efficient operation of theequipment, maintenance of the plant in a cleanand safe condition to achieve performanceobjectives, minimising hazards and nuisance tooperators, visitors or the public, andmaintenOanceof detailedoperational records by

means of which performance and plant loadingscan be verified. For detailed advice on theoperation and maintenance of plants, referenceshould be made to the Local Authority NationalTraining Group or Sanitary Services SewageTreatment Trainers Manual. Operatives shouldfollow the detailed requirements in that manualfor equipment maintenance and operation,reporting, safe working and hygiene practicesrecommended therein.

II

OverflowSplitter Chamber-- Flow Control Device

s' Screenings Washing/Removal

- GritWashing/Removal$' Grease/Fat Removal

9 low Measurement(Flow Control Device)and SplitterToSecondaryTreatment

FIGuRE1.2:TYPICALWASTEWATERPRELIMINARYTREATMENTPROCESSES

INCOMINGSEWERI

SettledStormwater ScreeningsPlant

GritPlantGrease/Fat Plant


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HYDRAULIC DESIGN, STORM OVERFLOWS ANDFLOW BALANCING 19

2.1 TREATMENT PLANTFLOWS2.1.1 FLOW TOTREATMENTA critical consideration in the design of awastewater treatmentplantis thedetermination ofpeak hydraulic loading or maximum flow to beaccommodated through theworks. Thismaximumflow determines the sizing of pipework and thehead lossesto be providedbetween each stageofthe works. Therefore, it influences the processselection and sizing, including the volume ofprocess units and the surface area ofsedimentation tanks. As such, the selection ofpeakdesign flow has asubstantial bearing on thesizeofaworks and therefore onitscost.For arge reatment plants, it is common to designthe plant for a peak flow of 3 times DWF. Insmaller plants, the scale of works and costimplications of a higher design coefficient areless significant and the figurenormally taken is 6times DWF, where the sewerage system iscombined or partially separate. This approach isapplied toall plants upto 2000p.e. and maywellbe desirable up to at least 5000 p.e. Anyadditional costs for smaller plants arelikelyto beoffset by savings in the omission of storm flowbalancing at the works inlet. Ultimately, peakdesign flow should be determined byoptimisationof the total system comprising the collectionnetwork and treatment plant.2.1.2 FLOW CONTROLSOVERFLOWSFlowcontrol requires that a flow control devicebe incorporated at the inlet works to restrict theforward flow to treatment. Where inlet flows arepumped, thepump capacity determines the flowregime. Flow balancing in conjunction withvariable speed drives and PLC controllers canreduce hydraulic loading on the treatment works.The key elements of flow control and overflowworks are:

a flow control device such as a measuringflume or other appropriate control (e.g.hydrobrake type orifice). The use of orifice

platesorpenstocks givesrisetoraggingwhichcan cause variations intheflowspassed. Theycan be used downstream of screens, whereappropriate. Table2.1 lists characteristics offlow control options, with advantages anddisadvantages;a storm overflow structure designed o satisfythe WRc guidelines (reference ER 304E) toensure efficient hydraulic control and solidsseparation/retention. The device must ensurethat maximum inflows can be accommodatedwith minimum increase in the permittedthrough-flow, while at thesame time avoidingoverflows until the design through-flow isachieved;the overflow should have effective debriscontainment and be amenable to safe accessformaintenance and inspection. The objectiveshould be retention of screenings in the flowrather than removal at thisstage; andon larger works or where the receivingwaterway is sensitive to spills, a level monitorshouldbe incorporated to providea record ofoverflow frequency and duration.

Details and dimensions of theoverflow structureshouldbedetermined inaccordance with the U.K.Water Research Centre (WRC) guidelines (Ref.17) in order to achieve relatively quiescentconditions, to minimise carryover of debris.However, it is imperative that a dry weatherchannel be provided n the invert oftheoverflowstructure withan appropriate profile and gradientto achieve self cleansing conditions duringnormal flows. The plant operator should ensurethat this channel and benching are maintainedclean and free from ragging and sludgedeposition. A typical overflow weir application(high side weir overflow) is shown in Fig. 2.1.Thismayrequire flushing afterastormevent.

AND


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Single or doublehigh sideweirs

Inflow

Overflow

Dry WeatherFlowChannel

Flow oTreatment

Plan

RectangularChamber

Overflow

FIGURE2.1: HIGHSIDEWEIROvERFlow

BafflePlate/Section

1r ThrottlePipeorOrifice


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HYDRAULIC DESIGN, STORM OVERFLOWS ANDFLOW BALANCING 21

Continuation Pipe

____Channel directingFlow to Intake

Vortex Regulator

Curve withVortex

NormalOrifice Curve

Vortex Forms

Qb) HeadDischargeCurve

Where H is Head, Q is FlowFIGuRE2.2:VORTEX REGULATORTHROTTLE DEVICE (REF.WRc304E)

a) VortexRegulator Throttle(Plan View)

H


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Overflow level1. Downstreammanhole

Chamber H0 Tailwaterlevel_________________________ IQ0 v v

L ___________________________________-Downstreamlevel

H0= 1.5V2/2g+V(V-V)/g+Sf.L

Where Ho = Head Loss (m)V = ThrottlePipeVelocity (mis)Vd = Downstream Velocity (mis)Sf = FrictionGradientL = ThrottlePipeLength(m)

Q0 = Continuation Flow(m Is)

FIGURE2.3:THROTTLE PIPEDESIGN(REF.WRc304E)


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TYPE

HYDRAULIC DESIGN, STORM OVERFLOWS ANDFLOW BALANCING23

TABLE 2.1: FLOW CONTROLDEVICESATSTORMWATEROVERFLOWS- OPTIONSPROPERTIES ADVANTAGES DISADVANTAGES

Flume Critical depth flumeQ=C BH3'2(Capprox. 1.805)

Accurate measurement Combines control with flowmeasurement

Good eliability Lowhead loss Facilitates proportional

sampling

Requires long approach channel(lO*W) Headmaynot suit the minimumweirheight, requiring separateoverflow control.

H=-+(\ iv)+SLPenstock Used inconjunctionwith low - (Adjustable) measurement -possibleautomatic

operation.Note: Q=Flow (m3Is);B=Width m);h1- h2 (Head difference (m));

Orifice

Weir

VortexRegulator(e.g.Hydrobrake)Fig.2.2

ThrottlePipeFig.2.3

AccurateHJQ Inexpensiveandeasy oinstall

Accurate l-IJQ Suitable forsmall flows Easyto install Relativelyconstant

discharge rate Accommodatesgrosssolids

Prone o ragging andobstruction Smallhead incrementgiveslargeflow ncrease Needs approach lengthof10xdia.

Largeobstruction and HeadLoss

Causebackup, siltation andragging

Unsuitableforpermanent se - Expensive, for maller works Nomeasurementof low

OrificePlate o equired diameterQ=CdA(2g (h1-h2))5Dia.>200mm

Rectangular:Q= l.744BH'5"V"Notch(90deg.)Q= 1.42 H25Vortexgenerated allowing air oreand high peripheral velocity.(Ref. Manufacturer's Catalogues).

Pipe, 200mmdia.orbigger;minimum length:Slope =0.002,L=16D

=0.004,L=25D=0.006,L=35D

Accommodated inconfinedspace

Lowmaintenance Simple installation Optimum foroverflows innetwork

Cancontrol downstreamflow tooptimise storage(real-time control)

Lessaccurate Potential forblockage Length maynot be available at

works

HighMaintenance Requires automatic controlsystemH=Head (m);C orCd=Coefficient,L=Length(m)


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2.2 COMBINED SEWER OVERFLOW -DISCHARGE CRITERIAThe excess flows spilled at the oserflo mayhave the potential to cause pollution of receivingwaters if discharged without restriction. Thisrequiresconsideration of thefollowing issues:

the water qualitycriteriaand objectives for thereceiving water:the nature of the spill, its characteristics,frequency andvolume;the ocation of the stormoverflow pipeoutfalland the aesthetic and general amenity impactsofdischargest:the bacteriological effect of discharges onreceiving waters if located in designatedbathing areas:the potential nutrient load (nitrates andphosphates) in thespillflow:andthe balancing storage provided.

The general criteria for wastewater effluentdischarges derive from the urban wastewatertreatment directive (91/27 I/EEC) and the nationalregulations S.l. 419. 1994. The Directive containsno specific criteria for storm sewage overflows.Appropriate criteria have been interpreted in thelight of the directive in a Department of theEnvironment memorandum entitled "Proceduresand Criteria in Relation to StormwaterOverflow". This document provides guidelines indetermining an appropriate design for combinedseweroverflows.2.2.1 STORMSETTING WATER OVERFLOW

The minimum overflow setting, above whichoverflows might be permitted. is defined as thatgiven by"FormulaA",following the report of theTechnical Committee on Storm Overflows andthe Disposal of Storm Sewage (HMSO 1970)(Ref. 19).This isdefined asfollows:Formula A =DWF + 1.36P + 2E m3/day,where DWF=PG+ I +E

Where "F" is the population served and "G" isthe average per capita water consumption(m3/hd-d), "E" is the ndustrial effluent flow and

'1'' is the rate of infiltration. The factor of 2Eshould he reconsidered \\here the industrialeffluent is a high strength waste s th potentiallytoxic impacts or shere it constitutes a significantproportion of the total floss In thesecircumstances, a higher factor would heappropriate. This floss should he regarded as aminimum setting and reference should he made tothe D.O.E. guidelines in determining theappropriate setting foreach site (Ref. 18).In general. the following criteria also require tobe satisfied:

overflows to minor watercourses should beavoided, where possible:as already stated, the storm overflowstructures should satisfy the criteria in WRcpublication ER3O4E (Ref. 17) with overflowstructures confined to high side eir.stillingbasin or vortexchamberoverflows:the outlet control at the overflow shouldmaximise the retained flow at a nearconstantrate within the system capacity and overflowspilling should not occur until the minimumsettings have been exceeded. Chamfered or,bevelled outlets should he used to minimiseragging:such an overflow should be designed foreffective containment ofdetritus and floatingdebris, oil and grease. It should be fitted withadequate baffle plates withadequate reeboardanddepth of immersion:overflow dischargepointsshould be discreetlylocated and coastal outfalls should be taken,where practical. tobelow ow water level:apreliminary assessmentshouldbecarriedoutto establish containment of "first foul flush"flows having regard to the nature of thecatchment runoff (time of concentration.extent ofsediment insewers, etc):andspecific performance objectives in terms ofspill frequency are required for discharges ocoastal waters, including bathing waters andrecreational amenity waters (3 and 7 spillsrespectively per bathing season) (Ref. 18).Similarly. spill frequency and volumelimitations may apply to other receivingwaters having regard to available dilutionsand water quality objectives. Frequencies of


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HYDRAULIC DESIGN, STORM OVERFLOWS ANDFLOW BALANCING25

16. 8 or 4 timesper yearon average mightbeset,depending on available dilutions.

It follows, therefore, that the sewer flows abovewhich discharges to watercourses or otherreceiving waters maybe permitted will generallyexceed the maximum flow permitted to thetreatment plant. This requires the provision ofstorage on the overflow to intercept the spilledflows for return to the inlet works, when stormconditions haveabated.2.3 STORM OVERFLOW STRUCTURESAs alreadystated, he WRc reviewed thedesigncriteria for stormsewageoverflows and provideddetailed guidelines for appropriate structures inits report on "A Guide to the Design of StormOverflow Structures", (Ref. 17).In arriving at recommendations for futurepractice, the WRc study identified the followingdifficultieswith traditional designs:

outlet obstructions are common wherediameters are less than 200mm or wheregradients are slack, or where they areinaccessible for maintenance;forward flows can exceed the safe limit instructures with restricted spill capacities orvariable headsuch as leapingweiror "hole inthe wall" types;poor solids separation is common. Traditionallow side weiroverflows can giverise to solidsbeing carried out in the overflow due tolongitudinal currents in the mainchannel.Thescumboards have little effect in suchsituations; andsuch overflows (low side weir, leaping weir,etc.,) commence overflowing before the fullforward flowis reached;

Recommended overflow structures include"HighSide Weir", "Stilling Pond" and "Vortex"overflows. Detailed design recommendations aregiven for eachtypeto achieve theobjectives of:controlled forward flow at near constantvalue;deferral of spilling until forward flow isreached; and

efficient solids andscreenings retention.Details of each type of overflow are showndiagramatically (Figs. 2.4 to 2.6), giving keydimensions for correct sizing. The characteristicsofeach typeare istedinTable2.2.Arising from the foregoing selectioncriteria, itmight be concluded that the high side weir orstillingpondoption would normally suffice wherestorage is provided. The storage would beexpectedto make up for any loss of settlementefficiency. However, where storage is notprovidedand the receiving water is particularlysensitive, for example, where it supports a highlevel of visual or general amenity, the vortexseparatorshouldbeconsidered.2.4 OPERATIONS AND MAINTENANCEOFOVERFLOWSProper operation andmaintenance ofoverflows isessential to satisfactory performance. Thisrequires attention tothefollowing aspects:. access foreasy inspection and maintenance atinlet andoutlet;

good ventilation is necessary and lightingshouldbe considered, ifoverflow iscovered;inverts should minimise risk of solidsdeposition and be selfcleansing;highpressure washing facilities are desirable;and

on largeoverflows, a bypass is desirable, withpenstock control, to allow maintenance ofchamberand throttle device.Overflows should be regularly checked to ensurethat outlets are clear and freefrom ragging.2.5 STORM FLOW BALANCINGHistorically, storm low balancingwas frequentlyprovidedforstorage ofthat proportionofflow inexcess of the through flow to treatment up to thepermitted flow level at which spills might bepermitted. Forexample,aweir would beprovidedto allow diversion to storage at 3 DWF with asecond weir spilling to overflow at 6 DWFsetting.


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TABLE2.2: RECOMMENDEDSTORM OVERFLOW STRUCTURES

HighSideWeir(Fig.2.4)

VortexOverflow(Fig.2.6)

CharacteristicsStilling Zone,OverflowZone,double weirs ofspecified height and lengthand storage zone. IncludeDWF Channel for lm/s at2* DWFExtendedStilling pondchamber, surchargeof nletsewer and transverse weiroverflow.Secondary currents naforcedvortexused toseparate solids.

Accommodates toexisting inverts Minimum surcharge ofu/s sewers Easy maintenance Higher eparationefficiency Shorter structure

High separationefficiency Works withhigh nletvelocity Circular anksuited odeep sites orpoor

Large structure (> l5D) Moderate separationefficiency Limitedoverflowscreening options.

Surchargeof nletsewer Greater gradientrequirement Outlet must not impede Significant dropininvert evel required. May surcharge upstreamsewer. Normally requiresHydrobrake typeoutletcontrol

However, many storms are of short durationwithconcentrated discharges whichwould give rise tospills of moderate volume but high frequency(with short durationintensityexceeding 6 timesDWF). Suchspillsarepotentially unsatisfactory,particularly during summer conditions, whenreceiving waters are most vulnerable. If fullydiverted to storage, many such overflows wouldbe contained without discharge and could bereturned back to the treatment works when thestorm has abated. Even where discharge isnecessary due to the severity of a storm, thebeneficial effectsofsettlement in the tank can beconsiderable.Therefore, the objective of storm balancingstorage should be to intercept all flows whichexceed the through flow to treatment up to thecapacity of the tank. Where the intensity andduration of the storm results in spill dischargeswhich exceedthe tank capacity, theexcess flowsare then permitted to be discharged to thereceiving waters subjectto effective containmentof floating debris. Suchflows have the benefit ofsubstantial dilution and potentially significantsettlement n thebalancing tank.Theoptimumsizingofstormbalancing storage isdetermined from modelling of the seweragenetwork combined with spill settings determined

for the receivingwaters. Historically, tanks weredesigned orup to 6 hoursDWF and this volumewould generally be satisfactory. Flow balancingmightalso be desirable to even out the effectsofpumping. A directmonitoring and control systemis recommended to optimise the use of the totalstorage volume.The control system may combine the followingelements:

timer control with return flow commencedlinked toaset ime delayinsmallerworks:flow proportional returnbased on inflow rate.For example, storm water return at DWFflow-rate could commence when inflow fallsbelow 2DWF;andthis flow proportional return can beoptimisedby use of variable speed pumps to optimisestormwater handling efficiency. This wouldalso permit flow balancing to the works toeven outhydraulic loading.

Type AdvantagesLow gradient

S

Disadvantages

StillingPond(Fig.2.5)

weir

ground


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HYDRAULIC DESIGN, STORM OVERFLOWS AND FLOW BALANCING27

D>D(D=0.815Q4m)

DryWeather Channel300mm. dia. mm.

'Stilling length

PlanWefrlen

0.lDtoO.15D0.8D

Benching nrangelin4tolinl2Fig.2.4 TypicalHighSideWeirCriteria (Ref WRc304E)

FIGURE2.4: TYPICAL HIGH SIDEWEIR CRITERIA (REF. WRc304E)

0.1Dto0.15D mm. 200mm)

4D I 3DStorage ength

I

BafflePlate

r.Jnvert gradeforself-cleansing velocity

Section


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D>DmOutlet Pipe

Flow toTreatment Plant

OverflowPipe t* 25Dmrn

Plan

Flow toTreatment Plant

FIGURE2.5: STILLING PONDOVERFLOWCRITERIA (REF. WRC304E)

7D mm. Weir

Section


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HYDRAULIC DESIGN, STORM OVERFLOWS ANDFLOW BALANCING

DrContinuationPipe

/K IOverspill____ Flow to=--- reatment

FIGURE 2.6:VORTEX CHAMBER NCIRCULAR SHAFT (REF. WRc304E)

Scumboard R=2D

Weir CrestO.36DIf

Weir levelwith inletpipe offit-11.8D

ChamberDiameter1< =4DSectionOverflow

CircularShaft

Scumboard

Flow to

Bypass

Overspill

Plan


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Flow balancing can be achievedby a variety ofmethods including thefollowing:in-line storage upstream of the overflow bymeans ofa ank sewerorlargediameter sewerin which the flows are backed up beforedischarge. Such systems have the benefit ofautomatic operation. Even though fitted withDWF channels, there is a tendency forretention ofsettled deposits onbenching whenstorm lows havereceded, whichmayresult nsubstantial maintenance;horizontal rectangular tank with sloping floorand eitherpumped or gravity facilities for thereturnflow.Traditionally,such tanksprovidedfor separate sludge draw off, but a modernsystemwould ideally incorporate facilities forresuspension of sediments to avoid separatesludge handling facilities. Tanks would besubdivided intoanumberofcells;andcircular radialflow tank with sloping conicalfloor and central sump fitted with scrapermechanism (similar to primarysedimentationtank).

Wheresome surcharge or backing up of the trunksewerto the reatment works is permitted, storagecan be achieved withinthe sewer systemto meetsomeor all ofthe storage requirement. This"in-line" storage is limited by the extent to whichsurcharging can be permitted without risk ofupstream flooding. Its impact on the flowmetering devices being utilised should also beconsidered.Off-line tanksprovidethe balance of the storagerequirement tomeettheoverflow spillobjectives.Fig. 2.7 illustrates schematically the nature of"on-line" and "off-line"storage. These tanks arecommonly constructed in cells with provision foroverflowing from one cell to the next. Thisensures that the more contaminated overflows arecollected in the initial cells with more dilutedischarge in downstream cells to the point ofultimate overflow. For smaller storms, only theinitial tankorcell isutilised, thereby reducing themaintenance requirement involved indesludging/cleaning.The design objective is that storm tanks arealways emptied completely each time they areused. This ensures that storage capacity isavailable for the next rainfall event and alsoprevents consolidation ofsludge in thebaseof thetank. The whole of the contents of the tanks

should be returned to the pretreatment works toprevent malodours and avoid sludge residues inthe tank. This may involve incorporatingmeasures for resuspension of settled sludge.Alternatively, the tanks can be designedso thatthe supernatant water is returned for treatmentand the sludge dealt withseparately.Where ground conditions are poor, relativelyshallow rectangular storm settlement tanks maybe most appropriate. Such tanks are particularlysuitable where a gravity return of the storedflowsis possible, for example adjacent to pumpingstations, where flows are returned o the wet well.Such tanks should incorporate the followingfeatures:

adry weather channel in each compartment toensure adequate velocity for self cleansing ofthechannel;facilities for resuspension of solids as thetanks are drawn down by means of pumpedrecycling, tank mixer unit or air/water scourpumps;facilities for cleaning of the tank floor ondraw-down of the contents. utilising "tippingbucket" flushing facilities, combined withhigh pressurepowerhoses; andwherepumped return is required, the optimumdesign isfor a circular radial flow tanksimilarto the tanks required for primarysedimentation. These tanks are equipped witha central hopper and scraper system. Thesludge is automatically drawn off from theconical sump at the invert of the tank, inconjunction with the returned flow.

The operational management of storm tankswithin he treatment works requires that:the tanks are drawn down by recycling offlows to the inlet works at the end of eachstorm event when flows have returned to asatisfactory level. For example, a plantdesigned for three times DWF couldaccommodate a return rate of DWF when theinflow rate has subsided to twiceDWF:storm tanks should be fully emptied at theconclusion ofeach storm event; andthe operator's duties should includethoroughly flushing out thestorm tankat the


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HYDRAULIC DESIGN, STORM OVERFLOWSANDFLOW BALANCING

conclusion of each storm cycle. Appropriatepower washing facilities should be providedto achieve this.Stormwater storage at the inlet to a sewagetreatment works is utilised for approximately 5%of the time. For this reason, there may be atendency to neglect or underestimate itssignificance as part of the treatment works. Theintermittent use of equipment may also lead tobreakdown, for example, of scraper mechanismsand pumping plant. Nevertheless, the properoperation of storm balancing storage is critical tothe satisfactory operation of the treatment plantand theprotection of receiving waters.Given the operational difficulties which mayoccur with storm balancing storage, it might beconsidered that treatment plants up to 5000 p.e.and possibly even of larger size (up to 10,000p.e.) would be designed to accommodate"Formula A" flows through the treatment plant,avoiding the need for balancing tanks. Thishydraulic capacity combined with well designedand well maintained overflow facilities couldachieve the required objectives without off-linestorage, except where higher standards arenecessary for the receiving waters. However, thisshould always be subjected to economic andtechnical appraisal in order to determine theappropriate option ineachcase.

31


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DWFChannelUpstreamSewer*

SECTION A - ATankSewer OverflowUpstream ThrottleA Sewer - A

1-7 .O\erfloWeirBaffleDWF Channel

OverspillPLAN

a) On-lineBalancing StorageBaffleorScreen

-- Weir

O%ersplllSECTIONS-B

BL 1 lB- - I Overspill

- GravityorUpstream Pumped ReturnSewer - --, r

- ThrottleOverflow

PLANb) Off-lineTankOverflow

FIGURE 2.7:ON-LINE ANDOFF-LINE STORAGE


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3.1 DEFINITIONScreenings comprise the coarse suspended andfloating solids which are present in a wastewaterstream and which are retained on bar racks orscreens. The smaller the screen opening, thegreater he quantity ofscreenings. In addition, themore screenings that are removed in them, themore organic putrescible content will bepresent.3.2 PURPOSE OF SCREENINGThe functions of screening equipment as part ofthepretreatment works are:

To protectdownstream mechanical plant fromdamage or obstruction due to largeobjects inthe wastewater flow;To separate and remove the larger materialwhich might interfere with the efficientoperation of wastewater treatment processes;and

treatment plant byremoving suspended solids andBUD. However, in municipal wastewatertreatment, theobjective should be to minimise theremoval of organic matter at the preliminarytreatment stage so that the screenings material forultimate disposal is less objectionable and lesslikely to give rise to odour nuisance at the plantordisposal site.3.3 SOURCES OF SCREENINGSThemainconstituents ofscreenings are:- rags,

paper,plastics,timbers,offal,and

To ensure the absence of unsightly floatingmatterat outfalls or in receiving waters.The protection of receiving waters from aestheticnuisance is increasingly the objective in selectionof screening plant. For bathing waters, S. . 84,1988 entitled "European Communities (Quality ofBathing Water Regulations), 1988" gives effectto the CouncilDirective No. 76/160/EEC. Thisrequires effective containment of screeningsdebris for discharges to bathing waters andsimilar standards are nowgenerally applicable toamenity waters.The National Strategy for Sewage SludgeManagement adopted by the Department of theEnvironment envisages re-useof sludge as far aspossible. E.C. Directive 86/278/EEC and theensuing National Regulations (SI. 183, 1991) setcriteria for the use of sewage sludge inagriculture. The successful implementation of are-use strategy nvolving land spreading requiresa consistent, high quality product. This requiresthat the sludge be free of rags, plastics and othernon-biodegradable debris, which would normallyberemoved byan efficient finescreening process.In some instances, fine screening can be used toreduce the pollution loadentering thewastewater

leaves.There are many additional elements ofscreeningswhich arise from the nature of the activitiesconnected to thecollection network. Forexample,building works tend to give rise to buildingdebris. Other materials arise from illicitdeposition of waste materials in sewers andmanholes.The amount of screenings will also vary withflow conditions in the sewer. Storm conditionswill tend to increase the quantity of screeningsarriving at the treatment works as additionalmaterial will be carried in via gulley traps,gratings, and resuspension of material trapped inbed sediments associated with increased sewervelocities. Therefore, screenings design mustcater for the maximum hydraulic load to theworks and have regard to the likely maximumscreenings loadassociated with this flow.The treatment works operator should maintainrecords of the quantities of screenings. Theoperator should also note any problems withscreening equipment or problems associated withparticularly heavy screenings loads. Theoccurrence of unusual or excessive quantities ofscreenings should prompt investigation of

33SCREENING


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potential sources both to alleviate the problem atthe works and to avoid potential problems in theupstream sewer network.Catchments with a substantial amount of storminflow and where sewer gradients are steep cangenerate large debris which will damage finescreens and screenings handling equipment. Suchcatchments may require the provision of coarsescreens upstream of fine screening to interceptboulders, timbers and similardebris.3.4 CHARACTERISTICS OFSCREENINGSThe characteristics of screenings are extremelyvariable anddependenton many factors includingthe nature of activities within the catchment, thenature of the sewer system (combined, partiallycombined or separate), industrial and commercialactivity and the natureof industrial effluents andtheirpretreatment prior todischarge.Coarse screens with bar spacings of the order of75-100mm are designed to intercept only thelargest materialsand these are generally heldbackin the flow to be manually removed. Suchmaterials will generally be rocks, branchesandlarge pieces of timber with little organiccontamination.Coarsescreening of the order of 20 mm spacinghave been foundto haveahigh rag content. Suchscreenings will have a relatively high volatilesolids content which can be up to 80% and willtypically have a dry solids contentin the order of15-25%.Fine screenings retained onscreens withaperturesof the order of 6mm will also have significantvolatile solids contents and are likely to include5-10% of influent suspended solids. Moisturecontents are likely to be somewhat greater thanfor coarse screenings. They will also containsignificantelementsofgrease and scum.Because of the high putrescible matter content.including faecal material, screenings requirecarefulhandling and disposal. In a raw state, theyarehighlyvolatile and will quickly giverise to anodour nuisance if they are stored on site for anysignificant length of time. Daily disposal istherefore necessary, together with washing downofstorage areas.

3.5 QUANTITY OF SCREENINGSThe quantity of screenings collected will varydepending on the typeof screen used, the size ofthe screen opening, the typeof sewer system andthe geographical location. Sewer gradients and.the resulting flow velocities can significantlyinfluence the typeofscreenings encountered.The predicted quantity of screeningsat any givenlocation is difficult to estimate as there are nodata available for wastewater treatment plants inIreland. However, the following textbook dataprovide an ndicative range.

Continental data suggest a figure of 10-15 litresof screenings per person per year for 6mmscreens. Fig. 3.1 provides indicative guidelinesfor screenings volume for a range of screen barspacings.In recent years. there has been a marked increasein the amount of plastic materials arriving at thetreatment plants. These plastics are difficult toremove by traditional screening methods andmaybe seen at various locations throughoutwastewater treatment plants. They have atendency to float longitudinally. prising throughtraditional barscreens.

3.6 TYPES OF SCREENUp to the mid 1970s. the basic type of screenused in sewage treatment in Ireland was amanually raked barscreen, having bar spacingsof20 - 25 mm. These screens have been overtakento a large extent by the advent of economicalmechanically raked screens and the need for finerscreening. The screen types may be defined asfollows:

ScreenType Aperture

Volume ofScreenings 0.01 -0.03m3/l.000p.e.perdayDensity ofScreeningsMoisture ContentVolatile Solids

600- 950kg/m3.75% -90%.65% -95%.

CoarseMediumFineStraining

> 50 mm.15-50mm.3- 15 mm.


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V

The following table lists the types of screenavailable, although each type of screen has asmany versions as there are manufacturers. Wherereference is made to proprietary equipment, this isintended as indicating a ypeof screen and is notto be interpreted as endorsing or excluding anyparticular manufacturer or system. In addition,thereareanumberofnew typesof screen now onthe market which do not come under thetraditional descriptions. Table3.1 lists the screentypes and their applications. Strainers have notbeen included in this list as these area form oftreatment and are not normally used in municipalwastewaterpretreatment.3.7 MANUAL BARSCREENSOn many of the older wastewater treatmentplants, theonly formof screening is the manualbar screen with a bar spacing traditionally ofapproximately 25 mm. These bar screens arenormally fixedon a guide rail inclined at 45 to60 to the flow and are fitted with a perforatedscreenings trough above the water level intowhich the screenings are raked at irregular

intervals. These screens are quite ineffective andblind easily. They should be replaced wherepossible with mechanically raked screens andretained on aby-passbasisonly.3.8 COARSE SCREENS3.8.1 TRASH RACKSCoarse screens, such as trash racks,arenormallyused only in large volume stormwater drainageschemes and upstream of largepumping stationswherethepumps needto be protected from largeobjects such as concrete blocks, ogs ofwood andother solid objects which could damage themechanical equipment. Pumps on such dutieswould normally be unaffected by medium sizedobjects. These trash racksarerarelymechanicallyraked. Theyaregenerally aligned at an angle tothe flow to minimise the risk of blinding duringfloodconditions.

35SCREENING

100

80

60

40

20

010 20 30 40 50 60

Openingbetween Bars(mm.)FIGURE 3.1:QUANTITIESOF SCREENINGSCOLLECTEDFROM MECHANICALLYCLEANEDBAR RACKS

(COURTESY NVIREX NC.)


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36 PRELIMiNARY TREATMENT

TABLE 3.1: TYPICAL SCREEN APPLICATIONSScreen Type ApplicationTrashRack Coarse Used at inletto arge stormwater orcombined flowpumping stationsorbefore fine screens on a argewastewater treatment plant. Thesemay bemanually or mechanically raked.RotatingBarInterceptor Coarse As for trash rack, but do not needto hecleanedexcept to(R.B.I.) take out large objects by hand. Useful for interception ofboulders, large imber segments. etc.Manually Raked Bar Medium Used at inlet to small wastewater treatment plantsor onby-pass tomechanical screen ordisintegrator.Curved BarScreen Medium Used at inlet tosmall to medium size wastewatertreatment plants. Intermittent raking. Shallowchannels.Verticalor Inclined Bar Fine or Usedat inlettosmall to large size wastewater treatmentScreen Medium plants.. Intermittent orcontinuous raking. Anydepth ofchannel.BandScreen (Stepped Fine Usedat nletto small to large size wastewater treatmentScreens) plants. Continuous cleaning. Shallow to mediumdepths ofchannel.Drum Screen Fine Used at inlet on large wastewater treatment plants.Continuous cleaning.Cup Screens Fine or Used at inlet to large wastewater treatment plants andMedium seaoutfafls. Continuous cleaning.Screezer. Rotomat Fine These machines combine screening. screenings removaland dewatering suitable for medium to large wastewatertreatment plants. Intermittent orcontinuous cleaning.Discreen Fine For use on stormwater overflows or in combination withadisintegrator. Continuous cleaning.Disposable Bags Fine Used at ouflet ofprimary settlers, screened outlets.overflows. etc.


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3.8.2 ROTATING BAR INTERCEPTORS(R.B.I.)Rotating bar interceptors are now used inpreference to static trash racks upstream ofpumping stations (Fig. 3.2). These consist ofrotating bars which, by virtue of their rotation,prevent the accumulation ofdebris(rags, etc.) onthe bars. They do, however, retain larger objectssuch as concrete blocks, large timber sections,animal carcasses and similar large debris whichwould be likelyto cause pumps blockage. In theevent ofjamming of the screen, the motors arereversed, reversing the direction of rotation andfreeing the obstruction.3.9 MEDIUM SCREENSMedium screens re bar screens having a spacingof 15 - 50 mmand generally in the range 20-25mm. Thesescreens were initially developed as an

upgrade from the manual screen and weregenerally fitted with mechanical rake or brushfacilities to convey screenings to a receivingtrough.The -operation of the raking mechanism isnormally intermittent, controlled by timerorheadloss measurement. Downstream ofpumps, it maybe activated by pump start-up with timercontrolled duration.Automatic bar screens must be fitted with limitswitches to prevent damage due to overloading orblockage. The controls alsogenerally providefora switch to ensure that the rake automaticallystopsmoving at apointoutside the screen area toavoid jammingatstart-up.

37SCREENING

FIGURE 3.2:ROTATING BAR INTERCEPTOR (JONESANDAlTWOOD)


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3.9.1 CURVED BAR SCREENSThe curvedbar screen issuitable for shallow nletchannels (i.e.. less than 2.5 ml (Fig. 3.3). It isfixed n the channel sloping away from the flow.A rotatingrake (normallydouble sided) s drivenb an electricmotor and follows the curvature ofthe screen with the tines interlocking with thescreen bars. A tine cleaning device at the top ofthe screen sweeps he screenings intoa collectiontrough. Curved bar screens, by virtue of theirshape.are verysuited to ow flows.3.9.2 VERTICALSCREENS AND INCLINED

or hack raked and are zeI1erall\ hvdraulicall\ orchain operated. The cleaning action mimics amanual raking action. The cleaning rake isnonall parked in the upper position and isactuated eitherb a timer or a water level signal.The cleaning rake will then travel down to thebottom of the screen v th the rake in adisengaged or retracted condition.At the bottomof he travel, the rake is engaged into the screenby hydraulic or mechanical means. The rakecollects the solids from the screen bars andelevates them to the discharge chute where ahinged wiper pushesthe solids into the receivingtrough.

The vertical and inclined bar screens arevariationsof each other and are used for deeperinlet channels. These screens can be either front

FIGURE3.3: CURVEDBAR SCREEN (JONESANDA1T WOOD)

Electrohydraulicoperatingunit Rake arm intop parkedposition,


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3.10 FINE SCREENSFine screening (3 - 15 mm) is becomingincreasinglycommon for hefollowing reasons:

advances in technology have made finescreens more reliable and moreeconomical tomanufacture;protection of bathing waters requires theadoption of fine screens for virtually allapplications, particularly where european blueflag standards apply;the quality requirements for sludge re-userequire effective fine screening to ensureremoval ofplastics and rags;development of improved and moreeconomical screen washing equipment offsetsthe extraorganic loadremoved byfinescreensby returning ittothe flow;andimproved dewatering and compactionequipment greatly improves thehandling andvolume reduction ofscreenings, offsetting heeffectsof increased quantity removed.

The decision of Her Majesties Inspectorate ofPollution (H.M.l.P.) in the U.K. to set amaximum spacingof6 mm foroutfallshas set thestandard forscreen manufacturers inBritain.In general, Irish practiceis likely to be similargiven the common standards used and the factthat equipment is commonly sourced in Britain.The Department of the Environment criteria forstorm overflows to Bathing Waters, for example,haveadoptedthe6mmstandard.Considerable reseaich and development inscreening is taking place in the development offine screening, washing and dewatering as aconsequence of their increased usage. Therefore,any discussion of screens isofnecessity confinedto thegeneral typesrather than the full range ofmodels available or under development. Thefollowing sections discuss the main types ofequipment on offeratpresent.3.10.1 INCLINED BAR SCREENSThese screens are, as the namesuggests, standardbar screens set at any inclined angleto the flow,having bar spacings of 5 mm upwards.Screens

are continuously frontorback rakedby meansofcleaning tines mounted on a chain mechanism.These tines continually lift the screenings fordischarge at theupperend, eitherbygravity orbymeans ofa brushedcleaning device(Fig.3.4).3.10.2 BAND SCREENSBand screens consistof a seriesofpanelswhichcontinually move up through the flow on a beltdrive collecting the screenings on the way. Ingeneral, these bandscreens are madeup in eitherof wo ways (Fig.3.5):

shapedplastic or stainless steel hooks,formedin rows orbands; andstep shaped perforated plates, similarlyarranged.

Theseare assembled to form a continuous beltwhich filters the effluent and, in fact, thecollected screenings provide further filtration.There are a number of methods of removing thescreenings from theband screen, eitherby meansofa brushsystemorabackwash system.3.10.3 DRUM SCREENSDrum screenshave been in existence for manyyears and basically consistof a large perforatedcylinder mounted horizontally in the flow andthrough which the wastewater hastopasstoreachthe inlet to the treatment works. The flow mustpass from outside to inside the drum. Thescreenings are maintained on the outside of thedrum which rotates into the liquid and they arecollected withina screenings pit from where theyare removed by means of a pump or otherelevator.3.10.4 ROTOMAT, SCREEZER, CONTRA-SHEARThere are now, however, a number of otherrotating screens which fall into this generalcategory, such as the HuberRotomat, theJones &Attwood Vertical Drum Screen and the Contra-Sheardrumscreen.Eachof these screens operates by having rotatingdrums immersed to some degree in the sewageflow. In the case of the Rotomat and Contra-Shear, the sewage enters the insideof the drumand flows through it to a collector channelexternally. The screenings are collected on the

39SCREENING


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FIGURE 3.4: INCLINED MECHANICALLYRAKED BAR SCREEN

FIGURE3.5: STEP SCREENS (INKA SYSTEMS)

TinePlatp

wMate


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FIGURE3.6: SCREEZER (JONESANDATTWOOD)

FIGURE3.7: DRUM SCREEN ROTAMAT TYPE (HANS HUBERGMBH)

SCREENING41


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inside of the drum and rotated upwards to bedropped out into a receiving chute in the centre.The Jones & Attwood Vertical Drum Screen is adirectreplacementfor the comminutor and, in thiscase, the sewage flows from outside to inside ofthedrum, dropping down from thecentreviaaninverted syphon. The screenings are retainedoutside the drumand are lifted o the surface by alifting tine or conveyor. The Jones & AttwoodVertical Drum Screen is ideal as an upgrade foran existing comniinutor installation, whereit canbereadily retro-fitted.Some of these screens, notably the Contra-shearscreen, require a significant level dropwhichmaynotbeavailable atagravity plantorin an existingworks. All of these machines are effective but arealsoexpensive (bothfor the mechanical plantandin terms of civil works) with the exception of the

Rotomat which can be fitted in a standardchannel.3.10.5 DISCREENThe Discreen sa variation on the Monomuncherdisintegrator and consists of a series of verticalshafts, each fitted with overlapping discs whichrotate in the same direction at different speedsaccelerating towards the downstream end (Fig.3.8). The screenings are continuously movedalong thescreen and kept in the flow, while liquidpasses through.The screen does not remove thescreenings but retains them in the flow. Thisscreen is suitable for use on stormwateroverflows, particularly at pumpingstations and isfitted before the sump overflow pipe.

FIGURE 3.8:DISCREEN (HO W.4STE-TEC)


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3.10.6 DISPOSABLE BAGSDisposable open meshbag screening is a recentand cheap option which has been marketed byCopa Sacs and takes the form of disposable openmesh bags which are mounted in the flow andcollect fine screenings. These screens are notsuitable for inlet application, but can be retro-fitted to unsatisfactory existing installations toprotect the overflow or after primarysedimentation to protect the distribution arms ofpercolating filters. They could alsobeused on theoutlet from a treatment plant to ensure that nofloating debrispassesout.Ifused, the sacks needto bechanged regularly.

3.11 SCREEN DESIGN3.11.1 SELECTION

SCREENING43

The specific screen to be selected will depend onthe application. In general, the approach as setoutinTable3.2 is suggested.In general, manually raked screens should not beinstalled on newplants,except in the caseofverysmall plants which would not justifymechanically raked screens. In this case, the areaof immersed bar screen must be higherthan thatfor a mechanically raked screen to avoidblockages.

TABLE3.2:SCREEN SELECTIONAPERTURE TYPE

50- 15 mm. TrashRack.R.B.I.LiftableCage.BarScreen.Curved Bar Screen.Vertical BarScreen.Inclined BarScreen.Inclined BarScreen.Vertical BarScreen.BandScreen.Inclined BarScreen.Vertical BarScreen.BandScreen.Screezer (V.D.S.).Rotomat.Contra-Shear.

Large Wastewater Treatment Plants(With SludgeTreatment) 15-50mm.(BeforeFineScreen) Vertical BarScreen.R.B.I.

BandScreen.DrumScreen.CupScreen.Screezer V.D.S.).Rotomat.Contra-Shear.

Overflows (Retain ScreeningsinFoulFlow) 5- 10mm. Discreen.J&A WeirMount.

APPLICATIONLargePumphousesSmall Pumphouses 50mm.

Small Wastewater Treatment Plants 15 - 25 mm.(Without SludgeTreatment)Small Wastewater Treatment Plant 5 - 10mm.(WithSludgeTreatment)Medium Wastewater TreatmentPlant 5 - 10mm.(With SludgeTreatment)

5 - 10mm.


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A standby or bypass channel should be providedfor all screening facilitiesto avoid the possibilityof flooding or discharge of unscreened anduntreated sewage due to a breakdown orcloggingof the inlet screen. A manually raked screenshould be fitted in this bypass channel. Dualscreen channels are appropriate only at largeworks, say 20,000 population equivalent andover, where a bypass channel should also beincorporated.3.11.2 STANDARDSDue to the large diversity of screen typesavailable, there is no standard method ofcomparison. If comparison is desired, then anumber of simple tests could be carried out.Thesetests are:

tracer test: a known quantity of mixed itemsof plastics such as plastic strips, papers.condoms, sticks. etc.. can be placed in thechannel a fixed distance upstream from eachscreen and the quantity of plastics capturedmeasured: and

mesh test: depending on the aperture size, aseriesof aluminium meshes may be placed inthe sewage flow both upstream anddownstream of the screen and the degree ofcapture on each mesh compared. These meshsizes would typically be in multiples of thedesign screen size.

3.11.3 DESIGNThe basicdesignof a bar screen shouldbe suchthat the velocity through the screen would hesufficient for matterto attach itself to the screenwithout producing an excessive loss of head orcomplete clogging of the bars. At the same time,velocities in the channel upstream should besufficient to avoid deposition of solids. In allcases. the shape of the bar shouldbe tapered fromthe upstream side so that any solids which passthe upstream face ofthe screen cannotbejammedin the screen, thereby causing a trip out of theraking mechanism.The following table gives the design factors forbarscreens:

TABLE3.3: SCREEN DESIGN FACTORSItem Manually Cleaned MechanicallyCleanedBarSize: Width (mm)Depth (mm)Aperture mm)Slope toFlow (Deg)

5- 1525 - 8020 - 50

5- 1525 -805 - 8()

VelocityThroughScreen (mis)

The minimum head loss which should be allowedfor through a screen is 150 mm but this will varywith screen type and design. Allowable head losswill oftendependonavailable head.The degreeof clogging of a bar screenwill varywith the size of the screen and the wastewaterquality. For mechanically raked bar screens, theclogging can be anywhere between 10% forsurface water and 30% for wastewater with highsolids content. For manually raked screens, thedegree of clogging will be greater due toinfrequent cleaning.

0.3 - 0.6 0.6 - 1.0 (Max. 1.4)

Because of the need to control flow velocitiesthrough the screen, approach velocities upstreamwill generally be slow, especially in the case offine bar screens with an open area of less than50%. This means that the channel widths will berelatively wide and channel deposition is difficultto avoid. The plant operators must flush suchchannels cleanon a dailybasis.It is important in the design of screeninginstallations to ensure that upstream velocities arekept sufficiently high to minimise deposition ofsediments which create nuisance.

450 - 60 18-90


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The following equations maybe used forstandardbar screens to calculate the width of channelrequired and the head ossthrough the screen:

Head Loss

1

2 2orcleanor IV Vpartially HL = 1.43cloggedscreens 2gfor clean B(A)creens HL = b h sinq

where:

H 2gC 1A

Q = MaximumFlow(m3Is)V = Velocity ThroughScreenmis)v = Velocity inUpstream Channel (mis)D = DepthofFlow (m)W = Width ofChannel (m)S = % ScreenOpenArea.HL = Head LossThrough Screen (m)g = 9.81 mis2(gravity).h = Head on Screen Upstream (m)A = SubmergedApertureArea(mm2)B = Bar Width (mm)9 = Angleof inclinationofbars.C = Coefficient which should becheckedwith themanufacturer.= Bar ShapeFactor.The valuesofbarshape factors for clean rack are summarised asfollows:

BarType.Sharp-edgedrectangular. 2.42Rectangular with semi- 1.83circularupstream face.Circular. 1.79Rectangular with semi- 1.67circularupstream anddownstream faces.Tearshape. 0.76Proprietary screens, such as the Rotomat,Screezer, Contra-Shear, etc., are not covered bythese formulae and head losses have to becalculated separately. Reference shouldbe made

to manufacturer's technical data sheets forappropriate design criteria. In general, the headavailable (together with the performancerequired) will tendtodictatethe typeofscreen obeused.3.12 SCREENINGS DEWATERINGThe disposalof screenings is made difficult bythe presence of faecal matter and otherundesirable materials. When medium to coarsescreening of sewage was the norm at plantsandthe screensweremanually raked, the quantities ofscreenings to be dealt with wasreasonably small.In small sewage treatment plants, these werenormally dealt with by burying on site, while onlarger plants they were transported to the locallandfill.With the improvement in screening techniquesand the adventof finer screens, the quantity ofscreenings to be dealt withhasgreatly ncreased.This increase will accelerate withtherequirementunder the Urban Wastewater TreatmentRegulations and Directive to providewastewatertreatment facilities atmany more towns, ncludingthe largercoastalconurbations. At the same time,finerscreening means that more faecalmatterandother undesirable objects are trapped in thescreenings making them unacceptable forhandling intheir raw state.Since the mid-1980's, much development hastakenplacein the design ofscreening dewateringdevices. These devices come under four mainheadings:

hydraulic press,screw compactor,washer/dewaterer, andcentrifuge.

Most of these devices include some form ofwashing to reduce the amount of faecal matter.The success of this washing is veryvariable. Onesuccessful approach is to liquefy the faecal matterby means ofpumping or maceration and washingin the flow prior to screening. Alternatively,equipment is available to disintegrate thescreenings in thepresence ofwater, after removalfromthe flow. This will liquefy he faecal matterfacilitating its return to the flow, leavingrelatively cleanscreenings.

45SCREENING

Width ofChannel IOOQV*D*S (3.])

(3.2)

(3.3)

(3.4)for ineperforatedplate screens


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The dryness of the screenings followingdeatering isalsoveryvariable, depending on themachine used and a further consideration is thecompactness of the finished product.3.12.1 HYDRAULIC PRESSIn the hydraulic press, the screenings aredeposited into the pre-pressing/wash stage via ahopper. The faecalmatter is washed out by meansof high pressure water ets and the hydraulic ramthen forces the washed screenings into acompression chamber where a constant pressureis maintained, thereby achieving a high degree ofdewatering. The washed and dewateredscreenings are discharged by means ofdisplacement by freshscreenings deposited in thepress. The mostcommon version ofthehydraulicpress on the Irish market is the Launder FeedPress manufactured by Jones & Attwood (Fig.3.9). This achieves a finished product of

approximately 5O moisture and can acceptscreenings from multiple screens A moreexpensive variation of the LaunderPress is theScreezer which is a dedicated hydraulic ram pressfitted to a vertical drum screen which replacesexistingcomminutors. in the larger sizes (25R.M.and 36R.Mj.3.12.2 SCREW COMPACTORSThere are very many versions of the screwcompactorwith extremely variable performances.Basically, all screw compactors operate in thesame way in that screenings are depositedontothe screw through a feedhopper where washing scarriedoutusingwaterjetswhich mayormaynotbe high pressure units. The screenings areconveyed from the washing area into acompaction area where more drainage occurs(Fig.3.10).

FIGURE 3.9: HYDRAULICSCREENINGSPRESS DETAIL (JONESANDAmVOOD)

ccic xivE

7


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Eo;s1 I __rainage TraySupport Legs'FIGURE3.10: SoLIDWASTE SCREENINGS- TOPFEEDPRESS (JONESANDArrwOoD)

Compactioncan take place either by a springloaded flap or merely by gravity on an inclineddischarge chute. Some screw compactorsincorporate ascrew withadecreasing pitchwhichaids thecompression ofthescreenings.In general, screw compactors do not removemuch ofthe faecalmatter fromthescreenings andthe compacted cake will have a moisture contentofbetween 60% and 70%.3.12.3 WASHER DEWATERERSThese machines incorporate a definitedisintegrating stage which breaks down andwashes out the sewage solids before thecompacting stage takes place. Compaction is bymeans of rollers or a screw compactor. Theversions of this type ofmachine in use in Irelandat present are the Washpactor from Jones &Attwood and the Parkwood WasherlDewaterer.The final productis cleanand inoffensive and hasamoisture contentofapproximately 60% -65%.3.12.4 CENTRIFUGEA screenings centrifuge process currentlyavailable is the Lisep Process (HaighEngineering). Screenings are first collected to amacerating pump sump from where they arepumped with maceration to the Lisep unit. Themaceration process has the effect of liquefyingthe faecal matter and separating it by washingfrom the other materials in a liquid stream. TheLisep device itself can be located eitheradjacent

to the screening installation or remotely in abuilding. The latter is advisable because he finalproduct i

epa water treatment manual preliminary

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