29.6 Construction 29/1029.6.1 Introduction 29/1029.6.2 Renovation of pipelines 29/11
29.7 Maintenance 29/11
Sewage treatment
29.8 Introduction 29/1129.8.1 Characteristics of sewage 29/1129.8.2 Sampling and analysis 29/1129.8.3 Ease of treatment 29/1229.8.4 Possible effects of industrial effluents 29/12
29.9 Effluent disposal 29/1229.9.1 Introduction 29/1229.9.2 Effects of water pollution 29/1229.9.3 Degree of treatment necessary 29/12
29.14 Advanced treatment 29/2029.14.1 Introduction 29/2029.14.2 Chemical coagulation and flocculation 29/2029.14.3 Ammonia stripping 29/2029.14.4 Recarbonation 29/2029.14.5 Granular activated carbon 29/2129.14.6 Membrane processes 29/2129.14.7 Ion exchange 29/21
29.15 Sludge treatment 29/2129.15.1 Introduction 29/2129.15.2 Character and amount of sludge 29/2129.15.3 Screening 29/2229.15.4 Sludge thickening 29/2229.15.5 Anaerobic sludge digestion 29/2329.15.6 Anaerobic sludge digestion 29/2329.15.7 Sludge dewatering 29/2329.15.8 Other sludge treatment processes 29/24
29.16 Sludge disposal 29/24
29.17 Intermediate technology 29/25
References 29/26
SEWERAGE
29.1 Introduction
29.1.1 Sewerage
The function of a sewerage system is to convey domestic andindustrial wastewaters, and runoff from precipitation, safelyand economically to a point of disposal.
Urban areas may be sewered by a combined system, aseparate system, or a partially separate system. In a combinedsystem, which is the most common in Britain, one network ofsewers collects foul sewage and stormwater. In a separate systemtwo sewer networks are used, one for foul sewage and the otherfor stormwater. A partially separate system is a compromiseallowing some of the precipitation, e.g. from the backs ofhouses, to flow into the foul sewer; the second sewer carries therest of the storm water.
29.1.2 Sewage
The term 'sewage' is applied to the contents of sewers carryingthe waterborne wastes of a community. The network of sewersin which the wastes are conveyed is known as the seweragesystem.
Domestic sewage is the discharge from water closets, sinks,baths, and washing machines in offices, schools, homes, factor-ies, etc. Industrial effluent is the waterborne waste of industry.Infiltration is the unintended ingress of groundwater into thesewerage system. Foul sewage is a term commonly used fordomestic sewage, but strictly includes any polluting wastewater,as distinct from stormwater. Storm sewage is foul sewagediluted by stormwater. It will readily be appreciated thatcombined and partially separate sewerage systems, carryingstormwater, must be designed for considerable variations inflow; in consequence it may be necessary to provide storm-sewage overflows as discussed below.
29.1.3 Disposal of stormwater and sewage
Runoff from precipitation, and certain other clean waters, isusually permitted by the pollution-control authorities to bedischarged directly to the nearest watercourse.
Wastewaters collected by sewerage systems are usually deli-vered to a works for treatment before disposal to an appropriatereceiving water. In combined and partially separate systems it isusually possible to limit the amount of wastewater passedforward for full treatment; the excess flow of storm sewage may,before disposal, require a lesser degree of treatment or evennone at all if it has been sufficiently diluted with rainwater. Inthe latter case, separation of storm sewage may be effected at themost appropriate location within the sewerage system, theoverflowed portion of the storm sewage passing directly, or viathe stormwater sewerage network, to an adjacent watercourse.
Storm sewerage is needed to limit physical damage andfinancial loss caused by flooding.
29.1.4 Statutory control
The discharge of wastewaters to surface and undergroundwaters in Britain is governed by Part 2 of the Control ofPollution Act 1974. This calls for the consent of the controllingauthority before any wastewater may be discharged to a receiv-ing water or to a public sewer, or before any change may bemade in an existing discharge.
In England and Wales the control is exercised by the ten waterauthorities. In Scotland, discharge to receiving waters is subjectto the consent of the ten river pollution prevention boards, while
discharge to public sewers comes under the regional and islandcouncils.
Normally, the consent will specify the quantity permitted andits quality. Industrial effluents, only, are subject to consent fordischarge to sewer, and a reception and treatment charge will bemade.
29.2 Design of storm sewers
29.2.1 'The Wallingford procedure'
A manual of practice1 for the design and analysis of urbanstorm-drainage systems was published in 1981. This is known asthe 'Wallingford procedure', and the five volumes not onlydescribe the general procedure and choice of method of analysisand design, but also include maps of Britain with meteorologicaland soil data, and computer programs. In addition, one volumeis devoted to the modified rational method, which is particularlysuitable for small systems (not exceeding 100 to 150 ha in area orwhere pipe sizes are not larger than 600 to 1000mm diameter).
29.2.2 Modified Rational method
This method is a development of the widely used Rational (orLloyd-Davies) method; it gives the peak discharge from theequation:
Q = 2.78CM (29.1)
where Q is the peak discharge in litres per second; C is adimensionless coefficient; / is the average rainfall intensityduring the time of concentration in millimetres per hour; and Ais the contributing catchment area in hectares.
The coefficient C may be regarded as a combination of twoseparate coefficients - for volumetric runoff (Cv) and a dimen-sionless routing coefficient (Cr).
The duration of a storm to give peak rate of flow in the seweris assumed to be equal to the time of concentration of thesystem. This is the sum of the time of entry and time of flowthrough the longest route of the system to the point underconsideration.
In the detailed calculation it is necessary to consider the timeof entry, which may vary from 3 to 10 min, according to size andslope of the catchment, and the severity of the storm. TheManual1 gives values for time of entry which are shown in Table29.1. The smaller values are applicable to subcatchments of lessthan 200 m2 and with slope greater than 1 in 30, whilst the largervalues are for subcatchments greater than 400 m2 with slope lessthan 1 in 50.
Table 29.1 Time of entry
Return period Time of entry(min)Small subcatchments Large subcatchments
5 years 3 62 years 4 71 year 4 81 month 5 10
The time of flow may be determined from pipe-full velocitiesobtained from design tables.2 For the design of new systems,trial determinations are necessary to find the approximate sizeand gradient of pipe or channel, generally at the natural slope ofthe catchment.
The selection of the design return period is an economic,rather than a meteorological decision. Longer return periods
will lead to systems with greater capacities, providing a higherstandard of drainage at greater cost. At one time, design wasfrequently based upon storm-return periods of 1 year; this is stillsatisfactory where surface flooding during storms of greaterseverity is acceptable. Where inhabited basements in buildingsare at risk, a design return period of once in 50 years or evenonce in 100 years should be considered.3
As a first approximation for a 1- and 5-year return period, therainfall intensities (mm/h) in Table 29.2 could be used. Averagerainfall intensities for a specific location in Britain and fordifferent return periods may be obtained from the Meteorologi-cal Office, Bracknell, or may be derived from a simple manualcalculation, which is set out in the appendix to Volume IV of theManual.1
Table 29.2
Time of concentration Rainfall intensity(min) (mm/h)
1-year 5-year
10 35-40 60-6520 23-25 40-4230 18-20 33-35
The volumetric runoff coefficient Cv may be defined as theproportion of the rain falling on the catchment which runs offinto the storm-sewer system. The value is affected by whetherthe whole catchment (impervious and pervious areas) is con-sidered, or the impervious areas alone. As a first approximation,if impervious areas alone are considered, the value of Cv couldbe taken as unity, although actual values may be within therange 0.6 to 0.9.
The routing coefficient (Cr) might be expected to vary with theshape of the catchment but examination of data led to therecommendation1 of a constant value for Cr of 1.3 for bothdesign and simulation.
29.2.3 Pollution from storm runoff
Urban storm runoff will be polluted to a greater or lesser extent.Several comprehensive studies of this pollution have been made,and are referred to in the Manual,1 which includes a summarytable to show the scale of the problem.
Accidental spillage of contaminants, e.g. in a road accident,can cause danger to watercourses, especially since it is commonpractice to remove such spillage by hosing into the surface-waterdrains. Where the result of such an accident can be particularlyserious, e.g. in contaminating a potable water supply, specialprotective measures may be necessary in the drainage design.
29.3 Sewage
29.3.1 Introduction
The various types of sewage have been defined in section 29.1.2above, and their polluting characteristics will be discussed insection 29.8 below. The current section is concerned with thevolumes of flow for which the sewers must be designed, and withmeans for dealing with peak flows. It also looks at design flowsfor the treatment works.
29.3.2 Dry weather flow
The dry weather flow (DWF) is the rate of flow of sewage(together with infiltration if any) in a sewer in dry weather,
usually defined as a period of 5 successive days and nightswithout measurable rain.
Different values of DWF will be obtained in summer andwinter, as a result of changes in infiltration caused by variationin the level of the water table, or domestic holidays, or changesin the industrial pattern of operation.
The DWF of sewage in a sewer, on arriving at a sewagetreatment works, is the sum of domestic flow, infiltration andindustrial flow. Values for the average daily domestic waterconsumption should be ascertained from local records. A typi-cal UK figure is 1851 per head-day but as little as 75 to 1001 perhead day may be appropriate in developing countries, and 400to 5001 per head • day is often consumed in areas such as NorthAmerica where air-conditioning, lawn watering, and automatedcar washes are in wide use.
Values of infiltration are best determined from sewer gaugingat night, when domestic flow is almost zero, and industrialdischarges are also least in number, and thus more readilycalculable or measurable. Typical values might be 15 0001 perday per kilometre of sewer and house connections, for averageconditions (sewer partly above water table and partly below).
Values of industrial discharge should be determined frommetered records, or by reference to agreements with the localauthority.
29.3.3 Storm sewage
Combined and partially separate sewers carry surface water inaddition to the normal foul sewage. These sewers are designedto carry peak flows far in excess of the peak flow of foul sewage,when storms or long periods of heavy rainfall occur.
It is not necessary or economical to treat the full peak flowconveyed by such sewers. Provided the sewage-treatment works,downstream of storm separation (see below), has adequatecapacity to treat fully the maximum contributory rate of flow offoul sewage, without bypassing in dry weather, it has beenfound that the remainder of the peak combined or partiallyseparate flow can be separated using a storm-sewage overflow.
In the past it had been commonly assumed that dilutionduring storm periods would allow the excess storm flow to bebypassed to the nearest watercourse without serious detrimentto its quality. This is not, however, the case and substantialpollution has been produced, not only from floating objectscommonly seen caught by riverside bushes, but also becausepolluting sediments in the sewers are resuspended by the stormflush.
No complete solution has yet been achieved, but a combi-nation of a suitably designed storm overflow structure with astorage basin, from which the first flush can be returned to thefoul sewer for later treatment, has produced an improvement.
For small overflows (0.15 to 0.85 m3/s) storage-type overflowsare suitable; control by throttle pipe and overflow weirs ispreferred. At least manual screening of the overflow should beprovided. For larger overflows only limited storage capacity ispracticable, and design should be concentrated on avoidingoverflow of the first storm flush. Control may be by throttlepipe, orifice, or flow regulator, and mechanically raked screensshould be provided.
The overflow must be set to operate at a predetermined rate offlow, designed to mitigate, so far as possible, the pollutiondischarged with the excess storm sewage. In Britain the overflowsetting (Q 1 per day) is given by the former Ministry of Housing:4
2 = DWF+1360P+2£,
where P is the tributary population and E is the industrialeffluent flow.
29.3.4 Design flow for sewage treatment works
The volume of foul sewage flowing in the sewer, downstream ofthe last storm water overflow, will be approximately 6DWF.Not all of this can be fully treated, if the rainfall continues forlong. It is recommended4 that 3 DWF is fully treated (noallowance being made for an increase of infiltration), theremainder being bypassed to storm tanks to receive gravitysettlement. It is usual, after the storm has ceased, to pump thecontents of the storm tanks back to the works inlet for fulltreatment.
The rate of flow to the treatment works will vary over the day(and also weekly and seasonally, if the proportion of industrialeffluent is substantial).
29.3.5 Pollution load
The inlet works, tanks, pumps, etc. on a sewage treatment worksmust be designed to deal with the design flow discussed in theprevious subsection. In addition, the treatment processes, es-pecially the secondary biological stage, must be designed for thepollution load, which is not necessarily affected by the actualfluid flow in the sewerage system.
It will be seen, from the discussion of sewage characteristics insection 29.8, that the principal parameter of pollution in domes-tic sewage is biochemical oxygen demand (BOD). The BODload may be readily calculated by multiplying the average DWFby the average BOD concentration. In the absence of suitablemeasurements, the values given in Table 29.3 may be used as afirst approximation.
Hydraulic load and pollution load are important concepts inboth the design of process units and treatment works, and in thedetermination of equitable charges to be applied to industrialusers of sewers and sewage-treatment processes.
The balance between industrial and domestic waste will beimportant for any given sewage. Whilst many industrial wastescan readily be treated in admixture with domestic sewage, someindustrial effluents prove difficult in terms of proportion, tem-perature, or BOD: N ratio. The concept of treatability is exa-mined in more detail in section 29.8.
29.4 Design of sewerage systems
29.4.1 Introduction
The design of sewerage systems calls for the optimization of thehydraulic, structural and constructional aspects to suit thedrainage area.
Very many factors affect this design and this section will
concentrate mainly on hydraulic calculations, selection ofmaterials, and structural design of the sewer line.
29.4.2 The Colebrook-White formula
Over past years many formulae have been developed for hy-draulic design of pipes and channels. The equation derived byColebrook in conjunction with White in 1939 is now regarded asthe most satisfactory basis for hydraulic design. The HydraulicsResearch Station at Wallingford has expressed the formula intabular and graphical form more suited to the designer's needs.5
The tables and charts present flow rates (1/s), flow velocities(m/s) and hydraulic gradients for pipe sizes from 0.025 to 2.5 mdiameter, and for roughness factors (&s) from 0.003 to 600 mm.Recommended roughness factors are listed. The tabulated fac-tors 'good', 'normal' and 'poor' relate only to the standard ofuniformity of the surface of the pipeline or conduit when cleanand new (unless otherwise stated). In the case of short pipelines,extra allowances must be made for discontinuities such aschanges in direction, sizes, junctions and valves.
Pipelines and conduits may become fouled if not correctlydesigned and constructed. Physical fouling is caused by settle-ment of particulate matter in the invert; transport of biologicalmatter present in wastewater results in sliming of pipelinesurfaces below water, but both can be significantly reduced bymaintaining high velocities. Grit need not be taken into con-sideration for new designs provided the pipeline has good self-cleansing characteristics.
Storm sewers may normally be considered as being in a cleanstate, whereas foul sewers become slimed, and necessitate theuse of a roughness factor higher than that for storm sewers.Generally, the factors diminish as the velocity increases; thisfeature also applies to sewage rising mains.
Within a treatment works it is usual to assume that the mainflow lines are 'sewers' until after the secondary stage of treat-ment. Gravity and pressure pipelines for sludge are specialcases; friction factors will be dependent upon the characteristicsof the sludge and may be up to 7 times that appropriate forsewage.
29.4.3 Design parameters
Experience has shown that a flow velocity of at least 0.75 m/sonce a day for an hour or so is usually sufficient to keep gravitywastewater sewers clean. Designing for a higher daily peakvelocity will also allow the use of a lower ks factor and hencemake possible decreased pipe size with improved conditions atminimum flow. Typical values for a concrete gravity slimedsewer are given in Table 29.4.
Table 29.4
Velocity Roughness k&(m/s) (mm)
0.5-1.0 6.01.0-1.5 1.5>1.5 0.6
A pumping main always runs full, and flow may be discontin-uous. Thus the cleansing velocity must be regularly achievedand sustained for a period sufficient to scour any settled solids.
Suggested minimum velocities are as shown in Table 29.5.Typical roughness factors for coated steel and iron sewage-
pumping mains are as shown in Table 29.6.
29.4.4 Sewer materials
As usual, the selection of the most appropriate material is acompromise between first cost and service life. The costs ofrelaying, and of the upheaval caused during this process, are,however, so great that the first cost of the material cannot be theprincipal criterion for choice.
The material chosen must resist aggression by the liquid beingcarried (or outside the sewer) and by matter in suspension, andalso by-products of biological degradation (e.g. sulphide). Itmust also be strong enough to withstand the internal andexternal loads. The following materials are in common use.
29.4.4.1 Clay ware
Clay pipes are suitable for nonpressure applications, and are notgenerally available in diameters greater than 1 m. Their chemi-cal inertness fits them for aggressive chemical wastes and sewageat high temperatures. Their main drawback is brittleness.
29.4.4.2 Cementitious
Pipes made from cementitious material are generally robust,reliable and relatively cheap. Unless expensive systems of pro-tection are applied, however, such pipes are vulnerable to attackby sulphate in groundwaters, and to acid attack from industrialeffluents or as a result of bacterial action in septic sewage.
Unreinforced concrete is available up to 1.4 m diameter, andis suitable only for gravity flow. Reinforced concrete pipes arewidely used for gravity sewers in temperate climates, in dia-meters up to 3 m; they can also be used for pressure pipelines upto about 4 bar. Polyvinylchloride liners have been developed toprotect the inner wall from septic sewage. Prestressed concretepipes have been used up to 7 m diameter, and are particularlysuitable for pressure sewers.
Asbestos cement pipes are widely available in diameters up toat least 2.5m, and for pressures up to 32 bar. More recentlyglass and steel fibres have been used to reinforce concrete pipes.
29.4.4.3 Ferrous
Ductile iron is widely used for sewage-pumping mains up to1.6m diameter. Steel pipes are less widely used, but are availablein larger diameters. Their suitability for welding means thatjoints capable of taking tensile loads can be made, making steelpipes suitable for long sea outfalls, river crossings, etc. Corru-gated steel pipes have been widely developed and used in theUS, particularly for storm and surface-water culverts. As com-plete pipes they are available up to 3 m diameter and, in sectionsfor assembly on site, they can be made in spans of 10 m or more.
29.4.4.4 Plastics
There is a major distinction between thermoplastics, whosestrength generally reduces markedly with temperature, andthermosetting resins (normally glass-fibre reinforced), whosestrength falls much less with temperature. Both groups havevery good chemical resistance, although this may be reducedwhen the pipe is stressed or strained.
There are two main groups of thermoplastics: the polyolefins,which include polyethylene (PE), polypropylene (PP) and poly-butylene (PB), and the vinyls, which include polyvinylchloride(PVC) and acrylonitrile butadiene styrene (ABS). Polyethylenepipe is the most widely used of these for sewerage. It is availablein medium and two main high-density forms (MDPE, HDPEland HDPE2). As extruded pipe it is made in diameters up to1.6m, suitable for pressures up to at least 12 bar at 2O0C. Inhelically welded form it is available up to 3m diameter forgravity sewers.
Polypropylene is available up to 1.2m diameter, and forpressures to 15 bar (2O0C). Polybutylene is made up to 600mmdiameter and 17 bar pressure (2O0C). Polypropylene and PBhave better high-temperature properties than PE, and PB isprobably the best of all thermoplastic pipe materials, havingparticularly good high-temperature strength, environmental-stress cracking resistance, abrasion resistance and low creep. Allthe polyolefin plastics can be welded by thermal fusion, makingthem suitable for the pulling of outfalls and river crossings andfor the slip-lining of old pipelines.
Of the vinyl-type thermoplastics PVC, in its unplasticizedform, is the more common. It has been quite widely used sincethe late 1950s and its reputation has sometimes suffered as aresult of its being the prototype for all plastic pipes. As a gravitysewer material, design can be carried out with confidence. Forpressure applications it is important that the pipe should bederated not only for temperature, if appropriate, but also forfatigue effects where the pressure varies cyclically.6 Acrylonitrilebutadiene styrene pipes are available only up to 300 mm dia-meter.
Pitch-fibre pipes, which may be regarded as plastics, arelimited to the even smaller diameter of 200mm. Reinforcedthermosetting resin pipes, variously known as GRP, FRP,RTRP, RPMP, etc. are now available in all sizes up to at least4 m and for pressures up to at least 25 bar. For gravity and low-pressure applications the pipes often contain one or more layersof unreinforced sand and resin (RPM pipes). Pipes containingessentially only resin and glass fibre are known as GRP inBritain and FRP in the US.
29.4.5 Jointing materials
Flexible joints for rigid pipelines normally employ a socket(bell) and spigot arrangement, or a double collar or sleeveassembly. Both these jointing systems rely on an elastomericsealing ring or gasket to ensure watertightness. Natural rubberhas been used successfully for such sealing rings, but, in certaincircumstances, may deteriorate as a result of microbial attack.
Probable particlesize (any pipediameter)
Grit up to 5.0 mmdia.Sand up to 2.5 mmdia.
Table 29.6
Velocity(m/s)
0.8-1.11.2-1.5>1.5
Settlingvelocity(m/s)
1.50
0.45
Pick-up velocity(m/s)
150mmpipe
1.2
0.6
300mmpipe
1.5
0.6
Suggested ks factor(mm)
3.01.50.3
600mmpipe
1.8
0.6
Table 29.5
Work at the Water Research Centre in Britain7 has led toethylene-propylene rubber and styrene-butadiene rubber beingthe preferred materials for ordinary sewer use. Where industrialeffluents are present consideration of other synthetic rubbersmay be necessary, in order to obtain the appropriate chemicalresistance.
29.4.6 Structural design of nonpressure pipes
Flexible joints and uniform beddings are used to minimizelongitudinal bending moments, allowing structural design toconsider only the two-dimensional case of the pipe cross-section. Two sources of loading are considered, that due to thebackfill and that due to any surcharge loads on the surface.Backfill loads on all flexible pipes and on rigid pipes in trenchesnot wider than, say, 1.75 pipe diameters can be taken as theweight of the prism of soil vertically above the pipe. Rigid pipesin wider trenches may experience up to 1.5 times the 'prismload'. Surcharge loads are calculated according to Boussinesq,assuming a pattern of point loads due to vehicle wheels. Figure29.1 shows the pressures at various depths, according to theusual British, American and German assumptions. Consider-able care is required in the selection of factors of safety.8
Rigid pipes, e.g. clay, concrete and asbestos, are specifiedaccording to their crushing strength in a two- or three-edge line-load test. The load distribution and support provided by thebedding of a buried pipe enable it to carry a load greater thanthe test load by a factor (FJ known as the bedding factor. (SeeFigure 29.2 for design values of F1n.) A factor of safety of 1.25 isnormally taken and the enhanced pipe strength (i.e. x Fm) mustprovide this.
Flexible pipes (metal and plastic) should be specified accord-ing to their stiffness (E?/\2D\ where E is Young's modulus, / isthickness of pipe wall, and D is pipe diameter). Table 29.7 listsvalues of the elastic constants for flexible pipes. (It should be
Rigid Pipes
Figure 29.1 Pressures at various depths
noted that the Young's modulus of thermoplastic pipes reduceswith time because of creep.)
The structural design of flexible pipes involves ensuring thatthe pipe neither collapses (by buckling) under the external loadnor deflects to such an extent that it loses too much cross-sectional area, causes its joints to open or is overstressed oroverstrained. Deflection is measured as percentage reduction ofvertical diameter, and 5% deflection is normally regarded as thelimit for loss of area or joint watertightness, with lower limitssometimes being imposed by stress or strain considerations. Asfor rigid pipes, the load-carrying capacity of flexible pipes is alsoinfluenced by the bedding. In this case it is the deformationmodulus E of the bedding, in resisting the outward deflection of
US Railway (Cooper E-72)UK main road (BS 153 Type HB)German main road (DIN 1072, SLW 60)US road (H-20)
300 mm
Concrete bedsGranular bedsTrench-bottom beds
Flexible Pipes
Filter fabric Rigid insert Trench sheeting left in
*ring tension fing bending tfcombined tension and bending
the sides of the pipe, which is used. Deflection is calculated asfollows:
Relative deflection (%) = F01 (g/fSg-) (29.2)
where FDL is deflection lag factor (increase in deflection withtime - see Figure 29.3), PE is the external pressure (backfill-+ surcharge) and 5p the pipe stiffness, as from Table 29.7.
The deflections corresponding to stress or strain limits for thepipe material are calculated as follows:
strain-limited deflection = eLD/Fct (29.3)
where eL is the limiting strain and FG the strain factor, whichtakes account of the geometry of the distortion, and for which avalue of 6.0 can be taken for design purposes. Where stresslimits apply, fJE may be substituted for eL in Equation (29.3),/Lbeing the limiting stress.
Design of flexible pipes to resist buckling involves ensuringthat the critical pressure (PCR) which will cause the buried pipeto buckle, exceeds the actual external loading pressure by asuitable factor of safety.
PCR = (V32£*Sp) (1 - 3 x relative deflection) (29.4)
The value of relative deflection inserted in Equation (29.4)should be that calculated according to Equation (29.2).
29.4.7 Structural design of pressure pipes
The circumferential tensile stresses set up in the pipe wall by thepressure within the pipe reduce the effective strength available toresist the external loads. With rigid pressure pipes the value ofthe crushing strength must be increased by a factor Fp given byEquation (29.5):
F9=II(I-PJPJT (29.5)
where P1 is the design internal pressure, Pu is the ultimatepressure capacity of the pipe, and n is 1 for reinforced concretepipes, 1/2 for asbestos-cement pressure pipes, 1/3 for prestressedconcrete pipes.
When designing flexible pressure pipes the stress or strain in thepipe wall, produced by the internal pressure, is added to thestress or strain induced by deflection, as indicated above. Suchtotal stress or strain must not exceed the limit given in Table29.7.
Where flexible pressure pipes may be subjected to sub-atmospheric pressures, e.g. during surging following pumpshutdown, the vacuum pressure should be added to the externalpressure and the factor of safety against buckling recheckedusing Equation (29.4) above.
29.4.8 River crossings and submerged outfalls
For these types of installations the stresses and strains in thecompleted pipeline are seldom great. Because, however, they areoften constructed by assembling long strings of pipes on landand then towing or pulling these into position, high stresses andstrains may be set up during construction. These stresses andstrains are likely to be due either to direct tension or tocurvature of the pipeline as it passes over supports. The tensileloading depends on the pulling force involved in the particularmethod of construction. Thus, a maximum pulling load may becalculated, for given pipe properties, and this must be specifiedas not to be exceeded in construction. Curvature of the pipelinemay occur both during construction and in its final position.The radius of curvature should not be less than a critical value,which will be governed by stress, strain or buckling.
29.4.9 Ancillary structures
Other than pumping stations, which are dealt with elsewhere,the structures associated with underground drainage systemsinclude manholes, drop chambers and storm water overflows.Sizing of these structures is controlled by their hydraulic design,and the need to provide adequate access for maintenance. Sincethey are often constructed below ground water level, concrete isusually required to overcome buoyancy, and thus becomes thebasic structural material. It should be noted that, in hotclimates, or where pumping mains discharge into manholes,hydrogen sulphide is often released, and severe corrosion ofconcrete manholes and chambers can occur. In these casesprotective systems are required - either coatings applied to theconcrete in situ or prefabricated linings such as PVC or GRP.Alternatively, chemical injection can be used to control thegeneration of sulphides.9
On pumping mains themselves, access chambers are requiredat air valves and washouts. Again, the possibility of hydrogensulphide corrosion should be considered. Pumping mainsshould also be provided with means to resist the thrusts gener-ated at changes of direction. Where flanged or welded joints areemployed it may be possible for the thrusts to be resisted by thetensile-load capacity of the pipe. If this is not feasible, thrustblocks should be provided to transmit the thrust to a satisfac-tory foundation, e.g. the undisturbed ground at the side of atrench.10
Both gravity and pressure mains should be provided withanchorages, if laid to steep gradients, in order to preventgradual sliding, permitted by closure of the joint gaps in thelower portion, leading to the disengagement of joints in theupper portion of the pipeline.
Figure 29.3 Modulus E'
29.5 Pumping sewage
29.5.1 Introduction
Pumping sewage presents a particular problem in the need tohandle the solids contained therein. It is common practice toassume that the smallest sewer within a sewerage system is100mm diameter and therefore may pass solids of almost thissize. Therefore pumps are specified as being capable of passing a90-mm diameter sphere, and the inlet and discharge connectionsmust not be less than 100mm diameter, and this is true forpumping mains. This limitation precludes satisfactory pumpingat rates below about 151/s.
The need to handle solids also dictates that end-suctionsingle-stage pumps are used, thus limiting the possible head thatcan be generated to about 75 m.
Sewage pumps are normally centrifugal or mixed-flowmachines. In smaller sizes, submersible sewage pumps aremanufactured. These pumps have a close-coupled, fully submer-sible electric motor fitted to the pump and are designed to belowered into the sewage.
If the flow rate required falls below 151/s, special devices arerequired. Various manufacturers can supply these and theydepend either on comminuting the solids or on some method ofhandling solids without passing them through a pump (e.g.solids diverter (R) or compressed-air ejector).
For lifting duties at sewage works large Archimedean screwsare being increasingly used. These devices are suitable for liftingsewage, but not for feeding into pumping mains under pressure.
Sewage sludges are of various consistencies. The ability ofcentrifugal sewage pumps to be used satisfactorily or the need touse positive displacement pumps are covered in a publicationissued by the Water Research Centre.11
29.5.2 Sewage pumping stations
Sewage and drainage installations differ from almost all othersin one important point. This is that once the installation hasbeen commissioned it is virtually impossible to close it down;sewage continues to flow in the sewers. All sewage installationsmust be designed with this in mind, particularly pumpingstations. For whilst it may be possible to bypass a part of the
treatment process, it may be essential to continue pumping in allcircumstances.
There are two facets of the need to maintain a pumpingstation so that it is continuously available for service or running.The first is that it should be possible for all routine maintenance,including major overhauls, to be carried out with the stationoperating, and the second is that machine breakdowns or othersimilar circumstances should be 'fail-safe'. The various ways ofmeeting these requirements underlie the remainder of thissection.
Sewage-pumping stations are almost always equipped withelectrically driven automatic pumps, operated from level-mea-suring devices or switches in the reception sump, enabling themto operate without full-time pump attendants.
The sump should be designed to allow easy flow to the pumpsuctions, and with sufficient benching to avoid undue settlementof solids.12 In major installations, two interconnecting sumps areoften provided to enable either to be drained for cleaning ormaintenance, without closing down the installation. Some in-takes to sumps are fitted with screens. However, there areconflicting views on the fitting of screens. If screens are fittedthere is the need to dispose of screenings; if they are not disposedof there is a risk of items reaching the pumps and blocking ordamaging them. Current practice may follow either view.
Major pumping stations are normally designed to be similarto that shown in Figure 29.4. Of particular note as being currentgood practice are the following features:
(1) The pump casings are below the invert of the incomingsewer, thus ensuring that the pumps require no specialpriming equipment.
(2) The nonreturn valves and the entries to the rising main areboth horizontal, thus avoiding some of the problems causedby deposition of solids when the pumps are not running.
(3) The electrical equipment is at a high level, thus obviatingdamage in the event of the pump well being flooded. It isalso normal to fit automatic cellar-drainage pumps to thiswell.
(4) Sufficient access and cranage is provided to ease mainten-ance as far as possible. Most designs provide access stairs tothe pump well rather than ladders to encourage mainten-ance staff to inspect the machinery regularly.
Native Soil Modulus £s
Bedding Modulus E0DIn terms of depth in metres (H)Overall Modulus £'
MaterialGravel
Coarsesand
Finesand
Material Peat Clay Silt Sand Gravel Rock
Compaction
KEY(1) Pump (8) Overflow(2) Nonreturn valve (9) Electric motor(3) Isolating valves (10) Switchboard(4) Rising main OD Overhead crane(5) Drainage channel (12) Sewer inlet(6) Air release pipework (13) Machinery access cover(7) Intermediate shaft support
Figure 29.4 Pumping station
The switchboard should be of such a design that individualpump starters, controls, etc. can be isolated for maintenance,whilst the board is live, and other pumps are running oravailable for service.
The ability to continue to pump or bypass sewage under allcircumstances is normally provided by several features. At leastone standby pump will be provided, and will have suitableautomatic controls for it to take over the duties of any pumpwhich fails from whatever cause. The number of pumps pro-vided will depend on the expected flow variation, length ofpumping main, lift and other similar design parameters.
The electricity supply may need to be secured, either byduplicating the connections to the public supply, or by provid-ing standby generating plant within the pumping station, orboth.
Despite these precautions it is wise to provide a high-leveloverflow to avoid flooding if there is a total breakdown.
The design of stations using close-coupled submersible sew-age pumps is similar but normally rather simpler. It is notnecessary to provide a building, so long as there is good accessto the well containing the pumps, and the electrical switchgear ishoused in a suitable weatherproof kiosk.
29.5.3 Rising mains
Sewage-pumping mains differ from those containing most other
fluids in that whenever the sewage therein becomes stationary,or falls to a low velocity, deposition of the solids will occur. Toensure that this deposition is not cumulative, it is good practiceto design the pipeline so that a velocity at which solids arepicked up is achieved on some occasion daily. This velocityshould be at least 1 m/s.
As in other systems, hydraulic surge will occur whenever thevelocity in a sewage pipeline is changed. Suitable precautionsshould be taken to ensure that this surge does not generate apressure which is likely to cause damage. The velocity of suchhydraulic surges is materially lowered by any dissolved gases inthe fluid;13 sewage normally contains gases. However, since onsome occasions the system might be filled with water, theprecautions taken should be effective with surge velocities bothfor water and for sewage.
29.6 Construction
29.6.1 IntroductionConstruction should aim to achieve the design objectives withthe greatest economy. The choice of materials may makediffering demands on the installation costs of buried pipelines.The achievement of the necessary pipe-bedding standard (seeFigure 29.2) is crucial and, because of the greater dependence offlexible pipes on their bedding, may invalidate cost comparisonsbased on material prices only. The influence of trench width andnative ground conditions on the design of flexible pipelines isignored in many published 'design methods'. The informationprovided in Figure 29.2 is intended to remedy this situation, andshows how trench width, bedding material and its degree ofcompaction must be considered together. The data relatingbedding moduli to the degree of compaction applied to variousmaterials are based on empirical relationships obtained from aconsensus of various published sources.
Gravel beds are frequently preferred because they can achieve90 to 95% MPD with minimal compaction. Where gravels areused they must be prevented from acting as groundwater drains,by the use of regular impermeable barriers, e.g. polyethylenesheeting. In some cases, e.g. where the native soil is fine sand orsilt, it may be necessary to enclose the whole of the bedding in animpermeable membrane or filter fabric to prevent groundwaterflows leaching out fine material and forming voids at the trenchside. A high-modulus bedding material may, however, still notachieve a high overall modulus if the native soil is soft. The useof wide trenches will improve this, but may be impracticable forlarge-diameter pipes, in which case resort should be made to oneof the special beddings.
For all types of pipeline the provision of uniform support bythe bedding is essential to prevent the development of unaccep-tably high shear forces or longitudinal bending moments. Wherethis cannot be achieved, e.g. where pipes are built into the wallsof underground structures, in areas of mining subsidence orabrupt transitions from rock to soil, closely spaced mechanicaljoints should be specified. Flexible pipes are often supplied inlong lengths, very long in the cases of welded steel or polyole-fins, and the basic flexibility of the pipe may not be sufficient toaccommodate differential settlements without the use of suchflexible joints.
Careful attention must be given to the manner of joininglateral sewers and house connections to main sewers.14 TheConstruction Industry Research and Information Associationhas recently issued an authoritative report on trenching.15
In the construction of above-ground pipelines, proper con-sideration must be given to possible thermal movements, and toeven load distribution at supports.
The construction of river and estuarine crossings, and ofsubmerged outfalls, favours the use of materials which can be
joined into long strings on land and then pulled into position.Steel and polyolefin plastics with welded joints are thereforeoften used. Glass reinforced plastic pipes, with hand lay-upoverwrap joints, have also been successfully used in this manner,as also has prestressed concrete.
29.6.2 Renovation of pipelines
The construction of new underground pipelines by trenching isexpensive in established urban areas, not only in direct cost butalso in the indirect costs of disruption. This has encourageddevelopment of so-called 'nondisruptive' construction methods.The techniques which have received most attention are minia-turized tunnelling, and sewer renovation.
Quite apart from the fact that many old urban sewers requirerenovation because they are structurally unsafe, constructionbased on renovation of the old sewer has the additional advan-tages that a pipeline route clear of other underground services isautomatically provided, and also that all lateral connections areautomatically located.
Renovation techniques have been reviewed extensively by theWater Research Centre,16 which has adopted the followingsystem of categorization:
Type 1: Included in this category are lining systems which arebonded to the fabric of the old sewer so as to form a compositerigid structure. Examples are glass reinforced concrete segmen-tal linings, and GRP segmented, or complete pipe, liningsroughened to provide the required bond.
Type 2: Lining systems in this category do not rely on theformation of a bond to the old sewer structure. The liner usuallyconsists of a polyethylene pipe inserted by sliplining, a rein-forced thermosetting resin liner, installed by inversion and curedin situ, or a plastic pipe liner formed by individual insertion ofGRP or polyolefin pipes. As with Type 1 linings, the annulus isgrouted, but no reliance is placed upon the formation of a bond,so that the liner is regarded as acting as a flexible pipe.
Type 3: Linings of this type, thin-walled GRP or in situ resin, arenot regarded as fulfilling any permanent structural role. Rather,they are considered as formwork left-in, with the annulus groutproviding the structural element.
Most of the renovation systems result in lateral connectionsbeing temporarily blocked, but several ingenious methods ofreopening laterals have been developed, e.g. by remote cuttingfrom the sewer, by remote cutting from the lateral or by'minimum excavation' techniques from the surface.
Pipe 'renovation' has developed to the stage that one tech-nique, polyethylene pipe sliplining, can provide an increase inthe diameter of the sewer. In a 1985 example, a 225mmdiameter clay pipe was 'relined' with a 350mm diameterpolyethylene pipe, inserted behind an impact mole which splitthe old pipe.
29.7 Maintenance
Given good design and construction, the correct choice ofmaterials should reduce sewer maintenance to the clearance ofoccasional blockages. Although good hydraulic design shouldminimize blockages, the fact that they may never be completelyavoided requires proper provision for maintenance to be in-cluded in the design. Easy and safe access for men and equip-ment is essential, but further consideration should be given tothe possible need to extricate injured workers. Thus space for atleast two men should be provided in all manholes, together withopenings permitting unobstructed lifts to the surface.
Since access arrangements in deep manholes should precludethe possibility of long falls, intermediate platforms are required.Since such platforms might interrupt full height vertical lifting,manholes on deep sewers should preferably have two surfaceaccess openings.
It is essential to ensure proper ventilation of sewer systems tominimize the generation of hydrogen sulphide and to dispose ofthis and other toxic gases.
SEWAGE TREATMENT
29.8 Introduction
29.8.1 Characteristics of sewage
Municipal sewage is mainly the wastewaters from homes, officesand shops, and, therefore, consists of human wastes and of thedischarges of man's domestic activities. Many industries uselarge quantities of water, which must also be disposed of afteruse. In industrialized countries, a very large proportion of theirindustrial effluents is discharged to the municipal sewers, andtreated with the domestic sewage; this may demand somepretreatment to ensure that it does not interfere with the normaltreatment process (especially the biological stage) or with thedisposal of sludge.
A partial analysis of a typical British domestic sewage is givenin Table 29.3. (The strength of sewage depends somewhat on thediet and other living habits of the contributory population, andmarkedly on the quantity of water used.) If this were to bedischarged to an inland stream in quantity it would causesubstantial pollution. The principal polluting matters in sewageare suspended solids (SS) and organic matter.
The suspended solids would be unsightly, and, being at leastpartly organic, would reduce the dissolved oxygen in the receiv-ing water.
The organic matter is partly carbonaceous and partly ni-trogenous. Both are oxidized by naturally occurring microorga-nisms in the receiving water, and so reduce the dissolved oxygen,which is essential for fish and other animal life in the water.Since we are primarily concerned with oxygen demand, carbo-naceous organic matter is normally measured as biochemicaloxygen demand (BOD), the oxygen consumed in 5 days at 2O0Cby microorganisms consuming the organic matter, or as chemi-cal oxygen demand (COD), a purely chemical parameter whichapproximates to the ultimate oxygen demand.
Nitrogen is commonly analysed in its various forms, and weare mainly concerned with ammoniacal nitrogen. This will beoxidised in the receiving water, and so will increase the oxygendemand. At high pH values, ammonia can also be poisonous tofish.
It has been found, in Britain, that each person contributes thefollowing pollution loads (grams per day): BOD 60, suspendedsolids 60, ammoniacal nitrogen 8. In the absence of morespecific data, these values may be used to assess the pollutionload to be removed in sewage treatment.
29.8.2 Sampling and analysis
Before selecting the method of disposal, and the appropriatetreatment of the sewage to permit disposal without pollution, itis necessary to sample and analyse the sewage.
Sampling should usually be carried out over the full 24 h,since flow varies greatly over the day, and the individual samplesmust be bulked in such a way as to give a properly weightedrepresentative sample. It is desirable that sampling should becarried out over the various seasons and in a range of weatherconditions, but this is often not possible.
The analyses should be carried out by the standardizedmethods,17'18 which have been laid down in the UK and US, andwhich are generally used throughout the world.
Although suspended solids, oxygen demand and ammoniaare the most important design parameters, it is essential, in thesepreliminary analyses, to seek also a wide range of substancesthat might cause danger or damage to sewer workers, to thesewerage system or to the treatment processes, and to ensurethat these are at acceptable levels.
For operational control the sewage works operator willanalyse routinely (probably daily on the larger works) for SS,COD, and ammoniacal and oxidized nitrogen in the raw sewageand in the effluent after various stages of treatment. (Although ittells little about the biological effect of organic matter, COD isroutinely used in place of BOD because the analysis for CODcan be carried out in about 2 h as compared to 5 days for BOD.)
29.8.3 Ease of treatmentThe final method of disposal, and the proper degree of treat-ment to allow this without environmental nuisance, is discussedin the next section. It is important, at an early stage, to be able toestimate the ease, or otherwise, with which the pollution can beremoved.
Generally, much of the suspended solids in domestic sewagecan be readily removed by gravity (sedimentation); in thisprocess some 60% of the SS may be removed, with a consequentreduction in the BOD of about 30%.
In normal sewage treatment the majority of the organicmatter is removed from the aqueous stream by biological meansduring the secondary stage (see section 29.12). It is essential,therefore, to be able to assess the ease with which the organicmatter can be oxidized, and to estimate the possible interferenceof toxic and other substances with the biological oxidation.
Actual tests of the 'treatability' of an effluent may be made bybench- or pilot-scale tests, the latter being the more reliable.Often, however, such tests are not possible, and judgement mustbe based on experience.
A crude estimate of 'treatability' in relation to a strictlydomestic sewage may be obtained from the CODtBOD ratio.For raw domestic sewage this ratio is usually about 2; theorganic matter in a wastewater will be more easily degradedbiologically if this ratio is less than 2, and less easily brokendown the more the ratio exceeds 2. The most easily degradedsubstances are broken down first and, in consequence, theCOD: BOD ratio of the aqueous stream increases as it passesthrough biological treatment.
29.8.4 Possible effects of industrial effluents
Many industries concerned with the manufacture and process-ing of food and drink use great quantities of water, and producecorrespondingly large quantities of effluent. The pollution car-ried by such an effluent is of much the same nature as domesticsewage (mainly organic matter), and can also be treated bybiological processes, some being more suitable than others. It isfrequently very much 'stronger' than sewage, and due allowancemust be made in design and operation.
Other industries discharge a very wide range of substancesinto their effluents, and many of these can interfere withtreatment processes. In such cases it is essential to pretreat theindustrial effluent to remove or neutralize the interfering sub-stance. The treatment of industrial effluents is far too extensive atopic to treat in an introductory chapter, and reference shouldbe made to recent books.19
Discharge of dangerous or inhibitory wastes is controlled bylocal ordinance, and local regulatory authorities have widepowers in respect of consent to discharge, inspection, with-
drawal of consent, or the penalizing of offenders. Discharge tosewers of petrol or cyanides, for example, is forbidden for safetyreasons.
Discharge of inhibitory matter, such as certain metallic ionsor phenol, has to be very closely controlled if treatment pro-cesses are not to be upset, and watercourses put at risk bycontamination. Trade-waste control is thus an extremely im-portant factor in the day-to-day operation of sewers and sewagetreatment works.
29.9 Effluent disposal
29.9.1 Introduction
A wastewater can be finally 'disposed of only into water, on toland and into the ground. The last of these is available inpractice only for small quantities of hazardous materials thatcannot be safely dealt with in any other way, e.g. into worked-out salt mines.
Until the early years of this century, discharge on to land(sewage farming) was the only method in Britain acceptable tothe Local Government Board. Generally, now, however, sewagefarming is of no more than historic interest, although theirrigation of growing crops with fully treated sewage is of greatinterest in parts of the world where water is short.
This chapter is therefore largely concerned with the disposalof sewage into surface waters, and this section briefly considersthe degree of treatment necessary to avoid danger, damage ornuisance resulting from such disposal.
29.9.2 Effects of water pollution
Wastewaters may contain substances which are poisonous toman and to plant and animal life in the water; sewage ought notto hold such substances, and it is essential to ensure that toxicmatters are not allowed to enter municipal sewers from indus-try.
More important for municipal sewage are its power to use upthe small amount of oxygen dissolved in the water, as theorganic matter and nitrogen are oxidized by aqueous microor-ganisms, and the possible aesthetic effect of floating and sus-pended substances.
Nitrogen is of significance in a number of ways. Organic andammoniacal nitrogen are oxidized in the receiving water, and soalso use up dissolved oxygen. The fully oxidized form (nitrate)in sufficiently high concentrations may cause methaemoglobi-naemia in very young infants. Above all, nitrogen is a plantfertilizer, and its presence in water, especially in standing bodiesof water, may promote undesirable weed growth.
Sewage will, of course, also contain faecal microorganisms,and some of these may be pathogenic. In urban, industrializedcommunities the main protection against waterborne diseases iswater treatment, with its accompanying disinfection. Neverthe-less, as will be seen from Table 29.8, full normal sewagetreatment reduces sewage bacteria very substantially.
It is usual to treat sewage to remove, partly or fully, sus-pended solids, oxygen demand (measured as BOD or COD - seesection 29.8) and nitrogen, to prevent the kinds of pollutingeffects mentioned in previous paragraphs.
29.9.3 Degree of treatment necessary
The effects of pollution outlined in the previous section areclosely related to the dilution available for the effluent dis-charged. The lower the dilution the greater will be the damagecaused. For this reason it is usual to prescribe the quality of
Table 29.8 Removal efficiencies of sewage treatment
sludge +final sed. 85-95 80-95 90-98Chlorination following full
biological treatment 98-99
effluent required for discharge to various types of receivingwater; some suggested values are shown in Table 29.9.
The various methods of treatment that may be given tomunicipal sewage are indicated diagrammatically in Figure29.5. The degree of removal that may be achieved by variouscombinations of treatment process is shown in Table 29.10.
It may be seen, by considering Tables 29.9 and 29.10 inconjunction, that full primary, biological and final treatmentwill be necessary for discharge to inland rivers; nitrificationfollowed by denitrification may be required, in addition, for lowdilutions. Equally effective treatment is likely also to be neces-sary for discharge to lakes, together with, in some cases,removal of the other important plant nutrient, phosphorus.Preliminary treatment alone is likely to be sufficient for oceandischarge, although it may sometimes be desirable also toprovide primary settlement.
During full treatment not more than about one-third of theincoming pollution is converted to relatively harmless sub-stances; the rest remains on the treatment works as solids fordisposal (sludge). This is a major problem and expense insewage treatment, and is discussed in sections 29.15 and 29.16.
29.10 Preliminary treatment
29.10.1 Introduction
The principal objective of preliminary treatment is to protectsubsequent treatment processes, by preventing blockage and
Table 29.9 Typical concentrations of pollution for discharge tovarious receiving waters (all values in mg/l, except pH)
Parameter Inland River dilution Estuary Open sea2
dilution more than 81
less than8l
BOD 10 20 150 -3
SS 15 30 200 2504
Ammoniacalnitrogen 10 — —pH 5-9 5-9 5-9
Notes:(1) With clean water.(2) The ocean outfall must be carried sufficiently far out to sea to ensure that
pollution is not brought back to the bathing beaches, etc. The end of theoutfall must be provided with a properly designed diffuser and be located in asufficient depth of water to ensure thorough mixing and dilution before theeffluent reaches the surface.
(3) Not usual to specify a BOD limit.(4) A SS standard is not always specified, but it is always desirable that floating
matter should be reduced to a practicable minimum.
Table 29.10 Treatment of organic wastewaters
BOD Method BOD loading(mg/l)
< 500 Single filtration or 0.1 kg/m3-dayactivated sludge 0.2 kg/kg MLSS-day
500 + Filtration with 0.15 kg/m3-dayrecirculation or 0.2 kg/m3-dayalternating doublefiltration
1000 Extended aeration 0.05-0.15 kg/kgMLSS-day
1000-1500 High-rate filtration (with Up to 5 kg/m3-dayrecirculation)followed bypercolating filters or 0.1 kg/m3-dayADF or 0.2 kg/m3-dayactivated sludge 0.2 kg/kg MLSS-day
1500+ Anaerobic treatment l-5kg/m3-dayfollowed by one or two (depending onstages of aerobic degree oftreatment removal)
Any Oxidation anaerobicponds first stage 7000 kg/ha-day(multi- aerobic finalstage) stage 250 kg/ha-day
damage to the plant, and to increase the reliability and efficiencyof the treatment process. Those objectives are achieved byremoval of large solids by screening, removal of grit, removal ofoil and grease, balancing of flow and/or load, pH control, andnutrient addition.
29.10.2 Screening
The quantity and nature of screenings vary, often substantially,between one plant and another. Relevant factors are socialconditions and habits, industrial contributions, the type ofsewerage system and the design of the screening plant. Thefollowing guidelines may be used to estimate quantities wherethere is no previous experience of local conditions.
The volume of screenings depends more on the character ofthe sewage than on the bar spacing; average volumes fordomestic sewage are in the range 1 to 3 m3 per day per 100 000population. Where there is an industrial effluent, the nature ofthe industry may suggest the additional screening load. Thepeak hourly rate of screenings removed is likely to be 4 to 6times the average, and with combined sewers the peak rateduring a storm following a dry period may be 10 to 20 times theaverage.
There are two basic approaches to handling large solids:
(1) To comminute in the flow or to remove, disintegrate andreturn to flow.
(2) To remove from the flow and dispose of elsewhere.
Comminutors are clean, innoffensive and relatively trouble-freemachines, which are normally left unattended. However, ragstend to be shredded rather than cut up and may 'ball up' in latertreatment stages, and scum volumes are increased by comminu-tion. Similar problems are encountered with disintegrators.
The alternative of permanent removal and separate disposalof screenings is preferable in relation to the operation of the
