29.Sewerage and Sewage Disposal

7/30/2019 29.Sewerage and Sewage Disposal

1/13

This page has been reformatted by Knovel to provide easier navigation.

Sewerage andSewage Disposal29Staff of Watson Hawksley, ConsultingEngineers

Contents

Sewerage

29.1 Introduction 29/3

29.1.1 Sewerage 29/3

29.1.2 Sewage 29/329.1.3 Disposal of stormwater and sewage 29/3

29.1.4 Statutory control 29/3

29.2 Design of storm sewers 29/3

29.2.1 The Wallingford procedure 29/3

29.2.2 Modified Rational method 29/3

29.2.3 Pollution from storm runoff 29/4

29.3 Sewage 29/4

29.3.1 Introduction 29/4

29.3.2 Dry weather flow 29/4

29.3.3 Storm sewage 29/429.3.4 Design flow for sewage treatment works 29/5

29.3.5 Pollution load 29/5

29.4 Design of sewerage systems 29/5


29.4.2 The Colebrook-White formula 29/5

29.4.3 Design parameters 29/5

29.4.4 Sewer materials 29/6

29.4.5 Jointing materials 29/6

29.4.6 Structural design of nonpressure pipes 29/7

29.4.7 Structural design of pressure pipes 29/8

29.4.8 River crossings and submerged outfalls 29/8

29.4.9 Ancillary structures 29/8

29.5 Pumping sewage 29/9


29.5.2 Sewage pumping stations 29/9

29.5.3 Rising mains 29/10

29.6 Construction 29/10


29.6.2 Renovation of pipelines 29/11

29.7 Maintenance 29/11

Sewage treatment

29.8 Introduction 29/1129.8.1 Characteristics of sewage 29/11

29.8.2 Sampling and analysis 29/11

29.8.3 Ease of treatment 29/12

29.8.4 Possible effects of industrial effluents 29/12

29.9 Effluent disposal 29/12


29.9.2 Effects of water pollution 29/12

29.9.3 Degree of treatment necessary 29/12

29.10 Preliminary treatment 29/13

29.10.1 Introduction 29/1329.10.2 Screening 29/13

29.10.3 Grit removal 29/14

29.10.4 Skimming, flocculation and preaeration 29/14

29.10.5 Flow/load balancing 29/14

29.10.6 pH control 29/14

29.10.7 Nutrient addition 29/15

29.11 Primary treatment 29/15

29.11.1 Sedimentation 29/15

29.11.2 Chemical treatment 29/15

29.11.3 Flotation 29/16

29.11.4 Septic tanks 29/16

29.12 Biological treatment 29/16


29.12.2 Percolating filters 29/17

29.12.3 Rotating biological contactors 29/17

29.12.4 Activated sludge 29/18

29.12.5 Oxidation ponds 29/19

29.12.6 Anaerobic treatment 29/19

29.12.7 Fluidized beds 29/19

29.12.8 Final sedimentation 29/19
http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/


2/13

This page has been reformatted by Knovel to provide easier navigation.

29.13 Tertiary treatment 29/19


29.13.2 Sand filters 29/19

29.13.3 Upward flow clarifiers 29/19

29.13.4 Microstrainers 29/20

29.13.5 Lagoons 29/20

29.13.6 Irrigation over grassland 29/20

29.13.7 Disinfection 29/20

29.14 Advanced treatment 29/20


29.14.2 Chemical coagulation and flocculation 29/20

29.14.3 Ammonia stripping 29/20

29.14.4 Recarbonation 29/20

29.14.5 Granular activated carbon 29/21

29.14.6 Membrane processes 29/21

29.14.7 Ion exchange 29/21

29.15 Sludge treatment 29/21


29.15.2 Character and amount of sludge 29/21

29.15.3 Screening 29/22

29.15.4 Sludge thickening 29/22

29.15.5 Anaerobic sludge digestion 29/23

29.15.6 Anaerobic sludge digestion 29/23

29.15.7 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
http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/http://19643_29b.pdf/


3/13

SEWERAGE29.1 Introduction29.1.1 SewerageThe function of a sewerage system is to convey domestic an dindustrial wastewaters, and runoff from precipitation, safelyan d 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 an d stormwater. In a separate systemtw o sewer netwo rks ar e 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 SewageThe term 'sewage' is applied to the contents of sewers carryin gthe waterborne wastes of a communi ty . 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-i e s , 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 fo rdomestic sewage, bu t strictly includes an y 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 sewageRunoff from precipitation, and certain other clean waters, isusually permitted by the pollution-control authorities to bedischarged d irectly to the nearest watercourse.Wastewaters collected by sewerage systems are usually deli-vered to a work s for treatment before disposal to an appro priatereceiving water. In combined and partially separate systems it isusually possible to limit th e a m oun t of wastewater passedforward fo r 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 th e sewerage system, theoverflowed port ion 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 controlThe discharge of wastewaters to surface and undergroundwaters in Britain is governed by Part 2 of the Control ofPollution Act 1974. This calls for the consen t of the con trollin gauthori ty before an y wastewater may be discharged to a receiv-in g water or to a public sewer, or before any change may bemade in an existing discharge.In En gland and Wales the control is exercised by the ten waterauthorities. In S cotland, discharge to receiving waters is subjectto the consent of the ten river pollution prev ention boards, w hile

discharge to public sewers comes under th e regional an d islandcouncils.Normally, the consent will specify the quan tity permitted an dits quality. Industrial effluents, only, ar e subject to consent fo rdischarge to sewer, and a reception an d treatment charge will bemade.29.2 Design of storm sewers29.2.1 'The Wallingford procedure'A manual of practice1 for the design and analysis of urbanstorm-drainage systems was published in 1981. This is kno wn asth e 'Wallingford procedure', and the five volumes no t onlydescribe the general procedure an d choice of method of analysisan d design, bu t also include maps of Britain with meteorologicaland soil data, an d computer programs. In addition, one volumeis devoted to the mo dified rational m ethod, which is particularlysuitable fo r 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 methodThis method is a development of the widely used Rational (o rLloyd-Davies) method; it gives the peak discharge from theequation:

Q = 2.78CM (29.1)where Q is the peak discharge in litres pe r second; C is adimensionless coefficient; / is the average rainfall intensityduring th e time of concentration in millimetres pe r hour; an d Ais the contributing catchment area in hectares.

The coefficient C may be regarded as a combination of twoseparate coefficients - for volumetric runoff (C v) and a dimen-sionless routing coefficient (C r).The du ration 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 th e 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 m in, according to size andslope of the catchment, and the severity of the storm. TheManual1 gives values for time of entry w hich are shown in Table29.1. The smaller values are applicable to subcatchments of lessthan 2 00 m 2 and with slope g reater than 1 in 30, whilst the largervalues are for subcatchments greater than 40 0 m 2 with slope lessthan 1 in 50.T a b l e 2 9 . 1 T i m e o f e n t r yReturn period Time of entry(min)Sma ll subcatchments Large subcatchments5 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 ar e necessary to find the approximate sizean d 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


4/13

will lead to systems with greater capacities, providing a higherstandard of drainage at greater cost. At one time, design wasfrequently based upo n sto rm- 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.3As 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-ca l Office, Bracknell, or may be derived from a simple manualcalculation, which is set out in the appendix to Volume IV of theManual.1

T a b l e 2 9 . 2Time of concentration Rainfall intensity(min) (mm/h)1-year 5-year10 35-40 60-6520 23-25 40-4230 18-20 33-35

Th e volumetric runoff coefficient Cv may be defined as theproportion of the rain falling on the catchment which runs of finto the storm-sewer system. The value is affected by whetherth e whole catchment (impervious an d pervious areas) is con-sidered, or the imperv ious areas alone. As a first approximation,if imperv ious areas alone are considered, the value of Cv couldbe taken as unity, al though actual values may be within th erange 0.6 to 0.9.The routing coefficient (C r) might be expected to va ry with theshape of the catchment bu t examination of data led to therecommendation 1 of a constant value for C r of 1.3 for bothdesign an d simulation.29.2.3 Pollution from storm runoffUrban storm runoff will be polluted to a greater or lesser extent.Several comprehensive studies of this pollution have been m ade,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,ca n cause danger to watercourses, especially since it is commonpractice to remove such spillage b y hosing into the surface-waterdrains. W here the result of such an accident can be particular lyserious, e.g. in contaminating a potable water supply, specialprotective measures may be necessary in the drainage design.

29.3 Sewage29.3.1 IntroductionThe v arious 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 mu st be designed, an d withmeans for dealing with peak flows. It also looks at design flowsfo r th e t rea tment w orks .

29.3.2 Dry weather flowThe 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 an d 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 an dindustrial 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 pe rhead 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 automatedca r washes are in wide use.Values of infiltration are best determined from sewer gaugingat night, when domestic flow is almost zero, an d industrialdischarges ar e also least in number, an d 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 an d partly below).Values of industrial discharge should be determined frommetered records, or by reference to agreements with th e localauthori ty.

29.3.3 Storm sewageCombined and partially separate sewers carry surface water inaddition to the normal foul sewage. These sewers ar e 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 wo rks,downstream of storm separation (see below), ha s adequatecapacity to trea t fully the maxim um 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, bu t also becausepolluting sediments in the sewers are resuspended by the stormflush.N o 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 fo r later t reatm ent , ha s produced an improvement .

For small overflows (0.15 to 0.85 m 3/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, an d design should be concentrated on avoidingoverflow of the first storm flush. Control may be by throttlepipe, orifice, or flow regulator, an d 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 th e excess storm sewage. In Britain th e overflowsetting (Q 1 per day) is given by the former Ministry of Housing:42= DWF+1360P+2,

where P is the t r ibutary populat ion an d E is the industrialeffluent flow.


5/13

29.3.4 Design flow for sewage treatment worksTh e 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 fo rlong. It is recommended 4 that 3 DW F is fully treated (n oallowance being made for an increase of infiltration), theremainder being bypassed to storm tanks to receive grav itysettlement. It is usual, after the storm has ceased, to pump thecontents of the storm tanks back to the works inlet fo r fulltreatment.The rate of flow to the treatment works will vary over the day(and also weekly an d seasonally, if the proportion of industrialeffluent is substantial).29.3.5 Pollution loadThe inlet works, tanks, pumps, etc. on a sewage treatment w orksmust 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 m ultiplying the average DW Fby the average BOD concentration. In the absence of suitablemeasurements, the values given in Table 29.3 may be used as afirst approximation.

T a b l e 2 9 . 3 S t r e n g t h o f s e w a g eSettledCrude sewage (mg/1) sewageweak medium strong (% removalson crude)

BO D 5 200 350 550 30- 0COD 350 600 950 30-40SS 200 350 500 50-70N H 3-N 25 35 60 -Org. N. 10 15 20 15-20Chloride (Cl) 70 100 130 -Org. C 140 210 300 3(MO

Hydraulic load an d pollution load ar e important concepts inboth the design of process units and treatment works, and in thedetermination of equitable charges to be applied to industrialusers of sewers an d 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 systems29.4.1 IntroductionThe design of sewerage systems calls for the optimization of thehydraulic, structural an d constructional aspects to suit th edrainage area.Very many factors affect this design an d this section will

concentrate mainly on hydraulic calculations, selection ofmaterials, an d structural design of the sewer line.29.4.2 The Colebrook-White formulaOver 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 an d graphical form more suited to the designer's needs.5The tables and charts present flow rates (1/s), flow velocities(m/s) an d hydrau lic gradients fo r pipe sizes from 0.025 to 2.5 mdiameter, and for roughness factors (& s) from 0.003 to 600 mm.Recommended roughness factors ar e 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 wate r, but both can be significantly reduced bymaintaining high velocities. Grit need not be taken into con-sideration for new designs provided the pipeline ha s good self-cleansing characteristics.Storm sewers m ay 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 w orks it is usual to assume that the mainflow lines ar e 'sewers' until after the secondary stage of treat-ment. Gravity an d pressure pipelines fo r sludge ar e specialcases; friction factors will be dependen t upon the characteristicsof the sludge and may be up to 7 times that appropriate forsewage.29.4.3 Design parametersExperience has s hown 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 k s factor an d hencemake possible decreased pipe size with improved conditions atminimum flow. Typical values for a concrete gravity slimedsewer are given in Table 29.4.

T a b l e 2 9 . 4Velocity 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 fo r coated steel an d iron sewage-pumping mains are as shown in Table 29.6.


6/13

29.4.4 Sewer materialsAs 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 fo r choice.

The m aterial chosen mu st 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 wareClay pipes are suitable for nonp ressure applications, and are n otgenerally available in diam eters 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 CementitiousPipes made from cementitious material ar e generally robust,reliable and rela tively cheap. Unless expensive systems of pro-tection ar e applied, how ever, such pipes ar e 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 fo r 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. P olyvinylchloride liners have been developed toprotect the inner wall from septic sewage. Prestressed concretepipes have been used up to 7 m diameter, and are p articularlysuitable 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 an d steel fibres have been used to reinforce concrete pipes.

29.4.4.3 FerrousDuctile iron is widely used fo r sewage-pumping mains up to1.6m diam eter. Steel pipes are less widely used, but are availablein larger diameters. Their suitability fo r welding means thatjoints capable of ta king tensile loads can be m ade, m akin g steelpipes suitable for long sea outfalls, river crossings, etc. Corru-gated steel pipes have been widely developed an d used in theUS, particular ly fo r storm an d surface-water culverts. As com-plete pipes they are available up to 3 m diameter an d, in sectionsfor assembly on site, they can be made in spans of 10 m or more.29.4.4.4 PlasticsThere is a major distinction between thermoplastics, whosestrength generally reduces markedly with temperature, an dthermosetting 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 m ain groups of thermoplastics: the polyolefins,which include polyethylene (PE), polypropylene (PP) and poly-butylene (PB), and the vinyls, which include polyvinylchloride(PVC) an d acrylonitrile butadiene styrene (ABS). Polyethylenepipe is the most widely used of these fo r sewerage. It is availablein medium and two main high-density forms (MD PE, HDPEland HDPE2). As extruded pipe it is made in diameters up to1.6m, suitable for pressures up to at least 12 bar at 2O 0C. Inhelically welded form it is available up to 3m diameter forgravity sewers.Polypropylene is available up to 1.2m diameter, and forpressures to 15 ba r (2O 0C). Polybutylene is made up to 60 0 m mdiameter and 17 ba r pressure (2O 0C). Polypropylene and PBhave better high-temperature properties than PE, and PB isprobably the best of all thermoplastic pipe materials, havingparticularly good high-temp erature strength, environm ental-stress cracking resistance, abrasion resistance and low creep. Allthe polyolefin plastics can be welded by thermal fusion, makingthem suitable for the pulling of outfalls an d river crossings an dfor the slip-lining of old pipelines.O f the vinyl-typ e thermop lastics P VC, in its unplasticizedform, is the m ore comm on. It has been qu ite 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 fo r temperature, if appropriate, bu t also fo rfatigue effects where the pressure v aries 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 G R P, F R P ,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 an d 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 materialsFlexible joints for rigid pipelines normally employ a socket(bell) an d spigot arrangement, or a double collar or sleeveassembly. Both these jointing systems rely on an elastomericsealing rin g 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 29.6Velocity(m/s)0.8-1.11.2-1.5>1.5

Settlingvelocity(m/s)

1.500.45

Pick-up velocity(m/s)

150mmpipe1.20.6

30 0 m mpipe1.50.6

Suggested k s factor(mm)3.01.50.3

60 0 m mpipe1.80.6

T a b l e 2 9 . 5


7/13


8/13

T a b l e 2 9 . 7 E l a s t ic c o n s t a n t s f o r f le x i b le p i p e sMaterial Ambient temperature Ambient temperature2O 0C 4O 0Cinitial long-term initial long-termE (GPa) E (GPa)/(MPa) E (GPa) E (GPa)/(MPa)MDPE 0.600 0.090 6.3 0.320 0.025 4.1HOPE 1 0.875 0.130 5.0 0.435 0.030 2.0HDPE 2 0.800 0.120 6.3 0.465 0.040 3.0PP 1.150 0.115 5.0 0.760 0.050 3.0PB 0.425 0.380 7.6 0.345 0.250 6.6ABS 1.650 0.550 7.5 1.500 0.500 5.5PV C 2.790 1.350 12.3 2.650 1.250 7.4D.IRON 165.0 165.0 150.0 165.0 165.0 150.0STEEL 200.0 200.0 85.0 200.0 200.0 85.0

e (%) e (%)* 0.25 0.20GRPf 17.5- 10.0- 0.35 15.0- 8.0- 0.30J40.0 25.0 0.40 35.0 13.5 0.35* 4.5-12.0 2.7-7.2 0.20 4.0-11.0 2.1-6.0 0.18RPM 6.0-15.0 3.6-9.0 0.18 5.0-13.5 2.7-7.3 0.15t 4.5-15.0 2.7-9.0 0.35 4.0-13.5 2.1-7.3 0.30t

*ring tension fing bending tfcombined tension and bendingthe 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/F ct (29.3)where eL is the limiting strain an d FG th e strain factor, whichtakes account of the geometry of the distortion, and for which avalue of 6.0 can be taken fo r design purposes. Where stresslimits apply,fJE may be substituted fo r eL in Equation (29.3),/Lbeing the limiting stress.Design of flexible pipes to resist buckling involves ensuringthat th e critical pressure (P CR) 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 pipesThe circumferential tensile stresses set up in the pipe wall by thepressure within th e pipe reduce th e effective strength available toresist the external loads. With rigid pressure pipes the value ofth e 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, an d n is 1 for reinforced concretepipes, 1/2 fo r 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 Table2 9 . 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 outfallsFor 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 landan d then towing or pulling these into position, high stresses an dstrains 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 pipelinem ay 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 structuresOther than pumping stations, which ar e dealt with elsewhere,the structures associated with underground drainage systemsinclude manholes, drop chambers and stormwater overflows.Sizing of these structures is controlled by their hydraulic design,and the need to provide adequate access fo r 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, an d 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.9On pumping mains themselves, access chambers are requiredat air valves an d washouts. Again, th e 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.10Both gravity an d 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.


9/13

F i g u r e 2 9 .3 M o d u l u s E '29.5 Pumping sewage29.5.1 IntroductionPumping sewage presents a particular problem in the need tohandle the solids contained therein. It is common practice toassume that th e smallest sewer within a sewerage system is100mm diameter an d 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, an d this is true fo rpumping mains. This limitation precludes satisfactory pum pingat 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 ar e normally centrifugal or mixed-flowmachines. In smaller sizes, submersible sewage pumps ar emanufactured. 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 comm inuting the solids or on some m ethod ofhandling solids without passing them through a pump (e.g.solids diverter (R) or compressed-air ejector).Fo r lifting duties at sewage works large Archimedean screwsare being increasingly used. These devices are suitable fo r 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 stationsSewage an d drainage installations differ from almost al l 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 continuo usly av ailable for service or runnin g.The first is that it should be possible for all routine maintenance,including major overhauls, to be carried out with th e stationoperating, and the second is that machine breakdowns or othersimilar circumstances should be 'fail-safe'. The various ways ofmeeting these requirements underlie th e 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, en abling themto operate w ithout full-time pump attendants.The sum p should be designed to allow easy flow to the pum psuctions, an d 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 th e 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 th e pumps an d blocking ordamaging them. Current practice m ay follow either view.Major pump ing stations are norma lly designed to be similarto that shown in Figure 29.4. O f particular note as being currentgood practice are the following features:(1 ) The pump casings ar e below th e invert of the incomingsewer, thus ensuring that the pumps require no specialpriming equipment.(2 ) The nonreturn valves and the entries to the rising m ain ar eboth 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 p u m p well being flooded. It isalso normal to fit automatic cellar-drainage pumps to thiswell.(4 ) Sufficient access an d cranage is provided to ease mainten-ance as far as possible. Most design s provide access stairs tothe pump well rather than ladders to encourage mainten-ance staff to inspect th e machinery regular ly .

N a t iv e S o i l M o d u l u s s

B e d d i n g M o d u l u s E 0DI n t e r m s o f depth in m e t r e s (H )v e r a l l M o d u l u s 'M a t e r i a lG r a v e l

C o a r s es a n d

F i n es a n d

M a t e r i a l P e a t C l a y Si l t S a n d G r a v e l R o c k

C o m p a c t i o n


10/13

K E Y( 1 ) P u m p ( 8 ) O v e r f l o w(2 ) N o n r e t u r n v a l v e ( 9 ) E l e c t r i c m o t o r( 3 ) I s o l a t i n g v a l v e s ( 1 0 ) S w i t c h b o a r d( 4 ) R i s i n g m a i n O D O v e r h e a d c r a n e( 5 ) D r a i n a g e c h a n n e l ( 1 2 ) S e w e r in l e t( 6 ) A i r r e l e a s e p ip e w o r k ( 1 3 ) M a c h i n e r y a c c e s s c o v e r( 7 ) I n te r m e d i a t e s h a f t s u p p o r t

F i g u r e 2 9 . 4 P u m p i n g s t a t i o nThe switchboard should be of such a design that individualpump starters, controls, etc. can be isolated fo r maintenance,whilst the board is live, an d other pumps ar e running oravailable fo r 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, an d will have suitableautomatic controls for it to take over th e duties of any pumpwhich fails from whatever cause. The n u m b er of pumps pro-vided will depend on the expected flow variation, length ofpumping main, lift an d other similar design param eters.The electricity supply may need to be secured, either byduplicating the connections to the public supply, or by p rovid-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-ag e 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 mainsSewage-pumping mains differ from those containing most other

fluids in that whenever th e 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. Th e velocity of suchhydraulic surges is materially lowered by any dissolved gases inthe fluid;13 sewage normally contains gases. However, since onsome occasions th e system might be filled with water , th eprecautions taken should be effective with surge velocities bothfor water and for sewage.

29.6 Construction29.6.1 IntroductionConstruction should aim to achieve th e design objectives withth e greatest economy. The choice of materials m ay m ak ediffering demands on the installation costs of buried pipelines.The achievement of the necessary pipe-bedding standard (seeFigure 29.2) is crucial an d, because of the greater dependence offlexible pipes on their bedding, m ay invalidate cost com parisonsbased on material prices only. The influence of trench width an dnative ground conditions on the design of flexible pipelines isignored in many published 'design methods'. The in fo rmat ionprovided 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 ar e frequently preferred because they ca n achieve90 to 95% MPD with minimal compaction. W here gravels areused they must be prevented from acting as groundw ater drains,by the use of regular impermeable barriers, e.g. polyethy lenesheeting. 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 form ing 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 fo rlarge-diameter pipes, in which case resort should be made to oneof the special beddings.For all types of pipeline th e provision of uniform support bythe bedding is essential to prevent the development of unaccep-tably high shear forces or longitudinal bending mom ents. Wherethis cannot be achieved, e.g. where pipes are built into the wallsof underground structures, in areas of mining subsidence orabrupt transit ions 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 withou t the use of suchflexible joints.Careful attention m ust be given to the mann er of joininglateral sewers and house connections to main sewers.14 TheConstruction Industry Research and Information Associationhas recently issued an authoritative report on trenching.15In th e construction of above-ground pipelines, proper con-sideration m ust be given to possible thermal movem ents, and toeven load distribution at supports.The construction of river and estuarine crossings, and ofsubmerged outfalls, favours the use of materials which can be


11/13

joined into long strings on land an d then pulled into position.Steel and polyolefin plastics with welded joints are thereforeoften used. Glass reinforced plastic pipes, with hand lay-upoverw rap joints, have also been successfully used in this man ner,as also ha s prestressed concrete.29.6.2 Renovation of pipelinesTh e 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 ar e minia-turized tunnelling, and sewer renovation.Quite apart from th e fact that many old urban sewers requirerenovation because they ar e 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, an d also tha t al l lateral connections areautomatically located.Renovation techniques have been reviewed extensively by theWater Research Centre,16 which ha s 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 th e required bond.Type 2: Lining systems in this category do not rely on theformat ion 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 an d 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 up on the formation of a bo nd,so that the liner is regarded as acting as a flexible pipe.Type 3: Liningsof this type, thin-walled GRP or in situ resin, arenot regarded as fulfilling an y permanent structural role. Rather,they are considered as fo rmwork left-in, with th e annulus groutproviding the structural element.Most of the renovation systems result in lateral connectionsbeing temporarily blocked, bu t several ingenious methods ofreopening laterals have been developed, e.g. by remote cuttingfrom th e sewer, by remote cutting from th e lateral or by'minimum excavation' techniques from th e surface.Pipe ' renovation' has developed to the stage that one tech-nique, polyethylene pipe sliplining, ca n provide an increase inthe diameter of the sewer. In a 1985 example, a 2 2 5 m mdiameter clay pipe was 'relined' with a 350mm diameterpolyethylene pipe, inserted behind an impact mole which splitthe old pipe.

29.7 MaintenanceGiven good design an d construction, the correct choice ofmaterials should reduce sewer maintenance to the clearance ofoccasional blockages. Although good hydraulic design shouldminimize blockages, th e fact that they m ay never be completelyavoided requires proper provision for maintenance to be in-cluded in the design. Easy an d 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 platform s 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 an d other toxic gases.

SEWAGE TREATMENT29.8 Introduction29.8.1 Characteristics of sewageMunicipal sewage is mainly the wastewaters from homes, officesan d 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, an dtreated with the domestic sewage; this may demand somepretreatment to ensure that it does not interfere with th e 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 an d other living habits of the contr ibutory population, an dmarkedly 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 natura lly occurring microorga-nisms in the receiving wate r, and so reduce the dissolved oxyg en,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 2O 0Cby microorganisms consuming the organic m atter, or as chemi-cal oxygen demand (COD), a purely chemical parameter whichapproximates to the ultimate oxygen demand.Nitrogen is comm only 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, amm onia can also be poisonous tofish.It has been foun d, 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 analysisBefore selecting the method of disposal, and the appropriatetreatment of the sewage to permit disposal with out pollution, itis necessary to sample an d analyse the sewage.Sampling should usually be carried out over th e full 24 h,since flow varies greatly over the day, and the individ ual 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.


12/13

The analyses should be carried out by the standardizedmethods,17 '18 which have been laid dow n in the UK and U S, andwhich are generally used throughout the world.Although suspended solids, oxygen demand and ammoniaare the most im porta nt design paramete rs, 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, an d ammoniacal an d oxidized nitrogen in the raw sewageand in the effluent after variou s stages of treatment. (Although ittells little a bout the biological effect of organic matter, C O D i sroutinely used in place of BOD because the analysis for CODcan be carried out in about 2 h as compared to 5 days for BO D.)29.8.3 Ease of treatmentThe final method of disposal, and the proper degree of treat-ment to allow this with out environme ntal 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 th e 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 th e ease with which th e organicmatter can be oxidized, and to estimate th e possible interferenceof toxic an d other substances with th e biological oxidation .Actual tests of the ' treatability' of an effluent may be made bybench- or pilot-scale tests, th e latter being th e 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 th e C O D t B O D 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 th e more th e ratio exceeds 2. The most easily degradedsubstances are broken down first and, in consequence, th eCOD:BO D ratio of the aqueous stream increases as it passesthrough biological treatm ent.29.8.4 Possible effects of industrial effluentsM an y industries concerned with th e manufac tu re an d process-ing of food an d drink use great quantit ies of water , an d producecorrespondingly large quantities of effluent. Th e 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.O ther industr ies discharge a very wide range of substancesinto their effluents, an d m an y of these ca n interfere witht reatment processes. In such cases it is essential to pretreat th eindustrial effluent to remove or neutralize th e interfering sub-stance. The treatment of industr ial effluents is far too extensive atopic to treat in an introductory chapter, an d reference shouldbe made to recent books.19

Discharge of dangerous or inhibitory wastes is controlled bylocal ordinance, an d 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, an d watercourses put at risk bycontamination. Trade-waste control is thus an extremely im-por tant factor in the day-to-day operation of sewers and sewagetreatment works.

29.9 Effluent disposal29.9.1 IntroductionA wastewater can be finally 'disposed of only into water, on toland and into the ground. The last of these is available inpractice only fo r small quantities of hazardous materials thatcannot be safely dealt with in any other way , e.g. into w orked-out salt mines.Unti l the early years of this century, discharge on to land(sewage farming) was the only method in Britain acceptable tothe Local Governm ent B oard. Generally, now, however, sewagefarming is of no more than historic interest, although th eirrigation 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, an d this section briefly considersthe degree of treatment necessary to avoid danger, damage ornuisance resulting from such disposal.29.9.2 Effects of water pollutionWastewaters m ay contain substances which ar e poisonous toman and to plan t an d animal life in the water; sewage ought notto hold such substances, and it is essential to ensure that toxicmatters are not allowed to enter m unicipal sewers from indus-try.More importan t for municipal sewage are its power to use upth e small amount of oxygen dissolved in the water, as theorganic matter an d nitrogen ar e 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. O rganic an dammoniacal nitrogen are oxidized in the receiving w ater, and soalso use up dissolved oxygen . Th e fully oxidized form (nitrate)in sufficiently high concentrations m ay cause methaemoglobi-naemia in very young infants. Above all, nitrogen is a plantfertilizer, and its presence in wa ter, especially in standing bodiesof water , m ay promote undesirable weed growth.

Sewage will, of course, also contain faecal microorganisms,and some of these may be pathogenic. In urban, industrializedcommunit ies th e main protection against waterborne diseases iswater treatment, with it s accompanying disinfection. Neverthe-less, as will be seen from Table 29.8, full normal sewaget reatment 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 necessaryTh e effects of pollution outlined in the previous section areclosely related to the dilution available for the effluent dis-charged. The lower th e dilution th e greater will be the damagecaused. For this reason it is usual to prescribe the quality of


13/13

Table 2 9 . 8 R e m o v a l e f f ic i e n c ie s o f s e w a g e t r e a t m e n tPercentage removal ofSS BOD Bacteria

Primary sedimentation 40-70 25-40 25-75Chemical precipitation 70-90 50-85 40-80Sedimentation + tricklingfilters+fina l sed. 70-92 80-95 90-95Sedimentation + activatedsludge +f inal sed. 85-95 80-95 90-98Chlorination following fullbiological treatment 98-99

effluent required fo r 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 Figure2 9 . 5 . The degree of removal that may be achieved by variouscom binations of treatment process is shown in Table 29.10.It may be seen, by considering Tables 29.9 an d 29.10 inconjunction, that full primary, biological and final treatmentwill be necessary fo r discharge to inland rivers; nitrificationfollowed by denitrification may be required, in addition, for lowdilutions. Equally effective treatment is likely also to be neces-sary fo r discharge to lakes, together with, in some cases,removal of the other important plant nutrient, phosphorus.Preliminary treatment alone is likely to be sufficient fo r oceandischarge, although it may sometimes be desirable also toprovide primary settlement.During full treatment no t 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 an d expense insewage treatment, and is discussed in sections 29.15 and 29.16.

29.10 Preliminary treatment29.10.1 IntroductionThe principal objective of preliminary treatment is to protectsubsequent treatment processes, by preventing blockage an dT a b l e 2 9 . 9 T y p i c a l c o n c e n t r a t i o n s o f pollution f o r d i s c h a r g e t ov a r io u s r e c e i v in g w a t e r s ( a l l v a l u e s in mg/l , e x c e p t p H )Parameter Inland River dilution Estuary Open se a2dilution more than 81

less than8 lBOD 10 20 150 -3SS 15 30 200 2504Ammoniacalnitrogen 10 pH 5-9 5-9 5-9Notes:(1) With clean water.(2) The ocean outfall must be carried sufficiently far out to sea to ensure thatpollution is not brou ght back to the bathing beaches, etc. The end of theoutfall m us t be provided with a properly designed diffuser and be located in asufficient depth of water to ensure thorough mixing and d ilution before the

effluent reaches the surface.(3 ) Not usual to specify a BO D l imi t.(4) A SS standard is not alway s specified, but it is always desirable that floating

Table 2 9 . 1 0 T r e a t m e n t o f o r g a n i c w a s t e w a t e r sBO D Method BO D loading(mg/l)< 500 Single filtration or 0.1 kg/m 3-dayactivated sludge 0.2 kg/ kg MLSS-day50 0 + Filtration with 0.15 kg/m 3-dayrecirculation or 0.2 kg/m 3-day

alternating doublefiltration1000 Extended aeration 0.05-0.15 kg/kgMLSS-day1000-1500 High-rate filtration (with Up to 5 kg/m 3-dayrecirculation)followed bypercolating filters or 0.1 kg/m 3-dayADF or 0.2 kg/m 3-dayactivated sludge 0.2 kg/ kg MLSS-day1500+ Anaerobic treatment l -5kg/m 3-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 plan t, and to increase the reliability and efficiencyof the treatment process. Those objectives are achieved byremoval of large solids by screening, removal of grit, removal ofoi l and grease, balancing of flow and/or load, pH control, andnutrient addition.29.10.2 ScreeningThe quantity an d 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 fo rdomestic sewage are in the range 1 to 3 m 3 per day per 100 00 0population. Where there is an industrial effluent, th e nature ofthe industry m ay suggest th e 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 f low.(2 ) To remove from the flow and dispose of elsewhere.Comminutors ar e clean, innoffensive an d relatively troub le-freemachines, which are normally left unattended. However, ragstend to be shredded rather than cut up and may 'ball up' in latertreatment stages, and scum volum es are increased by comm inu-tion. Similar problems are encountered with disintegrators.The alternative of permanent removal an d separate disposal

Documents

29.Sewerage and Sewage Disposal