Table 29.8 Removal efficiencies of sewage treatment

Primary sedimentation Chemical precipitation Sedimentation + trickling filters +final sed. Sedimentation + activated sludge +final sed. Chlorination following full biological treatment

Table 29.10 Treatment of organic wastewaters

Percentage removal of SS BOD Bacteria

BOD (mg/l)

Method

BOD loading

40-70 70-90

25-40 50-85

25-75 40-80

< 500

Single filtration or activated sludge

0.1 kg/m3-day 0.2 kg/kg MLSS-day

70-92

80-95

90-95

500 +

0.15 kg/m3-day 0.2 kg/m3-day

85-95

80-95

90-98

Filtration with recirculation or alternating double filtration

1000

Extended aeration

0.05-0.15 kg/kg MLSS-day

1000-1500

High-rate filtration (with recirculation) followed by percolating filters or ADF or activated sludge

Up to 5 kg/m3-day

98-99

effluent required for discharge to various types of receiving water; some suggested values are shown in Table 29.9. The various methods of treatment that may be given to municipal sewage are indicated diagrammatically in Figure 29.5. The degree of removal that may be achieved by various combinations of treatment process is shown in Table 29.10. It may be seen, by considering Tables 29.9 and 29.10 in conjunction, that full primary, biological and final treatment will be necessary for discharge to inland rivers; nitrification followed by denitrification may be required, in addition, for low dilutions. Equally effective treatment is likely also to be necessary 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 ocean discharge, although it may sometimes be desirable also to provide primary settlement. During full treatment not more than about one-third of the incoming pollution is converted to relatively harmless substances; the rest remains on the treatment works as solids for disposal (sludge). This is a major problem and expense in sewage 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 protect subsequent treatment processes, by preventing blockage and

Table 29.9 Typical concentrations of pollution for discharge to various receiving waters (all values in mg/l, except pH) Parameter

Inland River dilution Estuary dilution more than 81 less than 8l

Open sea2

BOD SS Ammoniacal nitrogen pH

10 15

20 30

150 200

-3 2504

10 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 the outfall must be provided with a properly designed diffuser and be located in a sufficient depth of water to ensure thorough mixing and dilution before the effluent 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.

0.1 kg/m3-day 0.2 kg/m3-day 0.2 kg/kg MLSS-day

1500+

Anaerobic treatment l-5kg/m3-day followed by one or two (depending on stages of aerobic degree of treatment removal)

Any

Oxidation ponds (multistage)

anaerobic first stage 7000 kg/ha-day aerobic final stage 250 kg/ha-day

damage to the plant, and to increase the reliability and efficiency of the treatment process. Those objectives are achieved by removal of large solids by screening, removal of grit, removal of oil and grease, balancing of flow and/or load, pH control, and nutrient addition. 29.10.2 Screening The quantity and nature of screenings vary, often substantially, between one plant and another. Relevant factors are social conditions and habits, industrial contributions, the type of sewerage system and the design of the screening plant. The following guidelines may be used to estimate quantities where there is no previous experience of local conditions. The volume of screenings depends more on the character of the sewage than on the bar spacing; average volumes for domestic sewage are in the range 1 to 3 m3 per day per 100 000 population. Where there is an industrial effluent, the nature of the industry may suggest the additional screening load. The peak hourly rate of screenings removed is likely to be 4 to 6 times the average, and with combined sewers the peak rate during a storm following a dry period may be 10 to 20 times the average. There are two basic approaches to handling large solids: (1) To comminute in the flow or to remove, disintegrate and return to flow. (2) To remove from the flow and dispose of elsewhere. Comminutors are clean, innoffensive and relatively trouble-free machines, which are normally left unattended. However, rags tend to be shredded rather than cut up and may 'ball up' in later treatment stages, and scum volumes are increased by comminution. Similar problems are encountered with disintegrators. The alternative of permanent removal and separate disposal of screenings is preferable in relation to the operation of the

remainder of the works, but the problems of handling, transporting and disposing of the screenings are not easily resolved. For most applications, mechanically raked bar screens are preferred for removing large solids from the flow. Curved bar screens are restricted to use in shallower channels, but the vertical or inclined bar types may be used in deeper channels and may use front or back raking. Bar spacing (clear opening between bars) depends on the required removal efficiency. To protect pumps or fine screen units, a spacing of up to 150 mm is acceptable, although 90 mm is usual for all but the largest pumps. Pump clearway dimensions must obviously be considered. For normal sewage works a spacing of 20 mm is common although the need for individual design to suit local sewage characteristics must be considered. Screenings are normally transported from the screen by mechanical conveyor or by launders. Whilst belt conveyors cannot be duplicated for standby they are attractive for smaller works. Launders are favoured for their simplicity, wherever the addition of transport water can be accepted, and are preferred on large works where a number of screens is to be installed. The use of a launder implies the need for a subsequent dewatering stage unless the screenings are disintegrated and returned to the sewage flow. In the past, screenings were allowed to drain and were then carted away for burial, but the trend now is towards mechanical dewatering followed by transport to a screenings burial area, a refuse disposal site or an incinerator. The dewatering of screenings by ram or roller press not only reduces the volume, but often also improves the appearance and odour. Generally, the drier the product the more unrecognizable and acceptable it becomes. On-site burial of screenings is normally practicable only on small to medium works. Specialized small-capacity incinerators can be considered for large works, but they are an expensive option and transport to a controlled refuse tip may be preferable. Incineration in combination with municipal refuse can be considered, but difficulties may arise from the different combustion characteristics of screenings and refuse. 29.10.3 Grit removal Grit is a mixture of heavy mineral particles in sewage, such as sand, gravel, cinders, glass, etc. and when removed from sewage it contains some organic matter. The quantity of grit in normal municipal sewage varies according to area, degree of separation of surface water, time of year, etc. Typical yields range from 0.02 to 0.2m3/ 100Om3 of sewage, with most data tending towards the lower values. Grit removal is normally preceded by screening, but comminutors, if used, are best installed after grit removal, with coarse bar screening upstream. Four methods of grit removal are commonly used: (1) detritors; (2) constant velocity channels; (3) spiral flow aerated channels; and (4) vortex-type chambers. The detritor, which is a short-period settlement tank with a mechanical scraper for grit removal, is a satisfactory method of grit removal, but in general requires more complex civil engineering structures and larger land areas than the spiral flow channel. Constant velocity channels are a simple design suitable for small and medium-sized works. They consist of long channels with a cross-section approximately parabolic. This maintains a relatively constant velocity over the full range of flow, when controlled by a flume or similar device. Settled grit is removed either manually or by suction dredgers. The spiral flow aerated channel combines the constant velocity principle of differential settlement with the washing action of air turbulence. Air injected into a rectangular channel at one side induces a rotational motion, which sweeps grit into a floor

hopper and maintains lighter particles in suspension. Spiral flow aerated channels are the most suitable method for large works. In vortex-type chambers a mechanically induced vortex in a conical tank produces secondary currents, which maintain the organic matter in suspension. A deep hopper at the bottom of the tank is used for grit collection and as a sump for grit pumping. The vortex-type trap has not given consistently good results, but has applications in some situations. Grit washing after extraction is essential with a detritor, and desirable with constant velocity channels. Grit from spiral flow channels should normally be sufficiently clean. Grit from vortex traps varies in quality. Various washing mechanisms are available, often combined with dewatering classifiers to ease handling of the final product. 29.10.4 Skimming, flocculation and preaeration Other pretreatment operations have been used to remove grease, oil and scum from sewage prior to primary sedimentation (see section 29.11) and to improve the treatability of wastewater. Skimming, flocculation and preaeration have been used for this purpose, although these techniques, other than perhaps preaeration, are not commonly practised in the UK. Skimming tanks to remove floating matter may be designed to provide retention periods of 1 to 15 min. The outlet, which is submerged, is opposite the inlet and at a lower elevation to assist in flotation and to allow any solids that may settle to pass on to subsequent treatment stages. Flocculation of sewage by mechanical or air agitation, although not commonly used, is sometimes given consideration when it is desired to increase the removal of suspended solids and BOD in primary sedimentation facilities, to condition wastewater containing certain industrial wastes, and to improve the performance of secondary sedimentation tanks following biological treatment processes, particularly the activated sludge process. Preaeration, i.e. aeration of sewage prior to primary sedimentation, may be practised to provide grease separation. It may also be used for a variety of other purposes including grit removal and flocculation, to prevent septicity and hence to control odours, to promote uniform distribution of suspended and floating solids to treatment units, and to increase BOD removals. Preaeration may be practised in purpose-designed tanks, sometimes as an extension to aerated grit channels. Depending on the particular objective, it may also be carried out using aerated channels, which also serve to distribute flows to subsequent treatment units. 29.10.5 Flow/load balancing Balancing of variations of flow and pollution load involves the damping of flowrate fluctuations so that a constant or reduced peak flow and pollution-load rate is achieved. It reduces the size of subsequent treatment units and reduces the risk of shock loadings (including elevated temperatures) affecting the performance of the main treatment units. If practised at all in sewage treatment it is normally coupled with primary sedimentation. Separate balancing with mixing facilities to maintain solids in suspension is normally considered only for industrial waste treatment applications or where the sewage has a high industrial waste content. Combined sedimentation/balancing tanks are discussed further in section 29.11.1. 29.10.6 pH control The pH of a wastewater is a key factor in the growth of organisms within biological treatment processes. Most organ-

isms cannot tolerate pH levels above 9.5 or below 4.0. Generally, the optimum pH for growth lies between 6.5 and 7.5. Where the sewage contains a high proportion of industrial wastes and pH control is not practised at the factory premises, pH correction facilities may have to be installed at the sewage treatment works to ensure the satisfactory operation of biological processes. Control of pH is also frequently necessary where chemical flocculation is practised (see section 29.11.2). pH correction, in the form of alkali addition, may also be necessary where a high degree of biological nitrification (ammonia oxidation) is required for sewages having low alkalinities and hence insufficient buffering capacity to absorb the reduction in alkalinity associated with this process. Chemicals used for pH correction are common inorganic acids and alkalis, including sulphuric or hydrochloric acid, lime, caustic soda or soda ash. Dosing can be adjusted automatically by pH probes linked to pH controllers. 29.10.7 Nutrient addition If a biological treatment system is to function correctly, nutrients must be available in adequate amounts. The principal nutrients are nitrogen and phosphorus. Assuming an average composition of cell tissue OfC5H7NO2, about 12.4% by weight of nitrogen will be required. The phosphorus requirement is normally assumed to be about one-fifth of this value. These are typical values rather than fixed quantities, since the distribution of nitrogen and phosphorus in cell tissue varies with the age of the cell and with environmental conditions. An alternative approach is to relate nutrient requirements to the waste BOD requiring removal; BOD:N:P ratios of 100:5:1 are often quoted for aerobic biological treatment, with lower N and P requirements being appropriate for anaerobic processes. Domestic sewage generally contains more than adequate nitrogen and phosphorus concentrations to satisfy all sewage treatment requirements. However, where a high proportion of industrial wastes is also present, a nutrient deficiency may result and consideration then has to be given to the addition of nutrient chemicals. Soluble ammoniacal and phosphate salts, urea or combinations of these chemicals may be used.

29.11 Primary treatment 29.11.1 Sedimentation Primary sewage treatment, following preliminary treatment, commonly takes the form of sedimentation for the removal of readily settleable suspended solids (SS) and associated BOD. Sedimentation tanks may be circular (radial flow), rectangular (horizontal flow) or pyramidal (upward flow) and are normally designed to remove 60 to 70% SS together with 30 to 40% associated BOD. They normally operate on a continuous flow basis, and include hoppers or troughs for collection of sludge and, in the case of circular and rectangular tanks, power-driven scrapers to move the sludge across the floor to the outlet. Facilities are also usually provided for collecting and removing surface scum and other floating material for subsequent treatment and/or disposal with the settled sludge. The design criteria for primary sedimentation are based primarily on the maximum flow to receive treatment which should be established, taking full account of the effects of the return of secondary sludge and works liquors. The two principal design parameters are surface hydraulic loading (m3/m2-h or m3/ m2-day) and retention period (h). Maximum surface loadings in the range 1.5 to 2m 3 /m 2 -h and minimum retentions of 1.5 to 2.0 h are commonly selected, due account being taken of the

need to provide a sufficient number of tanks, so that any one tank may be taken out of service without greatly affecting the sedimentation process. Consequently, even with a small works there should ideally be a minimum of two tanks. Somewhat higher primary-tank loadings than specified above (e.g. up to 3 m3/m2-h and 0.75 to 1 h minimum retention) are sometimes adopted, at the expense of slightly lower pollution removals, depending on the particular application; this is particularly applicable in warmer climates, where the retention of sewage and sludge can usefully be minimized to reduce risk of septicity, rising sludge and associated problems. In the extreme, high-rate sedimentation tanks (8 to 12m3/m2-h) are sometimes considered as an alternative to fine-mesh screens for removal of gross solids (including floating material) plus associated BOD (15 to 30% SS, 5 to 10% BOD) prior to discharge to sea (see Table 29.11).

Table 29.11 Suggested design upflow velocities (surface loading) for sedimentation tanks For settlement of

Design upflow velocity (surface loading)

Sewage - primary - primary before plastic (structural medium) Sewage - final -finalfollowing extended aeration With chemical treatment With ferric salts or alum Difficult solids following lime treatment

2 m/h at peak flow 3 m/h 1.5 m/h at peak Im/h Up to 2.5 m/h 1.5-2 m/h

Im/h

Note: The design upflow velocity can vary quite widely for various types of industrial wastes. It will not usually be suitable to use sedimentation for light solids that have upflow velocities less than 1 m/h; in such cases alternative clarification processes, such as flotation, should be considered.

Settled sewage usually leaves the sedimentation tank via a weir. Maximum weir loadings in the range 300 to 450 m3/m2-day are commonly specified by designers, although the significance of this parameter on the performance of primary sedimentation tanks is a subject of debate; weir loadings are of greater importance in the case of secondary sedimentation tanks following biological treatment. Where extreme diurnal variations in flow and strength of sewage reach a treatment works, it is sometimes necessary to consider combined sedimentation/balancing tanks to equalize the flow and pollution load passing to subsequent treatment units. Such tanks are not as efficient for removal of suspended solids as tanks designed for sedimentation only, but are to be preferred to the installation of separate balancing tanks. Sedimentation tanks are commonly sized to accept up to about 3 x DWF, with the higher flows arising during wet weather being diverted to storm tanks, which are normally allowed to overflow directly to the receiving watercourse when full. When flows reduce, storm-tank contents, including sludge, are then pumped back to the head of the works for further treatment. With small works, however, storm tanks are sometimes dispensed with and the capacity of the sedimentation tanks is increased in order to receive flows up to 6 x DWF or even higher. 29.11.2 Chemical treatment During the earlier part of this century, it was a common practice

for connection to a public sewer. A cesspool is simply a storage tank which should be emptied at regular intervals by a suitable tank-emptying vehicle. A septic tank is a continuous, horizontal-flow tank in which settled sludge is retained sufficiently long for the organic content to undergo anaerobic digestion. When sludge is eventually removed (maybe once or twice yearly), a portion is left in the tank as a seed to initiate further digestion. A septic tank therefore combines the operations of sedimentation and sludge digestion.

in the UK, especially for sewages containing a high industrial waste content, to add one or more chemical flocculants (often at controlled pH) in order to improve the removal of suspended solids and associated BOD. While such chemical treatment is sometimes practised today throughout the world, chemical treatment, because of high cost, is now normally used in the UK only as a temporary measure to relieve an overloaded works by improving the efficiency of primary sedimentation. However, chemical treatment may also be necessary where there is a need to reduce phosphate levels to meet strict standards for nutrients or as part of a tertiary treatment facility to produce a high-quality effluent for discharge to watercourse or for reuse. In the former case, flocculation with lime, iron or aluminium salts, often with polymer addition as a flocculant aid, is appropriate prior to primary sedimentation and subsequent treatment. Alternatively, variations on this practice may be adopted including chemical treatment following primary and secondary (biological) treatment or dosing directly into the biological treatment stage.

29.12 Biological treatment 29.12.1 Introduction Preliminary and primary treatment may remove two-thirds of the suspended matter in sewage, but no more than perhaps onethird of the organic pollution (BOD). If further reduction of BOD is necessary - and it usually is - it is essential to provide secondary treatment, in which the organic matter is oxidized by microorganisms, part being removed as such end-products as carbon dioxide and water, and part being converted to new microorganisms, which must be removed from the aqueous stream by final settlement. Normal biological treatment of domestic sewage, together with final sedimentation, can consistently achieve an effluent with less than 20 mg/1 BOD and with most of the ammoniacal nitrogen oxidized to nitrate. Very much better effluents can be produced only with additional treatment (see section 29.13). Biological processes may also be used for partial treatment where appropriate. Biological oxidation may be achieved with the microorganisms held in a fixed film, or suspended in the sewage, or within a fluidized bed (see Figure 29.5). Biological breakdown may also be carried out by anaerobic processes (in the absence of air). There is a very wide range of aerobic processes which may be

29.11.3 Flotation Flotation is a process used to separate solids or liquid particles from a liquid phase by introducing fine bubbles of gas (usually air) into the liquid phase. The bubbles attach to the particulate matter and the buoyant force of the combined particle and gas bubble causes the particles to rise to the surface. In sewage treatment, flotation may be used to remove suspended solids either at the primary solids-removal stage or following biological treatment; in practice, however, flotation has greater interest as a method of thickening biological sludge. In all cases, chemical flocculant addition is required prior to the flotation process. Flotation thickening for waste sludges is discussed more fully in section 29.15.4.2. 29.11.4 Septic tanks Cesspools and septic tanks are frequently used for receiving sewage from houses and other premises which are too isolated

SEWAGE Preliminary Primary Biological. . fixed-film .suspendedgrowth .fluidized bed Final Tertiary

SCREENING

COMMINUTION

GRIT-REMOVAL

Solids for disposal

SETTLING

CHEMICALSETTLING

FLOTATION

Solids, etc. for disposal

Anaerobic TRICKLING FILTERS ANAEROBIC FILTERS ROTATING CONTACTORS ACTIVATED SLUDGE OXIDATION POND ANAEROBICCONTACT ANAEROBIC

AEROBIC SETTLEMENT POLISHING

FLOTATION

NUTRIENT REMOVAL TREATED EFFLUENT to Receiving Water

Figure 29.5 Treatment of municipal sewage

Solids for

return disposal

DISINFECTION

used for treating organic wastewaters of a correspondingly wide range of strength. An indication of suitable processes is given in Table 29.10 (p. 29/13).

animals and insects, which may assist in controlling the build-up of biomass. Filter flies, however, can be an unpleasant local nuisance. Odour, too, can be troublesome, especially with highrate filters, and special steps, e.g. by exhausting the air and passing it through a scrubber, may be called for.

29.12.2 Percolating filters The oldest, and the most common, fixed-bed process is the percolating filter, which consists of a solid medium over which the sewage is sprinkled, and through which air passes freely, as the source of the essential oxygen. The medium may be stone, slag, gravel or specially made random-pack plastic; structural plastic media of various kinds have also been used. For treatment to take place both BOD and oxygen must diffuse into the biological film, whose active depth must, therefore, be relatively limited. Surface area is, accordingly, an important design requirement, and may be increased by using a small medium. On the other hand, the film increases in volume, as the microorganisms multiply, and so may block the interstices and, in turn, prevent the access of oxygen. Although the film continually drops off, and there may also be a seasonal sloughing, it is essential to select the size and nature of medium with care. Guidance is given in Table 29.12. It will be seen that single filtration removes the lowest amount of BOD, although it will produce an effluent of high quality. Performance may be improved by recirculating treated and settled effluent. In addition, filters may be used in-series, with the order of filtration being alternated. Both these modifications are useful for preventing the excessive increase of film. The more recently introduced plastic media, both structural and random pack, have at least two advantages over stone, slag, etc. They provide a very much greater specific surface, and their lower weight can allow a lighter containing structure, which may be less costly. Filters using the heavier media are commonly about 2 m deep. They must be contained in brickwork or concrete walls, and are underdrained in such a way that the ingress of air is encouraged. Plastic-packed filters may be much deeper. Settled sewage may be applied to the filter beds by fixed sprays or moving sprinklers. Many filters are circular, and their rotating sprinklers are often driven by the reaction of the sewage itself; this calls for intermittent application controlled by a siphon dosing chamber. The biological film is normally colonized by a number of

29.12.3 Rotating biological contactors The rotating biological contactor (RBC) is a secondary biological treatment system. It consists of a large-diameter corrugated plastic medium, or set of discs, mounted on a horizontal shaft and placed in a concrete or steel tank. The medium is slowly rotated while approximately 40% of the surface area is submerged in the wastewater. During operation, biological growths adhere to the surface of the medium and form a slime layer over the entire wetted surface area. The rotation of the medium brings about oxygen transfer, and maintains the biomass in an aerobic condition as it absorbs and degrades the soluble organic constituents of the wastewater. The rotation is also a mechanism by which excess slime growth (or humus) is sloughed off into the tank, while at the same time maintaining the solids in suspension before they are carried from the unit to a subsequant settlement tank. The attached biomass typically contains of the order of 50 000 mg/1 suspended solids. If these solids were removed and placed in the mixed liquor, the resulting mixed liquor-suspended solids concentration would be 10 000 to 20 000 mg/1, i.e. very significantly higher than is practicable for an activated sludge plant. Hence, a high degree of treatment is possible for a relatively short retention period; however, in view of this short retention period, suspended solids in the influent to the plant are bioflocculated in the mixed liquor tank but otherwise pass through to the final settlement tank largely unchanged. For industrial waste treatment applications, rotation of the% plastic medium is achieved by direct motor drive. For domestic sewage applications, the alternative of diffused air, released into the mixed liquor near the periphery of the immersed medium, is sometimes used to create rotation, while at the same time helping to maintain aerobic conditions, but air drive units are not normally recommended for treatment of industrial waste. The use of periodic supplementary aeration is, however, beneficial, particularly for industrial waste treatment, for controlling the thickness of the slime layer on the medium. This control

Table 29.12 Biological filtration Mode of operation

Straight filtration Straight filtration with recirculation Double filtration Alternating double filtration

Overall filter loading (kg BOD/m'-d)

Medium size (mm)

Specific surface area Strength of sewage (m2/m3) feed BOD (mg/1)

without nitrification

with nitrification

0.09-0.12

0.06-0.09

40

120

150-350

0.15-0.18 0.18-0.24

0.09-0.15 0.15-0.21

40-60 40-60

80-120 80-120

150-350 >350

40-60

80-120

>350

100-150

33-60

20(MOO

0.18-0.24

High-rate (slag) Up to 2.7 High-rate (structural plastic) with recirculation if necessary Up to 8 High-rate (random pack plastic) with recirculation if necessary Up to 10

85

Up to 1500

180

Up to 1500

helps to reduce overall power costs by minimizing the weight of biomass adhering to the rotating medium. Rotating biological contactor units are sized primarily on soluble BOD loadings per square metre of effective surface per day. Choice of loading will vary according to the type, strength and temperature of the wastewater, as well as on the degree of BOD removal required. When comparing differences among the various units marketed by RBC manufacturers, due account must be taken of differences in effective surface area of medium among the various designs. Rotating biological contactor plants are often installed within a building to protect the plant against adverse weather conditions. 29.12.4 Activated sludge In the activated sludge process the microorganisms responsible for breaking down the organic matter are suspended in the sewage. There are at least two basic requirements: (1) oxygen for the metabolism of the organisms; and (2) good mixing to ensure that the organic matter, the organisms and the oxygen are in intimate contact, and that the solids are kept in suspension. The agitation and oxygenation are usually combined. The two common processes are: (1) Diffused air, in which air is blown into the sewage through porous diffusers or orifices. (2) Mechanical aeration, in which the mixture of sewage and microorganisms is vigorously agitated mechanically, thus dissolving air. It is essential to return active microorganisms to the aeration stage (returned activated sludge). An important design parameter is the sludge loading rate (often called the food: microorganism ratio). The higher this rate, the lower will be the quality of the effluent. For a given BOD load, therefore, it is necessary to have a specified weight of active organisms, known as mixed liquor suspended solids (MLSS). There is a limit to which the concentration of MLSS can be increased in the aeration tank, which is governed by the process requirements. Suitable design parameters are shown in Table 29.13. During the aeration stage, part of the incoming BOD is converted to new biological cells, up to 1 kg or more of new cells being produced for each kg of BOD removed. As indicated

above (and as shown in column 3 of Table 29.13) some of this must be returned to the reaction vessel (aeration tank), but most of it must be discarded after final settlement. The quantities involved are shown in the last column of Table 29.13. It will also be seen from the table that the MLSS concentration is normally maintained at no more than 3000 to 4000 mg/1, with a consequent relatively low sludge loading rate. This is largely because transfer of oxygen to the microorganisms is limited with normal aeration systems. Therefore, the sludge loading rate must be relatively low to maintain a residual dissolved oxygen concentration in the mixed liquor of about 1 to 2 mg/1. Much higher oxygen-transfer rates are possible if pure oxygen is used in place of air, or if a deep shaft (50 to 15Om deep) is used as the aeration vessel. In both cases, higher oxygen transfer rates allow higher MLSS and sludge loading rates. The parameters for these two important modifications of the activated sludge process are shown in the bottom two lines of Table 29.13. The figures given in the table are especially applicable to settled domestic sewage (say, BOD 200 to 300 mg/1 and SS 100 to 150 mg/1), but the processes are suitable for a wide range of organic wastewaters, the sludge loading rates given in column 4 governing the size of aeration tank. Pumps for returning the activated sludge to the aeration tank must be carefully selected to avoid excess shearing forces on the sludge floes. It is usual, therefore, to use axial flow, torque flow or screw pumps. The extended aeration modification, shown in the table, can be applied simply in an oxidation ditch, a process first developed in the Netherlands. An aeration rotor both circulates the mixed liquor and aerates it. The ditch is sometimes also used as the final settling tank by intermittently discontinuing the operation of the rotor. Surplus sludge must be periodically withdrawn from a collecting sump. Retention periods will vary from 1 to 2h for the high-rate process, through 6 to 1Oh for the conventional process to 1 or more days for extended aeration. The removal of organic and ammoniacal nitrogen is becoming increasingly important. It will seem from the table that nitrification (i.e. oxidation of the ammoniacal nitrogen) calls for a low sludge loading rate, and hence a large aeration tank. Even oxidized nitrogen (NO3) is coming under fire for health reasons, as well as because it is an important plant nutrient which may produce eutrophic conditions in lakes. Recent

Table 29.13 Activated sludge MLSS

Sludge return (%)

Sludgeloading rate (kg BOD/ kg MLSSday)

Average oxygen requirement (kg O2/ kg BOD)

Excess sludge (kg/kg BOD)

1500-2500

25 +

1.0-2.0

0.5

1.2

1500-2500

33 +

0.35-0.45

0.8

0.8

2500-3000

50 +

0.20-0.30

1.2

0.7 0.6

300(MOOO

50 +

0.15

Extended aeration

2500-6000

100 +

0.05-0.15

1.2 + 4.5 x NH3 N-oxidized 0.8-1.0 2.0

Oxygen activated sludge Deep shaft

4500-8000 4000-6000

25 + 100

0.4^1.0 0.4^1.0

0.8-1.2 1.3

Mode of operation

(mg/1)

High-rate Conventional (30/20 effluent) Conventional (10/10 effluent) Conventional with nitrification

0.4^-0.75 0.5-0.85

modifications of the activated sludge process have introduced anoxic zones (oxygen free) in which the oxidized nitrogen is denitrified to gaseous nitrogen, and is so removed. 29.12.5 Oxidation ponds Where land is cheap, and especially where sunshine is plentiful and temperatures high, sewage may be inexpensively treated in oxidation ponds. These are large lagoons, normally up to 4 ha in area, and usually about 1.0 to 2.5 m deep. The biological oxidation may be carried out by the same sort of organisms as act in the aerobic processes described in sections 29.12.2 to 29.12.4 above. Anaerobic organisms may also be used, as may a combination of the two commonly called 'facultative'. Some typical design parameters are given in Table 29.14. There may be odour problems from the anaerobic and facultative ponds.

Table 29.14 Some typical design parameters for oxidation ponds Operation

Anaerobic Recirculation of aerated effluent series

Pond size (ha) 0.2-1 Depth (m) 2.5-5 Retention time (days) 20-50 Temperature range 0 6-50 (Q optimum (0C) 30 BOD reduction (%) 50-85 Effluent SS (mg/1) 80-160

'Facultative' Mixed surface layer series or parallel

Aerobic Intermittently mixed series or parallel

I^ 1-2.5

4 1-1.5

7-20

1(MO

0-50 20 80-95 40-60

0-30 20 80-95 80-140

new microorganisms, and must effectively be removed if an effluent of high quality is required. Final settlement is, therefore, essential. It is also required in order to separate the activated sludge solids for return to the aeration tank. Activated sludge does not always settle easily. For example, high concentrations of carbohydrates, e.g. in brewery effluents, may cause special problems. It is essential, therefore, to consider the requirements for final sedimentation in designing the biological treatment stage. The solids to be settled in final tanks are somewhat lighter than those removed in primary settlement. Accordingly, the design upflow velocity is commonly limited to 1.5 m/h at peak flow. It may be desirable to reduce this even further to 1 m/h following extended aeration.

29.13 Tertiary treatment 29.13.1 Introduction It is sometimes necessary to produce a final effluent quality significantly better than the 30 mg/1 SS, 20 mg/1 BOD, which may be achieved with biological treatment methods. Such further treatment, commonly referred to as tertiary treatment or 'polishing', usually involves reduction of residual suspended solids, and hence associated BOD. Sand filtration, upflow clarification, microstraining or lagooning are commonly employed for this purpose; irrigation over grassland is also feasible where sufficient land is available.

Fluidized beds have long been used in chemical engineering to promote rapid and complete reactions. They are now beginning to be applied to the much more dilute conditions of sewage treatment, using both aerobic and anaerobic organisms.

29.13.2 Sand filters Filtration may be carried out using slow sand filters, rapid downflow filters or upward flow filters. Slow sand filters are not often installed on modern works since they occupy large land areas and maintenance costs are high. Rapid downflow filters involve passing the sewage effluent downwards through a bed of graded sand (up to 1.5 m deep), under the influence of gravity or pressure, and these can cope with loadings up to 50 times that of slow sand filters. The gravity type, operated with 3 to 4 m head and at loadings up to 10 m3/m2-h, is normally used in the sewage treatment field, rather than fully enclosed pressure filters. Periodic backwashing with treated effluent (1 to 3 times daily), typically at 10 l/m2-s, is required to prevent clogging of the filter medium, backwash flows being returned to the head of the treatment works for further treatment. An air scour is also commonly used to aid the backwashing process. During backwashing, flows to the filter are usually diverted to standby or other duty filters. However, package rapid downflow filters are available on the market, where filtration through each filter can be maintained without interruption, sand from the base of the filter being air scoured during continuous airlifting to the top where washing takes place prior to the clean sand particles reentering the main filtration zone. Removals of 80% SS and 70% associated BOD can be obtained with rapid downflow filters. Rather higher loading rates can be achieved with upward flow sand filters (up to 17m3/m2-h) for similar pollution removals. Backwashing involves increasing the effluent feed rate to achieve expansion of the bed which has first been loosened by an air scour. Whichever type of filter is employed, performance improves the more stable the suspended solids in the effluent feed are.

29.12.8 Final sedimentation It cannot be too strongly emphasized that about one-half of the BOD applied to the biological treatment stage is converted to

29.13.3 Upward flow clarifiers Upward flow clarifiers are sometimes used at small sewage works for effluent polishing. The secondary effluent passes

(After Metcalf and Eddy, Inc. (1979) Wastewater engineering: treatment, disposal, reuse, (2nd edn), McGraw-Hill) Note: Examples quoted relate to pond systems in North America. In tropical climates, anaerobic pond loadings may be considerably higher in the range 100 to 400 g BOD/m3-day giving 1 to 2 days' retention. Similarly, facultative pond loadings up to 300 to 400 kg BOD/ha-day may be appropriate.

29.12.6 Anaerobic treatment Organic matter may also be broken down, usually less completely, in the absence of oxygen by anaerobic microorganisms. The end-products are mainly carbon dioxide, methane and water, and the gas produced has a high calorific value and can be a useful source of energy. Anaerobic processes have been used for very many years for treatment of sewage sludge before final disposal (see section 29.15.5). Recently such processes have been successfully applied to the treatment of strong organic effluents, e.g. from production of starch from wheat. 29.12.7 Fluidized beds

through a shallow bed of pea gravel, a mesh screen or other suitable medium which removes suspended solids by flocculation and settlement. The filter medium is normally incorporated within the final clarifier(s) and is kept clean by simple backwashing procedures. Maximum hydraulic loadings are normally in the range 1.3 to 1.8m3/m2-h. 29.13.4 Microstrainers Microstrainers (or microscreens) involve the passage of effluent through a stainless steel fabric which is retained around the periphery of a rotating drum. Flow enters the drum through a single open end and flows radially out through the steel mesh. Solids collected on the inside of the drum are then removed by a continuous filtrate spray applied along the drum at the highest point, the washings being collected in a trough and returned to the works' inlet. Best performance is achieved when the secondary effluent is well oxidized. Suspended solids removals of 30 to 80% and 25 to 70% associated BOD can be achieved depending on the mesh size employed (65 to 15 um openings) and the hydraulic loading applied (typically 12 to 30m3/m2 fabric-day). 29.13.5 Lagoons Provision of lagoons as a final stage of treatment allows further flocculation and settlement of suspended solids and removal of associated BOD. Some further biological oxidation, reoxygenation and removal of bacteria also take place; a reduction in nitrate content due to denitrification, and of phosphate due to uptake by algae may also occur. In order to minimize the growth of algae, retention times should not normally exceed about 2 days, divided among several lagoons in-series. For typical depths of the order of 1 m, algal growths tend to increase the SS content of the final effluent, if retention periods exceed about 2.5 to 3 days. Lagoons often support a fish population and attract wildlife and, hence, have appreciable amenity value. However, the availability and cost of land normally determines whether they are to be preferred to alternative tertiary-treatment methods. Improved performance may be expected with a number of lagoons rather than a single lagoon of the same capacity. More than one lagoon is in any case desirable to provide flexibility of operation, particularly if solids deposition exceeds sludge solubilization and degradation, and desludging is eventually required.

Chlorination and ozonation both provide a very effective means of destroying bacteria and viruses in wastewaters and, in addition, have a remarkable effect on the colour and clarity of the effluent. Chlorination tends to be significantly cheaper than ozonation. However, its suitability for treating sewage effluents should take full account of the final destination of the effluent, since certain residual compounds can combine with chlorine to form more objectionable compounds.

29.14 Advanced treatment 29.14.1 Introduction The object of advanced treatment is to remove those pollutants which persist after conventional secondary treatment, but need to be removed to render the water suitable for reuse. Such pollutants include suspended, colloidal or dissolved, organic and inorganic compounds, as well as bacteria and viruses. Many processes exist for advanced treatment, and a few of these are outlined in the following paragraphs. 29.14.2 Chemical coagulation and flocculation The removal of fine suspended solids may be improved by the addition of chemical coagulants. These act by destabilizing the colloidal particles, causing them to settle out more rapidly. In addition, the coagulant may also precipitate out soluble salts by chemically combining or physically adsorbing on to the floe. Chemical coagulants include alum (aluminium sulphate), lime (calcium hydroxide), and ferric chloride, but only the former two can be recovered from the coagulation step. Organic polymers (polyelectrolytes) may be used in conjunction with, or in the place of, the inorganic coagulants in order to increase flocculation. Addition of coagulants to wastewater takes place in rapid mix basins in which very high velocities are obtained and retention times are of the order 0.5 to 2 min. Flocculation is achieved by inducing velocity gradients (using mechanical stirrers or diffused air systems), into the wastewater to increase floe collisions. Typical retention times for flocculation are 15 to 40 min. 29.14.3 Ammonia stripping

29.13.6 Irrigation over grassland This method of tertiary treatment is practised at many small and some medium-sized UK works. The grass plot retains suspended solids, thus improving effluent quality. Loadings are typically in the range 2000 to 5000 m3/ha-day depending on climate and soil structure for secondary effluents of consistent quality. Effluent is applied via feed channels or by spraying. Periodical rest periods for the plots in rotation are necessary to reaerate the soil, and regular grass cutting is desirable to encourage fresh growth. Removals of 60 to 80% SS and 50 to 75% BOD can be achieved together with some removal of bacteria, nitrogen and phosphorus and some increase in dissolved oxygen.

This unit operation is one of mass transfer in a packed tower, using a countercurrent flow of air and water. Ammonia is transferred from the water to the airstream and may be exhausted to the atmosphere or recovered in an absorption column with acid to precipitate the corresponding salt. However, there are two limitations to the system: (1) the efficiency of the process is significantly reduced as the ambient air temperature falls below 10°; and (2) calcium carbonate scale may deposit on the tower packing and reduce the mass transfer efficiency. Scale formation has been found to be readily removed by frequent light hosing with water. Ammonia-stripping towers have been successfully employed with ammoniacal removals of over 90%, achieved in 7 m towers with an air throughput of 1560m3/m3 water. For higher removals greater airflows are necessary.

29.13.7 Disinfection The disinfection of sewage effluents (e.g. by chlorination or ozonation) is not extensively practised in the UK, although it is widely practised in many countries around the world, particularly where waterborne diseases or parasites are prevalent, and where effluent reuse for irrigation or other purposes is required.

29.14.4 Recarbonation The addition of lime to wastewaters (e.g. in phosphorus removal) increases the pH to a value of 10 or higher, which is not only outside usual discharge limits, but is also detrimental to the operation of downstream units such as carbon adsorption. Recarbonation is employed to lower the pH of wastewater and

render it suitable for discharge. The pH is lowered by introducing carbon dioxide into solution. Generally the source of CO2 is stack, or oven, gas, although underwater burners are also used. Two forms of recarbonation are in use: single- and two-stage. Since the solubility of calcium carbonate is a minimum at pH 9.3, much of the CaCO3 present precipitates out in the first tank, and enables the lime to be recalcined and reused. Single-stage recalcination does not afford this saving, since little CaCO3 is precipitated. However, substantial savings can be made in capital and operating costs. In the former type the pH is lowered to about 7 in one tank, whereas in the latter case, two tanks are employed with possible intermediate settling. In this case, the pH is reduced to around 9.3 and 7 in the first and second tanks respectively. Single-stage recarbonation systems have typical residence times of 5min and two-stage systems 15 and 45min for the first and second stages respectively. 29.14.5 Granular activated carbon Removal of organic materials from wastewaters may be successfully achieved by adsorption on to a granular activated carbon (GAC) bed. Powdered activated carbon (PAC) is seldom employed, since it can usually be used only once and is therefore costly, and presents dust and disposal problems. The advantage of GAC systems is that they can accept shock hydraulic and organic loadings without a reduction in removal efficiency, and may be regenerated in an on-site furnace. Operational experience has shown that this is a simple and effective process, giving reductions of total organic carbon of more than 70%. A secondary, but desirable application of activated carbon, is as a pretreatment for membrane processes. 29.14.6 Membrane processes The introduction of membrane processes into wastewater treatment is relatively recent. The technique is one of selective removal of molecules or ions by a semi-permeable membrane. Three variations of this technique are discussed below. 29.14.6.1 Reverse osmosis Reverse osmosis (or hyperfiltration) is the most popular of the membrane separation processes. The technique employs a pressure difference greater than the wastewater's osmotic pressure to force the solvent (water) through the membrane, producing a very clean water on one side of the membrane and a concentrated solution of impurities on the other. Although a small amount of dissolved solids will remain, bacteria, viruses and other pathogens are effectively removed. The water quality obtained from a RO pilot shows a reduction in excess of 90% in total dissolved solids and chemical oxygen demand. The major problem with membranes is their potential to clog with suspended solids, chemical deposition or biological growths. Cleaning the membranes is accomplished by air/water scouring, although some chemical treatment may be necessary too. With domestic sewage a large proportion of the fouling matter is present as fats and oils, which have been successfully removed using detergents in conjunction with scouring. 29.14.6.2 Ultrafiltration Ultrafiltration uses a membrane to discriminate between the size and shape of molecules, whereas the membrane in reverse osmosis operates in a selective manner for the transport of water. This is the fundamental difference between the techniques. Operationally the difference lies in the applied pressure here it is significantly lower (0.5 to 7 bar) with typical water fluxes of 1 to 2.25 m3/m2-day. This method is applicable for the

separation of solutes of high molecular weight (e.g. 500) such as bacteria, viruses, starch and proteins. 29.14.6.2 Electrodialysis Electrodialysis is a technique for separating the ions of dissolved salts in solution. The ions are separated according to charge by the application of a current between two electrodes immersed in the solution. As the ions migrate to their respective electrodes they pass through a series of semi-permeable membranes which are ion-selective. The membranes are arranged with alternative selectivity, forming alternating zones of clean water and ion concentrate. The wastewater is pumped through the cell to achieve a retention time of 10 to 20 s. Scaling on the membrane may occur, but can be reduced by maintaining a lower pH with sulphuric acid. As in all the membrane processes, pretreatment is often necessary and prudent to avoid membrane fouling. 29.14.7 Ion exchange The soluble ions present in wastewaters may be effectively removed by substitution by insoluble ions of a different chemical species. This is the basis of ion exchange which is frequently used to demineralize water. The resin, an insoluble natural or synthetic material, needs to be regenerated once it becomes saturated with ions from the aqueous phase. Chemical treatment is employed to restore the resin. Two methods of operation are currently in use: batch and continuous exchange. In the batch mode of operation, the resin is added to the water and stirred until the reaction is complete. Spent resin is allowed to settle out and is removed. In the continuous mode, the resin is packed in columns and the wastewater is passed through the bed. It is often prudent to have two columns so that one may be regenerated without halting the flow of water. Ion exchange has applications in the treatment of municipal wastes in the removal of nitrates and phosphates. Using a strong-base anion exchanger, phosphate removals greater than 97% can be achieved, producing an effluent level of 0.2 p.p.m. Similarly, nitrate levels may be reduced by 90%, and COD by 45% with typical water throughputs of 200 x bed volume.

29.15 Sludge treatment 29.15.1 Introduction The volume of liquid sludge produced at a sewage treatment works typically amounts to some 1 to 2% of the total sewage flow. However, its treatment and disposal are usually major operations, accounting for as much as 50% of the operating costs of the works. The purpose of sludge treatment is to render the sludge more amenable to disposal, and to minimize the cost of disposal. 29.15.2 Character and amount of sludge Where primary sedimentation is practised, as at most sewage treatment works, about 60 to 70% of the suspended solids is normally removed, together with 30 to 40% of the associated biochemical oxygen demand (BOD). This sludge normally contains an average of about 5 to 6% dry solids, although the concentration at any one works will vary according to the frequency of sludging practised. This is commonly between once and three times daily, but where primary sludge is subsequently thickened in continuous-flow tanks, sludging will be more

frequent (e.g. every hour) giving thinner sludges of the order of 2% dry solids. Biological processes will contribute to the overall sludge volume produced in the works, but quantities generated depend on the type of biological treatment process operated. Yields of biological sludge are expressed as kilograms dry solids per kilogram BOD removed per day; examples, including typical dry solids contents are given in Table 29.15. In practice, actual yields of sludge requiring treatment and/or disposal will be within the ranges given in Table 29.15, less the dry weight of suspended solids discharged in the secondary sedimentation tank effluent; however, this loss will be relatively small in the case of secondary effluents conforming to the common 30 mg/1 SS, 20 mg/1 BOD standard. Biological sludges are frequently returned to the primary sedimentation tanks (where these are provided) and co-settled with primary sludge, although recent trends have been towards separate thickening of surplus activated sludge.

Table 29.15 Yields of biological sludge Process

Biological filters low-rate high-rate Conventional activated sludge Extended-aeration activated sludge Rotating biological contactors following removal of gross primary solids only following conventional primary sedimentation

disposal. The resultant liquors produced are normally returned for further treatment with the main sewage flow. The commonest form of thickening is by gravity, but centrifuging or, particularly in the case of surplus activated sludges, gas flotation or the use of a belt thickener, are sometimes employed. 29.15.4.1 Gravity thickening Gravity thickening, often aided by a rotating picket fence, is normally carried out in the UK as a batch process, three 1-day retention tanks commonly being provided, one each for filling, quiescent settlement and emptying respectively; in this case, supernatant liquors are removed by floating or swivel arm, telescopic weir or a series of controlled outlets set at different levels. However, the tanks may also be designed for continuous flow operation and may be used as storage tanks. Continuous flow tanks are primarily designed on the basis of solid loadings per unit area per day; examples of some typical figures are given in Table 29.16. The degree of thickening achieved by fill-and-draw and continuous flow tanks is dependent on the type and age of the sludge, and on its initial dry solids content, as well as on the design of the thickening tank provided. Increases in dry solids contents as a result of gravity thickening are commonly in the range 1 to 3% dry solids.

Biological sludge yield (kg dry solids/kg BOD removed -day)

Dry solids (%)

0.25-0.5 up to 1.0

0.5-2.0 0.5-2.0

Sludge type

Solids loading (kg dry solids per m2/day)

0.6-0.8

0.5-0.8

100-150 60-100

0.8-1.0

0.5-1.0

Raw primary Mixed primary/humus Mixed primary/surplus activated

Table 29.16 Tank loadings - continuous-flow sludge thickeners

0.7-0.8

1.0-3.0

0.5-0.6

1.0-3.0

Where tertiary treatment is practised, the weight of dry solids removed at this stage, which is returned for treatment and disposal, should be added to the quantity of biological solids produced. Additional quantities of sludge will also be produced where chemical treatment is practised, e.g. for the removal of phosphorus. 29.15.3 Screening Sewage is normally screened or comminuted on arrival at the sewage treatment works. However, since such screening is never completely effective, and because shredded rags can tend to 'ball up' and cause blockages in sludge pipelines and in treatment equipment, sludge is also sometimes screened before treatment. Manually cleaned screens (19mm bar spacings) or, for biological sludge alone, mesh screens (3 to 5 mm) may be used, e.g. rotary brush type, are suitable; screenings should be disposed of together with other works sludges. The alternative of sludge comminution is also sometimes practised. 29.15.4 Sludge thickening Thickening of sludge is frequently carried out to reduce the volume of liquid sludge requiring subsequent treatment and/or

40-80

29.15.4.2 Flotation thickening Sludge thickening by gas flotation is sometimes considered, where separate thickening of surplus activated sludge is required. Activated sludges are difficult to concentrate by gravity alone, but flotation thickening to 4 to 5% dry solids can be achieved by dissolved air or electrolytic flotation, the former being the more common of the two processes. Polyelectrolyte flocculant (1 to 4kg/t dry solids) is usually added to achieve efficient thickening in the dissolved air process, but owing to charge neutralization effects, flocculant aids are not normally necessary for the electrolytic process. Dissolved air flotation includes dissolving air into a water stream under pressure, usually the subnatant liquors or works final effluent, and introducing a mixture of this stream and the raw surplus activated sludge into the bottom of a flotation tank. The associated release of pressure generates very small air bubbles and the air/sludge mixture rises to the surface of the tank, whence it is removed by surface scraper. The clarified liquor is returned to the main treatment works or to the recycle stream for saturation with air and reuse. Electrolytic flotation is achieved by passing the sludge between a grid of electrodes operating with an applied d.c. potential. The resulting electrolysis produces very small gas bubbles which enable thickening effects similar to those achieved with dissolved air flotation to be achieved. The sizing of both systems is based on solids loading per unit area of tank surface per hour, a figure of 10kg dry solids/m2-h commonly being used for design purposes.

29.15.4.3 Centrifuges Two types of centrifuge are available for thickening of sewage sludges; the nozzle bowl or disc stack type for thickening surplus activated sludge, and the solid-bowl type for either activated sludge or mixed co-settled sludges. The nozzle- or disc-type centrifuge is a vertical spindle machine rotating at 900 to 3300 rev./min. The internal bowl contains a stack of conical discs which provide a large surface area to aid settling of solid particles. To minimize wear and risk of blockage, screening of the sludge before pumping through the centrifuge is required. Centrifuges of this type can achieve thickened sludges of up to 6% dry solids without polyelectrolyte flocculant addition giving a 90% solids recovery. The solid bowl centrifuge comprises a horizontally mounted cylinder with a tapered section forming a 'beach' for the solids, up which they are conveyed by the scroll rotating at a speed slightly higher than that of the bowl. The performance of this type of machine depends on: (1) the type of sludge; (2) the type of polyelectrolyte being used; (3) the bowl/screw conveyor speed difference; and (4) the liquid level maintained in the centrifuge. It is more commonly used, in conjunction with polyelectrolytes, to dewater sludge to a stackable cake, but can be used to produce a thickened slurry if the liquid is maintained above the discharge port. Polyelectrolyte dosing is normally essential to achieve efficient solids recovery. 29.15.4.4 Belt thickening Gravity thickening of surplus activated sludge, with addition of polyelectrolyte, has recently been developed using belt machines such as the Aquabelt manufactured by Simon Hartley Ltd. Thickened sludges of 4 to 5% dry solids can be produced. This type of machine incorporates a travelling belt on to which flocculated sludge is discharged. As the sludge travels along the length of the machine, thickening occurs with sludge liquors passing through the porous belt cloth. This development is a modification of the free-draining zone incorporated at the front end of belt presses which are commonly used for dewatering sludges to give a sludge cake. 29.15.5 Anaerobic sludge digestion Anaerobic digestion of raw primary, or mixed primary/secondary sludges, is carried out in the absence of oxygen, and results in conversion of organic matter into soluble and gaseous products. The process changes a malodorous sludge into one which is relatively inoffensive, destroying grease and reducing numbers of certain pathogenic organisms. The process has developed from cold digestion in open tanks to the modern mesophilic system with covered tanks operated at 30 to 350C, utilizing the gas for heating or power generation. The sludge gas normally consists of about 65 to 70% methane, the remainder being mainly carbon dioxide with a small amount of other gases; typically the gross calorific value is 24 000 to 26 000 kJ/m3. Process efficiency depends on the type of sludge and the extent to which inhibitory substances may be present (e.g. heavy metals), the method of addition, the degree of internal mixing within the digester, the retention period and the operating pH and temperature. For a 20-day retention at 30 to 350C the organic content of the sludge is reduced by 40 to 50% (equivalent to 30 to 40% total solids reduction), and of the order of 1 m3 sludge gas per kilogram volatile matter-day is produced (equivalent to about 0.03 m3/hd-day). In the past, many heated digesters have been designed for longer retention periods (25 to 30 days) to provide a margin of safety, although in theory a digestion period of about 10 days is adequate. In practice, some safety margin is desirable to deal with inadequate mixing or sludge-

loading variations, but with improvements in available mixing and heating systems, retentions of 15 to 20 days can be quite satisfactory. Cold digestion is practised at many small treatment works. Mesophilic digestion at 30 to 350C is, however, preferred for improved efficiency. Thermophilic digestion (40 to 5O0C) is not practised in the UK. Digester mixing may be carried out by mechanical mixing, by internally or externally mounted gas-lift pumps (confined mixers), or by passing digester gas through floor-mounted diffusers (unconfined mixers). Mixing systems, including use of the gas, combine the advantages of good mixing efficiency with the benefit of having no moving parts within the digester, thus avoiding maintenance problems. Heat-exchange units can be incorporated within the sludgemixing system or as a separate circuit. Heat is supplied from a boiler which burns the methane gas or an alternative fuel. Gasholding capacity may be provided by means of floating covers on the digesters or by means of separate, normally floating-roof, gasholders. Sludge gas, surplus to that required for heating the digester contents, is commonly used for space heating or for power production. Sludge gas has also been used for incineration of screenings, sludge drying and, in an emergency, for vehicle propulsion. At large sewage treatment works, the total yield of sludge gas produced during anaerobic digestion is often used for power generation, the waste heat from the engines being recovered in the form of hot water for heating the sludge. The power produced may be used to generate electricity or for driving air blowers or pumps. Gas consumption typically lies in the range 0.45 to 0.55 m3/k\Vh. Following heated digestion, the digested sludge is generally passed to storage tanks or lagoons (commonly referred to as secondary digesters) prior to dewatering and/or off-site disposal. They enable the sludge to cool and allow some thickening to take place, supernatant liquors being returned to the sewage stream for further treatment. Retention periods vary from a few days to often more than 30 days. Rotating picket-fence mechanisms are often incorporated. 29.15.6 Aerobic sludge digestion The alternative of aerobic sludge digestion involves partial oxidation of sludges utilizing aerobic microorganisms supported by aeration. It serves the function of stabilizing the sludge to minimize odour nuisance and of reducing the solids content. The application of the process is limited within the UK, tending to be confined to use with small package sewage treatment plants. Under UK climatic conditions, surplus activated sludges and mixed primary/secondary sludges require a minimum aeration period of 10 to 15 days and around 20 days respectively to achieve a solids reduction of 30% and a relatively odour-free sludge. However, the resultant digested sludges often do not thicken readily and the overall dewatering properties of the sludge may also deteriorate. Aeration is normally carried out using mechanical or diffused-air systems. Electrical power consumption is high and solids concentrations should not exceed 1.5 to 2% to avoid problems of poor mixing and inadequate aeration. Oxidation rates increase with increasing temperature. Thermophilic aerobic digestion has been considered for certain applications, particularly where efficient removal of pathogenic organisms is required, but data currently available on this process are limited. 29.15.7 Sludge dewatering Sludge dewatering produces a readily handleable cake, normally

containing at least 15% dry solids and possibly up to 40% dry solids or more, depending on the type of sludge and method of dewatering concerned. In the early part of this century, drying beds, consisting of a sand and gravel bed overlying tile underdrainage, were in widespread use for dewatering sludge. In the UK, changing disposal strategies and the development of a range of mechanical dewatering equipment have resulted in land-intensive drying beds becoming less attractive in favour of direct disposal of sludge in liquid or mechanically dewatered form. 29.15.7.1 Filter presses Mechanical dewatering processes include filter plate and filter belt pressing, centrifuging and vacuum filtration. A filter plate press comprises a series of chambers formed between recessed plates. An appropriate filter cloth is fitted over the surfaces of each plate. The plates are closed together and held hydraulically, or by screws, to withstand the applied filtration pressure supplied by positive-displacement pumps/Filtrate passes out of the press via ports and is returned to the sewage stream for further treatment, leaving the sludge solids retained within the chambers as a cake. Lime with an iron salt, aluminium chlorohydrate or polyelectrolyte are commonly used to condition the sludge prior to feeding the press on a batch basis. Sludge cakes of 30 to 40% dry solids are normally achieved. With the press mounted in a building at first-floor level, cakes can readily be dropped into a lorry or skip. 29.15.7.2 Filter belt presses Filter belt presses (or band filters) involve the continuous discharge of liquid sludge, normally conditioned with polyelectrolyte (1 to 4 kg/t dry solids) on to a moving open mesh, endless roller-mounted belt (0.5 to 2.5 mm wide) which acts as filtering medium. This belt then converges with a second belt to produce pressure on the sludge layer. At the discharge end, a doctor blade lifts the sludge cake from the belt which is then washed with high-pressure water, or effluent sprays, before travelling back to the sludge inlet end of the machine. Since the early machines, more rollers have been introduced and, hence, better performance can now be achieved. Some designs include vertical belts, spring loaded rollers and caterpillar tracks. Sludge cakes typically lie in the range 15 to 35% dry solids, depending on the type of sludge, polyelectrolyte dose and machine design. 29.15.7.3 Centrifuges The use of centrifuges has already been described as a method of thickening sludges prior to further processing and/or disposal. The solid bowl (or decanter-type) centrifuge is most commonly used for dewatering sewage sludges. With polyelectrolyte dosing (2 to 5 kg/t dry solids) of mixed primary/secondary and digested sludges, sludge cakes averaging 20 to 25% dry solids can usually be achieved associated with solids recovery efficiencies of 90 to 99%. As is frequently the case with sludge-dewatering processes, performance improves the thicker and fresher the sludge feed. 29.15.7.4 Vacuum filters Rotary drum vacuum filters consist of a cylindrical drum covered with a filtration medium, normally cloths or stainless steel coils, and rotating partially submerged in a tank of sludge. With chemically conditioned sludge in contact with the external face of the filtration medium and the application of a vacuum to the internal face, liquor is drawn through leaving a dewatered sludge retained on the revolving outer surface, which can then be removed by a tine bar or doctor blade. Lime with an iron salt,

aluminium chlorohydrate or a polyelectrolyte can be used for conditioning the sludge, either of the latter two being more common today, since problems of scale formation giving increased wear and reduced efficiency of the vacuum are then minimized. Sludge cakes from 20 to 25% dry solids, and occasionally higher, can be produced depending on the particular application. A large number of rotary vacuum filters were installed in the UK during the 1960s; however, they are less popular today, important limitations being the relatively low dry solids content of the cake compared with plate presses and modern belt presses, high maintenance costs and the need for experienced operators to optimize performance. Vacuum disc filters differ in design from rotary vacuum filters, but operate on similar principles, and are simpler to operate. They are suitable for use at small sewage treatment works. In the UK, aluminium chlorohydrate has been the more usual conditioning aid, and using mixed raw sludge as a feed, sludge cakes typically in the range 15 to 18% dry solids can be achieved. 29.15.8 Other sludge treatment processes Elutriation, as a sludge conditioning process, is sometimes employed, although it is not common today for new works. The process involves washing the sludge to remove fine suspended solids which reduces the chemical conditioner requirements prior to mechanical dewatering. It also reduces the alkalinity of sludge (particularly by removing ammoniacal compounds) prior to anaerobic digestion. Heat treatment of raw sludge at temperatures up to 25O0C, with or without the introduction of air, is sometimes practised as a conditioning process. Although very effective, major disadvantages include the emission of strong odours, production of strong sludge liquors, which are difficult to treat, and high operational and maintenance costs. Following dewatering of sewage sludge to produce a cake (using drying beds or mechanical processes), thermal drying can be practised, where a granular product containing 85 to 90% dry solids is required. Oil, digester gas or other fuel is used to produce hot air for the drying process. The process is expensive and is normally considered only where the dried product can be sold, e.g. as a fertilizer or soil conditioner.

29.16 Sludge disposal Following treatment, or in a small number of situations without any treatment, sludge residues must be disposed of off-site. The method of disposal selected will usually depend upon the outlets available and on the characteristics of the sludge, in particular its toxic metal content. It is normal for the ultimate disposal route to influence the type of treatment practised prior to disposal, rather than vice versa. Sludge treatment and disposal are costly, and the objective is to obtain a balance between treatment and disposal, to discharge the sludge to the environment safely and at the minimum possible cost. The principal methods of sludge disposal are: (1) as liquid to agricultural land, usually and preferably after digestion; (2) as a liquid at sea, often after digestion; (3) as a cake to agricultural land or to tip; and (4) as cake to incineration, the ashes produced being dumped to tip. In the UK, disposal as a liquid to agricultural land is favoured where enough land is within reasonable reach of the treatment works and providing the sludge is of an acceptable quality. Disposal of liquid sludge to sea is also favoured, particularly for large conurbations (i.e. large amounts of sludge and more distant agricultural land) with access to a sea terminal. In both cases it is common practice to thicken the sludge prior to disposal to reduce its volume, and,

hence, the disposal costs. The disposal of sludge cake to agricultural land is less favoured because of the cost of dewatering to a cake, and the difficulty of spreading it. Incineration is costly and is generally practised only when the sludge is unsuitable for land disposal because of toxicity or due to the absence of a suitable dumping site. Dumping on sacrificial land is usually preferred to incineration for toxic sludges because it is cheaper. However, it is not always possible to establish environmentally safe dumping sites of adequate capacity within economical transport distance. In the past, sludge was disposed of to the environment with little concern for any adverse effects other than, perhaps, odour nuisance. An increasing level of environmental awareness, coupled with a fuller understanding of potential hazards, led to the introduction of controls upon sludge disposal in many countries. In Britain, sludge disposal to agricultural land is not subject to national legally enforced control at present. However, Regional Water Authorities and other responsible bodies do apply their own control measures working to guidelines laid down by the Department of the Environment/National Water Council20 and by the Ministry of Agriculture, Fisheries and Food.21-22 The guidelines specify the maximum load of toxic elements that can be added to agricultural land over a 30-year period. The load is calculated as individual elements and as 'zinc equivalent'; the latter concept permits the relative toxicity of the three most common and harmful toxic metals to be allowed for and quantified in a single figure. Zinc equivalent is the sum of the zinc content, twice the copper content and 8 times the nickel content of a sludge. The dumping of sludge at sea is practised according to two international Conventions as embodied in the Disposal at Sea Act 1974. Sludge discharged to sea via pipelines is effectively not controlled at present, although mechanisms exist. All sludge dumping activities are controlled by licence. The licence imposes pragmatically based limitations depending on the nature of the dumping grounds and the quantity of the sludge in mind. It is not possible to give typical standards, but the most stringent control is on persistent and recalcitrant materials, in particular mercury and cadmium. The dumping of sludge by tipping on sacrificial land is subject to normal solid waste disposal legislation in Britain, under which tipping sites are licensed and controlled by Waste Disposal Authorities. Controls on incineration relate to the dumping of the ash as a solid waste and the discharge of gaseous emissions to the atmosphere. Other countries have their own control mechanisms which may or may not impose stricter constraints upon sludge disposal activities. It is not possible to generalize. It is however, important to note that measures are being taken by the European Community to standardize the control of sludge disposal to agricultural land and to the sea. It may be expected that EEC Directives will be issued and that they will impose limitations and controls at least as stringent as those now applied in Britain.

29.17 Intermediate technology In many of the poorer developing countries there are large areas where not even a piped water supply exists, let alone a sewerage system. Sanitation practices are often based on open air defecation sites, which may be fields, waste land, or alleyways; it appears that no understanding exists of the link between faecal contamination and health. It is to these areas that the UN International Drinking Water Supply and Sanitation Decade is principally addressed. There is no possibility that conventional sanitation facilities could be introduced in all of these areas: the cost is unaffordable. In the more densely populated conurbations, some form of waterborne sewerage system could be

shown to be essential; elsewhere, consideration must be given to low-cost solutions to allow the benefits of sanitation to be enjoyed by the maximum number of people. The most common low-cost sanitation system in the past was the earth closet, usually associated with a nightsoil collection system in urban areas. Whilst this system is still widely used it cannot be commended. It offers a health hazard to those handling the containers and, as is often the case, when the crude excreta are used as fertilizer. Other methods, such as squatting huts built over fish ponds or excreta chutes to pig pens may be less unhygienic but do carry hazards and, in any case, cannot have a wide application. Biogas installations, which were originally developed in India in 1938, are being used in some countries with variable success. Here, biodegradable human, animal and vegetable wastes are anaerobically decomposed to produce a relatively harmless fertilizer and digester gas. The gas can be used for domestic cooking, heating and lighting. The concept is attractive, but has not gained widespread international favour, because of the relative complexity of design, the cost of the installation, the risk of process instability and the possibility of explosion or fire. In recent years much attention has been paid to privies of simple design and affordable construction. Privies vary from simple pits or shafts with a squatting plate above, to composting chambers into which both excreta and vegetable waste may be disposed. Refinement may be introduced by substituting the squatting plate by a water-sealed unit, hand-flushed with small volumes of water. Where pour-flushing cannot be afforded, or availability of water is inadequate, fly and odour nuisance should be minimized. This is best done by ensuring that light is excluded from the privy and that the collection pit is ventilated by a pipe. The pipe should be at least 75, and preferably about 200 mm, in diameter, should be painted black and sited in the sunniest position to encourage an upflow of air, and should contain a mesh filter to trap flies, since any entering the pit will tend to escape towards the light at the top of the pipe. Properly designed and maintained, ventilated pit latrines can be aesthetically and hygienically acceptable. There are usually two pits associated with these simple latrines, one in use and one standing full. Each pit should have at least 1 year's capacity and up to 15 years is used in practice. The full pit should not be emptied until it has been standing for a year or more, by which time its contents are a relatively harmless compost. Whether or not a pour-flush facility is installed it is desirable to ensure that liquid can soak away from the pit; otherwise it will fill too quickly and the contents will be less easy to dispose of safely. Where liquid can drain from the pit the risk of groundwater pollution must be avoided. Latrines are designed to accept only human excreta; sullage Cgrey water') must be disposed of separately. Often it may be discharged to a soakaway, or used for irrigation. However, in other than the most rural situation it is common to discharge sullage haphazardly to surface water drains. Since surface water systems are often crude and poorly designed the practice frequently leads to situations where health is at risk and which are aesthetically objectionable. When considering low-cost sanitation, safe sullage disposal must be given equal emphasis. Pit latrines are suitable only where water usage is relatively low; they should not be associated with normal toilet flushing practices. Where water is available for normal flushing, and the facility can be afforded, the lowest cost acceptable system is the septic tank. Septic tanks are designed to discharge an effluent usually to a soakaway. Again, groundwater pollution must be avoided. Where septic tanks exist, or are installed in relatively densely populated areas, they can be upgraded by collecting their effluents into a small-bore sewerage system for central treatment. However, whether or not this is done, a septic tank must

be sludged periodically, ideally once or twice a year, and the facility must exist to meet this relatively costly need. The overall concept of upgrading is important and must be borne in mind at design stage. As water availability improves, and a society becomes more affluent, the need will be perceived to move, perhaps, from simple pit latrines, to pour-flush latrines, to septic tanks and even eventually to main drainage. A long-term goal should be set appropriate to the area and each step towards it should be taken with the sequential development of the sanitation plan in mind. The need to generate reliable comparative costs is basic to the final evaluation of the options that are judged to be viable in technical terms. The World Bank has published a summary for the technologies of total annual economic cost per household;22-23 their data are reproduced in Table 29.17, which shows not only total costs but also a breakdown into on-site, collection and treatment costs. As the World Bank points out, the figures do not divide clearly into community and individual systems as might have been expected. However, it is evident that the choice among the three groups of technologies is clearcut, with large buffer areas available for up-grading any system within any one group.

Table 29.17 Average annual on-site, collection and treatment costs per household23 Average annual costs per household ($US

1978) Mean On-site total costs costs Low cost Pour-flush toilet Pit privy Communal toilet Vacuum truck cartage Low-cost septic tanks Composting toilets Bucket cartage

18.7 28.5 34.0

18.7 28.5 34.0

37.5

16.8

51.6

51.6

55.0 64.9

47.0 32.9

Medium cost Sewered aquaprivy Aquaprivy

159.2 168.0

High cost Septic tanks Sewerage

269.2 400.3

201.6

Collection Treatment costs costs

14.0

6.6

26.0

8.0 6.0

89.8 168.0

39.2

30.2

332.3

25.6 82.8

11.3 115.9

Intermediate technology is an important aspect of environmental engineering. Considerable practical experience now exists, and it is possible to select and design the system that will most closely meet local economic, geographic and ethnic needs. However, it must be emphasized that whilst low-cost sanitation systems may also be low-technology systems, this does not mean that optimum solutions can be established inexpertly. Low-cost systems must be selected with the same care and expertise that are required when working with more conventional technology.

References 1 Department of the Environment and National Water Council (1981) Design and analysis of urban storm drainage, vols 1-5. National Water Council, London.

2 Hydraulics Research Station (1977) Tables for the hydraulic design of pipes (3rd edn). HMSO, London. 3 Transport and Road Research Laboratory (1976) A guide for engineers to the design of storm sewer systems. Road Note No. 35 (2nd edn). HMSO, London. 4 Ministry of Housing and Local Government (1970) Technical committee on storm overflows and the disposal of storm sewage. Final report. HMSO, London. 5 Hydraulics Research Station (1978) Charts for the hydraulic design of channels and pipes (4th edn). HMSO, London. 6 Moore, D. R. and Gotham, K. V. (1978) The mechanical properties of uPVC in relation to pressure pipes.' The Pub. Health Engr, 6, 239. 7 Kirby, P. C. and Ridgway, J. W. (1982) 'Recent developments in rubber joint rings for water mains.' Proceedings of conference on the use of plastics and rubber in water and effluents, 6.1.-6.15. Plastic and Rubber Institute, London. 8 Olliff, J. L. (1982) Factors of safety in the structural design of large sewers. Proceedings of the 1 st International Seminar on Urban Drainage Systems, Southampton University. 9 Griffiths, I.W. 'Sulphide control in rising mains.' Water Pollution Control, 80, 5, 654-647. 10 Manganaro, C. A. (1968) 'Design for unbalanced thrust for buried water conduits.' /. Amer. Wat. Wks Assoc, 60, 6, 705-716. 11 Johnson, M. (1981) First report on the WRC sewage sludge pumping project. Technical Report No. TR162. Water Research Centre/British Hydromechanics Research Association. 12 Prosser, M. J. (1977) The hydraulic design of pumps, sumps and intakes. British Hydromechanics Research Association/Construction Industry Research and Information Association, London. 13 Frost, R. C. (1983) How to design sewage sludge pumping systems. Technical Report No. TR 185, Water Research Centre, Swindon. 14 Young, O. C. (1978) A review of practice and recommendations in making connections to pipe sewers. Occasional Technical Paper No. 1, National Water Council, London. 15 Irving, D. J. and Smith, R. J. H. (1983) Trenching practice. Report No. 97. Construction Industry Research and Information Association, London. 16 Parkinson, R. W. and Giles, R. G. (1983) Water Research Centre and Water Authorities Association. Sewerage rehabilitation manual. WRC Engineering. Swindon. 17 Department of the Environment and National Water Council (1977) Methods for the examination of waters and associated materials. London (1977 and continuing). 18 American Public Health Association, American Water Works Association and Water Pollution Control Federation (1981) Standard methods for the examination of water and wastewaters (15th edn) ALPHA, New York. 19 Callely, A. G., Forster, C. F. and Stafford, D. A. (eds) (1977) Treatment of industrial effluents. Hodder and Stoughton, London; Koziorowski, B. and Kucharski, J. (1972) Industrial waste disposal. Pergamon Press, Oxford. 20 Department of the Environment and National Water Council (1981) Report of the sub-committee on the disposal of sewage sludge to land. Standing Committee on the Disposal of Sludge, Report No. 20. 21 Ministry of Agriculture, Fisheries and Food (1971) Permissible levels of toxic metals in sewage used on agricultural land. ADAS Advisory Paper No. 10. 22 Ministry of Agriculture, Fisheries and Food (1978) The use of sewage sludge as a fertiliser. AF 51. 23 International Bank for Reconstruction and Development (1979) Appropriate sanitation alternatives: a technical and economic appraisal. World Bank, Washington DC.