Chapter 6 - Operation and maintenance of on-site sanitation
Any review of on-site sanitation shows that there are a large number of options to choose from. This is to be expected, since every project has different characteristics, requiring a different solution. Many of the alternatives are variations on, or combinations of, other designs and it is not possible to describe them all. Those planning on-site sanitation should adopt and combine the major options described in any way that will produce the most appropriate solution.
This chapter describes how the different types of latrine introduced in Chapter 4 work and discusses their relative merits. Details of construction of individual parts is given in Chapter 7 and design examples in Chapter 8.
The principle of all types of pit latrine is that wastes such as excrete, anal cleaning materials, sullage and refuse are deposited in a hole in the ground. The liquids percolate into the surrounding soil and the organic material decomposes producing:
In one form or another, pit latrines are widely used in most developing countries. The health benefits and convenience depend upon the quality of the design, construction and maintenance. At worst, pit latrines that are badly designed, constructed and maintained provide foci for the transmission of disease and may be no better than indiscriminate defecation. At best, they provide a standard of sanitation that is at least as good as other more sophisticated methods.
Simplicity of operation and construction, low construction costs, the fact that they can be built by householders with a minimum of external assistance, and effectiveness in breaking the routes by which diseases are spread, are among the advantages that make pit latrines the most practical form of sanitation available to many people. This is especially true where there is no reliable, continuous and ample piped water supply.
Unfortunately, past failures, especially of public facilities, discourage some sanitation field workers from advocating their widespread use. Objections to the use of pit latrines are that poorly designed and poorly constructed latrines produce unpleasant smells, that they are associated with the breeding of undesirable insects (particularly flies, mosquitos and cockroaches), that they are liable to collapse, and that they may produce chemical and biological contamination of groundwater. Pit latrines that are well designed, sited and constructed, and are properly used need not have any of these faults.
As a general rule, pits should be designed to last as long as possible. Pits designed to last 25-30 years are not uncommon and a design life of 15-20 years is perfectly reasonable. The longer a pit lasts, the lower will be the average annual economic cost and the greater the social benefits from the original input.
In some areas, ground conditions make it impractical to achieve such a design life. If the maximum possible design life is less than ten years, serious consideration should be given to using an alternating double-pit system. In such systems the pits must have a minimum life of two years. In the past, a minimum life of one year was considered sufficient for ensuring the death of most pathogenic organisms, but it is now known that an appreciable number of organisms can live longer (see Chapter 2). In any event the increased cost of designing a pit to last two years as compared to one designed to last one year is minimal because of decomposition and consolidation of the first year's sludge (see Chapter 5).
The depth of the pit to some extent affects the plan shape. Deep pits (deeper than about 1.5 m) are usually circular, whereas shallow pits are commonly square or rectangular. As the pit gets deeper the load applied to the pit lining by the ground increases. At shallow depths, normal pit linings (concrete, brick masonry, etc.) are usually strong enough to support the soil without a detailed design. Also square or rectangular linings are easier to construct. At greater depths, the circular shape is structurally more stable and able to carry additional loading.
Commonly, pits are 1.0-1.5 m wide or in diameter, since this is a convenient size for a person to work inside during excavation. The cover slab required is simple to design and construct, and cheap to build.
The emptying of single pits containing fresh excrete presents problems because of the active pathogens in the sludge. In rural areas, where land availability is not a constraint, it is often advisable to dig another pit for a new latrine. The original pit may then be left for several years
and when the second is filled it may be simplest to re-dig the first pit rather than to excavate a new hole in hard ground. The sludge will not cause any health problems and is beneficial as a fertilizer. However, in urban areas, where it is not possible to excavate further holes and where the investment in pit-lining and superstructure has been substantial, the pit must be emptied.
From the public health point of view, manual removal should be avoided. Where the groundwater level is so high that the pit is flooded or where the pit is sealed and fitted with an effluent overflow, the wet sludge can be removed by ordinary vacuum tankers. These tankers are the same as those used for emptying septic tanks or road gullies (Fig. 6.1). Hand-powered diaphragm pumps have so far proved to be very slow and laborious in emptying pits and have not been widely adopted.
Where pits are mainly dry, the greater part of the contents will have consolidated into solid material which conventional vacuum tankers cannot lift. In addition to this difficulty Boesch & Schertenleib (1985) summarized pit emptying problems as follows.
High-performance vacuum tankers able to deal with consolidated pit latrine sludge have been developed (Carol!, 1985; Boesch & Schertenleib, 1985) and are able to exhaust sludge over a horizontal distance of 60 m, thereby getting round problems of accessibility. However, considerable time is needed to set up and then dismantle and wash out the suction pipes.
As an alternative, the pump and tank may be mounted on a small, highly manoeuvrable site vehicle or on separate small vehicles in order to reach a latrine with limited accessibility. The disadvantage of using a smaller tank is that more journeys to the disposal point are required. Consequently, the suction pump is unused during this waiting period unless several small tankers are used with each pump. This can lead to a considerable increase in costs, particularly where disposal points are distant from the latrines. Larger-capacity transfer tankers may be employed to ensure best use of the costly vacuum pump.
Another approach involves the use of a container which can be manhandled close to an otherwise inaccessible latrine, even through the house where necessary. Small-diameter, clean vacuum lines connect the container to the distant tanker, providing the suction necessary to fill the container (Fig. 6.2). A fail-safe method of shutting off the sludge intake when the container is full is required to prevent sludge being carried through the air-line into the vacuum filter and engine. The containers have to be of such a size that they can be manhandled safely when full but also that the least possible number of container movements is required for each pit (Wilson, 1987).
All these systems are relatively expensive and require efficient mechanical maintenance to ensure reliability. The least sophisticated system should be used wherever possible for the majority of pit emptyings.
Simple pit latrines
The simple pit latrine (Fig. 6.3) consists of a hole in the ground (which may be wholly or partially lined) covered by a squatting slab or seat where the user defecates. The defecation hole may be provided with a cover or plug to prevent the entrance of flies or egress of odour while the pit is not being used.
The cover slab is commonly surrounded by some form of superstructure that provides shelter and privacy for the user. The superstructure design is irrelevant to the operation of the latrine but crucial to the acceptability of the latrine to the user. Superstructures range from a simple shelter of sacks or sticks to a building of bricks or blocks costing more than the rest of the latrine. The choice of superstructure will reflect the income and customs of the user.
The cover slab should be raised at least 150 mm above the surrounding ground to divert surface water away from the pit. Commonly, the cover slab sits directly on the lining, but if the lining is made of very thin material, such as an old oil drum, a concrete foundation beam may be necessary to distribute the load of the slab to the lining and surrounding ground (Fig. 6.4).
The simple pit latrine is the cheapest form of sanitation possible. Once constructed it requires very little attention other than keeping the latrine area clean and ensuring that the hole cover is in place when the latrine is not in use. Unfortunately the superstructure frequently becomes infested with flies and mosquitos and full of pungent odours because users do not replace the squat hole cover after use. Self-closing hole covers have been tried but are often disliked because the cover rests against the user's back. There may also be resistance to constructing new simple pit latrines because of their resemblance to existing, badly constructed, pit latrines.
Ventilated pit latrines
These are also known as ventilated improved pit (VIP) latrines (Fig. 6.5). The major nuisances that discourage the use of simple pit latrines - smell and flies - are reduced or eliminated through the incorporation of a vertical vent pipe with a flyscreen at the top (Morgan, 1977). Wind passing over the top of the vent pipe causes a flow of air from the pit through the vent pipe to the atmosphere and a downdraught from the superstructure through the squat hole or seat into the pit. This continuous flow of air removes smells resulting from the decomposing excrete in the pit and vents the gases to the atmosphere at the top of the vent pipe rather than through the superstructure. The flow of air is increased if the doorway of the superstructure faces the prevailing wind (Mare, 1984). If a door is fitted it should be kept shut at all times (except when entering or leaving) to keep the inside of the latrine reasonably dark, but there should be a gap, normally above the door, for air to enter. The area of this gap should be at least three times the cross-sectional area of the vent pipe.
The superstructure can be constructed in the form of a spiral (Fig. 6.6). This excludes most of the light whether a door is fitted or not. The defecation hole must be left open to allow the free passage of air. The vent pipe should extend at least 50 cm above the latrine superstructure except where the latter has a conical roof, in which case the pipe should extend as high as the apex. Air turbulence caused by surrounding buildings or other obstructions may cause reverse air flow, leading to foul odours and flies in the superstructure. If mean wind speeds are about 2 m/s, as is fairly common in rural areas, air speeds in the vent pipe are about 1 m/s (Ryan & Mara, 1983). Air flow may also occur at lower wind speeds because of solar radiation heating the air in the vent pipe, causing the air to rise. The vent pipe should then be placed on the equator side of the superstructure. It may be painted black to increase solar absorption, if the material of the pipe is not itself black.
In latrines relying on solar radiation for ventilation, foul odours are sometimes experienced in the superstructure at certain times of the day (usually early morning). This occurs where the outside air temperature is colder than the air in the pit, which may prevent the air circulating. Very little can be done to prevent this, other than sealing the defecation hole at night.
In addition to removing odours from the pit, the screened vent pipe significantly controls flies. In Zimbabwe, Morgan (1977) compared the number of flies leaving the squat hole of a VIP latrine with the number leaving a simple pit latrine. The results are shown in Table 6.1.
Flies are attracted to the pit by the odour coming from the vent pipe but are unable to enter because of the screen. A few flies enter the pit through the squat hole or seat, and lay eggs in the pit. New young flies attempt to leave the pit by flying towards the light. If the latrine superstructure is kept sufficiently dark, the major source of light is at the top of the vent pipe, but the screen prevents the flies from escaping there and they eventually fall back into the pit to die.
Well-constructed and maintained VIP latrines combat all the problems associated with simple pit latrines, except mosquitos. However, they are considerably more expensive than simple pits, since a ventilation pipe and full superstructure are required. Because the defecating hole is directly over the pit they accept any form of anal cleaning material without blocking. Routine operation is limited to keeping the superstructure clean, ensuring that the door (where fitted) is kept closed, occasionally checking that the fly-proof netting on top of the vent pipe is not blocked or broken, and pouring water down the vent pipe once a year to remove spiders' webs.
Table 6.1. Comparison of the numbers of files leaving the squat holes of a simple pit latrine and a VIP latrine a
Ventilated double-pit latrines
Although it is usually best to provide large deep pits, this may not be possible where rock or groundwater lie within one or two metres of the ground surface. A variation of the VIP latrine suitable for such situations has two shallow pits side by side under a single superstructure (Fig. 6.7). The pits are usually lined with bricks or blocks. Each pit may have its own squat hole or seat. Alternatively, slabs may be movable, one with a hole for the pit in use and a plain slab for the other pit. Whichever design is used, only one hole must be available for defecation at any time. The latrine may be provided with two ventilation pipes (one for each pit) but more usually only one is fitted, to the pit in use. The hole for the ventilation pipe for the pit not in use is sealed. As with single VIP latrines, the superstructure must be kept partially dark at all times to discourage flies.
One pit is used until it is filled to within about half a metre of the top. The defecation hole over the full pit is then sealed and the one over the empty pit opened. Where necessary, the ventilation pipe is moved from the full to the empty pit, and the vent hole in the slab of the full pit sealed. The second pit is then used until filled to within half a metre of the top. The contents of the first pit can now be removed and the pit reused. The pits must be large enough to allow each pit to be used for at least two years. This ensures that when the pit contents are dug out most of the pathogenic organisms have died.
Double-pit latrines can be considered as permanent installations. The small effective capacity (0.72 m³ for a family of six, using a sludge build-up rate of 60 litres per person per year, as suggested in Chapter 5) enables pits to be relatively shallow, and therefore easier to empty than deep pits. The pits should extend beyond the superstructure, either to the sides or at the back, with removable slabs for emptying. These slabs should be easy to lift, but should be sealed to prevent flies getting in or out. The central wall between the two pits should be made with full mortar joints and may be rendered with cement mortar on both sides.
As with the single-pit VIP latrine, the double-pit VIP latrine has the advantages of reduced smell and fly nuisance. Also the contents of the latrine dug out every two years or so are a valuable soil conditioner (see Annex 1). Double-pit VIP latrines are usually (but not always) more expensive than single-pit VIP latrines, and require a greater operational input from the user, particularly in changing over pits. Some societies have shown resistance to handling the decomposed contents of the pit but this can often be overcome with education and time. Allowing people to see (and handle) the contents of a pit as it is emptied is the strongest persuader for those concerned.
All projects involving the construction of double-pit latrines must allow for a prolonged support programme. Householders need to be reminded to change pits at the right time and should be assisted in doing so. This assistance will probably have to be available for at least the first two pit changes to ensure that the complete cycle is covered.
The problems of flies, mosquitos and smell in simple pit latrines may be overcome simply and cheaply by the installation of a pan with a water seal in the defecating hole (Fig. 6.8). Chapter 7 gives details of the design and fabrication of water seals. The pan is cleared by pouring (or, better, throwing) a few litres of water into the pan after defecation. The amount of water used varies between one and four litres depending mainly on the pan and trap geometry. Pans requiring a small amount of water for flushing have the added advantage of reducing the risk of groundwater pollution. The flushing water does not have to be clean. If access to clean water is limited, laundry, bathing or any other similar water may be used.
Pour-flush latrines are most appropriate for people who use water for anal cleaning, and squat to defecate, but they have also proved popular in countries where other cleaning materials are common. However, there is a likelihood of blockage where solid materials such as hard paper or corncobs are put in the pan. The placing of solid cleaning materials in a container for separate disposal is not generally recommended unless careful attention can be given to the handling of the waste and sterilizing of the container. Blockage may also be caused by material used by menstruating women. This should be disposed of separately, e.g., by burying or burning. Efforts to clear blockages often result in damage to the water seal.
In most cases, because of the small quantity of water required for flushing, pour-flush latrines are suitable where water has to be carried to the latrine from a standpipe, well, or other water source. There is no justification for the belief that the pit should be ventilated to prevent the build up of gases. A vent pipe adds to the cost of the latrine and any gases produced easily percolate into the surrounding soil.
Offset pour-flush latrines
An extension of the idea of the pour-flush pan with a water seal is for the pit to be outside the latrine building (Fig. 6.9). The contents of the pan are discharged through a short length of small-diameter pipe or covered channel with a minimum gradient of 1 in 30. PVC, concrete or clay pipes, 100 mm in diameter, are often used, but the diameter may be the same as the water seal (usually 65-85 mm). Masonry or brickwork channels with smooth circular concrete inverts have been adopted in some Asian countries. The channel is covered by precast concrete slabs or by bricks laid transversely across the top (Fig. 6.10). Pipes or channels should project at least 100 mm into the pit.
Generally speaking, an offset pour-flush latrine requires a larger volume of flushing water than a simple pour-flush latrine. The amount of water required depends on the pan design, pipe slope and roughness. As little as 1.5 litres has been recorded as necessary for each flush, but usually considerably more than this is required.
Offset pour-flush latrines are favoured by many because the superstructure can be permanent. When the pit is full, another pit can be dug alongside and the connecting pipe excavated and relaid to the new pit without damaging the superstructure (Fig. 6.11).
Another benefit is that the toilet can be located inside the house and the pit outside. If this layout is used, care must be taken to allow for movement of the pipe where it passes through the house wall. This can be achieved either by cutting a slot in the wall (Fig. 6.12) so that it does not bear directly on the pipe, or by installing two short lengths of pipe (Fig. 6.13) joining in the centre of the wall. Both systems allow movement of the wall without breaking the pipe. The distance of the pit from the house wall should be not less than its depth, to prevent the load from the wall causing the pit to collapse. If this is not possible, the pit may be located not less than one metre from the wall, provided that the pit is fully lined and the unsupported plan length parallel to the wall does not exceed one metre (Fig. 6.14).
Double-pit offset pour-flush latrines
As with VIP latrines there are occasions when two shallow pits are more appropriate than a single deep pit. Double pits with pour-flush pans and water seals have been successfully used in India (Roy et al., 1984) and elsewhere. The pit design is the same as in the double-pit VIP latrine but the two toilets are replaced by a single waterseal pan connected to both pits by pipes. An inspection chamber containing a Y junction is normally built between the pits and the pan so that the excrete can be channeled into either pit (Fig. 6.15).
Before a new latrine is brought into service, the inspection chamber is opened and one of the pipes leading to the pits is stopped off (a brick, stone, mound of clay or block of wood is quite satisfactory). The cover is then replaced and sealed to prevent gases escaping to the atmosphere. The latrine can now be used like an offset pour-flush toilet except that slightly more water may be required for flushing to prevent solids blocking the Y junction. Since one of the outlets from the chamber is blocked, all the contents of the toilet pan are directed into a single pit. When the first pit is full, usually after a couple of years, the inspection chamber is opened and the stopper blocking the outlet pipe removed and placed in the other outlet pipe. The cover is again replaced and sealed. The pan contents now enter the second pit.
In a further two years the contents of the first pit will have decomposed and nearly all of the pathogenic organisms will have died. The lid of the first pit is taken off and the contents of the pit removed and disposed of or reused (see Annex 1). After replacing and sealing the lid, the first pit can be used again if the stopper in the Y junction is returned to its original position. In this way, the twin pits can be used indefinitely, each pit in turn being used for two years, rested for two years, emptied and then used again.
The positioning and shape of the pits is determined to a large extent by the space available. Some options are shown in Fig. 6.16. If possible, the distance between the pits should be not less than the depth of a pit. This is to reduce the possibility of liquid from the pit in use entering the pit not in use. If the pits have to be built adjacent to each other, the dividing wall should be non-porous. It can also be extended beyond the side-walls of the pit, to prevent cross-contamination. Alternatively, the pit lining can be constructed without holes for a distance of 300 mm either side of the dividing walls.
As with double-pit VIP latrines, double-pit pour-flush latrines are most useful in areas where it is not possible to dig a deep pit or where excrete are to be reused.
For proper operation it is most important that the construction, particularly of the Y junction, is carried out properly, and the user is made fully aware of how the latrine should be operated. Long-term support facilities to remind and assist the user in changing and emptying pits will greatly improve operational success.
Raised pit latrines
Another way of dealing with the problem of difficult ground conditions close to the surface is to construct raised pit latrines. The pit is excavated as deep as possible, working at the end of the dry season in areas of high groundwater. The lining is extended above ground level until the desired pit volume is achieved.
If the pit extends more than 1.5 m below the ground there will probably be sufficient leaching area below ground for a pit latrine having a full depth of 3.5 m. In such cases, the lining above ground should be sealed by plastering both sides (Fig. 6.17). The minimum below-ground depth depends on the amount of water used in the pit and the permeability of the soil. Where insufficient infiltration area can be obtained below ground level, the raised portion of the pit can be surrounded by a mound of soil. The section of the lining above ground (excluding the top 0.5 m) can be used for infiltration provided the mound is made of permeable soil, well compacted with a stable side slope, and is thick enough to prevent filtrate seeping out of the sides (Fig. 6.18). Earth mounds are not recommended on clay soils as the filtrate is likely to seep out at the base of the mound rather than infiltrate the ground.
Raised pits can be used in combination with any other type of pit latrine (VIP, pour-flush, double-pit). A common application is where the groundwater level is close to the surface. A slight raising of the pit may prevent splashing of the user or blockage of the pit inlet pipe by floating scum.
Borehole latrines have an augered hole instead of a dug pit and may be sunk to a depth of 10 m or more, although a depth of 4-6 m is usual. Augered holes, 300-500 mm in diameter, may be dug quickly by hand or machine in areas where the soil is firm, stable and free from rocks or large stones. While a small diameter is easier to bore, the life of the pit is very short. For example a 300-mm hole 5 m deep will serve a family of five people for about two years.
The small diameter of the hole increases the likelihood of blockage, and the depth of the augered hole increases the danger of groundwater contamination. Even if the hole does not become blocked, the sides of the hole become soiled near the top, making fly infestation probable. However, borehole latrines are convenient for emergency or short term use, because they can be prepared rapidly in great numbers, and light portable slabs may be used.
The holes should be lined for at least the top half-metre or so with an impervious material such as concrete or baked clay. Because of the small diameter and short life, the full depth is not usually lined.
Septic tanks are commonly used for wastewater treatment for individual households in low-density residential areas, for institutions such as schools and hospitals, and for small housing estates. The wastewater may be waste from toilets only, or may also include sullage.
The septic tank, in conjunction with its effluent disposal system, offers many of the advantages of conventional sewerage. However, septic tank systems are more expensive than most other on-site sanitation systems and are unlikely to be affordable by the poorer people in society. They also require sufficient piped water to flush all the wastes through the drains to the tanks.
Wastes from the toilet, and possibly kitchens and bathrooms, pass through drains into a sealed, watertight tank, where they are partially treated. After a period - usually 1-3 days - the partially treated liquid passes out of the tank and is disposed of, often to the ground through soakpits or tile drains in trenches (Fig. 6.19). Many of the problems with septic tank systems arise because inadequate consideration is given to the disposal of the tank effluent.
A principal aim of septic tank design is to achieve hydraulically quiescent conditions within the tank to assist the settlement by gravity of heavy solid particles. The settled material forms a layer of sludge on the bottom of the tank which must be removed periodically. The efficiency of removal of solids by settlement can be high. Majumder et al. (1960) reported removal of 80% of suspended solids in three tanks in West Bengal; similar removal rates were reported in a single tank near Bombay (Phadke et al., undated). However, much depends upon the retention time, the inlet and outlet arrangements, and the frequency of desludging. Large surges of flow entering the tank may cause a temporarily high concentration of suspended solids in the effluent owing to disturbance of the solids which have already settled out.
Grease, oil, and other materials that are less dense than water float up to the liquid surface, forming a layer of scum which can become quite hard. The liquid moves through the tank sandwiched between the scum and sludge.
Sludge digestion and consolidation
Organic matter in the sludge and scum layers is broken down by anaerobic bacteria with a considerable amount of organic matter being converted into water and gases. Sludge at the bottom of the tank is consolidated owing to the weight of liquid and solids above. Hence the volume of sludge is considerably less than that of raw sewage solids entering the tank. Rising bubbles of gas cause a certain amount of disturbance to the liquid flow. The rate at which the digestion process proceeds increases with temperature, a maximum rate being achieved at about 35 °C. The use of ordinary household soap in normal amounts is unlikely to affect the digestion process (Truesdale & Mann, 1968). The use of abnormally large amounts of disinfectant causes bacteria to be killed off and thereby inhibits the digestion process.
Stabilization of liquids
The liquid in the septic tank undergoes biochemical changes, but there are few data on the removal of pathogens. Both Majumder et al. (1960) and Phadke et al. (undated) found that although 80-90% of hookworm and Ascaris eggs were removed by the septic tanks studied, in absolute terms very large numbers of viable eggs were contained in the effluent, with 90% of effluent samples containing viable eggs.
Since the effluent from septic tanks is anaerobic and likely to contain large numbers of pathogens which can be a potential source of infection, it should not be used for crop irrigation nor should it be discharged to canals or surface-water drains without the permission of the local health authority.
The guiding principles in designing a septic tank are:
Factors affecting design
The design method outlined below provides sufficient volume for both retention of liquid and storage of sludge and scum. The volume required for liquid retention depends upon the number of users, the amount of wastewater passed to the tank and whether sullage is accepted as well as waste from WCs. The volume for sludge and scum storage depends on the frequency with which the tank is desludged, the method of anal cleaning of the users and the temperature.
Estimating me volume of a septic tank
A sewage retention time of 24 hours is assumed to be sufficient. This should correspond to the situation immediately before the tank is desludged. After desludging the effective liquid retention time is greater because liquid then occupies the regions previously full of sludge and scum.
Codes of practice vary in their recommendations from a retention time of just less than 24 hours to about 72 hours. In theory, improved settlement results from a longer retention time, although the maximum rate of settlement is usually achieved within the first few hours. Settlement is impeded by flow disturbances caused by the inlet and outlet arrangements. The problem is likely to be greater in small tanks than large ones (whose hydraulic capacity is better able to damp out disturbances) and it is reasonable to assume that in large tanks correspondingly lower retention times can be used (Mare & Sinnatamby, 1986). The Brazilian code of practice (Associacao Brasileira de Normas Tecnicas, 1982) allows for reduced retention time in large tanks, such as those serving institutions or small communities. In summary, if the wastewater flowrate is Q m³ per day, it recommends that the retention time should be T hours, as follows:
Liquid retention volume
If the septic tank accepts sullage as well as toilet waste, the sewage flow from a house or institution usually represents a high proportion of the water supplied. If the water supply per person is known, the sewage flow may be taken as 90% of the water supply. If the water supply exceeds about 250 litres per person per day, the excess is likely to be used for watering gardens. In most developing countries, the maximum sewage flow may be assumed to be between 100 and 200 litres per person per day.
If only WCs are connected to the septic tank, the sewage flow is estimated from an assumption about the number of times each user is likely to flush the WC. For example, each person may flush a 10-litre cistern four times a day.
The minimum capacity required for 24 hours' liquid retention is:
Volume for sludge and scum storage
The volume required for the accumulation of sludge and scum depends upon the factors discussed in Chapter 5. Pickford (1980) suggested the formula:
Table 6.2. Value of the sizing factor F in determining volume for sludge and scum storage
Total tank volume
The total capacity of the tank (C) is:
In practice, there are limitations on the minimum size of tank that can be built; the guidelines described below are illustrated in the design examples given in Chapter 8.
Shape and dimensions of septic tanks
Having determined the overall capacity of the septic tank it is necessary to determine the depth, width and length. The aim is to achieve even distribution of flow so that there are no dead areas and no "short circuiting" (that is, incoming flow shooting through the tank in less than the design retention time).
A tank may be divided into two or more compartments by baffle walls. Most settlement and digestion may occur in the first compartment with some suspended materials carried forward to the second. Surges of sewage entering the tank reduce the efficiency of settlement but have less effect in the second compartment. Laak (1980) reported a number of studies in which septic tanks with more than one compartment performed more effectively than single-compartment tanks. His survey also indicated that the first compartment should be twice as long as the second. Any advantage of more than two compartments has not been quantified.
The following guidelines can be used to determine the internal dimensions of a rectangular tank.
These guidelines give the minimum size of tank. There is no disadvantage in making a tank bigger than the minimum capacity. It may be cheaper to build larger tanks using whole blocks, rather than cutting blocks. Examples of septic tank design are given in Chapter 8.
The construction of a septic tank usually requires the assistance and supervision of an engineer or at least an experienced construction foreman. The design of the inlet and outlet is critical to the performance of the tank. Careful checking of levels is particularly important for large tanks that include complicated inlet, outlet and baffle board arrangements.
For small household tanks, the floor is usually made of unreinforced concrete thick enough to withstand uplift pressure when the tank is empty. If the ground conditions are poor or the tank is large, the floor may have to be reinforced. The walls are commonly built of bricks, blocks or stone and should be rendered on the inside with cement mortar to make them watertight. Large reinforced concrete tanks serving groups of houses or institutions must be designed by a qualified engineer to ensure that they are structurally sound.
The tank cover or roof, which usually consists of one or more concrete slabs, must be strong enough to withstand any load that will be imposed.
Removable cover slabs should be provided over the inlet and outlet. Circular covers, rather than rectangular ones, have the advantage that they cannot fall into the tank when removed.
Septic tanks have been constructed from a variety of prefabricated sections, including large-diameter pipes. Experience has shown that the problems involved in fixing the inlet and outlet outweigh the advantages of using pipes. A number of proprietary designs of tank are manufactured from asbestos cement, glass-reinforced plastic and other materials and are sold commercially.
The sewage must enter the tank with the minimum possible disturbance to the liquid and solids already in the tank. Surges and turbulence reduce the efficiency of settlement and can cause large amounts of solid matter to be carried out in the tank effluent. Suitable inlet arrangements are shown in Fig. 6.21 and 6.24.
Surges are caused by flushing of the WC and emptying of sinks and baths. Their effect can be minimized by using drainpipes of not less than 100 mm in diameter and ensuring that the gradient of the pipe approaching the septic tank is flatter than about I in 66. Sizes and gradients of pipes between the building and the septic tank may be specified in local building regulations.
For septic tanks less than 1.2 m wide, a simple T-pipe arrangement can be used for the outlet. A removable cover above the T-pipe should be provided to permit clearance of any blockage. An alternative to the T-pipe is a baffle plate made of galvanized sheet, ferrocement or asbestos cement fitted round the outlet pipe (Fig. 6.22). A deflector may be provided below the outlet to reduce the possibility of settled sludge being resuspended and carried out of the tank. For tanks wider than 1.2 m, a full-width weir can be used to draw off the flow evenly across the tank. A scumboard should be fitted to prevent the scum washing over the weir (Fig. 6.23).
If a tank is divided into two or more compartments, slots or a short length of pipe should be provided above the sludge level and below the scum level, as shown in Fig. 6.24. At least two should be installed to maintain uniform flow distribution across the tank.
Ventilation of the tank
The anaerobic processes that occur in the tank produce gases which must be allowed a means of escape. If the drainage system of the house or other building has a ventilation pipe at the upper end, gases can escape from the septic tank along the drains. If the drainage system is not ventilated, a screened vent pipe should be provided from the septic tank itself.
The tank floor
Some codes of practice recommend that the floor of a septic tank should slope downwards towards the inlet. There are two reasons: firstly, more sludge accumulates near the inlet, so a greater depth is desirable; secondly, the slope assists movement of sludge towards the inlet during desludging. For a two-compartment tank, the second compartment should have a horizontal floor and the first compartment may slope at a gradient of 1 in 4 towards the inlet. When calculating the tank volume, it should be assumed that the floor is horizontal at the higher level. The effect of sloping the floor provides extra volume. The disadvantages of providing a sloping floor are that additional depth of excavation is required, the construction is made more complicated, and the cost of construction is increased.
Operation and maintenance
Starting up the tank
The process of anaerobic digestion of the sewage solids entering the tank can be slow in starting and it is a good idea to "seed" a new tank with sludge from a tank that has been operating for some time. This ensures that the necessary microorganisms are present in the tank to allow the digestion process to take place in a short time (McCarty, 1964).
Routine inspection is necessary to check whether desludging is needed, and to ensure that there are no blockages at the inlet or outlet. A tank needs to be desludged when the sludge and scum occupy the volume specified in the design. A simple rule is to desludge when solids occupy between one-half and two-thirds of the total depth between the water level and the bottom of the tank. One of the difficulties with septic tanks is that they continue to operate even when the tank is almost full of solids. In this situation the inflow scours a channel through the sludge and may pass through the tank in a matter of minutes rather than remaining in the tank for the required retention time.
The most satisfactory method of sludge removal is by vacuum tanker. The sludge is pumped out of the tank through a flexible hose connected to a vacuum pump, which lifts the sludge into the tanker. If the bottom layers of sludge have cemented together they can be jetted with a water hose (which may be fitted to the tanker lorry) or broken up with a long-handled spade before being pumped out.
If a vacuum tanker is not available, the sludge must be bailed out manually using buckets. This is unpleasant work which exposes the operatives to health hazards.
Care must be taken to ensure that sludge is not spilled around the tank during emptying. Sludge removed from a septic tank includes fresh excrete and presents a risk of transmission of diseases of faecal origin. Careful disposal is therefore necessary.
When a septic tank is desludged it should not be fully washed out or disinfected. A small amount of sludge should be left in the tank to ensure continuing rapid digestion.
An aqua-privy is a latrine set above or adjacent to a septic tank and is useful in situations in which there is a limited water supply (Fig. 6.25). Where the latrine is above the tank, a chute drop-pipe, 100 150 mm in diameter, hangs below the squat hole or latrine seat so that excrete drops directly into the tank below water level. The bottom of the pipe should be 75 mm below the liquid level in the tank, providing a seal which prevents gases escaping into the latrine superstructure and limits the access of flies and mosquitos to the tank. Alternatively the toilet may be fitted with a pan with a water seal. Where the latrine is adjacent to the tank, the pan with water seal is connected by a short pipe. Effluent from the tank goes to a soakpit, drainage trench or sewer. There is usually only a small flow of effluent and it is therefore very concentrated.
In order to keep a seal at the bottom of the drop-pipe it is essential that the water level in the tank is maintained. If the tank is completely watertight, a bucketful of water every day, used to clean the latrine, is sufficient to compensate for any losses due to evaporation. However, it has been found in practice that many tanks leak. In some places sullage is discharged into the tank (Fig. 6.26), but even this has not proved sufficient to ensure that the water level is above the bottom of the drop pipe at all times. In Calcutta, aqua-privies used by people who use water for anal cleaning have a water seal incorporated in the drop-pipe below the pan (Pacey, 1978).
The design capacity of aqua-privy tanks may be calculated by the same procedure as for septic tanks. Regular removal of sludge and scum is essential, so a removable cover for desludging is required. A vent pipe is usually provided.
Disposal of effluent from septic tanks and aqua-privies
A septic tank or aqua-privy is simply a combined retention tank and digester; apart from losses through seepage and evaporation, the outflow from the tank equals the inflow. The effluent is anaerobic and may contain a large number of pathogenic organisms. Although the removal of suspended solids can be high in percentage terms, the effluent is still concentrated in absolute terms, and the need for safe disposal of septic tank effluents cannot be too strongly stressed.
The effluent from large tanks dealing with sewage from groups of houses or from institutions may be treated by conventional sewage treatment processes such as percolating filters. Effluent from septic tanks and aqua-privies serving individual houses is normally discharged to soakpits or drainage trenches for infiltration into the ground. The infiltration capacities of the soil given in Table 5.4 (page 37) may be used to determine the required wall area of both soakpits and trenches.
Unfortunately it is not possible to predict the useful life of such disposal systems, which depend on the efficiency of the septic tank and the soil conditions. Pools of stagnant liquid often form when both toilet wastes and sullage are discharged to a septic tank and then to a drainage field which is too small or is clogged. This creates a potential health risk. Overloading of the drainage field may be avoided by allowing only toilet wastes to go to the septic tank. Sullage can be dealt with separately with fewer health risks than a mixture of partly treated toilet waste and sullage. Kalbermatten et al. (1980) proposed the use of a three-compartment septic tank, where sullage is introduced into the final compartment. It is suggested that the effluent infiltration rates may be double those for two-compartment tanks.
Pits used to dispose of effluent from septic tanks are commonly 2-5 m deep with a diameter of 1.0-2.5 m. The capacity should be not less than that of the septic tank.
Depending on the nature of the soil and the local cost of stone and other building material, soakpits may either be lined or filled with stones or broken bricks. Linings are generally made of bricks, blocks or masonry with honeycomb construction or open joints (Fig. 6.27), as for the linings of pit latrines which are described in Chapter 7. The infiltration capacity of the soil may be increased by filling any space behind the lining with sand or gravel (Cairncross & Feachem, 1983). Hard material such as broken rock or broken kiln-dried bricks not less than 50 mm in diameter may be used to fill an unlined pit (Fig. 6.28).
Whether the main part of the pit is lined or filled, the top 500 mm should have a ring of blocks, bricks or masonry with full mortar joints to provide a firm support for the cover. The ring may be corbelled to reduce the size of the cover. Covers are usually made of reinforced concrete and may be buried by 200-300 mm of soil to keep out insects.
The area required for infiltration should be calculated from the data given in Chapter 5, as illustrated in Example 8.6 in Chapter 8. Increasing the diameter of the pit results in a disproportionate increase in the volume of excavation and in the cost of the cover slab compared with the increase of wall area. Therefore, if the required infiltration area is large, it may be more economical to provide drainage trenches.
The disposal of the large quantity of effluent from septic tanks is often effected in trenches which disperse the flow over a large area, reducing the risk of overloading at one place. The trenches make up a drainage field. The effluent is carried in pipes which are normally 100 mm in diameter with a gap of about 10 mm between each pipe. Unglazed stoneware pipes (tile drains) are often used, either with plain ends or with spigot and socket joints. The upper part of the gap between plain end pipes may be covered with strips of tarred paper or plastic sheet to prevent entry of sand or silt. With spigot and socket pipes, a small stone or cement fillet can be placed on each socket to centre the adjoining spigot (Fig. 6.29).
Drainage trenches are usually dug with a width of 300- 500 mm and a depth of 600 1000 mm below the top of the pipes. A common practice is to lay the pipes at a gradient of 0.2-0.3% on a bed of gravel, the stones with a diameter of 20 50 mm. Soil is returned to a depth of 300 500 mm above the stones, with a barrier of straw or building paper to prevent soil washing down (Fig. 6.30).
If more than one trench is needed it is recommended that the drains be laid in series (Cotteral & Norris, 1969). Drains in series are either full or empty, allowing the soil alongside empty drains to recover under aerobic conditions (Fig. 6.31). If drains are laid in parallel, there is a tendency for all trenches to contain some effluent. Trenches should be 2 m apart, or twice the trench depth if this is greater than 1 m.
The length of trench should be calculated by dividing the flow of effluent by the infiltration rate, allowing for the area of both sides of the trench, as illustrated by the examples given in Chapter 8.
The value of composting excrete with dry organic matter is discussed in Annex 1. Composting toilets are of two types: those such as doublevault latrines, which use anaerobic bacteria, and continuous composting latrines, which make use of aerobic bacteria.
Each latrine consists of two chambers or vaults used alternately (Fig. 6.32). Initially a layer of about 100 mm of absorbent organic material such as dry earth is put in the bottom of one vault, which is then used for defecation. After each use, the faeces are covered with wood ash or similar material to deodorize the decomposing faeces and soak up excess moisture.
When the vault is three-quarters full, the contents are levelled with a stick and the vault is completely filled with dry powdered earth. The squat hole is then sealed. While the contents of the first vault are decomposing anaerobically, the second vault is used. When the second tank is full, the first one is emptied through a door near the bottom and the chamber is reused. The contents may be used as a soil conditioner.
Each vault should be large enough to hold at least two years' accumulation of wastes so that most pathogenic organisms die off before the compost is removed. Recommended vault sizes range from 1.1 m³ (Winblad & Kalama, 1985) to 2.23 m³ (Wagner & Lanoix, 1958).
Normally the superstructure is built over both vaults, with a squat hole over each vault. A cover sealed with lime mortar or clay should be fitted in the squat hole above the chamber not in use. A flyproof lid should be placed on the other hole when it is not being used for defecation. Flyproof vent pipes may be provided to avoid odour nuisance in the latrine, although covering the faeces with ash is reported to be sufficient to eliminate bad smells.
Control of the moisture content is vital for proper operation of the latrine. Consequently composting latrines are not appropriate where water is used for anal cleaning. It is usual to collect urine separately, dilute it with 3-6 parts of water and use it as a fertilizer (although this may cause a health hazard). Some latrines are constructed with soakpits below the vaults so that excess moisture can drain into the ground (Fig. 6.33). This allows for the disposal of urine into the vaults but with consequent loss of a valuable fertilizer and possible pollution of the groundwater. Wood ash, straw, sawdust, grass cuttings, vegetable wastes and other organic material must be put into vaults to control moisture content and improve the quality of the final compost.
Besides providing a reusable resource, the double-vault latrine has the added advantage that it can be built anywhere. Since the vault contents are kept dry, there is no pollution of the surrounding ground, even if the vault is buried. In rocky areas or where the water table is high the vaults may be built above ground. Walls and base should be watertight.
Double-vault composting latrines have been successfully used in Viet Nam (McMichael, 1976) and Guatemala (Buren et al., 1984). When tried elsewhere they have usually been unsatisfactory. Most of the disadvantages revolve round the problem of controlling the moisture content. Proper operation of the latrine is difficult to understand and considerable effort may be required to educate local people in its use. The contents are often allowed to become too wet, making the vault difficult to empty and malodorous.
Continuous composting toilets
These consist of watertight sloping chambers about 3 m in length. Excreta fall into the chamber from a toilet. Dry organic kitchen and garden waste is tipped in through a separate opening (Fig. 6.34).
Inverted U- shaped ducts and a ventilation pipe encourage the passage of air through the mass, preventing it from becoming anaerobic and allowing excess moisture to evaporate. As new material enters at the top of the chamber, older material gradually moves to the bottom and then slides into a smaller compartment from which it is removed periodically.
Such toilets have proved satisfactory in holiday homes and other isolated buildings in industrialized countries, where they are sometimes installed in a cellar beneath the latrine and kitchen.
Attempts have been made in Botswana and the United Republic of Tanzania to adapt the design to suit African materials and customs (Winblad & Kalama, 1985) using tanks made with concrete or sand and cement blocks. They were found to be inappropriate because of their high cost and sensitivity to user operation. Retaining the proper carbon-nitrogen balance and moisture content is crucial to proper operation. In practice, it has been found that moisture content is the most difficult to control. Fly and odour problems are also common, particularly soon after commissioning.
In some cultures there is a preference for separate latrines for men and women or adults and children. There is also a need for multiple latrines at places where large numbers of people meet, such as schools, restaurants, offices, etc.
Latrines fitted with a water seal may be connected to a common pit by drains (Fig. 6.35). VIP latrines may also be constructed over a common pit but the number of toilet holes using a single vent pipe should be limited to two. A multiple double-pit VIP latrine has been developed where each cubicle has two holes or seats (Fig. 6.36). These holes are used alternately in the same way as double-pit VIPs. The holes are used in such a way that the two holes which serve a pit are in use (or not in use) at the same time. The holes not being used are sealed. The dividing walls in the pit must extend to the full height of the pit.
The system in which excrete are removed from bucket latrines (also called nightsoil latrines or earth closets) is one of the oldest forms of organized sanitation. Bucket latrines are still found in many towns and cities in Africa, Latin America and Asia, because their low capital cost makes them attractive to underfunded local authorities.
In some rural and periurban areas, members of households take nightsoil to manure heaps or apply it directly to fields as fertilizer. In towns and cities, nightsoil is often collected by sweepers engaged by householders on contract, or by the local authorities. Buckets are usually emptied into larger containers near the latrine. In some places labourers carry these containers by hand or on their heads; hand-carts, animal-drawn carts, bicycles and tricycles are also used.
For the reasons given in Chapter 4, nightsoil collection should never be considered as an option for sanitation improvement programmes, and all existing bucket latrines should be replaced as soon as possible.
The number of bucket latrines is declining rapidly. However, for many years to come, some people will have to rely on bucket latrines as their only form of sanitation. The following paragraphs give suggestions for improvements to existing systems until they can be replaced by more acceptable forms of sanitation.
A container made of non-corrosive material is placed beneath a squatting slab or seat in the bucket chamber, with rear doors which should be kept shut except during removal and replacement of the bucket.
The bucket chamber should be cleaned whenever the bucket is removed. The squat hole should be covered by a flyproof cover when not in use. The cover of the seat should be hinged (Fig. 6.37) and the cover of the squatting slab should have a long handle.
At regular intervals (preferably each night) the container should be removed and replaced by a clean one. Full containers should be taken to depots or transfer stations where they are emptied, washed and disinfected with a phenol or cresol type of disinfectant. In some towns it is the practice to provide two buckets painted in different colours for each latrine. Containers should be kept covered with tight-fitting lids while in transit and the operators should be provided with full protective clothing. Proper supervision and management are essential. Defective buckets should be repaired or replaced and transport vehicles should be kept in good order.
In some systems, urine is diverted away from the buckets to reduce the volume to be dealt with. It is usually channelled to soakpits, but may be collected separately and used directly as fertilizer. Water used for washing latrines and bucket-chambers should pass to soakpits, and should not be allowed to pollute the ground around the latrines.
The practice of dumping nightsoil indiscriminately into streams or on open land is objectionable and causes health hazards.
Bucket latrines are sometimes found in towns that are partially provided with sewers, in which case it may be convenient to discharge the nightsoil into a main sewer. Tipping points on sewers require careful design to prevent contamination of surrounding areas and should be as near to the sewage works as possible. Extra water may have to be added to prevent blockage of the sewers.
Sewage treatment works
Nightsoil may be discharged into the sewage flow at the works inlet, at sedimentation or aeration tanks, or directly to waste stabilization ponds or sludge digestion tanks.
Trenches about 1 m deep and I m wide may be filled with nightsoil to within not less than 300 mm of the top. The trench is then backfilled with excavated soil, which should be well compacted to prevent the emergence of flies or the excrete being dug up by animals (Fig. 6.38). At the end of each day any exposed excrete must be covered with at least 200 mm of soil, well compacted. After backfilling, the trench should remain untouched for at least two years, after which it can be reexcavated for reuse and the contents used as fertilizer. The trenching site should be close to the collection area but away from residential areas. It should have deep and porous soil, be well above the water table, and not be subject to flooding.
Nightsoil can be used as a fertilizer after all pathogens have been destroyed. It may also be added to ponds for fish cultivation (see Annex 1).
Vault latrines are a way of overcoming the problem of frequent emptying needed with bucket latrine systems. A watertight tank or vault below or close to a latrine is used to collect faeces, urine and sometimes sullage. The capacity of the vault is often sufficient for 2-3 weeks' accumulation of excrete, after which time the vault is emptied. The system is satisfactory if collection is reliable and hygienic, and the vaults are properly flyproofed, vented and fitted with water-seal toilets.
In some places, the contents of vaults are bailed out by hand and taken away in tanks mounted on carts. This is highly undesirable. Trials with manually operated pumps to empty vault contents have not been very successful because with a low pumping rate (about 400 litres per hour) complete evacuation of the vault is a long and tedious operation. This method is obviously also undesirable.
Motorized vacuum tankers can provide safe removal but must be backed up by good institutional support for operation and maintenance. Most vacuum tankers cannot lift vault contents if the proportion of solids exceeds about 12%, but some have facilities for adding water to vaults before lifting the contents.
Sufficient extra space to allow for irregularities in collection time should be planned for in designing vault capacity. In communities where finance, spare parts and good maintenance are available, the additional space needed may be only 15-20%. However, where vehicle maintenance is poor, an allowance of 50% may be advisable.
The performance of vaults has been mixed, mainly dependent on the levels of finance and vehicle maintenance. Poorly constructed vaults are common, leading to problems with odour and flies, ground pollution and thickening of the vault contents. It is not recommended that new vault latrines be constructed.
Cesspits, like vaults, are watertight tanks with sealed covers (to keep out mosquitos). They differ from vaults in that they are usually located outside the premises and collect sullage as well as the wastes from water closets. The capacity may be sufficient for up to several months' use (Fig. 6.39). The cost of providing a regular removal service for all the wastewater from a house with a good supply of piped water can be very high, making cesspits an expensive form of sanitation.
Modern chemical toilets are normally of the following types:
The fluid is normally a chemical diluted with water which renders excrete harmless and odourless. When containers are full, the contents are tipped into pits or sewers, or pumped into storage tanks.
Chemical toilets are used in aircraft, long-distance coaches, caravans, vacation homes and construction sites. The chemical is expensive.
An overhung latrine consists of a superstructure and floor built over water (Fig. 6.42). A squat hole in the floor allows excrete to fall into the water. A chute is sometimes provided from the floor to the water. Overhung latrines should never be built in places where pit latrines can be provided. However, they may be the only possible form of sanitation for people living on land that is continuously or seasonally covered with water.
Wagner & Lanoix (1958) suggested that such latrines might be acceptable provided the following conditions are met.
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