The principle of all types of pit latrine is that wastes such as excreta, 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:
- gases such as carbon dioxide and methane, which are liberated to the atmosphere or disperse into the surrounding soil;
- liquids, which percolate into the surrounding soil;
- a decomposed and consolidated residue.
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 excreta 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.
Fig. 6.1. Vacuum tanker desludging a septic tank
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.
• The machinery may be too large to get to the latrines. Conventional vacuum trucks are too big to be driven into the centre of many ancient cities or urban/periurban unplanned or squatter settlements where pedestrian routes predominate.
• Maintenance of vacuum tankers is often poor. Their engines must be kept running all day, either to move the truck or to operate the pump when stationary. This causes rapid wear and makes them particularly susceptible to breakdown if preventive maintenance is neglected.
• Management and supervision of emptying services is often ineffective.
High-performance vacuum tankers able to deal with consolidated pit latrine sludge have been developed (Caroll, 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).
Fig. 6.2. Remote vacuum pump emptying system
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.