A pour-flush latrine utilizes a water seal to prevent odour and insects entering the latrine from the pit. This water seal may be part of the pan unit (Fig. 7.28) or may be connected immediately below the pan (Fig. 7.29). For on-site sanitation, flushing is normally carried out by the wash-down method where the force of the flush water thrown into the pan is enough to drive the excreta through the water seal.
Fig. 7.28. Combined pan and water seal for direct pour-flush latrine
Fig. 7.29. Pan and seal for offset pour-flush latrine
The pan may be of the squatting type or of the pedestal variety where the user can sit. The amount of water needed for flushing depends on the design of the pan or pedestal, the depth and volume of the water seal, and the minimum passage size through the seal. For a water seal directly above the pit about 1 litre of water is normally sufficient for flushing. Two litres may be required for an offset pit and a minimum of 3 litres for an improved pedestal pan and offset pit.
The depth of the water seal is measured as the depth of water that would have to be removed from a fully filled trap to allow the passage of air (Fig. 7.28). The seal volume is the amount of water held within the trap when the unit is not being used, and the minimum passage size is the opening through which the water must flow and which may be of a smaller diameter than the connecting pipe. The depth of the seal in a conventional WC is approximately 50 mm. However, the deeper the seal the more water is required for flushing. In pour-flush latrines, the depth of the seal is normally reduced to the minimum compatible with maintaining the seal in hot weather. The seal volume will be reduced by evaporation, the water loss being proportional to the time between consecutive flushings and the degree of exposure to direct sunlight and air movement. A minimum seal depth of 20 mm is considered reasonable with an optimum passage size of 70 mm (Mara, 1985b).
Water seals that can be removed during the dry season to minimize water usage are not recommended. It is likely that the seal would not be replaced at the beginning of the wet season and therefore the latrine would not work effectively.
Types of water seal
Where the water seal services a direct pit, the pan and the seal should be made as a single piece with a hemispherical bowl known as a "gooseneck" trap. This is designed to discharge into the centre of the pit and not against the pit lining where it might cause damage. These types of seal can easily be damaged by users trying to clear blockages with a rod where the thin cement of the seal is unsupported. As direct pits have become less popular because of emptying difficulties, the use of the gooseneck trap has also declined.
In many countries, the pan is made separately from the seal to facilitate manufacture and to give the installer greater freedom as to where the offset pit is located in relation to the pan. The normal system, which has an inclined outlet, is known as a P-trap, while the system with a vertical outlet is called an S-trap.
Pans and water seals may be produced by manufacturers or by project staff to standard specifications in a variety of materials. Ceramics, such as white vitreous china or other glazed earthenware, have traditionally been used for pans and pedestals. However, such items may be expensive to purchase and require careful attention to packing if they are to be transported safely. They may also be heavy and require a strengthened slab for a direct pit. Particularly because of the problems of transport and handling, the use of plastics for pans and water seals is becoming more common. Glass-fibre pans and high-density polyethylene (HDPE) water seals are light and easily transportable, even by bicycle, and are often preferred by users, even when more expensive than the cement-based systems described below.
The cheapest pans and seals are made from cement mortar (10-30 mm thick) close to the point of sale or delivery. They can be produced on a large scale without factory facilities, and can be repaired easily when damaged. Such units are likely to be rougher than manufactured pans and seals, and a reaction between urine and the cement normally leads to some staining of the surface and some odour from the trap. This can be minimized by the addition of marble dust and chippings to the cement mortar. When dry, the surface can then be rubbed down with carborundum stones to provide an attractive mosaic finish. Colourings may also be added to the mortar to give a more attractive appearance.
An alternative method of production uses casting boards to cast the pan and seal in two halves with a 1:2:2 concrete mix pressed around the form. After 24 hours the two sections can be removed from the moulds and joined together with neat cement, the inner surface also being smoothed off with neat cement. One disadvantage of having the pan and water seal in one piece is that the trap cannot be rotated in the direction of the offset pit.
The Thai model, which is now in use in about 3 million rural homes, employs a two-part mould and is cast in a single step, including the platform, without the need for grouting pieces together. The depth and angle of the seal are uniform. Large numbers of moulds can be cast quickly, thus facilitating production of pans and seals so that large numbers of households can have pour-flush latrines in a very short period of time (J. T. Visscher, personal communication).
Making the pan and trap separately enables very simple forms to be used. These may be built up from clay and husk or plastered brick or concrete which can be reused many times. A release agent is needed to break the bond between the mould and the new concrete. Proprietary agents are available, though used engine oil or even cow-dung wash have proved to be cheap and effective.
Pedestals designed for pour-flushing with small quantities of water (about 3 litres) are normally made of ceramic to ensure a smooth finish. Less efficient units may be made using cement-based methods with ferrocement, fibre-reinforced cement and concrete with marble chippings.
Pipes and junction chambers
The water seal may be connected to the offset pit by conventional pipework (see Fig. 6.9) or by a covered drain (see Fig. 6.10). Where double pits are in use, a junction chamber or inspection chamber (see Fig. 6.15) is required whereby the flow can be directed into one pit or the other.
The pipe or channel should be not less than 75 mm wide and should be as smooth and direct as possible. Any roughness or sharp bends will tend to slow the passage of excreta, eventually leading to a build-up of deposits and a blockage. The cheapest available non-pressure pipes will be adequate, whether in fired clay, plastic or asbestos cement. The minimum slope should be 1 in 30 for smooth pipes and 1 in 15 for rougher pipes or hand-shaped channels. If the slope is too steep there is a danger of solids being deposited in the pipe.
Special care must be taken where the pipe passes through the superstructure wall (see Fig. 6.12 and 6.13). If possible, some degree of flexibility is required at the pipe joints or in the channel so that differential settlement of the latrine superstructure or the pit lining will not cause damage. There is unlikely to be significant loading on the ground above the connecting pipe, but where there is any possibility of vehicles crossing the area between latrine and pit, conventional pipe-bedding and protection should be used.
The pipe or drain should extend some distance into the pit so that the wastewater discharges directly towards the centre and does not dribble down the pit walls, with a consequent build up of deposits.
Where a covered drain is used to connect a double-pit system, a simple Y-junction can be constructed to divert the flow. The junction in a pipework connection between pits and latrine requires a chamber which should be of sufficient size to allow for ease of construction of the concrete benching. It must also allow for the flow to be diverted from one pit to another with a temporary blockage in one or other arm of the Y-junction. A minimum internal dimension of 250 mm is recommended (Roy et al., 1984). The chamber cover slab needs to be removable to allow for access to divert the flow, but also has to be heavy enough and fixed in such a way that it is difficult for children to remove.