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 excreta can be channelled into either pit (Fig. 6.15).
Fig. 6.15. Double-pit offset pour-flush latrine
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.
Fig. 6.16. Some layout options for double-pit offset pour-flush latrines
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 excreta 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.