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.
Fig. 6.9. Offset pour-flush latrine
Fig. 6.10. Brick-covered drain
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).
Fig. 6.11. Moving the discharge pipe of an offset pour-flush latrine to a new pit
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).
Fig. 6.12. Pipe laid through a hole in an external wall
Fig. 6.13. Pipe fixed in place through a wall
Fig. 6.14. Minimum distance between a pit and the external wall of a house