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close this bookA Guide to the Development of on-site Sanitation (WHO; 1992; 246 pages)
View the documentPreface
open this folder and view contentsPart I. Foundations of sanitary practice
close this folderPart II. Detailed design, construction, operation and maintenance
open this folder and view contentsChapter 5. Technical factors affecting excreta disposal
close this folderChapter 6. Operation and maintenance of on-site sanitation
View the documentPit latrines
View the documentSimple pit latrines
View the documentVentilated pit latrines
View the documentVentilated double-pit latrines
View the documentPour-flush latrines
View the documentOffset pour-flush latrines
View the documentDouble-pit offset pour-flush latrines
View the documentRaised pit latrines
View the documentBorehole latrines
View the documentSeptic tanks
View the documentAqua-privies
View the documentDisposal of effluent from septic tanks and aqua-privies
View the documentComposting latrines
View the documentMultiple latrines
View the documentOther latrines
open this folder and view contentsChapter 7. Components and construction of latrines
open this folder and view contentsChapter. 8 Design examples
open this folder and view contentsPart III. Planning and development of on-site sanitation projects
View the documentReferences
View the documentSelected further reading
View the documentGlossary of terms used in this book
View the documentAnnex 1. Reuse of excreta
View the documentAnnex 2. Sullage
View the documentAnnex 3. Reviewers
View the documentSelected WHO publications of related interest
View the documentBack Cover

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.

Fig. 6.9. Offset pour-flush latrine


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Fig. 6.10. Brick-covered drain


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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


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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


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Fig. 6.13. Pipe fixed in place through a wall


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Fig. 6.14. Minimum distance between a pit and the external wall of a house


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