Latrine lining in black cotton soil
I have no direct experience of using oil-drums for lining pit-latrines. I suspect however that their life will be low – welding will damage any protective coating that there is and the conditions in a pit latrine, especially in the splash zone just above the liquid level will be ideal for corrosion. WEDC also suggest that the relatively small diameter means that the storage volume is low. I would be cautious! Properly burnt brick-lined pit latrines are indeed expensive but I would suggest sometimes that one has to accept that good sanitation has a cost, and that bad sanitation has an even higher one. If you think of a trial, perhaps in your own compound rather than with beneficiaries – if it goes better than I suspect, then people will come clamouring – if as bad as I suspect, then the example can be used to support the higher cost of bricks.
If these are domestic latrines and there is potential for the superstructure and slab to be moved then how about using shallow pits and digging new pits say every two years and then moving the superstructure over these and covering the pit with soil? However, ideally the pit should be lined for at least the top 0.5m to prevent cave-ins and entry of rodents and flies. Diverting any stormwater flows away from the pit by slightly raising it above the ground will help prevent surface water washing into the pit. It is also important to stop any rain flowing off the roof from eroding the soil and flowing into the pit.
An alternative is to use two shallow pits serving the same superstructure since this means that the costs of the lining are minimum and the pits can be reused. Each pit should have a capacity of two- year’s storage of faeces. One pit is used until it is full then the drop-hole to that pit is sealed whilst the other is used. After two years that one will be full and the contents of the first will be safe to remove and spread on the gardens/fields. Then the drop hole to the freshly-emptied pit can be opened and it can be re-used. Although the contents of the second pit are like rich humus the problem is that some people my culturally still not be willing to handle them.
I have seen oil-drum linings used (fairly successfully) in household pit latrines in northern India, but must admit that I don't know how the collapsible black cotton soil is likely to affect functionality.
Investing in a good pit lining (like fired bricks) is only worthwhile if you plan to re-use the (decomposed) contents; if you have very collapsible soils (obviously the case in Sudan); or if there is no space available for new pits. A lot of latrine designs don't cater for what happens when the latrine pit fills - when you need either to divert (or move) the latrine to a second pit, or to dig out the single pit (which now contains fresh excreta, thus presents a significant health hazard both for those digging it out, and for those that live near the site where the pit contents are dumped). In many cases, the investment in the pit lining is lost (as people often opt to build a new latrine rather than dig out the old one) ... which suggests that it is best to minimize the cost of the pit lining. Assuming that some form of pit lining is required, then oil drums are a reasonable alternative (if readily available). Obviously, there is a strong likelihood that the drums will corrode and have a fairly short life, but even a heavily corroded oil drum may provide enough support if the pits are not too deep. And if the latrine pits are going to be dug out at some stage, then there is always the option of adding some internal strengthening (timber, woven matting, other local materials) while the pit is empty? In South Asia, one of the most effective approaches for low-cost household latrines has been to dig a small, shallow pit (1.2m diameter, 1.2-1.5m deep); leave the pit unlined if possible - but use a disposable (woven bamboo or similar) lining if needed; then cover with some form of latrine slab (usually precast concrete). When the pit is full (usually in 6-18 months), the household dig another pit nearby, move and re-use the slab (plus rebuild a simple enclosure), and plant a fruit tree over the old pit. I guess this approach works less well in highly collapsible soils, unless the latrine pits are kept shallow and some local building material can be fashioned into a reasonably strong pit lining - what do people use to build their houses (there is a huge sustainability gain in using local building materials); is there any way that these local building materials can be used to weave/build some form of cheap lining? And I'm not sure why you plan to have such a deep latrine pit - presumably to lengthen the time taken for the pit to fill (or because this is the local practice). I always advise people to keep latrine pits shallow - deep pits are more difficult to dig; must resist higher forces (at the bottom of the pit); are more difficult to empty; and ... (more)
I'd avoid the oil drums because of low capacity and small infiltration surface per pit, even if you do a deep pit. And when they start to fail they are unlikely to resist the swelling and shrinking of the black cotton soil for long. I think Tim is right about the investment in a brick lining if, as Andy points out, the latrines are likely to be used for some time. Latrines on black cotton soils in Malawi in the late 80s were brick lined down to the limestone base about 2 m deep, with the base carefully cleaned of clay before commissioning to ensure maximum infiltration downwards, as horizontal movement of liquid was virtually nil.
I had thought about weldmesh and geotextile or even woven bamboo if available, but I'm not sure that a flexible lining like this would handle the soil movement well over time. Maybe if there was a band of sand and gravel, say 50 cm thick all around the flexible lining? This would have the advantage of increasing the infiltration surface but would start to get expensive too.
Try to find another place to put the latrines?????
As well as burnt brick lining, consider whether integrated soil stabilised blocks (ISSBs) are appropriate. If suitable soil is available locally, this is a relatively cheap, environmentally friendly (relative to burnt brick or cement blocks) durable solution, although black cotton soils are clearly not suitable. http://www.goodearthtrust.org.uk/
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