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eNote 6: Twin-pit latrines

Author(s): Reed, Bob  |  Shaw, Rod (ed)  |  Shaw, Rod (ill)

There will be times when it is not feasible to dig a deep pit. For example, when:

• the water table is very close to the surface;
• it is very difficult to dig more than 1.0 to 1.5 metres below water in a confined space;
• there is hard rock close to the surface; and
• the ground in which the pit is to be dug is so soft that it keeps collapsing before an adequate depth can be reached.

In such circumstances it is often easier to dig two shallow pits than one deep one. There are different arrangements for pour-flush and VIP latrines.

Twin-pit pour-flush latrines

For twin-pit pour-flush latrines, two pits are usually connected to the toilet by short pipes that converge at an inspection chamber. Here, wastes are prevented from entering one of the pits by a stone or block of wood. When the first pit used is full, the stone or wood is moved to prevent wastes from entering the full pit and allowing wastes to flow into the second, empty pit instead. The inspection chamber must have an air-tight cover to prevent flies and mosquitoes entering and odours escaping.

Using a twin-pit pour-flush latrine

1. Before use, the inspection chamber is opened and the outlet to one of the pits is blocked with a large stone or a block of wood. This forces all wastes from the toilet to pass into only one of the pits.
    
2. The first pit fills after about two years. The inspection chamber is reopened and the stone or wooden block is moved to cover the pipe leading to the full pit. The inspection chamber is closed again and the latrine returned to use. Wastes will now collect in the second pit.
    
3. When the second pit is full, the contents of the first pit will have been standing for at least two years and virtually all of the pathogenic organisms will have died. At this point, it will be safe to remove the pit cover slab and dig out the contents. By this time the pit contents will have broken down to leave an odourless material which can be safely spread on the ground to help improve the quality of soil.
    
4. After the first pit has been emptied the cover is replaced, the blockage in the inspection chamber is moved again and the first pit reused.
    
5. From then on, a pit is emptied every two years.

Shallow pits for family latrines typically have a capacity below the inlet pipe of about one cubic metre. This is sufficient for the accumulation of solids from a single family over a period of about two years. The amount of water required to flush double pits is slightly more than for a single offset pour-flush because the connection pipe is generally longer and bends.

Twin-pit VIP latrines

Two pits, each of about one cubic metre are dug side by side. The pits are covered by four cover slabs. The outer two slabs are fitted with handles so they can be more easily removed when the pits need emptying. The two centre slabs each contain two holes, one for the toilet and the other for the ventilation pipe. The two central slabs are covered by a superstructure which should be kept dark inside.

Using a twin-pit VIP latrine

1. The two holes over one of the pits are blocked with wooden or concrete covers. The ventilation pipe is fitted over the hole in the other pit slab. This pit can then be used as a latrine.
    
2. When the first pit is full the ventilation pipe is removed and fitted over the second pit and the two hole covers moved to the holes in the first pit. The second pit can then be used as a latrine while the contents of the first pit decompose.
    
3. When the second pit is full the removable side cover on the first pit can be opened and the pit contents emptied. The first pit can then be returned to service and the contents of the second pit allowed to decompose.

Advantages and disadvantages of twin-pit latrines

Double pits can be cheaper to build than single deep pits as the cost of excavation increases with pit depth. The difference is even more pronounced if single pits are to be lined to their full depth.

The main disadvantage of double pits is the length of time taken to introduce them. Experience has shown that communities are unwilling to accept the technology until they have seen for themselves how the pits are emptied. However, in some
circumstances they are the only viable option.

 

Keywords: Twin-pit

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