Although it is usually best to provide large deep pits, this may not be possible where rock or groundwater lie within one or two metres of the ground surface. A variation of the VIP latrine suitable for such situations has two shallow pits side by side under a single superstructure (Fig. 6.7). The pits are usually lined with bricks or blocks. Each pit may have its own squat hole or seat. Alternatively, slabs may be movable, one with a hole for the pit in use and a plain slab for the other pit. Whichever design is used, only one hole must be available for defecation at any time. The latrine may be provided with two ventilation pipes (one for each pit) but more usually only one is fitted, to the pit in use. The hole for the ventilation pipe for the pit not in use is sealed. As with single VIP latrines, the superstructure must be kept partially dark at all times to discourage flies.
Fig. 6.7. Double-pit VIP latrine
One pit is used until it is filled to within about half a metre of the top. The defecation hole over the full pit is then sealed and the one over the empty pit opened. Where necessary, the ventilation pipe is moved from the full to the empty pit, and the vent hole in the slab of the full pit sealed. The second pit is then used until filled to within half a metre of the top. The contents of the first pit can now be removed and the pit reused. The pits must be large enough to allow each pit to be used for at least two years. This ensures that when the pit contents are dug out most of the pathogenic organisms have died.
Double-pit latrines can be considered as permanent installations. The small effective capacity (0.72 m3 for a family of six, using a sludge build-up rate of 60 litres per person per year, as suggested in Chapter 5) enables pits to be relatively shallow, and therefore easier to empty than deep pits. The pits should extend beyond the superstructure, either to the sides or at the back, with removable slabs for emptying. These slabs should be easy to lift, but should be sealed to prevent flies getting in or out. The central wall between the two pits should be made with full mortar joints and may be rendered with cement mortar on both sides.
As with the single-pit VIP latrine, the double-pit VIP latrine has the advantages of reduced smell and fly nuisance. Also the contents of the latrine dug out every two years or so are a valuable soil conditioner (see Annex 1). Double-pit VIP latrines are usually (but not always) more expensive than single-pit VIP latrines, and require a greater operational input from the user, particularly in changing over pits. Some societies have shown resistance to handling the decomposed contents of the pit but this can often be overcome with education and time. Allowing people to see (and handle) the contents of a pit as it is emptied is the strongest persuader for those concerned.
All projects involving the construction of double-pit latrines must allow for a prolonged support programme. Householders need to be reminded to change pits at the right time and should be assisted in doing so. This assistance will probably have to be available for at least the first two pit changes to ensure that the complete cycle is covered.