Single Pit

The pit latrine (also called pit toilet) is one of the most widely used sanitation technologies. It is made out of a superstructure and a pit in which faeces, urine and anal cleansing material (water and/or solids) are disposed. Dry anal cleansing is advantageous to minimize water content and prevents fast fill up and groundwater pollution. To prevent people from falling into the pit and increase convenience and reduce odour, a slab with a hole should be used to cover the pit. A toilet seat can be installed over the hole. As the single pit fills, two processes limit the rate of accumulation: leaching and degradation. Urine and cleansing water trickle to the pit content (mixing up in a leachate) and percolate into the soil through the bottom of the pit and wall, while microbial action degrades part of the organic fraction. For that reason, underground of the latrine should be permeable. The pit latrine needs no water for its function which is an advantage in water scarce areas compared to flush toilets (see flush toilets, pour flush toilets or low-flush toilets). However, compared to other toilet systems such as UDDTS, composting toilet, twin-pits for pour flush, terra preta toilets, arborloo or fossa alterna, the treatment processes are limited (EAWAG/SANDEC 2008; TILLEY et al. 2008).

On average, solids accumulate at a rate of 40 to 60 L per person/year and up to 90 L per person/year if dry cleansing materials such as leaves or paper are used. The volume of the pit should be designed to contain at least 1,000 L. Typically, the pit is at least 3 m deep and 1 m in diameter. If the pit diameter exceeds 1.5 m, there is an increased risk of collapse. Depending on how deep they are dug, some pits may last 20 or more years without emptying. The longer a pit lasts, the lower will be the average annual economic cost and the greater the social benefits from the original input (WHO 1992). To prevent groundwater contamination, the bottom of the pit should be at least 2 m above groundwater level (rule of thumb). If the pit is to be reused, it should be lined. The lining also prevents the pit from collapsing and provides support to the superstructure. Pit lining materials can include brick, rot-resistant timber, concrete, stones, or mortar plastered onto the soil. If the soil is stable (i.e., no presence of sand or gravel deposits or loose organic materials), the whole pit need not be lined. The bottom of the pit should remain unlined to allow for the infiltration of liquids out of the pit.

As liquid leaches from the pit and migrates through the unsaturated soil matrix, pathogenic germs are sorbed to the soil surface. In this way, pathogens can be removed prior to contact with groundwater. The degree of removal varies with soil type, distance travelled, moisture and other environmental factors and, thus, it is difficult to estimate the distance necessary between a pit and a water source. A minimum horizontal distance of 30 m is normally recommended to limit exposure to microbial contamination. In densely populated areas with many pit latrines it is often not possible to keep these distances between pollution sources and water abstraction points due to very limited space. In these areas, the risk of groundwater contamination and health risks associated with pit latrines is high and other sanitation options should be considered.

When it is not possible to dig a deep pit (e.g. in rocky soil) or the groundwater level is too high, a raised pit can be a viable alternative: the shallow pit can be extended by building the pit upwards with the use of concrete rings or blocks. A raised pit can also be constructed in an area where flooding is frequent in order to keep water from flowing into the pit during heavy rain. Another variation is the unlined shallow pit that may be appropriate for areas where digging is difficult. When the shallow pit is full, it can be covered with leaves and soil, and a small tree can be planted (see Arborloo). Urine diverting dry toilets could be a suitable option for regions with high groundwater table, as they are generally built above ground. In regions which tend to get flooded, “hanging UDDTs” are recommended.

Besides using twin-pits or double-pits, an upgrade of the pit latrine increases the value of the toilet, is economical and very easy to implement. A ventilated improved pit ( single VIP or double VIP ) is slightly more expensive than a single pit, but greatly reduces the nuisance of flies and odours, while increasing comfort.

If a urine-diverting user interface is used (see urine diversion toilet slab) , only faeces are collected in the pit and leaching can be minimized. The separately collected urine as well as the dehydrated faecal material in the pit can bereused as a nutrient rich fertiliser in agriculture (see also use of urine in agriculture at small and large scale, use of dehydrated faeces and compost in agriculture).

Single pit latrines are very low costs, and are among the cheapest and easiest systems although almost no treatment is included. However, costs for relocation of emptying (see human powered and motorised) the pits need to be considered (WHO 1992).

A single pit is an improvement to open defecation; however, it still poses health risks:

• Leachate can contaminate groundwater;
• Stagnant water in pits may promote insect breeding;
• Pits are susceptible to failure and/or overflowing during floods.

Single pits should be constructed at an appropriate distance from homes to minimize fly and odour nuisances and to ensure convenience and safety. The emptying (see human powered and motorised) of pit latrines containing fresh excreta presents problems because of the active pathogens in the sludge. Manual removal should therefore be avoided (WHO 1992).

There is no daily maintenance associated with a single pit apart from keeping the facility clean. However, when the pit is full it can be a) pumped out and reused (see also human powered and motorised emptying, use of compost in agriculture) - in order to minimize the health risk the pit can also be treated before reuse (see e.g. drying beds) - or b) the superstructure and squatting plate can be moved to a new pit and the previous pit covered and decommissioned, which is only advisable if plenty of land area is available.