Cesspit

Cesspit (also known as cesspool)is a very general description for different on-site collection systems for liquid wastes, mostly blackwater or faeces. If a cesspit is sealed, it is also known as underground holding tank and can be used as a transfer station. Furthermore it is possible to set a cesspit below or beside a toilet. In this case it is very similar to a single pit. This system is still better than no sanitation from a point of view of the health risk for the user, but the treatment process in the cesspit is very limited. A secondary treatment of the collected sludge is necessary (see paragraph “Operation and Maintenance”). Therefore it is advantageous to use a more efficient system for treating human excreta (e.g. dry and semi-dry systems such as UDDT, composting toilets, or a water-based toilet system which is connected to a twin-pit for pour flush toilets, septic tanks ora semi-centralised secondary treatment system such as constructed wetlands (free surface, horizontal flow or vertical flow or non-planted filters.

Cesspits are simple pits collecting excreta, faeces and urine. They are well one of the most common on-site wastewater collection system in the absence of a sewered collection network. They typically serve one household or are shared by families, or neighbours of the same multiple apartment building. The size of the tank depends on the availability of land and on the construction costs. Their capacity might range from 5 to 50 m³. The periods between each complete evacuation of the contents of cesspits vary between a couple of months up to two years (WERNER et al. 2000; WHO 1992).

The cesspit is lined with bricks, concrete or plastic. Cess-pits are either water-tight (such as underground holding tanks, see also transfer stations or permeable (similar to soak pits). Treatment processes in permeable cesspits include wastewater settling (greywater and blackwater), anaerobic sludge digestion and percolation of liquid into the subsurface where microbial action degrades part of the organic fraction (MOREL & DIENER 2006; WERNER et al. 2000). As the effluent leaches from the permeable cesspit and migrates through the unsaturated soil matrix, faecal organisms are removed (similar as in soak pits). The degree of faecal organism removal varies with soil type, distance travelled, moisture and other environmental factors and thus, it is difficult to estimate the necessary distance between a pit and a water source (TILLEY et al. 2008).

If the groundwater table is high or the soil is very permeable, a cesspit can allow raw sewage to directly enter groundwater with minimal biological cleansing, which leads to groundwater contamination and undrinkable water supplies. Thus, in the case of high groundwater table, the cesspit should be constructed watertight. The disadvantage of a watertight cesspit is that it is filled up much faster. There are several pre-fabricated types made out of synthetic material (e.g. fibreglass) or concrete.

Cesspit is one of the simplest and cheapest methods to collect faeces. But still, the economic costs for a household that needs its cesspit emptied are high. That is why often cesspits are constructed for infiltration of the leachate into the soil, which leads to less frequent emptying and thus less costs. Comparing the costs for emptying by vacuum truck with an average income of people found in rural communities in developing countries, the cost for emptying cesspits are even more expensive (WERNER et al. 2000).

In some cases, untreated greywater is used to irrigate trees in backyards in order to minimise regular desludging of the cesspits in the household (MOREL & DIENER 2006). This could endanger the environment and the health of the locals around the irrigated area.

As already mentioned, a cesspit has to be emptied regularly by a vacuum truck (see also motorised emptying and transport) or manually (see human powered emptying and transport similar to a pit latrine. The faecal sludge needs to be correctly disposed and further treated (e.g. small or large composting, anaerobic digestion, or drying beds) because it contains still active pathogens. It is necessary that somebody takes care about the cesspool. The person responsible has to pay attention that that cesspit does not overflow and that it is well protected against external water infiltration (covered with a slab and the entrance of the manhole should be slightly higher than the surrounding).

When there is a high groundwater table, the leachate of the cesspit that does infiltrate into the soil can contaminate the groundwater. In the West Bank for example, this is why cesspits are considered to be the main source of groundwater pollution in the Palestinian Territories (WERNER et al.2000). ROY et al. (1984) give some guidelines for minimal distances between permeable systems and drinking water sources (chapter 4 and 5). In densely populated areas with many permeable cesspits or pit latrines, the risk of a groundwater contamination remains however extremely high (WHO 1992). In any case it remains very difficult to give some guidelines without knowing the type and hydraulic conductivity of the soil.

Manual removal should be avoided, because the risk of an infection is high. A cesspit is covered with a slab all the time. This prevents the spread of pathogens, bad odour, nuisance by flies or mosquitoes and contact of humans or animals with the collected sludge (WHO 1992).

It is dangerous to lift the slab by its own to check the level. Toxic gases from the cesspit can overcome the person very quickly. Therefore, an alarm should be installed to see check the level of the sludge (e.g. a float ball) (WTE Ltd. (Editor) (n.y.).