Low-cost sanitation in areas with shallow groundwater

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WELL has received technical queries from NGOs working in Angola, Mozambique, Bangladesh and South Africa on the subject of constructing low-cost, on-site sanitation facilities in areas with a seasonally high groundwater table, or in areas which are prone to flooding. This is clearly a widespread problem and the aim of this technical brief is to provide some practical guidance on sanitation options in these difficult conditions. More details on the options which are outlined below can be found in the references in the further reading section.

Raised or Step Latrines

Where there is a seasonally high water table, a raised or step latrine may be the most appropriate option for on-site sanitation. The pit for a raised pit latrine should be dug at the end of the dry season to maximise the available depth of unsaturated soil. In areas with a perennially high groundwater table this may be as little as 1 to 3 metres. The pit is lined with appropriate, locally available materials such as fired clay bricks, blockwork, porous concrete, large stones or pieces of rock, precast concrete rings or ferrocement. It is also possible to use 200 litre oil drums as a lining if these are readily available; these however make a pit with a very low volume which will have a short life. The lining is extended above ground level to provide the required pit volume as shown in Sketches 1 to 3. The excavated material can be used to build up a mound or embankment around the latrine. This embankment (excluding the top 1.5 m) can be used for liquid infiltration from the pit if; it is formed with permeable soil, is well compacted with a stable side slope not exceeding 1:1.5 and is thick enough to ensure that filtrate does not seep out of the sides of the mound. The slab should be constructed at least 0.5 meters above the highest water level.

In the case where there is a lack of suitable fill material available to build up an embankment (for example when excavating in rock) it may be necessary to make the lining impermeable by rendering with cement internally and externally.

Steps may be constructed to provide access to a raised latrine, although in some regions people may feel that they are ‘exposed’ when going up and down the steps.

The raised or step latrine is a relatively expensive option and in areas like Bangladesh which are prone to heavy flooding the pit may be rendered useless after filling up with silt during the rainy season. The owner then has to either empty out the latrine or construct a new one. In practice, people often resort to open defecation when faced with this situation and so it may be better to promote lower cost alternatives to the raised pit latrine e.g. a shallow lined or unlined pit.

Shallow Unlined or Lined Latrines

Shallow pit latrines of around 1.5 metres depth can be constructed relatively easily and cheaply; this may be the best option available to households in areas where latrines are prone to flooding and filling with silt. If land is readily available then an unlined pit would generally be abandoned when it becomes full and the household would dig a new one on their plot. If land is at a premium or if the pit is lined, then householders may consider emptying out a pit when it is full. There is obviously a health risk associated with manual excavation and disposal of the contents. However, even if the faeces are not adequately disposed of (i.e. buried), the single point source of pollution of the latrine contents is preferable to the multiple pollution of open defecation.

Aqua-Privy Latrines

An aqua-privy consists of a latrine constructed above, or adjacent to, a watertight tank which contains liquid effluent. The excreta drops into the tank through a vertical pipe. This pipe should extend at least 75 mm into the liquid so that a water seal is formed (see figure 4 below). In order to maintain the water seal, the fluid level in the tank must be maintained and this requires a bucketful of water each day to compensate for evaporation losses. The overflow pipe should be connected to a soakaway, drainage trench or sewer. Since this type of latrine has a very low water usage the volume of effluent discharging from the tank is small but very concentrated. The tank needs to be periodically desludged and so a removable cover for the tank must be provided.

The cost of constructing an aqua-privy is higher than building a raised or step latrine and it needs to be well designed and maintained to ensure that it has an adequate design life. There is also a risk that the tank may provide a breeding ground for mosquitoes unless it is perfectly sealed from the external environment.

Dry Composting Latrines

In areas where human excreta is regarded as a valuable resource for fertilizer a dry composting latrine may be an appropriate form of latrine to promote. A composting latrine can be constructed either above or below ground and so it is suitable for regions with shallow groundwater or flooding.

Composting latrines normally consist of a single or double vault construction with a system to ensure that urine is kept separate from faeces. The faeces are collected in the box and need to be mixed regularly with earth, wood ash or other organic waste material to deodorize and soak up excess moisture. The compost eventually formed in the vault can be a valuable fertilizer but these latrines needs to be well-managed to provide safe and useable organic material. If the moisture content is not properly controlled then the latrine will not form a useful compost and will be smelly and unsanitary.

Since the vault must be kept dry, composting latrines are not suitable for areas where water is used for anal cleansing. Some compost latrines are in use in China, Vietnam and Central America but unfortunately they are often abandoned in favour of indiscriminate defecation because households fail to master the art of managing the latrine properly.

Contamination of drinking water supplies

Where the source of drinking water is shallow groundwater there may be a risk of contamination from pit latrines. As a general rule of thumb, a downstream water abstraction point must be at least 15 metres from the point of pollution (i.e. the latrine). This varies according to the hydraulic gradient and the properties of the in-situ soil so each case needs to be considered individually to assess the potential risk. Pollution of groundwater from a pit latrine can be reduced by constructing an artificial sand barrier around the pit to create a schmutzdecke filter effect. However, this is an expensive solution and it would often be more practical to ensure that alternative drinking water sources are developed at a safe distance from the on-site sanitation facilities.

Sketches of design options

Figure 1: Raised pit latrine

(Source: Franceys, Pickford & Reed, 1992)

Figure 2: Mound latrine

(Source: Franceys, Pickford & Reed, 1992)

Figure 3: Raised pit latrine

Figure 4: Aqua-privy

(Source: Franceys, Pickford & Reed, 1992)

Author: Sarah Parry-Jones and John Picked

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