Soak Pits

If there is no intention or no need to reuse wastewater, collected stormwater or greywater, soak pits can offer a cost-efficient opportunity for a partial treatment of waste- grey- or stormwater from a primary treatment (e.g. septic tank, twin-pits for pour-flush toilets, biogas settler, anaerobic baffled reactor, etc.) and a relatively safe way of discharging it to the environment and therewith recharging groundwater bodies. As wastewater (greywater or blackwater after primary treatment) percolates through the soil from the soak pit, small particles are filtered out by the soil matrix and organics are digested by microorganisms. The wastewater effluent is absorbed by soil particles and moves both horizontally and vertically through the soil pores. Sub-soil layers should therefore be water permeable in order to avoid fast saturation. High daily volumes of discharged effluents should be avoided (HEEB et al. 2008). Thus, soak pits are best suited for soil with good absorptive properties; clay, hard packed or rocky soil is not appropriate. Soak pits are used the same way as leach fields, but require less space as well as less operation and maintenance. But they generally can also receive less influent and the groundwater pollution may be higher than with leach fields.

The soak pit , consisting basically of a simple pit (generally approximately 1 m³), should be between 1.5 and 4 m deep, but as a rule of thumb, never less than 2 m above the groundwater table. It should be located at a safe distance from a drinking water source (ideally more than 30 m). The soak pit should be kept away from high-traffic areas so that the soil above and around it is not compacted. It can be left empty and lined with a porous material to provide support and prevent collapse, or left unlined and filled with coarse rocks and gravel. The rocks and gravel will prevent the walls from collapsing, but will still provide adequate space for the wastewater. In both cases, a layer of sand and fine gravel should be spread across the bottom to help disperse the flow. To allow for future access, a removable (preferably concrete) lid should be used to seal the pit until it needs to be maintained.

As long as the soak pit is not used for raw sewage, and as long as the previous collection and storage/treatment technology is functioning well, health concerns are minimal. The technology is located underground and, thus, humans and animals should have no contact with the effluent. It is important however, that the soak pit is located at a safe distance from a drinking water source (ideally at least 30 m).

Since the soak pit is odourless and not visible, it should be accepted by even the most sensitive communities.

A well-sized soak pit should last between 3 and 5 years without maintenance. To extend the life of a soak pit, care should be taken to ensure that the effluent has been clarified and/or filtered to prevent the excessive build-up of solids.

The soak pit should be kept away from high-traffic areas so that the soil above and around it is not compacted. Particles and biomass will eventually clog the pit and it will need to be cleaned or moved. When the performance of the soak pit deteriorates, the material inside the soak pit can be excavated and refilled.

To allow for future access, a removable (preferably concrete) lid should be used to seal the pit until it needs to be maintained.