If there is no intention or no need to reuse wastewater or greywater, leach fields can offer an opportunity for a partial treatment of stormwater (precipitation water), greywater or wastewater effluents from a primary treatment (e.g. septic tank, twin-pits for pour-flush toilets, biogas settler, anaerobic baffled reactor, etc.) and a safe way of discharging it to the environment and therewith recharging groundwater bodies.
Compared to a soak pit, the leach field might be somehow safer, but is considerably more sophisticated in its construction.
Leach fields are a low-cost technology that allows for a secondary treatment of partly treated wastewater coming from septic tanks or another primary treatment unit, as well as for greywater or stormwater. It allows a controlled disposal of water into the environment. The pre-treated effluent flows by gravity or is pumped to a leach field for disposal. The wastewater effluent is absorbed by soil particles and moves both horizontally and vertically through the soil pores. The dissolved organic material in the effluent is removed by bacteria living in the top ten feet of the soil. The wastewater generally percolates downward through the soil and eventually enters a groundwater aquifer. A portion of the wastewater moves upwards by capillary action and either evaporates at the surface or is absorbed by plants.
The overflow effluent from the septic tank is fed into a distribution box, which directs the flow into several parallel channels. Either a small dosing system releases the pressurised effluent into the leach field on a timer (usually 3 to 4 times a day) or the water flows continuously by gravity. If the distribution is driven by gravity, the slope until the end of each leaching line must be sufficient in order to assure a good drainage. The intermittent pressurised injection ensures that the whole length of the leach field is utilised and that aerobic conditions are allowed to recover between each dosing. Each trench is 0.3 to 1.5 m deep and 0.3 to 1 m wide. The bottom of each trench is filled with about 15 cm of clean rock and a perforated distribution pipe is laid overtop. More rock covers the pipe so that it is completely surrounded. The layer of rock is covered with a layer of geotextile fabric to prevent small particles from plugging the pipe. A final layer of sand and/or topsoil covers the fabric and fills the trench to the ground level. The pipe should be placed 15 cm from the surface to prevent effluent from surfacing. The trenches should be dug no longer than 20 m in length at least 1 to 2 m apart.
Inevitably, a leach field will become clogged over time. However, a well-functioning pre-treatment technology guarantees proper functioning for many years. Effectively, a leach field should require minimal maintenance, although, if the system stops working efficiently, the pipes should be cleaned and/or removed and replaced. To maintain and protect the leach field, plants with important root systems or trees should not been planted above it. Also, any kind of heavy vehicles should not pass through the area, since the application of heavy weights might compact the soil and eventually damage the pipes.
Since the technology is underground and it requires little attention, users will rarely come in contact with the effluent and so it should pose no health risk. But to prevent contamination, the leach field must be kept as far away as possible from any potential potable water source (at least 30 m).
Leach fields require a large area and soil with good absorptive capacity to effectively dissipate the effluent. To prevent contamination, a leach field should be located 30 m away from a drinking water supply. Leach fields are not appropriate for dense urban areas. They can be used in almost every temperature, although there may be problems with pooling effluent in areas where the ground freezes. Homeowners who have a leach field must be aware of how it works and what their maintenance responsibilities are. Trees and deep-rooted plants should be kept away from the leach field as they can crack and disturb the tile bed.
This manual provides information on functionality of septic systems containing the modules septic tank and leach field and addresses the user at the household level. Construction, maintenance and operation are described.
TILLEY, E.; LUETHI, C.; MOREL, A.; ZURBRUEGG, C.; SCHERTENLEIB, R. (2008): Compendium of Sanitation Systems and Technologies. Duebendorf and Geneva: Swiss Federal Institute of Aquatic Science and Technology (EAWAG). URL [Accessed: 15.02.2010]. PDF
This compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.
See document in FRENCH
This document discusses several options for sewage collection and wastewater treatment in new subdivisions and low-cost housing developments.
MONVOIS, J.; GABERT, J.; FRENOUX, C.; GUILLAUME, M. (2010): How to Select Appropriate Technical Solutions for Sanitation. Cotonou and Paris: Partenariat pour le Développement Municipal (PDM) and Programme Solidarité Eau (pS-Eau). URL [Accessed: 19.10.2011]. PDF
The purpose of this guide is to assist local contracting authorities and their partners in identifying those sanitation technologies best suited to the different contexts that exist within their town. The first part of the guide contains a planning process and a set of criteria to be completed; these assist you in characterizing each area of intervention so that you are then in a position to identify the most appropriate technical solutions. The second part of the guide consists of technical factsheets which give a practical overview of the technical and economic characteristics, the operating principle and the pros and cons of the 29 sanitation technology options most commonly used in sub-Saharan Africa.
See document in FRENCH