Leach Fields

If there is no intention or no need to reuse wastewater or greywater, leach fields can offer an opportunity for a partial treatment of greywater or wastewater effluents from a primary treatment (e.g. septic tank, twin-pits for pour-flush toilets, biogas settler, anaerobic baffled reactor, etc.) or stormwater (precipitation water) and a safe way of discharging it to the environment and therewith recharging groundwater bodies. The pre-treated effluent flows by gravity or is pumped to a leach field for disposal. Compared to a soak pit, the leach field might be somehow safer, but is considerably more sophisticated in its construction.

Pre-settled effluent is fed into a piping system (distribution box and several parallel channels) that distributes the flow into the subsurface soil for absorption and subsequent treatment. 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. 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. A dosing or pressurized distribution system may be installed to ensure that the whole length of the leach field is utilized and that aerobic conditions are allowed to recover between dosings. Such a dosing system releases the pressurized effluent into the leach field with a timer (usually 3 to 4 times a day).

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 on top. More rock is placed to cover the pipe. A layer of geotextile fabric is placed on the rock layer 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 at least 15 cm beneath the surface to prevent effluent from surfacing. The trenches should be dug no longer than 20 m in length and at least 1 to 2 m apart. To prevent contamination, a leach field should be located at least 30 m away from any drinking water source. A leach field should be laid out such that it will not interfere with a future sewer connection. The collection technology which precedes the leach field (e.g., septic tank) should be equipped with a sewer connection so that if, or when, the leach field needs to be replaced, the changeover can be done with minimal disruption.

Since the technology is underground and requires little attention, users will rarely come in contact with the effluent and, therefore, it has no health risk. The leach field must be kept as far away as possible (at least 30 m) from any potential potable water source to avoid contamination.

A leach field will become clogged over time, although this may take 20 or more years, if a well-maintained and well-functioning primary treatment technology is in place. Effectively, a leach field should require minimal maintenance; however, if the system stops working efficiently, the pipes should be cleaned and/or removed and replaced. To maintain the leach field, there should be no plants or trees on it. There should also be no heavy traffic above it because this could crush the pipes or compact the soil.