Evapotranspiration / Absorption Septic Systems
This type of system is used less in this day and age, as the effluent produced is too contaminated for above ground or near surface application. Septic tank systems usually consist of septic tanks, which has a baffle dividing the tank, this baffle will hold back the solids, allowing for the settling of the solids which form a sludge layer on the bottom of the tank and floatation of fats and greases which form a scum layer on the surface.
The settling process takes about 24 hours; the natural bacteria in the septic tank begin to breakdown and digest the organic material found in the wastewater. Only the primary treated liquid (effluent) found in the centre level of the tank, flows out of the septic tank into an absorption field. The sludge and scum remain in the septic tank and must be periodically removed to ensure that the system operates properly.
Evapotranspiration/absorption systems are designed to disperse effluent exclusively by evaporation, transpiration and absorption. Evaporation is the movement of water from the soil to the atmosphere and is directly impacted by precipitation and climatic factors. Transpiration involves the removal of water by vegetation or how much water plants can soak using their roots. Absorption relies upon the percolation of water through the soil.
The Evapotranspiration/absorption trenches consist of perforated pipe (tunnel trench) and gravel, the trenches are normally about 600mm deep, and must be dug across slopes and be level. The trenches then have the tunnel trench laid out and backfilled around with the gravel. The gravel is then covered by a geotextile mating, and then covered by natural ground.
As this type of system has only primary treated the effluent, and is regarded as a higher risk to public health and safety. Most local governing authorities prefer to have wastewater treatment systems installed, as they secondary treat the effluent.
In most installations the septic tank is located upslope of the transpiration bed, so effluent will disperse through the transpiration trenches by gravity. In some installations a submersible pump is required to be installed in the effluent chamber of the septic tank to pump effluent to the transpiration field. Products such as splitter boxes may need to be installed to distribute effluent evenly, if a number of trenches have been installed.