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Concept
Septic drain field
Summary
Septic drain fields, also called leach fields or leach drains, are subsurface wastewater disposal facilities used to remove contaminants and impurities from the liquid that emerges after anaerobic digestion in a septic tank. Organic materials in the liquid are catabolized by a microbial ecosystem.
A septic drain field, a septic tank, and associated piping compose a septic system.
The drain field typically consists of an arrangement of trenches containing perforated pipes and porous material (often gravel) covered by a layer of soil to prevent animals (and surface runoff) from reaching the wastewater distributed within those trenches. Primary design considerations are both hydraulic for the volume of wastewater requiring disposal and catabolic for the long-term biochemical oxygen demand of that wastewater. The land area that is set aside for the septic drain field may be called a septic reserve area (SRA).
Sewage farms similarly dispose of wastewater through a series of ditches and lagoons (often with little or no pre-treatment). These are more often found in arid countries as the waterflow on the surface allows for irrigation (and fertilization) of agricultural land.
Many health departments require a percolation test ("perc" test) to establish the suitability of drain field soil to receive septic tank effluent. An engineer, soil scientist, or licensed designer may be required to work with the local governing agency to design a system that conforms to these criteria.
A more progressive way to determine leach field sizing is by direct observation of the soil profile. In this observation, the engineer evaluates many features of the soil such as texture, structure, consistency, pores/roots, etc.
The goal of percolation testing is to ensure the soil is permeable enough for septic tank effluent to percolate away from the drain field, but fine grained enough to filter out pathogenic bacteria and viruses before they travel far enough to reach a water well or surface water supply.
Official source
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