<P> 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 . </P> <P> A septic tank, the septic drain field, and the associated piping compose a septic system . The septic drain field is effective for disposal of organic materials readily catabolized by a microbial ecosystem . 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 hydraulic for the volume of wastewater requiring disposal and catabolic for the long - term biochemical oxygen demand of that wastewater . </P> <P> Sewage farms are similarly used to 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 . </P> <P> Many health departments require a percolation test ("perc" test) to establish suitability of drain field soil to receive septic tank effluent . An engineer or licensed designer may be required to work with the local governing agency to design a system that conforms to these criteria . </P>

How deep is a septic tank leach field