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.
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.
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.