Conventional Onsite Wastewater Systems

In the U.S., a conventional onsite wastewater system consists of a septic tank and a subsurface soil absorption system (commonly called a dispersal field, a drain field, or a leach field).  Typically, these effluent dispersal systems consists of several narrow trenches filled with a porous media such as gravel.  The gravel media functions to maintain the structure of the trenches, to distribute the effluent to the soil infiltrative surfaces, to provide storage capacity during peak discharges, and may help provide a limited amount of effluent treatment.
(Kreissl, J. F. 1982.  Onsite wastewater disposal research in the United States.  Alternative wastewater treatment. D. Reidel Publishing Company, Boston.)

Dispersal trenches are designed to allow the applied septic tank effluent to infiltrate into the soil below and around the trenches for treatment and transport it away from the drainfield area.