Drainfield

The septic tank effluent is allowed to flow to the drainfield by gravity or is dosed by pump or siphon. The effluent enters the soil and is treated as it percolates to the groundwater.

The soil acts as biological filter to remove nearly all harmful substances including disease-causing bacteria and viruses, toxic organics and other undesirable wastewater constituents remaining in the septic tank effluent.

The drainfield provides final treatment of the wastewater and releases the treated water into the groundwater recharge. The drainfield is typically built as a series of trenches or as one larger bed, and is usually one to three feet below ground level.

The drainfield must be constructed in permeable soils, have a level bottom, and be two or more feet above the groundwater table. While there are many types of drainfield systems such as aerobic, lagoon, sand filters or gravelless chamber technology, we will describe here a typical gravel and pipe system.

The excavated trench or bed is filled with six to twelve inches of gravel. The gravel exposes a soil infiltrative surface and provides storage for the wastewater.

A perforated pipe is laid over the gravel to distribute the partially treated liquid, called effluent, from the septic tank over the bottom of the drainfield. The gravel and pipe are covered with synthetic fabric to help keep soil particles out of the system and the area is backfilled with soil to cover the system.