Solids heavier than water settle at the bottom of the tank forming a layer of sludge. This leaves a middle layer of partially clarified wastewater. The layers of sludge and scum remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. The sludge and scum that cannot be broken down are retained in the tank until the tank is pumped. The layer of clarified liquid flows from the septic tank to the drainfield or to a distribution device, which helps to uniformly distribute the wastewater in the drainfield.
Perforated pipes run through the trenches to distribute the wastewater.
The drainfield treats the wastewater by allowing it to slowly trickle from the pipes out into the gravel and down through the soil. The gravel and soil act as biological filters. If you own a septic system, it is important that it be properly maintained. Do not plant trees or shrubbery in the absorption field area, because the roots can get into the lines and plug them. Do not cover the absorption field with hard surfaces, such as concrete or asphalt.