Typical concrete septic tank cross section.

Septic tanks

The system's first treatment and a critical component

 A septic system consists of two main parts – a septic tank and a drainfield. The septic tank is a watertight box, usually made of concrete or polymer (plastic, fiberglass), with an inlet and outlet pipe. Wastewater flows from the home to the septic tank through a sewer pipe. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to significantly separate. The wastewater forms three layers inside the tank. Solids lighter than water (such as greases and oils) float to the top, forming a layer of scum. Solids heavier than water settle at the bottom of the tank, forming a layer of sludge. This leaves the 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.