What is the difference between a septic tank, a sewage treatment plant and a cesspool?

A septic tank is a passive, multi-chamber tank – typically made from fibreglass, brick, block or cast concrete – into which raw sewage flows and settles. Heavier solids sink and lighter materials float; a partially clarified liquid is then discharged to a drainage field in the ground. A septic tank cannot legally discharge to a watercourse.

A sewage treatment plant (or package treatment plant) takes the process further by actively introducing oxygen into the tank – through a blower and aeration system – to support aerobic bacteria that break down sewage to a much higher standard. A correctly designed and maintained sewage treatment plant can discharge treated effluent to a suitable watercourse or, in some cases, to ground.

A cesspool (or cesspit) is a sealed tank with no outlet. It is designed to hold all incoming raw sewage and must be regularly emptied by tanker. Cesspools are typically used as a last resort where ground conditions, space or watercourse access make any form of discharge impractical or illegal.