A septic tank separates household sewage while allowing residual water to overflow out into a drain field or connected cesspool. In a cesspool, the wastewater is slowly filtered out through the cesspool walls into the surrounding soil.
Septic tanks are typically small, watertight concrete pits that provide on-site sewage treatment. Cesspools, also known as leaching pools, are similarly constructed and may exist as a standalone system or connected to a septic tank. Both systems are buried in the ground.
As sewage flows into the septic tank or cesspool, light waste such as soap and grease floats to the surface and forms scum. Natural bacteria degrade the scum over time while heavy waste sinks and forms sludge. Between the scum and sludge lies residual water called greywater. In a septic tank the greywater overflows out of the tank, whereas in a cesspool the greywater filters through the walls. Both systems require regular maintenance to prevent sewage overflow.