Household sewage flows directly into the septic tank, which is a water-tight structure; usually cement.

Inlet baffles slow the incoming water and outlet baffles prevent floating solids from escaping the tank. Heavy solids (sludge) will settle to the bottom while the lighter particles (scum) will rise to the surface.

Bacterium present in the tank will decompose organic solids found in the setic tank's waste water. The sludge that is not organic gradually accumulates over time and must be pumped out periodically (every one to two years) to insure that the solids will not overflow out of the tank and clog the leaching area.