Septic System Brochure: What is A Septic Tank System?
Cross section of a septic tank.

A septic tank system consists of a large, watertight tank that
receives wastewater from the home plumbing system. The tank is
followed by an underground drainfield consisting of a network of
perforated pipe or chambers for distributing partially treated water
from the septic tank to the soil for final treatment and disposal.