Although designs vary, most septic tanks consist of a watertight, below ground, tank that has one or two manhole covers (buried sometimes one to three feet below ground in older systems) to provide access for cleaning and inspection. Many of the newer tanks have risers with lids at the ground surface to allow easy location, inspection, and pumping of the tank. A riser can be added to an older deeper tank to allow for easier maintenance. Effluent from the house flows into the tank through an inlet pipe near the top on one side. It flows out through a discharge or overflow pipe on the other side. The pipe may end in a large tee-fitting or into a baffle (wall) preventing the effluent from flowing straight across from one pipe to the other. The incoming effluent will be diverted downward with a minimum of splashing, allowing the solids to sink to the bottom.

Outgoing effluent is drawn from several feet below the top layer of the floating waste (grease, oil, crust) so that only liquid waste or solids that have been liquefied by the bacterial action going on at the bottom of the septic tank are discharged out into the drainage field.