Although designs vary, most septic tanks consist of a watertight, below ground, the tank that has one or two manhole covers (buried a few inches below ground) to provide access for cleaning and inspection. 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 at 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, scum) so that only liquid waste or solids that have been liquefied by the BACTERIAL ACTION going on at the bottom of the septic tank (which we will come back to this point later) are discharged out into the drainage field.