Home wastewater enters the septic tank, which separates solids from liquids. Solids are held in the septic tank and liquids are conveyed to the final soil treatment site. The septic tank is a “bioreactor” where micro organisms break down organic matter in the wastewater to liquids, gases and solids. Gases are vented off through the house vent stack. Solids are composed of both scum and sludge. Scum is lighter than water and floats to the surface in the septic tank. The solid parts are heavier than water and sink to the bottom of the tank. Bacteria feed on the wastes and the fraction that can’t be decomposed is “sludge.” Sludge accumulates in the bottom of the septic tank and must be removed periodically.