FUNCTION OF THE SEPTIC TANK While relatively simple in construction and operation, the septic tank provides a number of important functions through a complex interaction of physical and biological processes.
The essential functions of the septic tank are to: receive all wastewater from the house separate solids from the wastewater flow cause reduction and decomposition of accumulated solids provide storage for the separated solids (sludge and scum) pass the clarified wastewater (effluent) out to the drain field for final treatment and disposal.
As stated, the main function of the septic tank is to remove solids from the wastewater and provide a clarified effluent for disposal to the drain field. The septic tank provides a relatively quiescent body of water where the wastewater is retained long enough to let the solids separate by both settling and flotation. This process is often called primary treatment and results in three products: scum, sludge, and effluent.
Scum: Substances lighter than water (oil, grease, fats) float to the top, where they form a scum layer. This scum layer floats on top of the water surface in the tank. Aerobic bacteria work at digesting floating solids.
Sludge and Effluent
Sludge: The "sinkable" solids (soil, grit, bones, unconsumed food particles) settle to the bottom of the tank and form a sludge layer. The sludge is denser than water and fluid in nature, so it forms a flat layer along the tank bottom. Underwater anaerobic bacteria consume organic materials in the sludge, giving off gases in the process and then, as they die off, become part of the sludge. Effluent: Effluent is the clarified wastewater left over after the scum has floated to the top and the sludge has settled to the bottom. It is the clarified liquid between scum and sludge. It flows through the septic tank outlet into the drain field.