ing that goes down any of the drains in the house (toilets, showers, sinks, laundry machines) TRAVELS first to the septic tank. The septic tank is a large-volume, watertight tank which provides initial treatment of the household wastewater by intercepting solids and settleable organic matter before disposal of the wastewater (effluent) to the drain FIELD.FUNCTION OF THE SEPTIC TANKWhile 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.Primary TreatmentAs stated, the main function of the septic tank is to remove solids from thewastewater and provide a clarified effluent for disposal to the drain field.The septic tank provides a relatively quiescent BODY of water where thewastewater is retained long enough to let the solids separate by bothsettling and flotation. This process is often called primary treatment andresults 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 watersurface in the tank. Aerobic bacteria work at digesting floating solids.Sludge: The "sinkable" solids (soil, grit, bones, unconsumed food particles)settle to the bottom of the tank and form a sludge layer. The sludge isdenser than water and fluid in nature, so it forms a flat layer along thetank bottom. Underwater anaerobic bacteria consume organic materials in thesludge, giving off gases in the process and then, as they die off, becomepart of the sludge.Effluent: Effluent is the clarified wastewater LEFT over after the scum hasfloated to the top and the sludge has settled to the bottom. It is theclarified liquid between scum and sludge. It flows through the septic tankoutlet into the drain field.