Everything 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 waste water by intercepting the solids and settable organic matter before disposal of the waste water (effluent) to the drain field.
The septic tank provides a relatively quiescent body of water where the waste water is retained long enough to let 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: 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 waste water 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.