A septic system is an onsite wastewater treatment system that processes and purifies household waste (effluent). The effluent consists of black water (toilet wastes) and grey water (kitchen sink, bathtub, and laundry wastes).
There are two main components to the septic system, the septic tank and the drain field. Primary treatment occurs in the tank where bacteria digest organic matter in the waste water. Secondary treatment occurs in the drain field where bacteria complete the digestion and purification of the effluent as it slowly infiltrates the soil.
The septic tank is a watertight underground treatment tank constructed of concrete, plastic, or steel. It is approximately eight feet long, four feet wide, and five feet deep. The tank is used to retain, store, and treat solids, as well as release effluent to the drain field. The tank size is determined by the number of bedrooms in the dwelling.
As waste water flows into the tank, the heavy solids settle to the bottom and become a sludge layer; while fats, oils, and greases float to the top forming a layer of scum. A zone of clear liquid between the layers of scum and sludge is called a clarified zone.