A septic tank is the component of a septic sewage treatment system that holds waste water long enough to allow solids to settle out. Its capacity is measured in gallons. A 1,500-gallon septic tank will hold 1,500 gallons of sewage.
Septic systems are used in rural areas where connection to a larger sewage treatment facility is not practical. A septic system consists of a pipe to conduct waste water from a home or building to the septic tank; the tank; a drain field consisting of trenches containing perforated pipes covered with gravel, mesh and dirt; and soil. Septic tanks for home systems are sized according to the number of bedrooms in the home.