A septic tank is a large chamber that stores household waste from sinks, toilets, laundry and drains. Waste is separated into three components, sludge, solids that settle in the bottom of the tank, scum, which is the floatable grease, and the relatively clear water that rises to the top and into the drainfield. It is recommended that a tank be pumped out at least every 3 years per 1,000-gallon tank.