A septic system is a sewage treatment and disposal system buried in the ground. It is composed of a septic tank and a leach field or trench. Household sewage (waste water from sinks, toilets, showers, washing machines, garbage grinder, and dishwashers) generally flows by gravity into the septic tank. There, heavier particles settle to the bottom and scum rises to the top. Bacteria in the tank help break down some of the solids. The sludge and scum remain in the tank and the effluent (liquid) flows to the leach field where it is distributed over crushed gravel or absorbent soil. The liquid typically includes contaminants such as nitrates, phosphorous, disease causing bacteria and viruses, dissolved metals, detergents, and solvents. The septic tank and leach field provide a minimal treatment for these contaminants. Generally phosphorus and dissolved metals are bound by the soil – although sandy and gravelly soil may not remove these compounds. Nitrates and solvents are diluted in the groundwater. Bacteria and viruses are filtered by the soil or die off. Septic systems can fail due to poor design or construction, overloading or inadequate maintenance.