Septic systems for homes are essentially on-site wastewater treatment systems. They receive water from showers, bathtubs, sinks, toilets and washing machines. The water flows into the septic tank where settling and anaerobic processes reduce solids and organics. Solids remain in the tank, as the septic leach field needs to be protected from solids, and the water flows out to a distribution box. This box evenly divides the water to flow into the drain field, where it seeps into the ground and is filtered by the soil. Most leach fields consist of a row of perforated pipes installed in a level bed of gravel. The water drains into the gravel and is absorbed by the earth.