Another step up the ladder one finds the septic tank, a common method of human waste disposal in rural and suburban areas of the United States. In this system the turd is deposited into a container of water, usually pure drinking water, and the whole works are flushed away (see Figures 5.6 and 5.7).
After the floating turd travels from the house inside a sewage pipe, it plops into a fairly large underground storage tank, or septic tank, usually made of concrete and sometimes of fiberglass. In Pennsylvania (US), a 900 gallon tank is the minimum size allowed for a home with three or fewer bedrooms.1 The heavier solids settle to the bottom of the tank and the liquids drain off into a leach field, which consists of an array of drain pipes situated below the ground surface allowing the liquid to seep out into the soil. The wastewater should be undergoing anaerobic decomposition while in the tank. When septic tanks fill up, they are pumped out and the waste material is supposed to be trucked to a sewage treatment plant (sometimes they're illegally dumped).
Source: The Humanure Handbook. Jenkins Publishing, PO Box 607, Grove City, PA 16127. To order, phone: 1-800-639-4099.