A septic tank, the key component of a septic system, is a small scale sewage treatment system common in areas with no connection to main sewage pipes provided by private corporations or local governments. (Other components, typically mandated and/or restricted by local governments, optionally include pumps, alarms, sand filters, and clarified liquid effluent disposal means such as a septic drain field, ponds, or peat moss beds.) Septic systems are a type of On-Site Sewage Facility (OSSF). In North America approximately 25% of the population relies on septic tanks; this can include suburbs and small towns as well as rural areas.
(Indianapolis is an example of a large city where much of the city's neighborhoods are still on separate septic systems) In Europe they are generally limited to rural areas only. The term "septic" refers to the anaerobic bacterial environment that develops in the tank and which decomposes or mineralizes the waste discharged into the tank. Septic tanks can be coupled with other on-site wastewater treatment units such as biofilters or aerobic systems involving artificial forced aeration.
Periodic preventive maintenance is required to remove the irreducible solids which settle and gradually fill the tank, reducing its efficiency. In most jurisdictions this maintenance is required by law, yet often not enforced. Those who ignore the requirement will eventually be faced with extremely costly repairs when solids escape the tank and destroy the clarified liquid effluent disposal means.
A properly cared-for system, on the other hand, can last for decades and possibly a lifetime.