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A holding tank, also called a waste water holding tank or black (water) tank, is a container for storing sewage in vehicles equipped with toilets. Vehicles that employ holding tanks include recreational vehicles (RVs), trucks (U.S. usage) or lorries (UK usage), railroad trains, boats, aircraft, and even spacecraft. The contents are emptied into a dump station, which discharges the raw sewage into a sewage treatment system.
Holding tank can also refer to a sealed, buried tank used to hold waste from a house or other building. Such a tank is referred to in the U.K. as a cesspit, and as a septic tank in the U.S. A Holding Tank Sewage System (HTSS) is an alternative to a conventional on-site sewage system with very special and limited applications. Simply, the HTSS provides a means to collect and temporarily store sewage from a facility or dwelling, for subsequent removal and transport to an approved treatment and disposal site. The required storage capacity of a holding tank depends upon two items: daily sewage flow, and available or optimal pumping service frequency.