Abstract: A used water reuse system includes a gray water holding tank to which gray water is supplied up to a maximum level through a filter and gray water drain line. Gray water sources such as kitchen and bathroom sinks, clothes and dishwashers, tubs and showers, and the like have drains connected through valves to the gray water drain line and to a conventional sewer line. Gray water is transferred, upon demand, from the gray water holding tank to noncritical water use devices, such as a toilet tank, for reuse and subsequent discarding. A sensor detects achievement of a maximum level in the gray water holding tank and causes the valves at the gray water sources to disconnect the latter from the gray water drain line, so as to discard any further gray water from the sources to the sewer line, and also closes a fail-safe valve to preclude flow from the gray water line to the gray water holding tank. Upon reduction of tank water level the sensor reopens the valves.