Abstract: A waste tank cleaning system in a preferred embodiment includes a flowmeter interposed in the line carrying cleaning fluid to the tank. A pair of solenoid valves are controlled by an electronic counter coupled to the flowmeter. In response to predetermined volumetric flows of fluid, the solenoid valves alternatively deliver fluid to a hydraulically operated drain valve and to a spray/rinse outlet in the waste tank. In one alternative embodiment, a flowmeter is mechanically coupled to a valve structure which alternatively applies fluid to the drain valve and to a spray/rinse outlet in the tank, directly. In other embodiments, the volume of flow can be determined by liquid level sensors in the tank and a predetermined cleaning sequence can be carried out based on the signals sent by the sensors. Power can be furnished by a rechargeable power supply.