Abstract: An apparatus (18) for receiving rain water from a water collecting means (20) and treating this water according to a treatment cycle. The apparatus has a stop tank (24) to hold incoming water while a treatment cycle is being executed in a main tank (32), so there is no mixing of the treated water with water to be treated. The main tank has various reservoirs (42, 52, 62), valves (30, 40, 50) and buoyant elements (68, 70), and the water flow is strictly controlled by gravity and the buoyancy of the water. Treating solutions such as a mineral solution and a chloride solution are added in predetermined quantities to the water during the treatment cycle, and the treated water is stored into a storage tank (22).