The supply water inlet connection o

The supply water inlet connection of a conventional reverse osmosis water purifying unit is connected to a pressurized water supply pipe, and the blowdown water connection of the unit is connected to a pressurized enclosure having, when stabilized, a pressure equal to that of the water supply pipe, such as another water supply pipe or a pressurized vessel. The unit is free from flow restriction between its inlet water and blowdown connections, and when pressure is lowered relatively, either in the pressurized supply pipe or the pressurized enclosure, a blowdown surge occurs toward the lowered side and through the unit between the inlet connection and the blowdown connection. Successive surges dispose of sufficient blowdown water, either to the inlet pipe or to the pressurized enclosure to make the system operative without other waste disposal facilities, and without noticeably contaminating the water in the pipe or pipes into which the saline waste water is thus discharged.