an economical and commercially practical reverse - osmosis water purifier system which has no back pressure acting against the semipermeable membrane , so that the filtering rate is maximized . furthermore , the supply of tap water to the system is shut off automatically as soon as the pure water storage tank is full , use of water thus being greatly reduced . these , and other major advantages , are achieved despite the fact that there is only a single tube extending to the faucet , automatic ice - maker , refrigerated drinking fountain or other point of use . the system uses rugged , long - lasting and economical diaphragm valves , which eliminates the need for sliding seals . thus , it becomes highly practical to injection - mold the valves of plastic . the long - standing major problem of plugging or fouling of the constricted outlet for waste water is eliminated at low cost and with very little use of water . what water is used provides not only cleaning of the constriction region but also a partial flushing of the filter membrane . these results are achieved by means of a self - cleaning constrictor valve having a predetermined flow area which may be made as small as the manufacturer desires . in one embodiment , a valve is provided to automatically stop the outflow of squeeze water while the faucet is in open condition . this saves additional water and also makes it possible to build up a relatively high pure - water pressure even when the inlet pressure is less than average .