A reverse osmosis apparatus compris

A reverse osmosis apparatus comprises an elongated pressure container housing a plurality of semipermeable membrane cartridges in end-to-end relationship. A separate tap from the product water collector of the semipermeable membrane cartridge or cartridges nearest the inlet of the pressure container for introduction of pressurized feed liquid, produces a high quality water product. A tap from the product water collector of the remainder of the semipermeable cartridges in the pressure container produces a lower quality water product. A minor amount of high quality product water and a major amount of lower quality product water may readily be produced. The process and apparatus may advantageously be employed in the second or subsequent or final step of a multistage process for converting sea water to potable water.