on the outside of the bladder in each work exchange vessel sea water can be introduced into one side of each of the lower work exchange vessels through respective valves 21 and 23 in the drawing these valves are shown as check valves though in certain applications it may be advantageous to utilize controlled valves e g solenoid or hydraulically operated valves it is an advantage of the overall system described herein that the valves which control the flow of sea water and brine in the present system do not need to open or close in the presence of a significant pressure differential thus certain design requirements for the valves are very relaxed the same side of each of the lower work exchange vessels can also provide sea water to the inlet of the reverse osmosis unit 11 through respective valves designated by reference characters 25 and 27 brine leaving the reverse osmosis unit 11 can be selectively admitted to one side of either of the upper work exchange vessels 15 and 19 through respective controlled valves 31 and 33 brine ejected from these work exchange vessels can be discharged out through a common outlet through respective controlled valves designated by reference characters 35 and 37 as illustrated.