Abstract: A pump assembly to move water past a reverse osmosis membrane, said pump assembly having a first pump (10A) and a second pump (10B) each pump including: a bore having a longitudinal axis and surrounding a chamber; a first partition member extending longitudinally of the chamber; a second partition member (17) also extending longitudinally of the chamber and moveable relative to the first partition member, and dividing said chamber into a first sub chamber (18) and a second sub chamber (19); a shaft (15) attached to said second member to cause angular movement thereof about said axis to change the volumes of said first and second sub chambers; and ducting communicating with said first and second sub chambers to provide for the flow of water with respect thereto as the volumes are varied; and wherein the shaft of said first pump is coupled to the shaft of said second pump so that the first pump second partition (17A) angularly oscillates in phase with the second pump second partition (17B).