In ro, feed water is pumped at high pressure through permeable membranes, separating salts from the water. Feed water is pre-treated to remove particles that would clog the membranes. The quality of the water produced depends on the pressure, the concentration of salts in the feed water and the salt permeation constant of the membranes. Product water quality can be improved by adding a second pass (module) of membranes, whereby product water from the first pass (module) is fed to the second pass (module).