Reverse Osmosis Membrane Reverse osmosis is a process in which water is forced through a semi-permeable membrane by applying very strong pressure to the feed – water side of the RO membrane, enabling the water molecules to pass through the membrane to other side. the diameter of the pores or apertures of the high technology RO membrane is 0.0001 micron. This permits only the passage of the water molecule while micro-organism and other solid or dissolved pollutants in the water are denied entry and hence effectively eliminated. The water produces is thus 99.99% pure.