Reverse Osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. IF normal osmosis takes place, the fresh water will cross the membrane to dilute the concentration solution. in reverse osmosis, pressure is exerted on the side with the concentrated solution to forece the water molecules across the membrane to the fresh water side. The pore size of the membrane is less than 1/10,000 micrometer and can remove suspended solids, particulates, salts, bacteria, total organic carbons.