Reverse osmosis is a process which uses a semi-permeable membrane under pressure to separate relatively pure water from a less pure water solution. It also applies to other solvent. Refer to diagram below, when tap water and pure water are separated by a semi-permeable membrane, water passes through the membrane in the direction towards the tap water which is more concentrated as a result of osmotic pressure. This process is referred to as Osmosis process. By applying sufficient pressure ( greater then the osmotic pressure ) on the tap water, the flow of water will be reversed in which pure water will flow into the pure water chamber. This processor is known as Reverse Osmosis.

In the Reverse Osmosis process, water molecules can form hydrogen bonds in the RO membrane and fit into the membrane matrix. The water molecules that enter the membrane by hydrogen bonding can be pushed through under pressure. Most organic substances with a molecular weight over 100 are sieved out, i.e., oils, pyrogens and particulates including bacteria and viruses (13).  

Salt ions, on the other hand, are rejected by a mechanism related to the valence of the ion. Ions are repelled by dielectric interactions; ions with higher charges are repelled to a greater distance from the membrane surface. Monovalent ions such as chloride ions will not be rejected as efficiently as, for example, divalent sulfate ions. The nominal rejection ratio of common ionic salts is 85 - 98%. Reverse Osmosis can produce up to 99% pure drinking water.