The process by which water is forced through a semi-permeable membrane while under pressure, in order to filter out unwanted contaminants.

This process was first demonstrated in 1959 at UCLA. Since then, cities have used reverse osmosis to provide potable water to its inhabitants by converting brackish or saltwater to drinking water. The bi-products of city-wide reverse osmosis are sometimes disposed of through deep-well injection. Aside from city use, reverse osmosis is used in desert regions such as northern Africa and the Middle East where water is scarce. Not to mention, small reverse osmosis systems are now being sold to individual homes in more developed regions.