Reverse osmosis has been used for the production of demineralised water for over 30 years.[1] In reverse osmosis, water is transported across a membrane under high pressure, leaving a product that consists mainly of water and a concentrate containing most other components such as minerals and unwanted residues.[12] Many contaminants, including iron, manganese, ammonium, traces of pesticides and medicines, organic micro pollutants, and radioactive particles can be removed with reverse osmosis alone.[3][12] This efficiency has made reverse osmosis the primary method of water purification, often being used in combination with other methods, as a final treatment. A downside to reverse osmosis is that the removing of most minerals from water can have negative effects on its taste.[12] Minerals are sometimes added to drinking water that was already filtered to improve its taste.