A potentially cheaper technolgy called membrane desalination may expand the role of desalination worldwide, which today accunts for less than 0.2 percent of water withdrawn from natural sources. Membrane desalination relies on reverse osmosis-a process in which a tin, semi permeable membrane is placed between volumes of salt water and a volume of fresh water. The water on the salty side is highly pressurised to drive water molecules, but not salt and other impurities, to the pure side. In essence, this process pushes fresh water out of salt water.