Reverse Osmosis is the reverse, of the natural separation process of Osmosis, when the water in a saline solution of a lower concentration passes through a semi-permeable membrane over to a higher concentration saline solution and diluting it. The process occurs because in nature the solution of different saline concentrations, will attempt to become equal. The salt ions in the solution don't pass through the membrane but the water does. The Osmosis process in nature is an important mechanism in the operation of plant and animal cells. That's way the tree always gets fresh water even in saline environment.
RO membranes have been used commercially since the 70's and they have improved to become very efficient. Today there are thousands domestic RO systems, in thousands of home around the world to produce purified drinking water and multimillion dollar desalination plants in many major cities.
