Reverse Osmosis in desalination is a process that forces salt water directly onto membranes at high pressure. These membranes are semi-permeable, biological or synthetic polymeric membranes that allow certain molecules or ions to pass through.
Osmosis is the movement of high-concentration water molecules through a membrane to a solution with a lower concentration of water molecules. In other words, osmosis is the watering down of another solution. In the most simple terms, adding water to a cordial syrup is an example of osmosis.
Reverse Osmosis is the movement of low-concentration water molecules through a membrane to a solution that will have a higher concentration of water molecules.
Reverse Osmosis (RO), therefore, is the water treatment process by which contaminants (or, for example, salt) are removed. The result is a solution that has a higher concentration of water molecules because most other non-water particles have been stopped by the membrane.