Reverse Osmosis :   A water purification device that forces feed water under pressure through a semi permeable membrane, allowing highly purified water with reduced solute content to  pass out as the product - filtering out salts or total dissolved solids (TDS). This when two channels of water are separated by a semi permeable membrane. Water will flow from the side of Low solute concentration to the side of higher solute concentration. If there are solute molecules only on one side of the system the pressure that stops the flow is called osmotic pressure. The solute molecules generate osmotic pressure on all the solution boundaries including the membrane, but when water flows between the two sides, it is the pressure on the moving free surface of the solution which is pushing it upward and is responsible for water pumping from the other side.