National University of Singapore
Food Science & Technology
Reverse osmosis membrane systems
Osmosis is defined as the movement of water molecules from a region of high concentration to a region of low concentration across a membrane (which only allows water molecules to diffuse through). Most of the time this will mean that there is a chemical potential across the membrane due to a difference in solute concentration across the membrane. This creates a chemical gradient resulting in the movement of water molecules. The below animation will show how osmosis works.
As a result, an external pressure that exceeds the osmotic pressure is needed to reverse the direction of water flow, a process known as reverse osmosis. Reverse osmosis membrane systems are commonly used to remove water from a water-solute mixture such as concentrating juices.
(Left) Animation of the effects of a system with a different solute concentration across the membrane.