The principle of reverse osmosis


Osmosis in nature

Osmosis is based on a natural physical process by which plants draw moisture out of the ground with their root cells. Our own metabolic processes in cells is based on osmosis as well.

Definition of osmosis

Osmosis is the targeted diffusion of water through a semipermeable membrane or diaphragm.

Semi-permeable membranes have special properties. They are permeable to water molecules, while larger molecules, such as salt particles, cannot pass the membrane.

Example: Salt solution

Osmosis balances the concentration of solutions separated by a semi-permeable wall.


Function of osmosis 

Water tries to reach a balanced solution on both sides of the membrane.
This happens until the concentration on both sides is the same or the water pressure of the salty side equals the osmotic pressure.


Reverse osmosis 

Our salt solution is subjected to pressure and pressed against the semipermeable membrane.


Applied pressure > osmotic pressure!


This makes water molecules move from the higher concentrated to the less concentrated solution* - the reverse to osmosis.


Example: Salt solution

The concentration of the two sides is not balanced by the applied pressure.

* but only water molecules and no other molecules.