Membrane separation processes


Driving force

Range (pm) particle size

Function of membrane

Electrodialysis Electrical poten- <0.1 tial gradient

Selective to certain ions


Concentration <0.1

Selective to solute

Reverse osmosis Pressure

Selective transport of water and small ions



Selective to molecular size and shape

However, one important difference is that the osmotic pressure, which is very small in ordinary filtration, plays an important role in reverse osmosis. Second, a filter cake with low moisture content cannot be obtained in reverse osmosis, because the osmotic pressure of the solution increases with the removal of solvents. Third, the filter separates a mixture on the basis of size, whereas reverse osmosis membranes work on the basis of other factors. Reverse osmosis has sometimes also been termed hyper-filtration to be distinguished from ultrafiltration, where dissolved ions and other inorganic molecules are not separated.

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