Range (pm) particle size
Function of membrane
Electrodialysis Electrical poten- <0.1 tial gradient
Selective to certain ions
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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