**3. Classification of membranes**

The comparison between membranes of the same type requires the evaluation of the performance of these membranes under real conditions.

Using a few criteria such as: selectivity, permeability and lifetime, one can consider that one membrane is more adequate than another to be separated [27]. The information provided by the membrane manufacturers is insufficient to compare the membranes. To understand these membranes, the acquisition of this information is essential in order to master the transfer mechanisms and improve their performance.

Two categories of parameters are often sought:

	- Permeability.
	- Retention.

The concentration factor.

	- The membrane thickness.
