**Classification according to morphology**

According to their structure, the membranes can be classified:


There are two subtypes of asymmetric membranes:


Skin: a layer with a very low thickness, this layer is responsible for the selectivity of the membrane.

Support layer: it is a layer with a much greater permeability than that of the skin layer, which is thicker and which retains the mechanical resistance.

In order to increase the permeability of the membrane, the majority of commercial membranes are manufactured with an asymmetrical structure [39].
