2. Basic concepts and mechanisms

2.1 Definitions

Permeate: fluid flowing downstream of the membrane.

Retentate: fluid retained by the membrane.

Asymmetrical membranes: membranes composed of a thin layer (skin) for selectivity and a thicker support for mechanical strength (composite membranes, heterogeneous membranes). Symmetrical membranes do not have this characteristic.

Composite membrane: membrane composed of several layers of different materials.

Dense membrane: non-porous membrane (reverse osmosis, electrodialysis,...