Membrane filtration (RO, NF and UF)

Filtration membranes (reverse osmosis RO, nanofiltration NF, ultrafiltration UF, microfiltration MF) are semi-permeable (permselective) membranes that retain solutes or particles in a solvent. Membranes are usually composed of a selective layer ensuring separation, combined with a support reinforcing mechanical resistance. Their performance, which corresponds to that of the selective layer, is characterized by their solvent permeability (see [J 2 790] ) and selectivity.

The latter characteristic is linked to membrane pore size, expressed in terms of cut-off (molar mass of the compound retained at 90% (cf. ) (UF, NF), salt retention (NF, OI) or micrometer size (MF). Figure 1 shows schematically the correspondence between membrane filtration operations, pore size and retained compounds.

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In practical terms, a membrane must have high permeability combined with high selectivity, and thermal, chemical and mechanical resistance, at a controlled installation and renewal cost in line with the product application in question.

Permeability is related not only to pore size and density, but also to the thickness of the selective layer, as shown by Poiseuille's law (see