Membrane filtration (RO, NF, UF) - Presentation of membranes and modules

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 (cf. ) and their 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 micrometric size (MF). Figure 1 shows schematically the correspondence between membrane filtration operations, pore size and retained compounds.

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 to pore size and density, but also to the thickness of the selective layer, as shown by Poiseuille's law (cf. ). High permeabilities are therefore achieved, for a given pore size or selectivity, by producing an extremely thin selective layer. Figure 1 gives an indication of the thicknesses of selective membrane layers as a function of process.

Thermal and chemical resistances are associated with the nature of the material(s). They play a part in ageing mechanisms and therefore condition the frequency with which membranes need to be renewed. Mechanical...