Membrane filtration (RO, NF, UF) - Membrane characterization

Membrane filtration is a physical separation process. This operation, which takes place in the liquid phase, aims to purify, concentrate or fractionate species dissolved or suspended in a solvent by passing them through a membrane. At the end of this operation, we obtain the retentate, also known as the concentrate, which is made up of the molecules and/or particles retained by the membrane, and the permeate.

In the case of reverse osmosis, nanofiltration and ultrafiltration, the driving force is a pressure difference. The membranes used are permselective, meaning that they favor the transfer of certain molecules or particles from the concentrate to the permeate over others. The pore diameters of these membranes decrease progressively as we move from ultrafiltration to nanofiltration, then to reverse osmosis. It should be noted, however, that in the latter case, the membrane used is not a porous membrane, but a dense membrane with no apparent porosity, whose selectivity results from a solubilization-diffusion mechanism.

Determining the characteristics of a membrane is intended not only to help in the choice of membrane for a given application, but also to gain a better understanding of how its performance evolves during use. The methods used enable us to access macroscopic or microscopic quantities, characteristic of the membrane structure and the chemistry of the material. Some of these techniques are specific to membrane or separation processes, while others call on the field of polymers or are much more general.

When choosing a membrane, the structural and transfer characteristics (hydraulic permeability and selectivity curve) are the most important, as they tell us about the membrane's performance for a chosen separation: the permeate flow rate we can expect and the size of the molecules likely to be retained by the membrane.

Physical-chemical and chemical surface properties (charge, hydrophilic-hydrophobic character, chemical composition) also play a role in the choice of membranes. To a certain extent, they help predict clogging phenomena and interactions between the different types of molecules on the membrane surface. They may also play a role in transport mechanisms.

For a description of the membranes, their suppliers and some economic data, please refer to the dossier. and "For further information .