Membrane filtration (RO, NF, UF) - Installation and performance

Membrane filtration operations (nanofiltration NF, ultrafiltration UF, microfiltration NF), which include reverse osmosis (RO), although the membranes used there are not porous, can be presented as three-pole separators which divide a feed stream into two: the retentate and the permeate (which is made up of the liquid, solvent and part of the solutes, which has passed through the membranes). The special feature of membrane filtration, compared with conventional filtration, is that the liquid to be filtered is circulated tangentially to the membrane, in such a way as to limit the accumulation of retained matter by hydrodynamic shear in the wall: this latter mechanism, when it develops in solution, is called concentration polarization and, when it results in a deposit of matter on the membrane, becomes a clogging of the latter. For this reason too, filtration membranes can handle suspensions whose components are much smaller in size than those involved in conventional filtration. This translates into much smaller pore diameters in the membranes than in the filter media.

Table 1 summarizes the main features of the separation operations concerned by this project.