Flux reduction

The build-up of a fouling layer on the surface of a membrane is one of the most ser ious problems in membrane processes. The term ‘ fouling, ’ rather than the more familiar ‘ filter cake,’ arose from the origins of membrane processes in molecular separations, where macromolecular proteins would separate on to the membrane surface as a slimy, gel layer, which rapidly reduced the flux through the membrane. Fouling layers have to be removed periodically by cleaning, but much ingenuity is employed by membrane system designers to minimize the formation of a fouling layer in the first place.

The extent to which cross-flow successfully prevents the surface of the membrane being fouled by deposited particles is dependent on a variety of factors, especially the cross-flow velocity. Chemical and/or mechanical procedures are usually required to clean (and sterilize) the membrane, which must be able to withstand the associated mechanical, chemical and thermal stresses .

Another operating problem, concentration polarization, affects the membrane processes dealing with suspended or dissolved species. The molecules to be separated (i.e. kept in the retentate) diffuse through the liquid close to the membrane surface and
become much more concentrated at the surface, creating a different kind of barrier to liquid flow, and so reducing flux. In the same way, the particulate matter accumulates in the liquid as it approaches the boundary layer, creating a similar resistance to liquid flow.

As well as local shear, there are two other types of method employed to reduce fouling and/or concentration polarization, and so increase flux rates:

● changes in the surface characteristics of the membrane, and
● conditioning of the feed slurry/solution.

The surface of the membrane needs to be as smooth as possible, and the slurry or solution as free as possible of material that will foul the surface. Operational modifications are generally designed to create some kind of shearing or scouring of the
fouling layer. Some of these are mechanical, while the use of a two-phase (gas/ liquid) flow is growing in importance.

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