Modes of Membrane Fouling

There are three modes by which membranes foul:


Case A

Case A fouling consists of pore narrowing or constriction involving the adsorption of particles onto the surface of the membrane. The extent of this mode of membrane fouling is highly dependent upon the morphology of the membrane. As previously mentioned, the material of construction of the membrane is what dictates the surface characteristics of the membrane. In general, the more porous a membrane, the more likely particles are to adsorb to the surface of the membrane or to become entrained in the membrane. Thus more porous membranes have an increased tendency to foul. Pore narrowing/constriction contributes significantly to membrane fouling during protein ultrafiltration, as the protein molecules are approximately the same size as the ultrafiltration membrane pores. However, since the average membrane pore size of a microfiltration membrane is much larger than an average protein particle size, the flux across the membrane is only slightly reduced due to this mode of fouling during microfiltration.

Case B

Case B, pore plugging, also contributes significantly to membrane fouling during protein ultrafiltration, again as the protein molecules are approximately the same size as the ultrafiltration membrane pores. Pore plugging, however, does not appear to contribute significantly to flux decline during microfiltration of protein solutions, as the average protein particle size is much smaller than the average membrane pore size.

Case C

Case C, gel/cake layer formation, appears to have a significant effect upon the decline in filtrate flux during both ultrafiltration and microfiltration of protein solutions. This effect tends to occur when the proteins form aggregates, which is discussed in detail later in this paper. The aggregates are essentially protein molecules which have "flocculated" and deposit at the surface of the membrane, forming a cake layer. When the cake layer is present, filtrate flux declines rapidly and significantly. During microfiltration, the proteins in solution are inhibited from passing through the cake layer. This occurrence is known as "sieving" of the proteins. If the protein is the desired product, this action is unwanted, as it decreases the yield of the protein.