Self-cleaning membranes for ultrafiltration

• Main Author: Velicangil, O.
• Published: Swansea University 1980
• Subjects: 620.106
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.639307

During separation and concentration of macromolecular solutions flux losses have traditionally been ascribed to the well-known phenomenon of concentration polarization with the formation of a gel layer that is subsequently compacted. In the present investigation ultrafiltration was modelled in three separate stages with distinctive time constants. It was shown that in the first stage listing less than five seconds a quasi-steady state concentration profile is reached. In the second stage of one to ten minutes solute adsorption on the membrane surface including the pores controls the permeation rate. The rate of flux decline is highest at this phase. The third stage is governed by a reaction mechanism which produces a surface gel causing flux decline at a much slower rate than in the previous adsorption step. This polymerization of the protein to a gel on the membrane was shown to be second order in the surface protein concentrations. The differences in the flux pattern between single protein solutions and natural protein mixtures were attributed to a change of order of the polymerization mechanism. Another model was developed to estimate the properties of anisotropic membranes as well as their very high water permeation rates which cannot be predicted with the earlier formulae used for homogeneous membranes. A reproducible and inexpensive method has been developed to attach food-grade proteases onto the UP membranes which then retard the rate of formation of gel on the membrane. This was achieved by ultrafiltering-the enzyme solution for the first ten minutes instead of the protein to be separated and hence producing a primary adsorbed layer of protease molecules. 25% to 78% improvement in cumulative permeate yield was obtained in a standard 22 hr run when processing 0.5% albumin or haemoglobin. It was also demonstrated that the flux enhancements were due to the biochemical action of the adsorbed protease and not to its physical effect as a prefilter coat. Economics of the system were shown to be favourable, improving the rate of return on capital investment up to 50% by reducing the total membrane area of the plant. The enhanced fluxes with self-cleaning membranes were modelled by modification of the original model, i.e. by incorporating an enzyme activity term to counteract the deposition of gel on the membrane surface.