Ultrafiltration rejection mechanisms

Ultrafiltration (UF) refers to the removal of high molecular weight colloids (10,000 MW) up to particles less than 0.05 pm in diameter [10]. Like MF, UF places a mechanical barrier into the flow stream to separate the solid and liquid phases. The most common UF is a cross-flow hollow fiber type whereby a UF module contains hundreds of hollow microfibers. Whether or not the medium to be filtered flows inside or outside, the microfibers depends on the characteristics of the waste stream. UF is different from MF not only because UF can filter very small particles and some colloids, but also because of the cross-flow dynamics inside the UF module that keeps the surface of the hollow libers clean. Because of the cross flow, UF modules require a reject stream as well as a permeate stream. In other words, 100% of the liquid that enters the UF module does not exit as permeate. Figure 19.1 shows the differences between the UF and microlilters with respect to flow path. 

Liquid separation. Separation can take place between solvents and solutes, macromolecules or particles or between species in liquid media by the effect of size exclusion. That is, those molecules or colloids larger than the size of the membrane pores will be retained or rejected while those smaller ones can pass through the membrane. The size exclusion mechanism predominates in pressure driven membrane processes such as microfiltration, ultrafiltration and even nanofiltration which has a molecular selectivity on the order of one nanometer. 

Examination of the results for the conventimial parameters tested in the feed and permeate (Table 3) provides some insight into the separation mechanism. High concentrations of oil and grease found in the feedwater suggests that cmisiderable oil remained in a dispersed or colloidal form. This oil would be removed by a membrane with ultrafiltration or hyperfiltration characteristics. Since the PAHs are more soluble in oil than in water, concurrent removal of the PAHs entrained within the oil may have occurred. The phenols with relatively high solubility in water are, also as expected, removed more poorly. This also is reflected in the poor rejections calculated for TOC and COD. Other contaminants, not quantified by the semivolatile analysis, also may contribute to the high TOC and COD in the permeate. 

Aromatic tri-functional acid and amine monomers are used to obtain reticulated polyamides, which have better mechanical and chemical stability and, for that reason, they are preferred for nanofiltration and reverse osmosis membrane materials. In these membranes, a thin polyamide layer (less than l jm thickness) is fabricated by interfacial polymerization on the top of a porous support (normally an ultrafiltration polysulfone membrane), which usually presents a non-woven reinforcement for mechanical stability as can be seen in Figure 8. Despite its small thickness, the polyamide dense layer is the main regulator of the rejection/transport of water and ions across the membrane. 

Cho, J., Amy, G., and Pallegrino, J. (2000). Membrane filtration of natural organic matter Factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane. J. Membr. Sci. 164, 89-110. 

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