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3. ULTRAFILTRATION Ultrafiltration is a step smaller than microfiltration, for particles larger than 0.04 urn or with a molecular weight larger than 10,000 Daltons. The membranes used for ultrafiltration are anisotropic, or asymmetric. They are made by either a thermal inversion process, or by laminating a thin layer of polymeric membrane material onto a supporting porous substructure of polysulfone. Zirconium and aluminum or titanium oxide are used for ceramic ultrafilters. Because they are primarily used in cross-flow filtration mode, much of what applies to cross-flow microfilters also applies to ultrafrlters, with some exceptions. 3.1 Characterization of Ultrafilters Unlike microfilters, ultrafiltration membranes do not have pores made by mechanical processes and consequently, do not have a sharp cutoff in pore size. Ultrafilters are larger in area and are thinner than microfilters, which may result in a higher defect rate. For these reasons, the pore size of ultrafilters is difficult to pinpoint accurately. By definition, the pore size of an ultrafilter ranges from 10 to 1000 A (0.001 to 0.1 pm). Another way of indicating pore size is with the molecular weight cutoff, usually given in Daltons or gram-molecular weight. Instead of using micro-organisms, as with microfilters, globular proteins of various molecular weights are used to estimate the size of molecules retained by the membrane. The definition of molecular weight cutoff (MWCO or NMWCO, nominal MWCO) generally used is the molecular weight of globular proteins that are 90 percent retained by the membrane (Porter, 1990). Manufacturers produce ultrafilters with a range of “nominal” MWCOs and then specify the percent rejection of proteins with a similar range of molecular weights for each membrane. Table 3.1 is a typical pore size specification list for a range of ultratilters. Rejection percentages decrease with increasing NMWCO. Because the specified MWCO is only an estimate of the effective pore size, the bubble point method described for microfilters is used to determine the maximum pore size and give an indication of pore size variation. Broken fibers or bubbles in the seals will allow particles to get through the membrane. The shape of the particles being separated also affects the effective pore size. Figure 3.1 illustrates how straight chain or branched molecules with molecular weights substantially over the NMWCO can slip through where a much smaller globular shaped molecule cannot. This process depends on the alignment of molecules as they contact the membrane, and the flow of water through the pores.PDF Image | DESALTING AND WATER TREATMENT MEMBRANE
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