Aromatic polyamide membranes were first developed by DuPont in a hollow fiber configuration. Like the cellulosic membranes, these membranes also have an asymmetric structure with a thin (0.1-1.0 mm) dense skin and a porous substructure. Polyamide membranes have better resistance to hydrolysis and biological attack than do cellulosic membranes. They can be operated over a pH range of 4-11, but extended use at the extremes of this range can cause irreversible membrane degradation. They can withstand higher temperatures than cellulosic membranes. However, like cellulosic membranes, they are subject to compaction at high pressures and temperatures. They have better salt rejection characteristics than cellulosic membranes as well as better rejection of water-soluble organics. A major drawback of polyamide membranes is that they are subject to degradation by oxidants, such as free chlorine.
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