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treatment is still needed. Systems that use chlorine with thin film composite membranes require dechlorination just ahead of the RO unit. Other methods of disinfection will be discussed later. Here, we will discuss how exposure to halogens, chlorine dioxide, and ozone affect cellulosic membranes and those made with synthetic polymers. 4.4.4.1 Halogens. - Polymer oxidation by halogens depends on pH as much as the presence of oxidants. The halogens chlorine, bromine, and iodine may be used for disinfecting water supplies. When mixed with water, they dissociate by the following reaction (X represents the halogen atom): X,+~H,O4HOX+H++X- (5) HOX%H++OX- 1’3 Table 4.2 lists the proportion of each species, Xs, HOX, and OX-, for chlorine, bromine, and iodine at pH 3.0, 5.6, and 8.6. The HOX species is the most damaging to polyamide and polyether urea membranes. Cellulosic membranes are resistant to halogens within their normal operating pH range of 4 to 6. Polyvinyl alcohol membranes are resistant over a pH range of 2 to 8. Two different manifestations of membrane oxidation occur in the presence of halogens, One is an increase in both salt and water flux. The other is a decrease in water flux in which the salt flux remains constant or even decreases. The first set of symptoms indicates breakdown of the polymer. In scanning electron micrographs, holes are shown scattered over the membrane, allowing feed water to enter the product side of the membrane. The second set suggests that halogen atoms may be forming cross-links in the polymer matrix that prevent both water and salt from passing through. If conditions continue, the membrane will become brittle, resulting in a dramatic increase in water and salt flux. In McCutchan and Glater’s study (1980), the homogenous aromatic polyamide membrane was susceptible to the second form of decay when exposed to chlorine and bromine. Under the same conditions, polyether urea and aromatic polyamide composite membranes demonstrated the fmt symptoms. Both conditions were exacerbated at lower pH levels.PDF Image | DESALTING AND WATER TREATMENT MEMBRANE
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