Sodium Metabisulfite for Dechlorination

Dechlorination of feed water to polyamide composite membranes is necessary as a polyamide membrane polymer cannot tolerate oxidizers of any kind. The options for dechlorination include activated carbon, sodium metabisulfite chemical feed, and UV radiation. Carbon has its own set of difficulties, as described previously, and UV radiation can be capital intensive. 

Sodium metabisulfite is the most commonly used technique to dechlori-nate RO influent. In water, the sodium metabisulfite forms sodium bisulfite  

The sodium bisulfite then reduces hypochlorous acid as follows  

Theoretically, 1.34 mg of sodium metabisulfite is required for every 1 mg of free chlorine. In practice, however, it is recommended that 2 mg 

Food-grade sodium metabisulfite that is free of impurities should be used in RO systems. The compound must not be cobalt-activated, as cobalt can catalyze the oxidation of the polyamide composite membrane in a manner similar to iron and manganese (see Chapter 7.6). Further, while the shelf life of solid sodium metabisulfite is 4-6 months, in solution, the shelf life depends on the concentration, as shown in Table 8.9.  

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