The major portion of cyanide-containing wastewater comes from metal finishing and metal plating plants. Photo-processing plants also contribute significantly. Cyanides are extremely poisonous, especially at acidic pH levels, where they are present as hydrocyanic acid, a powerful poison. Cyanide-containing wastewater should be treated prior to discharge into sewer lines, streams, or rivers. Treatment processes usually involve either partial oxidation of the cyanide to the substantially less toxic cyanate or complete oxidation to carbon dioxide and nitrogen. Frequently used oxidizing agents include chlorine, ozone, and electrolytic oxidation. The cyanide concentra-

FIG. 8.3.1 Chromate recovery by ion exchange.

tion in the effluent should be less than 0.2-1.0 ppm when the receiving body is a sewer line, stream, or river.

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