| 摘要 |
PROBLEM TO BE SOLVED: To easily apply reduction treating method of hexavalent chromium- containing waste water by the chemicals injection control of ferrous salt by a DO meter to the existing equipment adopting sulfite reduction method and also to reduce an administration work of the chemicals by decreasing the kind of the chemicals to be used. SOLUTION: In the method in which a ferric ion is added to the waste water containing the hexavalent chromium under the condition of pH 4 or above so that dissolved oxygen may be <=2 mg/L to reduce the hexavalent chromium to tervalent chromium, then resulting waste water is subjected to flocculating treatment, the ferous ion soln. prepared by adding the sulfite to a part of the ferric ion soln. is added to the waste water to reduce it, and the other part of the ferric ion soln. is added to the waste water after reduction treatment, and the metal hydroxide formed by adjusting pH with calcium hydroxide is subjected to flocculating and settling treatment.
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