METHOD AND APPARATUS FOR MAINTAINING CONCENTRATION OF REDUCING AGENT CONTAINED IN WASTE WATER AFTER REDUCTION TREATMENT OF HEXAVALENT CHROMIUM-CONTAINING WASTE WATER AT PROPER VALUE

PROBLEM TO BE SOLVED: To provide a method and apparatus for maintaining the concentration of a reducing agent contained in waste water after reduction treatment of a hexavalent chromium-containing waste water, which can prevent reoxidation of the resultant trivalent chromium to hexavalent chromium at a proper value by maintaining the concentration of a reducing agent contained in a waste water after hexavalent chromium reduction treatment at a proper value. SOLUTION: While maintaining a hexavalent chromium-containing waste water at a low and given pH value, the addition amount of an acid or a reducing agent is regulated so that the redox potential of the hexavalent chromium-containing waste water in a reduction tank is brought to a value lower than a value at which the redox potential is rapidly lowered around the end point of a redox reaction. The concentration of the reducing agent in the waste water in a residence tank after hexavalent chromium reduction treatment is quantitatively determined to determine whether or not the reducing agent concentration is a proper value. When the reducing agent concentration is not a proper value, the correction amount of the reducing agent added to the reduction tank is determined and a correction signal is output. When the correction signal is output, the reducing agent in an amount corresponding to the correction amount determined based on the correction signal is added to the reduction tank. COPYRIGHT: (C)2008,JPO&INPIT