In coal-fired power plants, chemical oxygen demand (COD) components, which originated from the flue gases, are contained in the wastewater from flue-gas desulfurizer. By chemically analyzing these COD components in the wastewater, it was elucidated that low-molecular organic matters such as formaldehyde and inorganic matters such as hydroxylamine mainly acted as COD. Furthermore, basing on the relation between the concentration of hydroxylamine-formed matters and oxidation/reduction potential (ORP) of the desulfurizer absorbent, it was experimentally confirmed in a practical plant that by adjusting air volume for oxidation, the ORP value was .modified so that the COD originated from hydroxylamine matters could be drastically reduced.%石炭火力発電所の脱硫排水には，石炭燃焼後の排ガスに由来する化学的酸素要求量（COD)成 分が含まれている。脱硫排水を分析した結果.ホルムアルデヒドなどの低分子有機化合物とヒドロ キシルァミン類などの無機化合物が主なCOD成分であることを解明した。さらに.排水中のヒド ロキシルァミン態化合物濃度と脱硫装置吸収液の酸化還元電位（ORP)の関係をもとにして.酸 化空気量を調整してORPを変化させることにより.ヒドロキシルァミン態化合物に由来するCOD が顕著に低減できることを実プラントで確かめた。
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