PURPOSE:To economize the injection of methanol and to stably maintain a denitrification rate by always accurately measuring the concn. of the methanol in a denitrification tank and injecting the methanol into the denitrification tank so that the concn. thereof attains a prescribed value. CONSTITUTION:A means 20 for detecting the concn. of an org. carbon source consisting of a biosensor (e.g., enzyme sensor, microorganism sensor) is provided to the denitrification tank 2 which subjects the nitrified liquid subjected to a nitrification treatment to a denitrification treatment in the presence of the org. carbon source (e.g., methanol) supplied from the outside. The injection of the org. carbon source into the denitrification tank is controlled in a means for controlling the injection of the org. carbon source consisting of a controller 18 and a methanol injector 19, etc., so that the concn. of the org. carbon source detected by the detecting means 20 attains a prescribed value. As a result the excessive or deficient injection of the org. carbon source into the denitrification tank is prevented, by which the economy for the injection of the org. carbon source is improved and the denitrification is stabilized.
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