PURPOSE:To make it possible to obtain good treated water, by providing a means which calculates the amount of air to be fed, from the signals for values measured by a means for measuring the amount of air fed and for values measured by a dissolved oxygen- concentration meter, such that the air exposure time in the subsequent treating step is approximately the same as a standard air-exposure time, and which controls the air-feeding means. CONSTITUTION:The amount of the air which is being fed from an air-feeding means to a means for exposure to air is measured by a means 3 for measuring the amount of the air fed. At the same time, the concentration of the oxygen dissolved in the original water in a treating tank 1 is determined by means of a dissolved oxygen-concentration meter 6. Signals for these values are inputted to a calculating and controlling means 7, which calculates the amount of air to be fed such that the air exposure time in the subsequent treating step is approximately the same as a predetermined standard air-exposure time, and which controls the air-feeding means 3 so that air is fed in an amount equal to the amount calculated. As a result, even if the quality of original water changes, properly treated water can always be obtained. This is so because one treating process is divided into a standard step of exposing to air and a step of anaerobic treatment; nitration and denitrification are allowed proceed simultaneously in the former step as in the beginning, and denitrification is allowed to efficiently proceed in the latter step.
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