PURPOSE:To make it possible to obtain good treated water, by providing a means for controlling air-exposure time which controls the air exposure period such that the period is within the range of air exposure-adjusting periods which have been set before and after a standard air exposure period, thereby ensuring a minimum air-exposure and anaerobic steps. CONSTITUTION:The concentration of oxygen dissolved in the original water in a treating tank 1 is determined by means of a concentration meter 4. Signals for this determined value are inputted to a means 5 for controlling air-exposure period. This means 5 controls an air-supplying means 3 which supplies air to be used for air exposure to a means for exposure to air. The control of the means 3 by the means 5 is done such that when the determined dissolved oxygen concentration value reaches to a predetermined value before the beginning of the air exposure-adjusting periods which have been set before and after a standard air exposure period, the air exposure period is prolonged to the beginning of this air exposure- adjusting period, and that when the concentration does not reach to the predetermined value during the air exposure-adjusting period, the exposure to air is stopped at the time when the air exposure-adjusting period is terminated. As a result, even if the concentration meter 4 produces an error, nitration and denitrification can be allowed to proceed simultaneously in a step of exposing to air and denitrification in the anaerobic step can be successfully performed as in the beginning, thereby obtaining treated water having quality equal to that of the treated water obtained in the beginning.
展开▼
