Water treatment system and method for controlling amount of aeration

A water system (1) comprises a series of biological reaction cells (10) for performing water treatment on the basis of the activated sludge method that have an aerobic cell (5) in which an aeration device (9) is disposed; a first ammonia gauge (32) for measuring the ammoniacal nitrogen concentration of the activated sludge of the aeration cell (5); a second ammonia gauge (33) for measuring the ammoniacal nitrogen concentration of the treated water after treatment in the series of biological reaction cells (10); an aeration calculation part for producing the standard operating volume of the aeration device (9); and an aeration control part for controlling the amount of aeration of the aeration device (9) on the basis of the standard operating volume. The aeration calculation part produces standard operating volume signals on the basis of the difference between the ammoniacal nitrogen concentration of the activated sludge mixture of the aerobic cell (5) and the ammoniacal nitrogen concentration setting for the activated sludge mixture of the aerobic cell (5) that has been adjusted in response to the ammoniacal nitrogen concentration of the treated water.