SEWAGE TREATMENT SYSTEM

The objective of the present invention is to perform a proper control of water quality corresponding to a situation even if the situation incapable of allowing water quality in a biological reaction tank to arrive at a target value level is brought about. The objective can be realized by the following. A water-quality control target value judging device 24 judges whether the water-quality control target value, inputted from a water-quality control target value setting device 22 is attainable, on the basis of the measured data from an inflow flowmeter 27 and a total nitrogen concentration meter 28 and a nitrifying bacteria concentration estimate value. A judge result practice device 25 alters the target value set to the water-quality control target value setting device 22 to an attainable level when a judge result is unattainable or holds the operation quantity of a controller 23 over a blower 13 to a predetermined level or below in the case of unalterability.