NOVELTY - Aeration of activated-sludge liquid mixture in a state of dissolved oxygen concentration of 0.5 mg/l or less is performed in an aerator, and biological oxygen demand and nitrogen content in wastewater are simultaneously removed in the activated sludge process equipment. After aerating strongly and temporarily and raising the dissolved oxygen demand in the aerator by an ultra-low dissolved oxygen demand process, aeration is stopped at preset conditions, in the control of aeration amount in the activated sludge.    USE - Control of aeration amount in activated sludge used for controlling reducing agent addition amount to denitrification process of biotic denitrifying method of single tank sludge system, digestive-fluid circulation system and batch type denitrifying system.    ADVANTAGE - The method enables effective control of aeration amount in activated sludge even at narrow aeration amount region conditions, and control of biological oxygen demand and denitrification, without modifying activated sludge process equipment.    DETAILED DESCRIPTION - Aeration of activated-sludge liquid mixture in a state of dissolved oxygen concentration of 0.5 mg/l or less is performed in an aerator, and biological oxygen demand and nitrogen content in wastewater are simultaneously removed in the activated sludge process equipment. After aerating strongly and temporarily raising the dissolved oxygen demand in the aerator by an ultra-low dissolved oxygen demand process, aeration is stopped. The time change dC(t)/dt of dissolved oxygen demand vale is given by the formula: dC(t)/dt is KLa(Cs-C(t))-Rr, where KLa is overall mass transfer coefficient, Cs is saturated dissolved oxygen concentration and Rr is oxygen consumption coefficient of the aerobic conditions of an activated-sludge liquid mixture, and Rr is calculated based on C(t) change in dissolved oxygen demand reduction process when aeration is stopped. C(t) change in dissolved oxygen demand raise process when balanced dissolved oxygen demand value is given by the formula: C1(dC(t)/dt is 0, C(t) is C1) when strongly aerated. The iterative operation in which KLa or C1 is changed based on the relation: KLa(Cs-C1) is Rr. The amount of aeration during strong aeration (G1), oxygen dissolution efficiency (Ea1) at that time, the amount (G0) of aeration when the oxygen consumption coefficient of oxygen supply capability by aeration and liquid mixture are equal satisfy the relation: G0 is ((Cs-C1)/Cs)x (Ea1/Ea0)x G1. The amount (G2) of appropriate aeration during ultra-low dissolved oxygen demand process is calculated by an iterative operation using the relation: G2 is kx ((Cs-C1)/Cs)x (Ea1/Ea2)x G1, where Ea0 and Ea2 are oxygen dissolution efficiency at the time of amount (G0 and G2) of aeration, respectively, and k is a proportionality coefficient which induces the amount (G2) of appropriate aeration in the ultralow dissolved oxygen process conditions from the calculative amount of appropriate aeration and is obtained using the relational expression of Ea and G. The amount of appropriate aeration is previously measured in various process conditions, and optimum value of k with respect to each process condition is obtained and applied corresponding to the ultra-low dissolved oxygen demand process conditions. The operation which sets the amount of aeration in ultra-low dissolved oxygen demand process to G2 is suitably performed in an ultra-low dissolved oxygen demand process, and the amount of aeration in ultra-low dissolved oxygen demand process is appropriately maintained. Thus, the control of aeration amount in the activated sludge is enabled.