A system for controlling the aerobic phase of a wastewater treatment cycle in a sequencing batch reactor, comprising a tank, an air blower, at least one sensor for measuring the concentration of dissolved oxygen in the tank, and means to control the length of time that the air blower introduces air into the wastewater and to maintain at a constant level the volume of air introduced per time by the air blower. The length of time that the air blower introduces air is determined by the change in the slope of the curve representing the concentration of dissolved oxygen in the wastewater. The volume of air to be introduced per time by the air blower is determined based on the average concentration of dissolved oxygen from the previous wastewater treatment cycle.