Automatic control of mixed liquor aeration in activated sludge processes is being applied in increasing numbers of wastewater treatment plants. Potential benefits of such control are minimization of aeration energy costs, improved process performance, and a resulting reduction in effluent variability. The development and gradual acceptance of the membrane-type polarographic dissolved oxygen (DO) probe has provided the means for continuous in situ measurement of mixed liquor DO.1,2 In its simplest form, automatic aeration control is accomplished by automatically adjusting the air flow to the mixed liquor to maintain the measured level of DO equal to a desired set-point value.3 In many plants where such control has been implemented, specific power savings and process performance improvements have been reported.4-6 DO control system designs vary between plants, depending mainly on the type of aeration equipment in use. Reported benefits also vary, depending on equipment, the nature of the wastes being treated, and the process flow scheme.