In wastewater treatment plants, the amount of ammonium is one example of a measure to determine the quality of the effluent wastewater. Ammonium is regarded as a hazardous chemical for aqueous ecosystems and can cause eutrophication due to its high nitrogen content. The ammonium content in the treated wastewater is controlled by aeration of the biological treatment stage, in which ammonium is converted to nitrate. The aeration process often accounts for the largest energy consumption of the wastewater treatment plant, which motivates automatic control solutions that can both aid in reducing the discharge of ammonium in the effluent and improve the energy efficiency of the aeration process. One such control technique currently used by several large municipal wastewater treatment plants in Sweden is ammonium-based aeration control. In this technique, the aeration process is controlled based on measurements of the effluent ammonium concentration. The purpose of the thesis was to study an extension of ammonium-based aeration control that could better adapt to daily, and often large, fluctuations in the influent load. The proposed method is to use an iterative algorithm to tune the set-point of the ammonium feedback controller. The objective is to, over a given time interval, achieve a flow-proportional mean of the effluent ammonium concentration close to a desired value for a wide range of influent loads. The method was tested by extensive simulations, and the results indicate that the iterative set-point tuning algorithm has the potential to offer a superior ability to achieve a desired flow-proportional mean at the end of a given evaluation period and, in some instances, energy savings compared to standard ammonium feedback control.