Anaerobic-anoxic-oxic (A2/O) is one of the most widely used processes in municipal wastewater treatment plants for simultaneous biological nitrogen and phosphorus removal. The A2/O process has many advantages, such as simple configuration and short hydraulic retention time (HRT), etc. In addition, it is easy to operate. Therefore, A2/O will be continuously chosen as the main option in all kinds of newly designed and built wastewater treatment plants. Though the A2/O process has been used widely, it has some inherent contradictions which are difficult to overcome. For example, the...