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 contradiction between substrate competition and SRT makes the high nitrogen and phosphorus removal unable to be achieved simultaneously. As a result, the removal efficiency of the system cannot be further improved. In the past 10 years in China, the wastewater quality very obviously changed with the improvement of living conditions. At present, municipal wastewater with a low C/N ratio is rather common in most countries in the world. The lack of a carbon source will make the inherent contradictions of the A2/O process becomes serious. Therefore, the conventional design parameters of the A2/O process are needed to adjust or the configuration of the A2/O process should change to fit the change of the wastewater quality. According to this, this book systematically describes how to improve the nitrogen and phosphorus removal efficiency of municipal wastewater with low C/N ratio, and effectively utilize the carbon resource in the influent of wastewater. This publication is useful for students, researchers and engineers whose major focus is municipal and the environment.