Objectives:In order to improve the stability of the ANAMMOX process, a three-stage ANAMMOX process combined with denitritation/nitritation process was operated, and effect of internal recycle ratio on the nitrogen removal efficiency was investigated.Methods:For the treatment of supernatant from anaerobic digester, a three-stage ANAMMOX process which combined as anoxic/oxic reactor and subsequent ANAMMOX reactor was installed. The reactor was designed with a working volume of 2.5 m³. The internal recycle ratio was considered as the main operating parameter, and the performance of nitrogen removal in the ANAMMOX process was evaluated.Results and Discussion:As the internal recycle ratio was increased from 0 to 1.5Q, reduction of the nitrogen load increased to 23.5 ± 3.8%, and the nitrogen load reduced by internal recycle was 0.22 ± 0.04 kgN/m³/d. Since the ratio of NO₂-N/NH₃-N introduced in the ANAMMOX process is very important, considering this together, the most suitable internal recycle ratio was determined to be 1.5Q. As a result of introducing the effluent from the A/O process into the ANAMMOX process, the nitrogen removal rate could be stably maintained at 1.27 kg/m³/d.Conclusions:In the ANAMMOX process in conjunction with the A/O process including internal recycle, the A/O process represented an effect of reducing the influent nitrogen load and nitrite concentration, and the ANAMMOX process was able to effectively remove residual nitrogen.