This study tested the feasibility of plug-flow integrated fixed-film activated sludge (IFAS) reactor in applying sewage partial nitritation/anammox (PN/A) process. The IFAS reactor was fed with real pretreated sewage (C/N ratio = 1.3) and operated for 200 days. High nitrogen removal efficiency of 82% was achieved with nitrogen removal rates of 0.097 +/- 0.019 kg N/(m(3).d). Therefore, plug-flow IFAS reactor could be an alternative to applying sewage PN/A process. Besides, it was found that the stability of sewage PN/A process was significantly affected by residual ammonium. Nitrate accumulated in effluent and PN/A performance deteriorated when residual ammonium was below 1 mg/L. On the contrary, long-term stable PN/A operation was achieved when residual ammonium was over 3 mg/L. (C) 2017 Elsevier Ltd. All rights reserved.