This study examined long-term effect of MnO2 powder (average diameter of 4-7 mu m) on nitrogen removal in anammox process. Two lab-scale up-Flow anammox reactors were operated for 380 days, one with and one without MnO2 powder addition. During the period when only substrate concentrations varied, the maximum nitrogen removal rate in the reactor with MnO2 addition reached 920.9 g-N/m(3)/d. This value was 2-folds higher than that (464.6 g-N/m(3)/d) of the reactor without MnO2 addition. The crude enzyme activities of the anammox biomass from the two reactors was measured as 0.531 +/- 0.019 and 0.298 +/- 0.007 mu mol cytochrome c reduced/mg protein/min, respectively. Transmission electron microscopy observation demonstrated more undefined particles existing inside anammox bacterial cell in the reactor with MnO2 powder addition. Furthermore, filament-like structures inside anammoxosome were observed, which formed a net-like structure with particles as the connecting nodes. The experiment results demonstrated that MnO2 improved nitrogen removal performance of anammox process. (C) 2012 Elsevier Ltd. All rights reserved.