An anaerobic rotating biological contactor was fed with inorganic synthetic wastewater for anammox. Besides biofilm, granular biomass with average diameter of approximately 5 mm formed. NH4+, NO2-, NO3- and pH microelectrodes were used to probe microbial activity in situ within the granules. At a sufficient substrate concentration, the anammox reaction was observed in the upper layer of granules, and the most active zone was found to be in the surface of 200-400 mu m. The in situ anammox activity increased with increasing substrate concentration, and a maximum ammonium consumption rate of 83.3 mu mol cm(-3) h(-1) was obtained at an ammonium concentration of 1000 mu mol L-1. Under an ammonium-limited condition, denitrification activity was observed in the inner layer, and the most active zone was limited to 700-1000 mu m. This study revealed that denitrification bacteria coexisted with anammox bacteria within inorganic anammox granules. (C) 2015, The Society for Biotechnology, Japan. All rights reserved.