BACKGROUND: Completely autotrophic nitrogen removal over nitrite (CANON) could decrease energy consumption and CO2 release compared with conventional nitration-denitrification. Trace NO2 addition could enhance the activities of aerobic and anaerobic ammonium oxidation. RESULTS: An aerated upflow sludge bed (AUSB) reactor inoculated simultaneously with aerobic and anaerobic ammonium oxidizing sludge was operated to cultivate granular sludge capable of carrying out CANON. The results showed that the efficiency and rate of total nitrogen (TN) removal reached 61% and 0.114 kgN, respectively (m(-3) day(-1)) for DO = 0.5-0.6 mg L-1. Batch tests indicated that trace NO2 addition could increase the CANON activity of sludge. The TN removal rate and efficiency of the reactor was increased to 0.234 kgN m(-3) day(-1) and 63%, respectively, when the reactor was aerated with air containing 2.7-3.3 mmol m(-3) NO2 and DO was at 0.5-0.8 mg L-1. CONCLUSIONS: Trace NO2 addition provides an alternative to increase the capacity of a CANON system at low DO concentration. (c) 2009 Society of Chemical Industry