The nitrogen removal performance and microbial characteristics of four completely autotrophic nitrogen removal over nitrite (CANON) biofilters were investigated. These four reactors were simultaneously seeded from a stable CANON biofilter with a seeding ratio of 1:1, which were fed with different ammonia levels. Results suggested that with the ammonia of 200-400 mg L-1, aerobic ammonia-oxidizing bacteria (AerAOB) and anaerobic ammonia-oxidizing bacteria (AnAOB) could perform harmonious work. The bioactivity and population of the two groups of bacteria were both high, which then resulted in excellent nitrogen removal, while too low or too high ammonia would both lead to worse performance. When ammonia was too high, the bioactivity, biodiversity and population of AerAOB all decreased and then resulted in the lowest nitrogen removal. Nitrosomonas and Candidatus Brocadia were detected as predominant functional microbes in all the four reactors. Finally, strategies for treating sewage with different ammonia levels were proposed. (C) 2014 Elsevier Ltd. All rights reserved.