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.