Completely　autotrophic　nitrogen　removal　over　nitrite　(CANON)　process　was　considered　as　one　of　the　most　efficient　and　economical　nitrogen　removal　processes,　which　was　suitable　for　treating　wastewater　with　low　ratio　of　carbon　to　nitrogen.　In　this　study,　an　enlarging　start-up　strategy　for　CANON　process　was　proposed,　and　a　40-L　CANON　reactor　was　successfully　started　by　seeding　2-L　mature　biofilm　containing　both　aerobic　ammonia-oxidizing　bacteria　(AerAOB)　and　anaerobic　ammonia-oxidizing　bacteria　(AnAOB).　The　effects　of　dissolved　oxygen　(DO),　ammonia　loading　rate　and　the　ratio　of　air　inflow　to　water　inflow　(Q(air)/Q(water))　on　nitrogen　removal　performance　were　investigated.　The　distribution　of　AerAOB　and　AnAOB　was　analysed　using　fluorescence　in　situ　hybridization　(FISH)　technique.　The　system　reached　a　maximum　NRR　of　3.11　kg　N　m(-3)　d(-1)　with　a　removal　efficiency　of　89.5%,　and　the　average　value　in　steady　state　was　2.42　+/-　0.26　and　(83.07　+/-　6.89)%,　respectively.　Analysis　of　influence　factors　showed　the　important　role　of　high　DO　(around　5　mg　L-1),　for　the　high-rate　nitrogen　removal,　and　the　Q(air)/Q(water)　should　be　controlled　at　28-40　for　stable　operation.　FISH　results　suggested　that　AerAOB　and　AnAOB　predominated　in　the　reactor,　with　proportions　of　46.8%　and　39.3%,　respectively.　This　study　demonstrated　that　the　biofilter　operated　with　high　effluent　DO　was　a　feasible　setup　for　CANON　process.