The　kinetic　characteristics　of　aerobic　ammonia　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB)　under　different　influent　NH4+-N　loading　and　free　ammonia　(FA)　concentration　were　studied　by　biofilm-activated　sludge　composite　process　(IFAS).　The　contributions　of　different　microbial　aggregates　(suspended　sludge　and　attached　biofilm)　to　the　removal　of　NH4+-N　were　investigated,　and　the　biological　adsorption　and　biodegradation　were　analyzed　quantitatively.　The　amount　and　spatial　distribution　of　total　bacteria,　AOB,　and　NOB　were　observed　by　fluorescence　in　situ　hybridization　(FISH).　The　experimental　results　showed　that　with　the　increased　of　influent　NH4+-N　concentration,　the　effluent　NO3--N　concentration　decreased　and　the　NO2--N　accumulated　a　lot.　When　the　influent　NH4+-N　concentration　was　480mg/L,　the　NH4+-N　removal　rate　and　the　nitrite　accumulation　rate　(NAR)　were　stable　above　95%　and　80%,　respectively,　while　the　FA　increased　from　(2.77　±0.07)　mg/L　to　(16.35　±0.3)　mg/L,　and　the　NAR　increased　from　9.42%　to　83.31%,　which　achieved　the　inhibition　of　NOB.　In　the　process　of　NH4+-N　removal,　biossorption　and　microbial　degradation　accounted　for　3.4%　and　88.1%　of　NH4+-N　removal,　respectively.　The　ratio　of　AOB　in　suspended　sludge　and　biofilm　increased　from　27.4%　and　10.3%　to　41.3%　and　18.1%,　respectively,　indicating　that　suspended　sludge　was　more　beneficial　to　enrichment　of　AOB　than　biofilm.　©　2020,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.