In　order　to　assess　the　feasibility　of　achieving　nitritation　in　the　anaerobic/anoxic/oxic　(A2O)　system　through　the　strategy　of　free　nitrous　acid　(FNA)　sludge　treament,　the　activities　of　ammonium　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB)　activities　was　studied.　Besides,　the　variations　of　nutrient　removal　performance　and　microorganism　community　structures　in　the　A2O　reactor　after　long-term　FNA　sludge　treatment　were　investigated.　Results　showed　that　the　activities　of　AOB　and　NOB　decreased　by　60%　and　86%　respectively　after　the　sludge　subjectd　to　FNA　treatment　at　1.12mg　HNO2-N/L　for　3h.　The　average　effluent　concentrations　of　NH4+-N,　NO2--N　and　NO3--N　in　the　A2O　system　were　12.76,　1.56　and　7.82mg/L　respectively.　The　nitrite　accumulation　rate　(NAR)　was　around　20%,　indicating　that　nitritation　in　the　A2O　system　was　not　achieved　by　the　FNA　sludge　treament.　Furthermore,　High-throughput　sequencing　and　qPCR　results　showed　that　AOB　abundance　was　greatly　decreased　from　2.05×108copies/gVSS　to　3.96×105copies/gVSS　and　the　ammonia　oxidation　process　was　negatively　affected.　In　addition,　Nitrobacter　and　Nitrospira　were　also　inhibited,　while　Nitrotoga　accounted　for　1.66%　of　total　sequences,　for　which　the　nitritation　in　the　A2O　system　was　not　achieved　by　FNA　sludge　treatment　strategy.　©　2017,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.