In　order　to　assess　the　feasibility　of　restoring　sewage　partial　nitritation/anammox　(PN/A)　process　using　free　nitrous　acid　(FNA)　to　treat　flocculent　sludge,　the　effect　of　different　concentrations　of　FNA　on　the　activities　of　ammonium　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB)　was　studied.　Besides,　the　variations　of　nutrient　removal　performance　in　the　SBR　reactor　by　twice　FNA　treatment　were　investigated.　The　results　showed　that　NOB　activity　could　be　inhibited　when　the　flocculent　sludge　treated　by　FNA　of　0.45mgHNO2-N/L　and　the　nitrite　accumulation　rate　(NAR)　reached　88.8%,　but　NOB　activity　recovered　from　the　8th　day.　While　NOB　activity　could　be　significantly　inhibited　when　the　flocculent　sludge　treated　by　FNA　of　1.35mgHNO2-N/L,　and　the　NAR　reached　89.1%.　Meanwhile　the　activities　of　AOB　was　also　inhibited,　and　NH4+-N　conversion　rate　decreased　to　6.8%.　By　enhancing　the　ratio　of　aerobic　time　to　anoxic　time　(t/t:　from　0.4~2.7)　and　increasing　dissolved　oxygen　(DO:　from　0.2~1.5mg/L),　the　AOB　activity　recovered,　and　NH4+-N　conversion　rate　reached　77.8%.　But　the　activities　of　NOB　was　not　recovered　within　150days,　and　the　NAR　achieved　98.1%.　With　the　stable　achievement　of　nitritation,　the　nitrogen　removal　performance　of　the　system　gradually　restored,　the　average　effluent　total　inorganic　nitrogen　(TIN)　was　8.2mg/L,　and　the　average　TIN　efficiency　was　84.1%.　Therefore,　the　strategy　by　first　treating　flocculent　sludge　with　higher　FNA　to　inhibite　both　AOB　and　NOB,　then　increasing　t/t　and　DO　to　restore　AOB　activity,　could　achieve　the　restoration　of　PN/A　process.　©　2019,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.