This　study　presented　a　strategy　for　recovering　partial　nitritation-anammox　(PN/A)　of　real　sewage　from　nitrate　accumulation　using　free　nitrous　acid　(FNA)　treatment.　Sewage　PN/A　was　successfully　achieved　in　an　integrated　fixed-film　activated　sludge　(IFAS)　reactor　but　effluent　nitrate　gradually　increased.　For　recovering　the　system　performance,　flocculent　sludge　of　the　reactor　was　collected　and　treated　with　FNA　of　1.35　mg/L　for　24　h.　After　FNA　treatment,　effluent　nitrate　decreased　from　17.6　to　6.1　mg/L　with　an　increase　of　total　nitrogen　removal　efficiency　from　29.1%　to　63.1%　within　32　days.　The　improvement　of　nitrogen　removal　was　mainly　due　to　the　selective　suppression　of　FNA　on　nitrite-oxidizing　bacteria.　Its　relative　abundance　decreased　from　0.32%　to　0.08%　and　the　activity　declined　from　9.05　to　2.42　mg　N/(g　MLSS.h).　Meanwhile,　ammonium-oxidizing　bacteria　and　anammox　bacteria　were　barely　affected.　Overall,　IFAS　reactor　combined　with　FNA　treatment　potentially　provided　a　promising　technology　for　stable　operation　of　one-stage　sewage　PN/A.