Starting　up　or　recovering　partial　nitritation　is　a　major　challenge　for　achieving　or　maintaining　stable　partial　nitritation/anammox　(PN/A)　during　mainstream　wastewater　treatment.　This　study　presents　a　novel　strategy　for　recovering　the　nitrite　pathway　by　selectively　reviving　ammonium　oxidizing　bacteria　(AOB)　after　thoroughly　inhibiting　AOB　and　nitrite　oxidizing　bacteria　(NOB)　using　free　nitrous　acid　(FNA).　A　sequencing　batch　reactor　was　operated　for　PN/A　to　treat　real　domestic　wastewater　for　423　days,　during　which　twice　FNA　treatment　was　temporarily　implemented.　Results　showed　that　with　a　single　0.45　mg/L　FNA　treatment　on　flocculent　sludge,　the　NO3--N　concentration　during　the　aerobic　period　showed　an　uptrend　again　and　the　partial　nitritation　performance　was　deteriorated.　In　contrast,　1.35　mg/L　FNA　treatment　induced　the　inhibition　of　both　AOB　and　NOB　leading　to　regressive　ammonium　oxidation,　but　a　subsequently　higher　DO　(1.5　mg/L)　and　longer　aeration　duration　recovered　partial　nitritation.　For　the　relative　abundances　of　the　acquired　biomass　related　to　nitrogen　conversion,　Nitrosomonas,　Nitrospira　and　Nitrolancea　increased　to　9.65%,　10.27%　and　4.35%,　respectively,　at　the　beginning　of　the　1.35　mg/L　FNA　treatment,　and　Nitrospira　and　Nitrolancea　decreased　to　2.80%　and　0.03%　whereas　Nitrosomonas　declined　to　8.71%　after　76　days.　Ca.　Brocadia　showed　less　resilience　after　the　1.35　mg/L　FNA　treatment,　with　the　relative　abundance　decreasing　from　13.38%　to　0.62%　due　to　insufficient　nitrite.　Molecular　ecological　network　analysis　indicates　that　among　anammox　taxa,　Ca.　Kuenenia　and　Ca.　Brocadia　formed　important　links　with　other　N　cycle　processes.　Moreover,　the　proposed　strategy　shows　operational　flexibility　because　it　can　be　easily　used　to　control　NOB　in　mainstream　PN/A　applications　offered　by　flocculent　sludge　systems.