Achieving　nitritation　is　a　prerequisite　to　promote　nutrients　removal　and　save　energy,　but　emission　of　nitrous　oxide　as　a　greenhouse　gas　cannot　be　ignored.　This　study　established　the　nitritation　in　a　continuous-flow　MUCT　process　and　investigated　the　mechanism　of　N2O　generation.　The　nitrite　accumulation　ratio　(NAR)　reached　95%　by　controlling　the　low　DO　of　0.3-0.5　mg/L　and　short　HRT　of　8　h.　The　N-15-isotope　tracer　experiment　indicated　that　the　percentage　of　nitrifier-denitrification　(ND)　pathway　increased　by　12.7%　under　the　limited-aeration　mode,　improving　the　stable　operating　of　nitritation.　Meanwhile,　the　autotrophic　anammox　pathway　increased　with　the　contribution　ratio　of　14.7%　to　N-2　emission　under　the　nitritation　mode.　The　N-15-DNA-SIP　revealed　that　the　Nitrosomonas　executed　the　ND　pathway　and　the　Planctomycetes　conducted　the　anammox　process,　respectively.　The　integration　of　autotrophic　and　heterotrophic　process　based　on　nitritation　technique　has　potential　to　solve　the　carbon-limited　issue　for　total　nitrogen　removal　in　mainstream　WWTPs.