Mainstream　partial　anammox　provides　a　cost-and　energy-efficient　alternative　for　wastewater　treatment.　This　study　provided　a　new　strategy　to　achieve　mainstream　partial　anammox　in　a　single-stage　suspended　sludge　system.　The　novel　method　coupling　external　excess　sludge　fermentation　with　simultaneous　partial　nitritation-anammox-denitrification　process　(SF-SPNAD)　was　established　for　202　days　in　an　anaerobic-aerobic-anoxic　sequencing　batch　reactor　(AOA-SBR)　with　real　sewage　and　actual　sludge　fermentation　products.　Under　the　condition　of　low　DO　(0.6　+/-　0.2　mg/L),　short　oxic　and　long　anoxic　hydraulic　retention　time　(HRToxic　=　6.5　h,　HRTanoxic　=　8　h),　the　average　total　inorganic　nitrogen　(TIN)　concentration　in　the　influent　and　effluent　during　110-day　operation　were　61.0　and　3.4　mg/L,　respectively,　and　the　TIN　removal　efficiency　was　94.56%.　Under　the　inhibitory　effect　of　continuous　sludge　fermentation　products　addition,　nitrite　accumulation　ratio　reached　99.1%　and　the　external　sludge　reduction　ratio　reached　38.75%.　N-15-stable　isotope　tracing　tests　showed　the　great　potential　of　nitrogen　removal　by　anammox　pathway　in　the　system.　High-throughput　sequencing　confirmed　that　Candidatus　Brocadia　(not　detected　to　0.50%)　and　Candidatus　Kuenenia　(not　detected　to　0.06%)　were　successfully　in-situ　enriched.　Nitrogen　conversion　pathways　based　on　stoichiometry　and　cycle　tests　show　that　34.69%　of　the　TIN　removal　was　obtained　by　simultaneous　nitritation　denitrification　and　anammox　under　oxic　stage　and　35.21%　of　the　TIN　removal　was　carried　out　by　anammox　under　anoxic　stage.　Overall,　the　SF-SPNAD　process　provides　a　new　possibility　for　coupling　autotrophic　and　heterotrophic　nitrogen　removal　with　excess　sludge　utilization.　(C)　2021　Elsevier　B.V.　All　rights　reserved.