At　present,　how　to　economically　and　effectively　realize　the　deep　denitrification　of　low　C/N　domestic　sewage　is　still　a　major　challenge　faced　by　sewage　treatment　plants.　Theoretically,　the　endogenous　denitrification　in　post　anoxic　zone　can　achieve　advanced　nitrogen　removal.　However,due　to　lacking　external　carbon　source,　it　was　often　difficult　to　meet　the　discharge　standard　and　saving-energy　at　the　same　time.　In　this　study,　a　novel　sludge　double　recirculation-anaerobic/aerobic/anoxic　(AOA)　process　was　set　up.　This　process　was　conducive　to　the　enrichment　and　cultivation　of　denitrifying　glycogen　accumulating　organisms　(DGAOs)　and　dominated　by　Candidatus　Competibacte,　which　could　develop　and　utilize　of　intracellular　carbon　sources.　The　TIN　removal　efficiency　of　91.81%　and　the　TIN　concentration　in　effluent　was　4.36mg·L-1　in　the　long-term　treatment　of　low　C/N　ratio　(3.2)　real　municipal　wastewater.　In　addition,　the　specific　endogenous　denitrification　rate　was　increased　with　a　higher　MLSS　in　the　anoxic　zone　after　setting　secondary　sludge　recirculation.　This　was　the　mainly　reason　for　achieving　advanced　nitrogen　removal.　With　the　improvement　of　the　nitrogen　removal　efficiency,　the　sufficient　intracellular　carbon　sources　storage　in　anaerobic　zone　was　increase,　and　the　endogenous　denitrification　enhanced.　The　nitrogen　removal　performance　showed　a　positive　cycle.　Finally,　the　additional　carbon　source　generation,　which　was　from　the　bottom　of　the　secondary　sedimentation　tank　and　introduced　to　the　system　through　the　secondary　sludge　recirculation,　further　enhanced　the　nitrogen　removal.　This　study　clarified　the　mechanisms　and　key　factors　for　advanced　nitrogen　removal　in　sludge　double　recirculation-AOA　system,　and　provided　feasibility　for　the　increased　nitrogen　removal　efficiency　in　low　C/N　municipal　wastewater　treatment.　©　2022　Chemical　Industry　Press.　All　rights　reserved.