In　this　study,　the　filamentous　bulking　(FB)　with　moderate　and　excessive　levels　were　demonstrated　to　induce　anammox　failure　by　inhibiting　nitrogen　(N)　removal　and　biomass　retention.　The　low　external　mass　transfer　resulted　from　high　liquid-surface　friction　and　low　turbulence　of　filamentous　surface　was　considered　the　“trigger”　of　anammox　failure,　which　decreased　flux　of　nitrogen　flow　toward　granular　surface　and　directly　limited　N-removal　loading,　which　meanwhile　exposed　granules　with　N-scarcity　environment　and　indirectly　inhibited　N-removal　bio-activity.　Low　bio-activity　performed　poor　extracellular　polymeric　substances　secretion　further　destroyed　bio-aggregation　with　low　suface　hydrophobicity,　which　acted　as　“accelerator”　for　granule　disintegration　and　biomass　washout,　ultimatly　leading　to　anammox　failure.　Fortunately,　incresing　hydraulic　shear　stress　could　eradicate　FB＇s　negative　effects　without　inhibiting　FB　itself,　which　promoted　re-granulation　and　N-remval　restore　by　enhancing　external　mass　transfer　more　than　hydraulic　detachment.　Enhancing　mechanical　stirring　with　FB　level　was　necessary　to　maintain　stable　operation　of　granular　anammox　system.　©　2024　Elsevier　Ltd