The　emerging　nitrogen　removal　process　known　as　CANDAN　(Complete　Ammonium　and　Nitrate　removal　via　Denitratation-Anammox　over　Nitrite)　has　been　developed　in　Sequencing　Batch　Reactors　(SBRs).　Yet,　starting　up　and　maintaining　stability　in　continuous-flow　reactors　remain　challenging.　This　study　explores　the　feasibility　of　transitioning　the　CANDAN　process　from　an　anammox-dominated　process　by　introducing　appropriate　external　organics　to　facilitate　indigenous　nitrite-producing　denitrification　community　in　an　Upflow　Anaerobic　Sludge　Blanket　(UASB)　reactor.　150-day　operation　results　indicate　that　under　feeding　rates　of　domestic　wastewater　at　0.54　L/h　and　nitrate-containing　wastewater　at　1.08　L/h,　excellent　N　removal　was　achieved,　with　effluent　TN　below　10.0　mg　N/L.　Adding　external　sodium　acetate　at　a　COD/NO3−-N　=　2.0　triggered　denitratation,　ex-situ　denitrification　activity　tests　showed　increased　nitrite　production　rates,　maintaining　the　nitrate-to-nitrite　transformation　ratio　(NTR)　above　90　%.　Consequently,　anammox　activity　was　consistently　maintained,　dominating　Total　Nitrogen　(TN)　removal　with　a　contribution　as　high　as　78.3　±　8.0　%.　Anammox　functional　bacteria,　Brocadia　and　Kuenenia　were　identified　and　showed　no　decrease　throughout　the　operation,　indicating　the　robustness　of　the　anammox　process.　Notably,　the　troublesome　of　sludge　flotation,　did　not　occur,　also　contributing　to　sustained　outstanding　performance.　In　conclusion,　this　study　advances　our　understanding　of　the　synergistic　interplay　between　anammox　and　denitrifying　bacteria　in　the　Anammox-UASB　system,　offering　technical　insights　for　establishing　a　stable　continuous-flow　CANDAN　process　for　simultaneous　ammonium　and　nitrate　removal.　©　2024　Elsevier　B.V.