The symbiosis of anaerobic ammonium oxidation bacteria and heterotrophic denitrification bacteria in a size-fractioned single-stage partial nitrification/anammox reactor

•A hybrid reactor combining flocs and granules was set up for nitrogen removal.•Coexistence of AAOB and heterotrophic denitrification bacteria was observed.•AAOB was enriched in granules and HDB was enriched in flocs.•DO concentration significantly affected abundances of AAOB and HDB.•Heterotrophic denitrification in flocs improved nitrogen removal.