Holdup of sub-granules towards enhanced production of anammox sludge in a dual circulation sludge bed reactor

•The anammox sludge production was greatly enhanced by 96.2% in the Ducan reactor.•Improved properties of new-born AnGS were responsible for enhanced AnGS production.•Sub-granules with size of 100 μm were the direct structural units of new-born AnGS.•The dual-systolic tubes in the Ducan reactor improved SG profiles for granulation.•A three-step mechanism was proposed for the enhanced production of AnGS.