In-situ cultivation and process dynamics of algal-bacterial granular sludge in a continuous-flow self-circulation...

This study successfully cultivated algal-bacterial granular sludge (ABGS) in-situ from floccular activated sludge, using a continuous-flow self-circulating reactor (namely Zier process). Over 180 days of operation, the system demonstrated stable and efficient removal of chemical oxygen demand (COD) (95 %) and ammonia nitrogen (NH₄⁺-N) (90 %) at hydraulic retention times of 4-11 h and organic loading rates of 0.6-2.6 kg-COD/(m³·d). Ammonium removal was contributed by nitrifying bacteria (≈52.1 %) and algal assimilation (≈47.9 %). Despite nitrogen source competition between algae and bacteria, the system maintained stable performance. Under the unique hydrodynamic conditions that promoted biomass densification and excellent settleability (SVI₃₀ ≈40 mL/g). Algal-bacterial synergy (O₂ supply, N assimilation, nitrification) and protein-dominated extracellular polymeric substance (EPS) drove granulation, evidenced by a 31 % increase in >0.2 mm particles. This system operated with a high non-aerated to aerated volume ratio (2:1), highlighting its energy-efficient potential and providing a theoretical basis for scalable continuous-flow ABGS processes.