Deciphering the granulation process in aeration self-circulating reactors: A robust strategy for continuous-flow...

The application of continuous-flow aerobic granular sludge (AGS) is primarily limited by the lack of effective in-situ selection mechanisms, which challenges biomass retention and long-term stability. This study evaluated an integrated self-circulating reactor (Zier), where internal circulation was driven passively by hydrostatic pressure and density gradients. This configuration decoupled the aeration zone from the selection zone to provide consistent hydrodynamic shear. The granulation trajectory and process robustness were investigated using two separate reactors (R1 and R2) inoculated with anaerobic granular sludge and excess sludge, respectively. The systems were fed with synthetic wastewater characterized by a chemical oxygen demand (COD) of approximately 300 mg/L, ammonia nitrogen (NH₄⁺-N) of 50-65 mg/L, and a C/N ratio of 4.6-6.0. Under a hydraulic retention time (HRT) of 0.52-0.70 d and a circulation ratio of 70-150, the Zier process facilitated a rapid transition to granular dominance, with the granular fraction (>0.2 mm) exceeding 80% and maintaining stable settling properties for over 170 days. Granular sludge was sustained at upflow velocities below 15 m/h, effectively balancing biomass retention and organic removal (>85% COD and >70% TN). These findings demonstrate that the self-circulating configuration provides a structurally simplified and robust strategy for long-term continuous-flow aerobic granulation.