Underlying mechanisms of pollutant removal enhancement through the formation of low levels of granular sludge in an innovative continuous-flow reactor.

In situ cultivation and long-term stabilization of continuous-flow aerobic granular sludge (AGS) pose significant challenges for the sustainable advancement of wastewater technology. Herein, we demonstrated the successful 330-day operation of a novel continuous-flow self-circulating AcOA-Zier reactor. Aeration-driven liquid recirculation achieved recirculation-to-influent (R/I) ratios of 26-70, optimizing dissolved oxygen gradients and enabling exceptional contaminant removal of 96 % for chemical oxygen demand (COD) and 95 % for total inorganic nitrogen (TIN). High hydrodynamic shear promoted granulation, yielding an average particle size of 369.7 μm, with >83 % of the granules in the optimal 200-600 μm range being used to ensure operational stability. Microbial community profiling revealed Proteobacteria (80 %), Chloroflexi (7.1 %) and Bacteroidota (10.2 %) as keystone taxa underpinning granule formation, structural integrity, and pollutant degradation. Metagenomics identified narG, nirK, norBC, nosZ and nxrB as core nitrogen cycling genes, with Methylotenera and unclassified_c_Betaproteobacteria serving as the dominant functional microorganisms. In this work, we established granulation dynamics and particle stability as pivotal factors for scalable AGS systems, providing a framework for optimizing energy-efficient, high-performance wastewater treatment processes.