Engineering Applications and Mechanistic Studies of the Combination of Aerobic Granular Sludge and the AAO Process in a Full-Scale Wastewater Treatment Plant

Abstract A full-scale wastewater treatment plant (WWTP) (4 × 104 t/d) was employed as the experimental site to investigate the granulation process and application of aerobic granular sludge (AGS) in an anaerobic–anoxic–oxic system via bioaugmented inoculation (AAGS). This study addresses the lack of a practical low-interruption retrofit route for implementing AGS in full-scale continuous-flow AAO systems and the limited understanding of how bioenhancer-assisted nucleation works together with sludge selection and hydraulic regulation during granulation. The results demonstrated that the bioenhancers facilitated initial nucleation through surface adsorption, forming a filamentous framework that promoted microbial aggregation, as the enhancers offered attachment sites, accelerating cell interactions and retaining slow-growing microbes. Meanwhile, tightly bound extracellular polymeric substances enhanced granule structural stability via hydrophobic interactions, minimizing disintegration under shear. Within the system, hydrodynamic shear force fostered the development of anaerobic–aerobic microenvironments inside the granules, thereby enabling niche differentiation between Nitrospira and denitrifiers, due to layered oxygen gradients supporting simultaneous nitrification–denitrification. The production-scale trial achieved AGS granulation within 30 days, with average COD, TN, and TP removal efficiencies of 87.4%, 79.4%, and 93.3%, respectively, through bioaugmentation and hydraulic optimization, demonstrating the potential of this technology for retrofitting continuous-flow WWTPs.