Process Intensification of Partial Denitrification Promotes Structural Reorganization and Enhancement of HAP-Coupled Granular Sludge.

Partial denitrification (PD) coupled with hydroxyapatite (HAP) mineralization offers a promising pathway for simultaneous nitrogen removal and phosphorus (P) recovery. However, the mechanisms linking alkalinity generation, mineralization pathways, and granule structural evolution remain poorly understood. Herein, an SBR was operated under varying cycle frequencies and pH conditions to regulate alkalinity sources and elucidate their role in shaping PD-HAP granules. Results showed that reducing cycle frequency effectively suppressed complete denitrification and enhanced nitrite accumulation (nitrate-to-nitrite transformation ratio (NTR): 52.7 ± 3.3% to 75.0± 4.4%). Meanwhile, alkalinity source plays a regulatory role in HAP precipitation. Morphological and compositional analyses showed that granules gradually transitioned from a homogeneously distributed biomass-mineral structure to a compact layered architecture. Consequently, the granules showed enhanced P removal capacity and crystallinity, along with improved granule settling velocity (80.7 ± 9.0 m/h) and mechanical strength (integrity coefficient (IC): 97.8%). Importantly, this study reveals a self-reinforcing coupling mechanism between PD metabolism and HAP mineralization: Alkalinity-regulated mineral deposition promotes the remodeling of the granule structure, thereby selectively enhancing the PD process. In turn, the enhanced PD performance further drives the transformation of the granule structure. This study provides links between microbial metabolism and mineral formation, offering a feasible strategy for optimizing granular sludge systems for low-carbon treatment and resource recovery.