Microbially induced hydroxyapatite improves simultaneous nitrogen and phosphorus removal in aerobic granular sludge

Aerobic granular sludge (AGS) is a promising technology; however, its potential for deep simultaneous nitrogen and phosphorus removal is constrained by competition for organic carbon among functional microbial communities. In this study, biologically induced phosphorus precipitation (BIPP) was implemented in AGS systems operated under aerobic (O_SBR) and anaerobic-aerobic (AO_SBR) regimes to elucidate its formation mechanisms and ecological impacts. The results showed that hydroxyapatite (HAP) was the dominant precipitate in both O_SBR and AO_SBR systems, although the induction mechanisms differed between the two operational modes. In the O_SBR, denitrification-driven localized pH elevation triggered HAP nucleation, whereas in the AO_SBR, phosphate release by polyphosphate-accumulating organisms created favorable ionic conditions for precipitation. Establishment of the BIPP process substantially improved phosphorus removal, achieving approximately 75% PO₄^3--P removal in the EBPR-absent O_SBR and stable removal exceeding 99% in the EBPR-present AO_SBR. Notably, mineral accumulation in both operational modes was accompanied by shifts in carbon and electron allocation patterns, characterized by increased electron flux toward denitrification pathways and concurrent improvement in nitrogen removal performance in the AGS systems. Through analysis of carbon, nitrogen, and phosphorus fluxes, this study provides a carbon allocation perspective linking mineral accumulation with microbial metabolism and pollutant removal. The findings not only elucidate the distinct induction mechanisms of HAP formation under different operational regimes, but also highlight the coupling between mineral accumulation and microbial metabolism within diffusion-limited granular structures. These findings provide mechanistic support for understanding and regulating AGS processes toward deep simultaneous nitrogen and phosphorus removal.