Hydroxyapatite Mineralization-Driven Granulation Enables Rapid Start-Up and Stable Operation of the One-Stage Partial Nitritation/Anammox Process.

Severe biomass washout remains a major bottleneck limiting the rapid startup and long-term stability of one-stage partial nitritation/anammox (PN/A) processes. In this study, a novel hydroxyapatite-based partial nitritation/anammox (HAP-PN/A) strategy was developed to enhance biomass retention and improve operational stability through HAP mineralization-driven granulation. An HAP-PN/A reactor was established using anammox sludge and partial nitrification sludge as inocula, and the effects of different Ca/P ratios on reactor startup, nitrogen removal performance, sludge characteristics, and microbial community succession were systematically investigated. The results demonstrated that Ca/P regulation promoted HAP precipitation and facilitated granular sludge reconstruction, enabling successful startup within 40 days at a Ca/P ratio of 3. The HAP-PN/A system achieved a maximum nitrogen removal efficiency of 85.24% at a nitrogen loading rate of 1.25 g N/(L·d), while simultaneously achieving an average phosphorus removal efficiency of 59.52%. The developed HAP-based granules exhibited excellent settleability, with an average particle size of 346 μm and a sludge volume index of 15.11 mL/g. Microbial analysis revealed that Ca. Brocadia became the dominant anammox genus after reactor acclimation, accounting for 9.6-13.9% of the microbial community, indicating successful enrichment of functional AnAOB within the HAP matrix. Overall, this study demonstrates that HAP mineralization-driven granulation provides an effective approach for accelerating PN/A startup, enhancing microbial retention, and achieving simultaneous nitrogen removal and phosphorus recovery from ammonia-rich wastewater.