Chemical and microbial mechanism governing simultaneous nitrogen and phosphorus removal by anammox-hydroxyapatite granules

Anaerobic ammonium oxidation (anammox)-hydroxyapatite (HAP) represents a novel granular sludge exhibiting simultaneous nitrogen and phosphorus removal, demonstrates remarkable potential in treating eutrophic wastewater. While the anammox-HAP granulation process has been documented, quantitative descriptions of organic biofilm and inorganic core formation associated with nitrogen and phosphorus removal remains lacking. We investigated an expanded granular sludge bed (EGSB)-anammox reactor's performance in treating partial nitrified effluent. Stable operation for 360 days achieved 90.4 % TN removal efficiencies at 6 g N/L/day. Elemental and stoichiometric analyses of granule biolayers validated the stoichiometric anammox equation: NH₄ + + 1.104 NO₂ − + 0.063 HCO₃ − + 0.054H + → 0.063 CH 1.72 N 0.15 O 0.84 + 0.113 NO₃ − + 0.991 N 2 + 2.00H₂O. The biomass (0.132 ± 0.003 g VSS/g N) and experimental HAP yields (5.81 ± 0.003 g ASH/g P) corresponded to a granule organic–inorganic yield ratio of 0.672. The governing strategy considered the initial conditions for both stable anammox ( Candidatus Kuenenia) bio-enrichment and HAP formation. Subsequent nitrogen removal was optimized by controlling the NO₂ − /NH₄ + ratio, using effluent nitrogen as indicator. A Ca/P ratio of 2.17–3.51 promoted HAP formation, enabling calcium and phosphorus removal as HAP cores. Overall, we propose a governing mechanism incorporating feedback control for anammox-HAP system in efficient simultaneous removal of nitrogen, calcium, and phosphorus, minimizing wastewater pollution to enhance water resource utilization efficiency. • Excellent N, Ca and P removal were sustained in anammox-HAP reactor. • The anammox stoichiometric equation was identified based on experiments. • The organic-to-inorganic ratio (R OI ) in HAP granules was proposed. • The progressive-inorganification observed in granules has been explained. • Governing mechanism of N and Ca/P for anammox-HAP system was suggested.