A novel two-layered hydroxyapatite-strengthened anammox-partial denitrification/anammox reactor for advanced nitrogen removal : Performance and mechanisms

Anaerobic ammonium oxidation (anammox) is a low-carbon nitrogen removal technology. However, its practical application remains constrained by nitrogen removal ceiling of 89%. This study developed a novel two-layered hydroxyapatite (HAP)-strengthened anammox-partial denitrification/anammox (A-PDA) reactor for enhanced nitrogen removal. By exploiting the up-flow hydrodynamics of expanded granular sludge bed (EGSB) reactor, carbon source was directionally transported to the upper zone, thereby establishing a two-layered configuration consisting of a bottom anammox zone and an upper PDA zone. The A-PDA reactor exhibited an excellent total inorganic nitrogen removal efficiency (TINRE) of 96.2% at inorganic nitrogen loading rate (NLR) of 4.4 g N/L/d. Directional carbon source transport enriched Thauera in the PDA zone, with its abundance increasing from 0.3 to 6.1% and its specific partial denitrification activity (SPDA) increasing from 0.019 to 0.148 g N/g VSS/d. Meanwhile, Candidatus Kuenenia remained stable in the anammox zone, with its relative abundance maintained at 22.8-23.6% and its specific anammox activity (SAA) of 0.09-0.10 g N/g VSS/d, whereas the abundance of Thauera remained below 1%. The formation of HAP improved settleability of granules, resulting in settling velocities of 349 m/h for anammox-HAP granules and 41 m/h for PDA-HAP granules, both values were substantially higher than the reactor up-flow velocity (2.39 m/h). Moreover, the distinct settling velocities of the two granule types also contributed to maintaining two-layered structure. Overall, this study clarified both the feasibility and coupling mechanism of the A-PDA reactor for deep nitrogen removal, providing support for its practical application.