Simultaneous nitrogen removal and phosphorus recovery in granular sludge-based partial denitrification/anammox-hydroxyapatite precipitation (PD/A-HAP) process under low C/N ratio and dissolved oxygen limitation

[Display omitted] •HAP crystallization integrated with PD/A for simultaneous N and P removal.•PD/A-HAP granules achieved 94.13% nitrogen and 73.6% phosphorus recovery.•Calcium addition enhanced sludge settleability and phosphorus crystallization.•Candidatus Brocadia increased from 1% to 26.1%, boosting anammox activity.•PD/A-HAP coupling provides an efficient system for wastewater N and P recovery. This study integrates partial denitrification/Anammox (PD/A) with hydroxyapatite (HAP) crystallization in a single reactor, achieving simultaneous nitrogen and phosphorus removal along with phosphorus recovery. By adjusting pH, sludge concentration, low COD/TN ratio, and applying moderate dissolved oxygen stress, the system operated stably and promoted the synergistic growth of HAP and biomass. Results showed a nitrogen removal efficiency (NRE) of 94.13 % and a phosphorus removal efficiency (PRE) of 73.6 %. Metagenomic analysis revealed that under dissolved oxygen stress, The abundance of Candidatus Brocadia increased from 1 % to 26.1 %, significantly boosting anammox activity. indicating enhanced microbial activity. The upregulation of related genes (sdh, suc, hzs) further boosted AnAOB activity. HAP was identified as the main inorganic component of the granule. This process shows strong potential for nitrogen and phosphorus removal with resource recovery in wastewater treatment.