Simultaneous hydroxyapatite-based phosphorus recovery and partial denitrification-anammox-based nitrogen removal during sludge leachate treatment

Anammox has been widely applied for treating high ammonia-nitrogen leachate, but challenges remain due to the phosphorus in the wastewater and by-products nitrate (NO3-). Therefore, we combined partial denitrificationanammox and hydroxyapatite crystallization within a single UASB reactor. This method achieved an effluent with a 4.27 +/- 0.45 mg TN/L and a maximum PO43- removal efficiency of 68.31 +/- 5.42 %. The addition of excess calcium led to an increase in phenylalanine-like substances, the humification index (HIX) of extracellular polymeric substances (EPS), and the MLVSS/MLSS ratio. These changes indicated that hydroxyapatite coated the initial granular sludge, causing it to crack and re-granulate, but the MLVSS didn't decrease significantly. Therefore, calcium addition didn't significantly impact the function of biomass. Metagenomic analysis revealed that by-products NO3- was removed through partial denitrification using in-situ refractory dissolved organic matters in SL. The formation of hydroxyapatite crystals is the main pathway for PO43- removal, and biological process also contributed to some PO43- removal. Overall, our study provided a viable solution for the simultaneous removal of nitrogen and recovery of phosphorus from high ammonia-nitrogen wastewater.