Hydroxyapatite Formation in a Single-Stage Anammox-Based Batch Treatment System: Reactor Performance, Phosphorus Recovery, and Microbial Community

Simultaneous ammonium-nitrogen ( NH4+-N) removal and orthophosphate-phosphorus (PO4-P) biomineralization was studied in two granular sludge sequencing batch reactors treating urban sidestream centrate through the one-stage partial nitritation-anaerobic ammonium oxidation (anammox) process. By adding an external source of calcium, concomitant conversion rates of up to 0.32 g NH4+-N/(L.days) and 12 mg PO4-P/(L.days) were measured while reaching water- phase removal efficiencies of ca. 80% of the NH4+-N and 70% of the PO4-P loaded. The mineral cores formed inside the granules were mostly composed of hydroxyapatite (HAP), which is a recoverable phosphate salt. Yet, the high mineralization of the sludge and its excessive purge from the bioreactor limited the microbial activity in the long term. In this sense, the densest granules, accumulated at the bottom of the bioreactor, were the most susceptible to being harvested because they were the richest in HAP, but at the same time, they contained the highest percentages of anammox bacteria.