Bioinduced phosphorus precipitation in granular sludge undergoing denitrifying biological phosphorus removal: Phosphorus recovery from sewage as hydroxyapatite

Recovering phosphorus from mainstream wastewater treatment systems by leveraging microbial metabolism without the addition of extra chemicals effectively streamlines the steps involved in phosphorus recovery from low-strength wastewater, thereby increasing the economic feasibility of the phosphorus recovery process. Hydroxyapatite (HAP, Ca₅(PO₄)₃OH) can be formed directly via bioinduction in biological treatment systems, serving as a potential substitute for phosphate rock. HAP formation in sewage with low phosphate and Ca²⁺ concentrations is challenging. Denitrifying polyphosphate-accumulating organisms (DPAOs) can regulate phosphate ions and pH to facilitate HAP formation in sewage. In this study, the denitrifying biological phosphorus removal (DPR) system was established, achieving phosphorus and nitrate removal efficiencies of 99.3 % and 45.1 %, respectively. In the anaerobic and anoxic sections, the saturation index for HAP was greater than zero, indicating favourable conditions for HAP formation. Inorganic cores, identified as HAP through chemical composition and structural features, were formed in the DPR granular sludge, contributing approximately 66 % to phosphorus removal. The HAP‒DPR granular sludge, located at the bottom of the reactor, consisted of 80 wt % inorganic matter and 15.4 wt % total phosphorus, 86.1 % of which was chemical phosphorus precipitation. Microstructural analysis of HAP cores revealed poly-pellet structures with nanoscale wires. The growth of HAP minerals was not inhibited by intracellular polyphosphates. The presence of HAP cores promoted a differentiated spatial distribution of granular sludge and contributed to a differentiated microbial community structure. DPAOs were located mainly in small-sized granular sludge (Type A), whereas denitrifying glycogen-accumulating organisms were found mainly in large-sized granular sludge (Type B), where HAP formation primarily occurred. Granular sludge with higher inorganic and chemical phosphorus contents (Type C) likely originated from the disintegration of Type B. In conclusion, the HAP‒DPR system has potential for phosphorus recovery in the form of HAP directly from low-strength wastewater.