AbstractExcessive phosphorus in aquatic environments can lead to eutrophication, posing a significant deterioration of water quality. In this study, modified steel slag was used as a novel filter media in the biological aeration filter (BAF) to achieve a bioinduced phosphorus crystallization process for simultaneous removal and recovery of phosphorus from municipal wastewater. Phosphorus removal efficiency could reach 80% during 4 days of operation due to high adsorption capacity of the modified steel slag in the single BAF reactor, but the phosphorus removal efficiency decreased to 30% after 30 days of operation. The phosphorus removal efficiency of the BAF system increased to 70% under alternating anaerobic-aerobic (A/O) operation condition. Concentration of PO43−-P in anaerobic effluent reached 19.9 mg/L at 68 days of operation and exceeded that in influent (10.6 mg/L), indicating that phosphorus accumulating organisms (PAOs) were successfully enriched in the alternating A/O BAF system. Hydroxyapatite (HAP) crystals formation was confirmed based on analysis of scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD). The proposed phosphorus removal mechanism suggests that the release of phosphorus from PAOs and the release of Ca2+ and OH− from the modified steel slag is beneficial to HAP crystals formation. These results indicated that the bioinduced phosphorus crystallization process achieved to improve phosphorus removal efficiency of the alternating A/O BAF system. This finding is a significant improvement to both biological and crystallization phosphorus removal processes, and has significant potential application for removal and recovery of phosphorus in wastewater treatment.