Spatial zoning-driven enhancement of simultaneous nitrogen and phosphorus removal: performance evaluation and microbial-metabolic responses

To enhance the sustainability of wastewater treatment and mitigate water eutrophication, we developed a coupled partial nitritation/anammox (PN/A) and hydroxyapatite (HAP)-induced crystallization system. Conventional chemical crystallization for phosphorus removal relies heavily on reagents, leading to high operating costs. By inducing HAP crystallization biologically, this PN/A-HAP system reduces external chemical demand and lowers operating costs. All experiments used a defined synthetic wastewater at 35 ± 1 °C. The PN/A-HAP system started up in 52 days, reaching 76.0 % total nitrogen and 56.6 % total phosphorus removal at a nitrogen loading rate of 0.9 kg-N/m³/d and an influent phosphorus concentration of 20 mg/L. Candidatus Brocadia enrichment increased from 1.65 % to 8.82 %, while AnAOB activity exceeded 0.6 g-N/g-MLVSS/day, demonstrating the successful initiation of anammox and a consequent reduction in sludge cultivation costs. Sludge morphology changes, Candidatus Brocadia enrichment, and computational fluid dynamics analysis confirmed a sludge recirculation pathway within the reactor. The PN/A-HAP system offers a sustainable, energy-efficient approach to wastewater treatment by integrating partial nitritation/anammox with biologically induced HAP crystallization. This integration shortens start-up, lowers operational costs, and enables resource recovery, thereby providing a valuable reference for future optimization and potential scale-up of single-stage PN/A systems.