Fe(II) complexation-regulated iron allocation supports coupled nitrogen and phosphorus removal in an anammox-FePs granular system.

Iron stimulates nitrogen metabolism and concurrently facilitates phosphate mineralization in anammox systems. However, this dual contribution directs iron into competing biological and mineral pathways that remain poorly understood. Here, Fe(II) complexation-regulated dosing was applied to control iron availability and balance metabolic enhancement with Fe-P precipitation in an anammox-FePs granular system. A four-phase regulation strategy revealed that balanced iron allocation was critical for efficient nitrate reduction and simultaneous phosphorus sequestration, achieving 99.5% total nitrogen and 80.3% phosphorus removal. FePs distribution within sludge granules was also reshaped by Fe(II) complexation regulation, promoting internal deposition while alleviating surface crusting. Multi-omics analysis showed that nitrate-dependent anaerobic ferrous iron oxidation-specific bacteria remained at low abundance, while biological nitrate removal was predominantly supported by Nap-mediated taxa in coordination with the nitrogen metabolic functions abundant in Kuenenia. Intracellular iron homeostasis was associated with Dps and ferritin-like proteins. The iron-storage-oriented metabolism and Fe(II) complexation likely contributed to stable community under supra-threshold iron dosing while enhancing NP removal. Overall, these findings provide a process-oriented basis for iron-assisted integrated NP removal.