Biomineral self-organization into tree-ring granules enables stable nitrite supply and phosphorus recovery

Low-carbon biological nitrogen removal via anaerobic ammonium oxidation (anammox) is often limited by unstable nitrite supply and the lack of integrated phosphorus (P) removal. Here, partial denitrification (PD) metabolism is demonstrated to drive in-situ hydroxyapatite (HAP) biomineralization, promoting the self-organization of bio-mineral granules with hierarchical tree-ring architecture. The system achieved stable and efficient nitrite accumulation (nitrate-to-nitrite transforming ratio (NTR) > 60% in in-situ activity tests) and P removal (phosphorus removal efficiency (PRE) > 80% of final-phase) during long-term operation. And combining long-term reactor operation with X-ray microtomography, crystallography, and microbial profiling, a transition is revealed from porous, biomass-rich hydrogel granules (honeycomb-like granules, HG) to tight granules (tree-ring-like granules, TG) featuring a dense mineral core enveloped by a thin, metabolically active microbial shell. This structure sustains rapid settling (326 ± 15 m/h) and stable size distribution, while concentrating HAP (14.5% P by mass, ≈33% P₂O₅) and minimizing biomass carryover (10.9%). Mechanistically, localized alkalinity generated by PD induces preferential HAP precipitation at the granule periphery. Progressive mineral accumulation restricts internal mass transfer, leading to microbial decay and inward mineral filling, thereby driving the self-organization of HG into TG with a dense mineral core and an active denitrifying outer shell. This spatial organization enables stable, high-rate nitrite production (NTR > 90% in ex-situ activity tests) under progressive mineral stress and simultaneous efficient P removal, yielding high-content HAP for resource recovery. By resolving the persistent trade-off between nitrite stability and P management, this biomineral self-assembly strategy establishes a generalizable framework for designing resilient, multifunctional PD/anammox systems for integrated, low-carbon nitrogen and phosphorus treatment in wastewater.