Mineralization-driven functional zonation of a PD-FAP granular system for coupled nitrite supply and fluoride removal

Industrial wastewaters containing nitrate and fluoride are commonly treated by separate biological and physicochemical processes, resulting in high energy demand and operational complexity. However, whether progressive mineralization can drive spatial stratification and functional differentiation within a single granular sludge system remains poorly understood. Here, a single-sludge system integrating partial denitrification (PD) with fluorapatite (FAP) crystallization was developed for coupled nitrite supply and fluoride removal. During 80 days of operation, stable PD was maintained, with residual NO₃⁻-N below 10 mg/L and average NO₂⁻-N accumulation of 43.56 mg/L, while F⁻ decreased to 3-5 mg/L (50-70% removal) with concurrent phosphate consumption. Progressive FAP accumulation was associated with granule densification, enhanced settling, and pronounced vertical differentiation. Granule characterization showed that the upper layer remained biomass-dominated, whereas the bottom layer became highly mineralized with markedly enhanced settling properties. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD) analyses supported an inward-to-outward mineralization pattern, from intragranular nucleation to partial surface encrustation. Ex-situ batch assays verified distinct functional zonation: upper layer granules exhibited superior nitrite supply kinetics, while bottom layer granules achieved enhanced fluoride and phosphate removal via sustained FAP crystallization. Despite progressive mineral loading, the nitrate-to-nitrite transformation ratio (NTR) remained above 85%, accompanied by microbial succession and extracellular polymeric substance (EPS) remodeling. A lifecycle-based conceptual model is proposed, in which progressive FAP accumulation is associated with density differentiation and vertical redistribution of granules under the prevailing hydraulic conditions. Overall, these findings identify biomineralization-associated density evolution as a potential internal structuring mechanism for coupling biological nitrogen transformation with mineral-mediated fluoride removal, providing a basis for developing more compact and functionally integrated single-sludge reactors for complex nitrate-fluoride wastewater treatment.