Coupled Partial Denitrification–Fluorapatite (PD–FAP) Granular System for Deep Defluoridation and Efficient Nitrite Production

Fluoride contamination requires deep removal to meet discharge limits, but conventional calcium fluoride precipitation is limited by residual solubility. This study developed a coupled partial denitrification–fluorapatite (PD–FAP) granular system that simultaneously achieved fluoride removal and nitrite production in a single reactor. At the optimal influent Ca:P:F molar ratio of 150:75:10, fluoride decreased from 10 mg/L to below 1.5 mg/L within 30 min, while stable nitrite accumulation was maintained (effluent NO2–-N ≈ 50 mg/L; nitrate-to-nitrite transformation ratio >70%). The granules showed excellent settleability (sludge volume index at 5 min = 22.6 mL g–1) and adaptive morphological evolution, including initial enlargement, surface mineralization, and subsequent size reduction that preserved mass transfer and microbial activity. Fluoride immobilization was mainly associated with fluorapatite crystallization, as indicated by the ΔP:ΔF ratio of approximately 5:1 and mineralogical evidence, with additional surface-associated retention on mineralized granules. The transformation of extracellular polymeric substances toward fulvic acid-like components likely promoted Ca+2 complexation and mineral nucleation. This integrated PD–FAP process provides a compact and resource-efficient pretreatment strategy for simultaneous deep defluoridation and nitrite supply for downstream anaerobic ammonium oxidation treatment of fluoride-containing industrial wastewater.