Fluoride-induced stress shapes partial denitrification granules to sustain microbial metabolism

The presence of fluoride ions (F^-) in nitrogen-rich wastewater from photovoltaic and semiconductor industries introduces a significant challenge to biological treatment processes, particularly for the innovative partial denitrification (PD) process, which supplies nitrite for anaerobic ammonium oxidation (Anammox). This study provides the first comprehensive and systematic investigation of the effects of F^- stress on the granule-based PD process through batch tests and long-term operation. Results indicate that PD activity remains resilient to F^- shock up to 1.5 g/L but is markedly impaired at concentrations of 2.0-3.0 g/L, despite maintaining a nitrate-to-nitrite transformation ratio (NTR) of approximately 80 %. Under long-term F^- stress at 0.5 g/L, NTR gradually reduces to 50 %, but subsequently recovers to and maintains at 70 %. The increased secretion of loosely bound extracellular polymeric substances and proteins likely enhances the resistance of PD granules to F^- stress, though excessive amounts degrade their settling properties. F^--induced microbial community succession shapes a predominance of medium granules (1.0 < d < 2.0 mm of 60.2 %) by enhancing aggregation of smaller granules and disintegration of larger ones. This enhances the mechanical strength and microbial activity of PD granules, aiding in resistance to F^- stress to sustain microbial metabolism. Thauera is selectively enriched under long-term F^- stress, with upregulated nirBDS genes contributing to the reduced NTR. Additionally, increased electron metabolism activity and a robust antioxidative response help to maintain higher microbial metabolic activity, mitigating F^--induced oxidative stress. These findings advance our understanding of the resilience and adaptability of the PD process under F^- stress, providing critical insights for optimizing biological wastewater treatment systems in challenging environments.