Insights into the Responses of the Partial Denitrification Process to Elevated Perfluorooctanoic Acid Stress: Performance, EPS Characteristic and Microbial Community

This study aimed at investigating the potential impacts of perfluorooctanoic acid (PFOA) exposure on the partial denitrification (PD) system. Our results indicated that nitrite accumulation rates were significantly decreased to 67.94 ± 1.25%–69.52 ± 3.13% after long-term PFOA exposure (0.5–20 mg/L), while the nitrate transformation ratio was slightly impacted. The PFOA removal efficiency gradually decreased from 67.42 ± 3.39% to 6.56 ± 5.25% with an increasing PFOA dosage, indicating that the main PFOA removal pathway was biosorption. The average EPS contents increased by two folds, which suggested that exposure to PFOA significantly stimulated EPS secretion. Excitation emission matrix analysis revealed that PFOA exposure promoted the secretion of tryptophan protein-like, humic acid-like, and aromatic protein II-like substances, which may act as a protective barrier against PFOA toxicity. Moreover, significant changes in characteristic peaks after PFOA exposure were shown as indicated by Fourier transform infrared spectroscopy. High-throughput sequencing suggested that PFOA significantly decreased bacterial richness and increased evenness, indicating that toxicity effects of PFOA were more pronounced for abundant species (e.g., Thauera) than rare species. Thauera was the most dominant genus responsible for nitrite accumulation, whose abundance significantly decreased from 35.99 ± 2.67% to 18.60 ± 2.18% after PFOA exposure. In comparison, the abundances of common denitrifiers, such as Denitratisoma, Bdellovibrio, and OLB8, significantly increased, suggesting that these genera were potential PFOA-resistant bacteria. This study presents new insights into the effect of PFOA on a PD system.