Ecotoxicity of HFPO-DA to Anammox systems at low temperatures: Hidden risks of fluorochemical substitutes behind functional resilience
Hexafluoropropylene oxide dimer acid (HFPO-DA) is a common substitute for perfluoro and polyfluoroalkyl substances (PFAS), yet its ecological safety in wastewater treatment systems remains questionable. Anaerobic ammonium oxidation (Anammox) is an efficient nitrogen removal technology with broad application potential, however, its response to HFPO-DA under low-temperature conditions remains unclear. This study evaluated the effects of HFPO-DA (0, 1, and 10 mg/L) on the anammox system through long-term exposure experiments. Results indicated that HFPO-DA was less amenable to removal than PFOS. The nitrogen removal efficiency remained at 86.36%, while microorganisms dominated by Pseudomonadota alleviated acute toxicity through extracellular polymeric substance (EPS) secretion, which increased by 111.6% at 1 mg/L HFPO-DA. Microbial community and network analyses demonstrated that Candidatus_Kuenenia gradually adapted to the dual stress of HFPO-DA and low temperature. However, 10 mg/L of HFPO-DA significantly weakened overall network connectivity (decreased by 30.14%), with Pseudomonadota (accounting for 45.42% of nodes) dominating local microbial interactions during the stress process. Meanwhile, the system depended on the succession of various bacterial groups, including Pseudomonadota, Chloroflexi, and Bacteroidota, to counteract the interference caused by HFPO-DA. This study reveals the impact of HFPO-DA on the Anammox system under low-temperature conditions and raises concerns about its safety as a 'green' PFAS alternative.