Advanced nitrogen removal from secondary effluent using the SAD/A process under the toxicity of PFOA

In response to increasingly stringent wastewater discharge standards, the need for advanced nitrogen removal from secondary effluent has become urgent. The sulfur autotrophic denitrification (SAD) coupled with anaerobic ammonium oxidation (Anammox) process (SAD/A), holds significant promise for this application due to its advantage of requiring no external organic carbon. However, its resilience to emerging contaminants, such as per- and polyfluoroalkyl substances (PFAS), which may persist in secondary effluent, remains unknown. This study investigated the long-term impact of perfluorooctanoic acid (PFOA) on a SAD/A system treating synthetic secondary effluent. Results indicated that exposure to 1 μg L^-1 PFOA inhibited the SAD pathway, reducing the total nitrogen removal efficiency (TNRE) to 49.68%. This inhibition was alleviated after replacing of the sulfur packing material. In contrast, exposure to a higher concentration of 1 mg L^-1 PFOA caused stronger and irreversible inhibition of the SAD pathway, even after sulfur replacement. Microbial community analysis revealed a significant decrease in the abundance of key sulfur-oxidizing bacteria, such as Thiobacillus, under PFOA stress. Furthermore, PFOA was transformed within the SAD/A system, generating shorter-chain products including perfluorohexanoic acid (PFHxA). Toxicity assessment suggests that some transformation products may pose higher ecological risks than the parent compound, highlighting the need to consider the potential hazards associated with contaminant transformation in such treatment systems.