Enhanced performance of algal-bacterial granular sludge in treating wastewater under PFOA stress: Synergistic effects...

The ubiquitous distribution of perfluorooctanoic acid (PFOA) in wastewater poses significant challenges for wastewater treatment systems, yet its impact on granular sludge systems remains poorly understood. This study investigated the performance of aerobic granular sludge (AGS) and algal-bacterial granular sludge (ABGS) systems in terms of stability, nutrient and PFOA removal, enzyme activity, and microbial communities under PFOA stress. Both systems demonstrated tolerance and adsorptive removal of PFOA with enhanced total nitrogen (TN) and slightly decreased total phosphorus (TP) removals. Notably, ABGS showed superior performance with approximately 19 % and 80 % improved removal for TN and PFOA compared to AGS under 1 mg/L PFOA stress. Microbial community and metagenomic analyses revealed that the proliferation of nitrogen-removal bacteria (e.g., Nitrospirae increased by around 360 %) and the increased abundance of denitrification-related genes (including genes associated with electron donor pathways) boosted TN removal. Under PFOA stimulation, the synergistic interaction between bacteria and algae in the ABGS system secreted about 12 % more extracellular polymeric substances (EPS). The higher concentration and diverse composition of EPS offered abundant adsorption sites for PFOA while serving as a protective barrier that restricted its penetration into cells. This protective mechanism was further supported by the enhanced antioxidant defenses in ABGS as evidenced by the lower increase in reactive oxygen species and lactate dehydrogenase levels compared to AGS. These findings deepen our understanding of the stress responses and adaptive mechanisms of granular sludge systems under long-term per- and polyfluoroalkyl substance exposure and highlight the potential of ABGS in treating PFOA-laden wastewater.