Microbiome adaptation of aerobic granular sludge allows process resilience during the treatment of seawater-based wastewater with pharmaceuticals

Coastal wastewater treatment plants often face multiple stressors (e.g., pharmaceuticals and oscillating seawater levels) simultaneously, and their combined effects on biological treatment systems are still largely underestimated. In this study, an aerobic granular sludge (AGS) reactor was challenged over a four-month period with wastewater that had daily fluctuations in seawater content (7.5 to 22.5 g/L) and occasionally contained two pharmaceuticals, venlafaxine (VNF) and tramadol (TRA), and their metabolites ( O -desmethylvenlafaxine and O -desmethyltramadol), a combination that closely mimics real-world conditions. Over time, pharmaceuticals removal improved, especially for VNF and TRA. For VNF, monitored using an enantiomer-discriminating method, non-enantioselective removal was observed, indicating that the removal most probably occurred through adsorption. Despite the pharmaceuticals' presence in wastewater, the chemical oxygen demand removal was efficient (89 ± 3 %), and ammonium removal was complete, with nitrate as the main nitrification end-product. During the period of sporadic pharmaceuticals presence in wastewater, extracellular polymeric substances (EPS) content within AGS increased up to 196 ± 5 mg/g TSS, possibly contributing to the improvement of pharmaceuticals' adsorption and AGS functional stability. The AGS core microbiome comprised several functional taxa sustaining the system performance under stress exposure. The AGS bacteriome steadily adapted to the changes in wastewater composition, presenting distinct bacterial signatures in each stage. Paracoccus , an EPS-producing genus, was enriched during pharmaceuticals load, which may have been crucial for the system's stability. The adaptable and versatile microbiome of the AGS under multiple wastewater stressors contributed to the system's resilience, expanding its applicability for wastewater treatment in vulnerable areas. • AGS system effectively managed pharmaceuticals and seawater oscillations in wastewater • Pharmaceuticals and metabolites were most likely removed from wastewater through adsorption • The AGS system kept stable COD and ammonium removal irrespective of pharmaceutical shock loads • EPS production occurred due to long-term exposure to multiple stressors in wastewater • Shifts in the AGS microbiome were driven by variations in wastewater composition.