Granulation and stability of microalgal–bacterial granular sludge under single and combined exposure to polycaprolactone microplastics and sulfamethoxazole

The increasing occurrence of microplastics and antibiotics in wastewater poses emerging challenges for biological treatment processes. Microalgal-bacterial granular sludge (MBGS) has emerged as an efficient algae-based biotechnology for wastewater treatment; however, its response to simultaneous exposure to these contaminants remains largely unexplored. Here, the effects of degradable polycaprolactone microplastics (PCL MPs) and sulfamethoxazole (SMX) were evaluated at 1 mg/L during granulation and long-term operation. PCL alone caused only minor changes, whereas SMX impaired granule stability and reactor performance. Under combined PCL + SMX exposure, the strongest deterioration was observed, including a 5.2% decrease in ammonium nitrogen (NH 4 + -N) removal and a 48.9% decrease in phosphate removal relative to the control. Effluent nitrite nitrogen (NO 2 − -N) and nitrate nitrogen (NO 3 − -N) concentrations increased to 8.9 and 31.6 mg/L, respectively, under combined stress. These changes were accompanied by lower microbial activities, with the specific nitrate reduction rate (SNRR) and specific phosphorus uptake rate (SPUR) decreasing to 31.6 mg N/g mixed liquor volatile suspended solids (MLVSS) and 10.3 mg P/g MLVSS, respectively. Extracellular polymeric substances (EPS) analysis further showed depletion of tightly bound proteins. Microbial analysis revealed community restructuring under SMX-containing conditions (R3 and R4), including enrichment of Nakamurella (up to 36.5% in R3) and Micropruina (up to 18.9%), alongside a decline in polyphosphate-accumulating genera such as Microlunatus . Overall, MBGS showed resilience to PCL alone, whereas SMX (especially in combination with PCL) induced significant structural and functional instability.