Granulation and stability of microalgal-bacterial granular sludge under single and combined exposure to...

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 (NH₄⁺-N) removal and a 48.9% decrease in phosphate removal relative to the control. Effluent nitrite nitrogen (NO₂^--N) and nitrate nitrogen (NO₃^--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.