Impact of different hydrodynamic flow regimes on antibiotic removal in anaerobic fixed-bed bioreactors: operational, ecotoxicological and microbiological aspects

Literature has reported antibiotic overuse and the spread of antibiotic-resistant bacteria, which require better wastewater treatment. The influence of reactor hydrodynamics on antibiotic elimination processes and related environmental hazards is yet inadequately comprehended. This study assessed the impact of different flow regimes— plug-flow (PF) and continuous-stirred (CS)—on the elimination of 12 antibiotics (five fluoroquinolones, six sulfonamides, and trimethoprim; 10.0 μg L −1 each) in anaerobic fixed-bed bioreactors (AnFBR). Analysis of residence time distribution indicated that the PF-AnFBR functioned under almost ideal conditions (Peclet number > 1000; dead volume < 2.5%), but the CS-AnFBR had significant non-idealities, characterized by substantial dead zones (54–72%) and hydraulic short-circuiting. Despite both systems attaining elevated COD removal rates (>94%), the patterns of antibiotic removal varied considerably. Plug-flow settings promoted biodegradation-dominated elimination of sulfonamides and trimethoprim (47–100%), while the continuous-stirred arrangement improved the removal of strongly sorbing fluoroquinolones, primarily through adsorption mechanisms. Ecotoxicity assays showed reduced toxicity to Chironomus sancticaroli after treatment, but increased toxicity was observed for Allonais inaequalis and Ceriodaphnia silvestrii , suggesting the formation of toxic transformation products. The microbial analysis revealed a diverse population in both bioreactors, with key genera such as Aeromonas , Pseudomonas , and Methanothrix cleaving aromatic compounds, playing significant roles in antibiotic biodegradation, and stability in the performance of anaerobic bioreactors. These findings indicate that the hydrodynamic regime is a crucial design component influencing antibiotic behavior and environmental efficiency in anaerobic wastewater treatment.