Removal of residual antibiotics from aquaculture effluents by denitrifying woodchip bioreactors

The aquaculture industry lacks effective and economical treatment processes to reduce the risk of antibiotic residues in its effluents. In this study, the removal performance of end-of-pipe denitrifying woodchip bioreactors on four commonly used antibiotics was determined. Laboratory batch experiments were performed to investigate the abiotic adsorption capacity of woodchips, while laboratory continuous-flow tests and monitoring of full-scale woodchip bioreactors in the field were performed to investigate antibiotic removal by woodchips during treatment of aquaculture effluents. Adsorption of oxolinic acid was most effective, with the highest measured adsorption capacity of 21 μg g⁻¹ achieved at an equilibrium concentration of 15 μg L⁻¹ in the water. Binding of trimethoprim and sulfadiazine was well described by Langmuir isotherms with maximum adsorption capacities of 106 μg g⁻¹ and 27 μg g⁻¹ respectively. In contrast, florfenicol was poorly adsorbed by woodchips. The results from continuous-flow experiments demonstrated the removal efficiencies for trimethoprim, sulfadiazine, and oxolinic acid to be 89 %, 64 %, and 63 %, respectively, while florfenicol was not effectively removed.
During the monitoring of a full-scale bioreactor, removals of 77 % and 93 % were observed for trimethoprim and oxolinic acid, respectively. In contrast, the concentration of sulfadiazine increased by 141 % in the bioreactor due to hydrolysis/deconjugation of its metabolite acetyl sulfadiazine. Overall, this study demonstrates that end-of-pipe denitrifying woodchip bioreactors can act as safety barriers to the potential discharge of certain antibiotics from recirculated freshwater aquaculture systems, providing substantial reductions, particularly for oxolinic acid and trimethoprim, which are commonly used for disease treatment in aquaculture.