In-situ removal of residual antibiotics (enrofloxacin) in recirculating aquaculture system: Effect of ultraviolet photolysis plus biodegradation using immobilized microbial granules

Abstract

The misuse of antibiotics and ineffective treatment in aquaculture cause serious environmental problems. The closed-loop design of recirculating aquaculture systems (RAS) enables the effective use of antibiotics and rapid removal of antibiotic residues. Hitherto, few studies have been carried out on these. This study aims to examine the feasibility of rapid in-situ removal of residual antibiotics in RAS using integrated process and characterize the operational parameters. The fate of the selected antibiotic enrofloxacin (ENR) in different units of a laboratory-scale RAS was investigated with photolysis pretreatment followed adsorption-biodegradation of photolysis intermediates by immobilized microgranules. The degradation kinetics was also characterized. The results indicated that the ultraviolet (UV) device in RAS played an important role in ENR removal. The wavelength and light intensity were crucial for the removal efficiency of ENR. The F- produced by defluorination could reproduce the effect of the decomposition of ENR by UV photolysis. The UV photolysis products of ENR had a positive relationship with photolysis time, indicating incomplete decomposition of ENR. Under optimal condition (UV 80 W/254 nm and flow rate of 60 L/h), the ratio of BOD5 to COD (B/C) after photolysis increased from 0.041 to 0.28, which supported biodegradation. The photolysis products were partially adsorbed by activated carbons in the immobilized microbial granules and then biodegraded by microbes of the granules. Based on this study, a four-step process was proposed for the control of residual antibiotics in RAS.