Antibiotic residues from human treatment and animal breeding are ultimately discharged to wastewater treatment plants (WWTPs) in the form of various wastewater. Engineered biological treatment systems are the mainstream treatment process of WWTPs. However, the antimicrobial activity produced by refractory antibiotics will seriously inhibit the biological activity and function of wastewater treatment plant, and further lead to the risk of transmission of antibiotic resistant bacteria (ARB) and genes (ARGs). Therefore, based on the noval concept of source control of ARGs in antibiotic-containing water, it is a promising way to eliminate the antibacterial activity of antibiotics by abiotic pretreatment technology before subsequent biological treatment to ensure the stability of wastewater treatment system and prevent the transmission risk of ARGs.
Compared with hydrolysis and electro-Fenton oxidation, UV photolysis was selected as an appropriate pretreatment technology for antibiotic-containing wastewater. It could decompose many kinds of antibiotics and was not inhibited by the coexisting organics in wastewater. The antibacterial activities of five kinds of antibiotics, which were eliminated with UV irradiation, exhibited a significantly positive correlation with their parent compound concentrations. The photodecomposition of the main functional groups in antibiotics contributed to the elimination of antibacterial activity. Defluorination was the main pathway to eliminate the antibacterial activity of antibiotics containing a fluorine substituent (e.g., florfenicol and ofloxacin), while the photoinduced opening of the β-lactam ring was the most efficient route to eliminate the antibacterial activity of β-lactam antibiotics (e.g. cefalexin, amoxicillin and ampicillin). These results demonstrated that UV photolysis could be adopted as an efficient and promising pretreatment strategy for the source control of antibiotic antibacterial activity by the decomposition of antibiotic functional groups before the biological treatment unit.
In order to verify the antibiotics without antibacterial activity will also lose the potential of inducing ARGs, a combined system for the treatment of florfenicol (FLO)-containing synthetic wastewater with different UV irradiation doses coupled with aerobic or anaerobic biological process was constructed. It was found that UV radiation pretreatment significantly diminished the diversity and abundance of detected ARGs in both aerobic and anaerobic bioreactors, especially the chloramphenicol ARGs closely related to FLO. Among them, after 2 hours or more of UV irradiation pretreatment, the relative abundance of floR in aerobic and anaerobic biological treatment decreased by two orders of magnitude, which was at the same level as the negative control group (10–3 copies/16S rRNA copies). Meanwhile, the relative abundance of floR under aerobic condition was generally higher than that under anaerobic condition, and the abundance of floR on plasmid DNA was higher than that on genomic DNA under the same condition. UV photolytic pretreatment also effectively reduced the relative abundances of intI1 on both genomic and plasmid DNA, keeping them at the level of blank control (< 1.0 10–1 copies/16S rRNA copies), which reduced the potential risk of horizontal transfer of floR. Moreover, UV irradiation pretreatment with a sufficient dose shaped the structure and composition of sludge microbial communities closer to those of the FLO-free groups. During this process, the relative abundances of Pseudomonas and Escherichia-Shigella working as the potential hosts of ARGs were significantly reduced with the increase of UV irradiation dose in aerobic and anaerobic bioreactors, respectively.
Considering the problems of long treatment time and low efficiency in the elimination of antibacterial activity by UV photolysis pretreatment, the dechlorination and defluorination performance of FLO photolysis process were improved by adding sulfite to introduce reducing free radicals. The correlation between antibacterial activity and defluorination rate of FLO was higher than that with the corresponding dechlorination rate. After 12 min UV/sulfite (UV/S) photolytic pretreatment, the relative abundance of floR was controlled at the level of blank control group (6.52 ± 0.60 10–4 copies /16S rRNA copies), which was equivalent to that of 20 min UV photolysis pretreatment. In addition, the conditions with excessive dissolved oxygen orpH > 8 were not conducive to the decomposition of FLO. The coexistence of humic acids above 5 mg L–1 significantly inhibited the decomposition of FLO, while other coexisting substrates (protein, glucose and HCO3–) did not affect the decomposition of target antibiotics. Moreover, the inhibitory effect of NO2– as eaq– and H• quehcher on the decomposition of FLO was significantly higher than that of NO3– as eaq– quencher at the same concentration, which indicated that there was a reductive dechlorination and defluorination process dominated by H• in UV/S photolytic system. Furthermore, UV/S system could also be used to decompose other chloramphenicols and fluoroquinolones. Compared with the UV/H2O2 and UV/peroxydisulfate systems in the same concentration of additives, the decomposition rate constant of FLO in UV/S system was the highest, and it was not affected by coexisting organic compounds.