Impacts of norfloxacin on sewage sludge anaerobic digestion: Bioenergy generation and potential environmental risks

Norfloxacin is a broad-spectrum antibiotic which has been frequently detected in the sewage sludge. However, whether norfloxacin in sewage sludge would affect the anaerobic digestion for bioenergy generation and induce environmental risks remained unclear. Therefore, this study investigated the impact of norfloxacin on sewage sludge anaerobic digestion and evaluated potential environmental risks by exploring related antibiotic resistance genes (ARGs) and norfloxacin transformation pathways. The results showed that norfloxacin at environmental relevant concentration ranges would not significantly affect methane production (p < 0.05) during the sludge anaerobic digestion, as the sludge solubilization and hydrolysis is unaffected. However, the individual methanogenesis process was enhanced with the increase of Methanosaeta abundances. Meanwhile, the increase of norfloxacin concentration will not decrease the removal efficiencies of fluoroquinolones-family ARGs by the anaerobic digestion process. However, the norfloxacin concentration is found to be increased during the anaerobic digestion process, which could probably be due to the desorption from the sludge due to cell lysis. In addition, 13 norfloxacin transformation metabolites were identified with 5 potential norfloxacin degradation pathways being constructed. The metabolites of levofloxacin/ofloxacin could be generated through Eschweiler-Clarke reaction, which may be more toxic than norfloxacin. Thus, the results of this study suggested that although norfloxacin would not affect the bioenergy generation and ARG removal during sludge anaerobic digestion, measures should be taken to deal with the potential environmental risks caused by norfloxacin release and formation of more toxic metabolites.