Insights into the combined effect of ofloxacin and humic substances on sewage sludge anaerobic digestion.

Humic substances (HS) and antibiotics are present simultaneously in various environments. However, the influence path and consequences of HS on antibiotics behaviors in complex anaerobic microbial systems are rarely known, hindering the understanding and control of antibiotics risks. This study for the first time investigated the combined effects of ofloxacin (OFL) and HS in sewage sludge anaerobic digestion system. Experimental results showed that OFL alone reduced the cumulative methane production and the maximum methane production rate by 14.6 % and 33.5 %, respectively. The methane production curves showed step by step adaption, which might be related with the increase of antibiotics resistance genes and their potential hosts. The coexistence of low concentration (6 % of sludge volatile solid) HS could alleviate the inhibition of OFL on hydrolysis-related bacteria and genes to a certain extent, thereby enhanced the methane production by 4.8 %. However, the coexistence of high concentration (12-24 % of sludge volatile solid) HS intensified the inhibition on hydrolysis-related bacteria and genes, and had more potential to combine with organic matters to prevent sludge solubilization, macromolecular organics hydrolysis and OFL degradation, thereby further decreasing the methane production by 7.6-15.9 %. Besides, the coexistence of OFL and high concentration HS increased the antimicrobial resistance and pathogenicity risks of digested sludge, by enhancing the residual level of verified pathogens, antibiotics resistance genes and virulence factor genes. This study provides new insights into the environmental risks of combined antibiotics and HS pollution, and offers a basis for strengthening the safe treatment and disposal of sewage sludge.