Coexistence of microplastics alters the inhibitory effect of antibiotics on sludge anaerobic digestion

Microplastics and antibiotics are emerging pollutants that inevitably enter the sludge anaerobic digestion system. Current studies mainly focused on the individual effects of microplastics or antibiotics on methane generation. However, there may also be complex reactions between them and their combine effects on anaerobic digestion remain unclear. This study revealed the combined influencing mechanism of polyamide and ofloxacin through batch and semi-continuous tests. Brief exposure to ofloxacin was detrimental to methane production. The coexistence of polyamide enabled partial adsorption of ofloxacin, which effectively mitigates the inhibitory effect of ofloxacin at low concentrations. Polyamide also facilitated the colonization of hydrolytic bacteria and acidogens, such as Hydrogenispora, and promoted the conversion of organic matter to acetate. However, polyamide could not effectively alleviate the inhibitory effect of excess ofloxacin on anaerobic digestion. Ofloxacin at high concentrations stimulated the release of humic acid, reduced methane production by more than 80%, and changed the predominant methanogenesis pathway from acetoclastic methanogenesis to hydrogenotrophic methanogenesis. Acidogenesis and methanogenesis pathways were restructured to further explore the effect of ofloxacin on the metabolic potentials of microorganisms. Metagenomic analysis revealed that most of functional genes associated with glycolysis, amino acid metabolism and methanogenesis were down regulated under long-term stress of high concentration of ofloxacin. This study elucidated the microbial process of organic matter conversion inhibition by ofloxacin, shed light on the combined effects of polyamide and ofloxacin on anaerobic digestion and provided new insights for assessing the impact of multiple pollutants on the safe disposal of sludge.