Nonmonotonic effect of CuO nanoparticles on medium-chain carboxylates production from waste activated sludge

•CuO NPs displayed dosage-dependent enhancement-to-inhibition effect on MCCs production.•CuO NPs at 10–50 mg/g-TS inhibited MCCs production, and ROS was the key factor.•CuO NPs at environmentally relevant concentrations stimulated MCCs production.•Metaproteomic and molecular analyses were combined to reveals the changes of pathways.•Low CuO NPs doses stimulated both WAS solubilization and transport/metabolism of carbohydrate. The growing applications of CuO nanoparticles (NPs) in industrial and agriculture has increased their concentrations in wastewater and subsequently accumulated in waste activated sludge (WAS), raising concerns about their impact on reutilization of WAS, especially on the medium-chain carboxylates (MCCs) production from anaerobic fermentation of WAS. Here we showed that CuO NPs at 10–50 mg/g-TS can significantly inhibit MCCs production, and reactive oxygen species generation was revealed to be the key factor linked to the phenomena. At lower CuO NPs concentrations (0.5–2.5 mg/g-TS), however, MCCs production was enhanced, with a maximum level of 37% compared to the control. The combination of molecular approaches and metaproteomic analysis revealed that although low dosage CuO NPs (2.5 mg/g-TS) weakly inhibited chain elongation process, they displayed contributive characteristics both in WAS solubilization and transport/metabolism of carbohydrate. These results demonstrated that the complex microbial processes for MCCs production in the anaerobic fermentation of WAS can be affected by CuO NPs in a dosage-dependent manner via regulating microbial protein expression level. Our findings can provide new insights into the influence of CuO NPs on anaerobic fermentation process and shed light on the treatment option for the resource utilization of CuO NPs polluted WAS. [Display omitted]