Long-term effect of ZnO nanoparticles on waste activated sludge anaerobic digestion

The increasing use of zinc oxide nanoparticles (ZnO NPs) raises concerns about their environmental impacts, but the potential effect of ZnO NPs on sludge anaerobic digestion remains unknown. In this paper, long-term exposure experiments were carried out to investigate the influence of ZnO NPs on methane production during waste activated sludge (WAS) anaerobic digestion. The presence of 1 mg/g-TSS of ZnO NPs did not affect methane production, but 30 and 150 mg/g-TSS of ZnO NPs induced 18.3% and 75.1% of inhibition respectively, which showed that the impact of ZnO NPs on methane production was dosage dependant. Then, the mechanisms of ZnO NPs affecting sludge anaerobic digestion were investigated. It was found that the toxic effect of ZnO NPs on methane production was mainly due to the release of Zn2+ from ZnO NPs, which may cause the inhibitory effects on the hydrolysis and methanation steps of sludge anaerobic digestion. Further investigations with enzyme and fluorescence in situ hybridization (FISH) assays indicated that higher concentration of ZnO NPs decreased the activities of protease and coenzyme F420, and the abundance of methanogenesis Archaea.

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► The impact of ZnO nanoparticles (NPs) on sludge digestion has never been reported.
► The released Zn2+ was responsible for the toxicity of ZnO NPs.
► The increase of reactive oxygen species was responsible for the toxicity of ZnO NPs.
► The steps of hydrolysis and methanation of sludge digestion were inhibited.
► ZnO NPs decreased the activities of protease and F420, the abundance of archaea.