Comprehensive meta-analysis reveals the impact of non-biodegradable plastic pollution on methane production in anaerobic digestion

• Toxicity of plastics to methane production was reviewed through a meta -analysis. • The impact of plastics was dependent on the particle size, type, and concentration. • Nanoplastic had a higher impact on the physicochemical parameters than microplastic. • Plastics affected reactive oxygen species production and organic matter dissolution. Plastics as emerging contaminants have been heavily accumulated in organic wastes (e.g. waste activated sludge and food waste), which have a dramatically different impact on the resource recovery from these organic wastes through anaerobic digestion. However, the reported studies differ significantly from each other, and a comprehensive analysis to reveal the complex effects of plastic pollution in organic wastes on methane production is still lacking. In this study, 28 articles were selected from three citation databases for meta -analysis according to the preferred reporting items for systematic reviews and meta -analyses guidelines. Subgroup analysis was then performed to determine the effects of plastic type, particle size, and concentration on methane production and multiple physicochemical parameters. The meta -analysis showed that the mean effect size of plastic pollution on methane production was 0.93 [0.89, 0.98] ( p < 0.05). The results also revealed that the presence of plastics negatively affected the organic content and enzyme activity, as well as increasing the reactive oxygen species. In addition, the effect of nanoplastics on these physicochemical parameters was more significant compared to microplastics, like highlighted by most studies. Finally, structural equation modelling quantified that plastic pollution affected methane production by two main pathways: inhibition of organic solubilisation and induction of reactive oxygen species. This information is helpful to a more profound understanding the underlying toxicity mechanisms of plastic pollution to methane production and provide guidance for future research.