Evaluation the impact of polystyrene micro and nanoplastics on the methane generation by anaerobic digestion

The widespread existence of microplastics in wastewater has caused great concern. As the exposure time of microplastics in the environment increases, the microplastic leaching solution (i.e.,chemical additives) may be released into the environment causing toxic effects. In this study, the effect of polystyrene (PS) microplastics on the anaerobic digestion system was investigated. The results showed that the exposure to 80 nm and 5 μm polystyrene microplastics with the concentrations of 0.2 g/L or lower did not significantly affect the cumulative methane production (P ≥ 0.05). On the other hand, 80 nm and 5 μm PS microplastic level of 0.25 g/L led to a decrease in methane production by 19.3% (P = 2 × 10^-5) and 17.9% (P = 4 × 10^-5), respectively. The 80 nm PS nanoplastics therefore had slightly higher inhibition capacity on methane production than 5 μm PS microplastics. The pH of all groups remained stable at 6.7-7.5. Volatile fatty acids (VFAs) concentration and ammonium-nitrogen concentration had no obvious relationship to PS micro and nanoplastics addition. Further investigation showed that PS micro and nanoplastics concentration of 0.25 g/L or higher could inhibit acidification and methanation stage of anaerobic digestion. However, the main negative influence of PS micro and nanoplastics on methane production was due to the severe inhibition on the methanization stage.