Systematic study of microplastics on methane production in anaerobic digestion: Performance and microbial response

Microplastics as a typical emerging pollutant has frequently been noted in wastewater treatment, yet its associated effect on anaerobic digestion have rarely been systemically investigated. In this study, the effects of different microplastics category, concentration and size on the performance of anaerobic digestion were evaluated under batch mode. The results determined that polyethylene microplastics had the pronounced negative effect on the anaerobic digestion and was selected as the model microplastic. Methane production decreased from 5.19 ± 0.03 to 4.20 ± 0.02 mmol as the polyethylene concentration increased from 0.002 particles/g TS to 0.2 particles/g TS. Meanwhile, when the size of polyethylene was reduced from 2 mm to 50 µm, the methane formation was reduced from 4.20 ± 0.02 to 2.62 ± 0.01 mmol. Moreover, microplastics can adversely affect extracellular polymers secretion, leading to loss of cytoskeletal structure and ultimately to cell lysis and death. The analysis of microbial community revealed that the addition of polyethylene microplastics increased the abundance of Firmicutes by 3.04%, while Bacteroidota and Chloroflexi decreased by 0.63% and 1.01%, respectively. In addition, microplastics reduced microbial activity and damaged cells leading to a reduction in the diversity of methanogens and thus affecting methane production. The results indicated that polyethylene microplastics had a significant effect on microorganisms abundance and species, probably because the toxic substances released by microplastics affected the metabolism of microorganisms, which in turn affected the anaerobic digestion process. And the results can be used as a theoretical reference for anaerobic digestion under continuous flow mode. The effect of microplastics on methane production in anaerobic digestion was systematic studied. PE microplastics significantly inhibited the methane production compared with other typical microplastics. Concentration and particle size of PE microplastics had different inhibition capacity on methane production. PE microplastics had an insignificant effect on the acidification of anaerobic digestion. PE microplastics inhibited the methanogenesis due to the reduction of methanogens diversity.