Insights into the impact of polyethylene microplastics on methane recovery from wastewater via bioelectrochemical anaerobic digestion

Bioelectrochemical anaerobic digestion (BEAD) is a promising next-generation technology for simultaneous wastewater treatment and bioenergy recovery. While knowledge on the inhibitory effect of emerging pollutants, such as microplastics, on the conventional wastewater anaerobic digestion processes is increasing, the impact of microplastics on the BEAD process remains unknown. This study shows that methane production decreased by 30.71% when adding 10 mg/L polyethylene microplastics (PE-MP) to the BEAD systems. The morphology of anaerobic granular sludge, which was the biocatalysts in the BEAD, changed with microbes shedding and granule crack when PE-MP existed. Additionally, the presence of PE-MP shifted the microbial communities, leading to a lower diversity but higher richness and tight clustering. Moreover, fewer fermentative bacteria, acetogens, and hydrogenotrophic methanogens (BEAD enhanced) grew under PE-MP stress, suggesting that PE-MP had an inhibitory effect on the methanogenic pathways. Furthermore, the abundance of genes relevant to extracellular electron transfer (omcB and mtrC) and methanogens (hupL and mcrA) decreased. The electron transfer efficiency reduced with extracellular cytochrome c down and a lower electron transfer system activity. Finally, phylogenetic investigation of communities by reconstruction of unobserved states analysis predicted the decrease of key methanogenic enzymes, including EC 1.1.1.1 (Alcohol dehydrogenase), EC 1.2.99.5 (Formylmethanofuran dehydrogenase), and EC 2.8.4.1 (Coenzyme-B sulfoethylthiotransferase). Altogether, these results provide insight into the inhibition mechanism of microplastics in wastewater methane recovery and further optimisation of the BEAD process.