Dissecting the effects of co-exposure to microplastics and sulfamethoxazole on anaerobic digestion

Microplastics (MPs) and antibiotics are frequently and simultaneously detected in sewage and sludge, raising global concerns in recent years. However, their combined effects on anaerobic digestion (AD) remain unclear. Herein, we evaluated the effects of the combinations of different MPs (i.e., polyethylene, polystyrene, polyvinyl chloride and polyethylene terephthalate) with sulfamethoxazole (SMX) on AD performance and microbial communities. The combined stress slightly decreased the chemical oxygen demand removal rate and total gas/methane production. Furthermore, co-exposure to MPs and SMX visibly changed the anaerobic sludge morphology during AD, reduced the methanogen activity, and increased the residual propionic acid concentration versus a control. The decreased relative abundances of Euryarchaeota ranged from 1.88% to 4.63% in the experimental groups compared with CK, suggesting that the microbial communities were inevitably affected by exposure to SMX alone or combined MPs/SMX. Interestingly, among the top 50 genera, only two were negatively related to a few antibiotic resistance genes, implying that sludge exhibited widespread multiple resistances. The correlation analysis between the MPs and microbial communities suggested that the MP properties, such as the aperture-desorption of MPs, may impact the microbial variations. This study will contribute to a deeper understanding of the impact of coexisting MPs/SMX on AD.