An emerging threat to anaerobic digestion: microplastic impacts on syntrophic volatile fatty acids metabolism in methanogenic systems

The presence of microplastics in anaerobic digestion (AD) substrates has emerged as a significant concern that cannot be overlooked. However, evaluating the methanogenic process remains challenging due to our incomplete understanding of how microplastics influence the syntrophic metabolism of volatile fatty acids (VFAs) during methanogenesis, particularly within unstable systems where VFA accumulate. This study investigates the impact of polypropylene microplastics (PP-MPs) on VFA metabolism in AD and assesses the AD performance during VFA accumulation. Our findings indicate that PP-MPs broke the syntrophic metabolism of VFA, resulting in increased propionate accumulation (from 0.086 to 2.320 g/L), reduced methane efficiency (a 38.7% decrease compared to controls), and prolonged periods of acidification (from 7.12 to 8.47 days). This is associated with increased apoptosis and altered community structure. Furthermore, PP-MPs impeded the transformation of inorganic substrates into VFAs by inhibiting homoacetogenesis and hindering acetogenesis, impacting acetoclastic methanogenesis rather than hydrogenotrophic pathway, through affecting key enzyme activities. This research provides a novel perspective on the role of microplastics in the AD utilizing PP as a case study. As the effects may vary among polymer types, further research is needed to clarify these differences.