Microplastics divert carbon flow in anaerobic digestion: a meta-analysis reveals product-specific effects

Microplastics (MPs) are pervasive contaminants in organic waste streams destined for anaerobic digestion (AD), yet their net effect on resource recovery remains a contentious issue with contradictory findings. To resolve this ambiguity, we conducted a robust three-level random-effects meta-analysis, synthesizing data from 55 studies, for disentangling the product-specific impacts of MPs on AD performance. Our analysis revealed that MPs shift carbon flow within AD: they suppress methanogenesis (-6.29% methane yield) while promoting acidogenesis (+23.73% total VFAs), with propionic, butyric, and valeric acids increasing by 12.74%, 14.55%, and 22.18%, respectively. This demonstrates that MPs do not simply inhibit AD but rather redirect its carbon flow, and this carbon rerouting is determined by polymer type. The effects of MPs on AD process could be conceptualized as novel hypothetical "Toxic Stress vs. Substrate Perturbation" models. Conventional polymers (e.g., polyvinyl chloride, polyethylene terephthalate) exert toxic stress, inhibiting methanogens and causing upstream VFA accumulation. Conversely, biodegradable polymers (e.g., polylactic acid, polybutylene adipate terephthalate) act as slow-release carbon sources, perturbing substrate availability to enhance methanogenesis. Furthermore, these effects are strongly modulated by MP concentration, size, and operational conditions like temperature, which can even reverse the inhibitory effects. This study reframes MPs from being simple inhibitors to active biogeochemical agents within engineered ecosystems. By clarifying MPs' product-specific effects, this study provides theoretical foundation of MPs affecting AD process, improving resilience and efficiency of AD-based resource recovery.