Metagenomics Reveals the Particle Size Effects of Polyethylene Terephthalate Microplastics on Anaerobic Digestion

Polyethylene terephthalate microplastics (PET-MPs) are among the most prevalent microplastics in activated sludge; however, the mechanism underlying its impact on anaerobic digestion (AD) remain unclear. Through batch experiments and simulated substrate experiments, this study found that PET-MPs exhibited particle size-dependent effects across the various phases of AD. Results indicated that 50 μm PET-MPs particles enhanced methane production, whereas 200 μm particles inhibited the process. Specifically, 50 μm PET-MPs primarily accelerated the acidogenesis and methanogenesis phases, while 200 μm PET-MPs predominantly affected the dissolution and hydrolysis phases. Scanning electron microscopy (SEM) coupled with metagenomic analysis revealed that the surface of PET-MPs became roughened during AD, leading to an increased specific surface area. This morphological change selectively enriched hydrogenotrophic methanogens such as Candidatus_Methanoculleus_thermohydrogenotrophicum and activated their corresponding metabolic pathways, including mthB and mcrA. Mechanistic analysis indicated that PET-MPs enhance methane production by (i) providing a biointerface for hydrogenotrophic methanogen colonization, (ii) promoting protein-derived substrate fluxes that elevate H2 and acetate availability, and (iii) enabling coactivation of acetoclastic and hydrogenotrophic pathways. This study provides new insights into microplastic removal and the enhancement of sludge resource utilization.