Effect of Microplastics on Anaerobic Digestion Process with Rapidly Degradable Organic Matter

The increasing presence of microplastics (MPs) in wastewater sludge raises concerns about their potential interference with anaerobic digestion (AD), a key process for energy recovery and sludge stabilization. This study investigated the impact of three common MPs, polystyrene (PS), polyethylene terephthalate (PET), and high-density polyethylene (HDPE), on the anaerobic degradation of a synthetic, rapidly biodegradable substrate under controlled batch conditions with the biomass from an anaerobic digester as inoculum. Biogas production, intermediate metabolic parameters, and microbial community dynamics were comprehensively assessed. The results showed a moderate inhibition of methane yield in the presence of MPs, with HDPE causing the most significant reduction (up to 24%) in biogas generation. PS exhibited the lowest impact, independent of the concentration added (0.5 and 1.0 g·L−1). The microbial community structure demonstrated robustness, with Firmicutes and Bacteroidota maintaining dominance and methanogenic populations largely unaffected, except in the presence of HDPE. Raman spectroscopy indicated that none of the MPs underwent substantial structural degradation, but the subtle spectral shifts—particularly in PET—suggested the initial stages of physicochemical alteration. These findings offer new insights into the short-term resilience and adaptability of anaerobic microbiomes in the presence of MPs while revealing potential signals of process disruption.