Resilience of anaerobic digestion to polypropylene microplastic contamination: Kinetic and structural evidence.

The increasing occurrence of microplastics (MPs) in organic waste streams raises concerns about their impact on anaerobic digestion (AD). This study examined the effect of polypropylene MPs (PP-MPs) on methane production during AD of food waste for 150 days under batch conditions. PP-MPs were added at 10-300 mg g-1 total solids (TS), covering reported MP levels in food waste and food packaging materials and extending to worst-case scenarios. Methane yields ranged from 310.2 to 324.4 mL CH4 g-1 volatile solids (VS) across treatments versus 334.3 ± 5.2 mL CH4 g-1 VS in the control, with no significant differences (p = 0.634). Kinetic modeling confirmed no consistent inhibitory trends. FTIR and SEM analyses indicated minor surface oxidation and cracking on PP-MPs, while the polymer backbone remained intact, suggesting only superficial aging. These results provide critical assurance for waste-to-energy facilities processing plastic-contaminated organic waste streams. Although PP-MPs do not impair AD performance, their persistence and potential fragmentation pose environmental risks. These findings provide critical insight into the resilience of AD systems and emphasize the need for strategies to mitigate secondary MP formation in biogas production from contaminated waste streams.