Enhanced Polylactic Acid (PLA) Degradation under Anaerobic Digestion by Microbial Acclimation: the Effect of Thickness.

The increasing use of PLA bioplastics in single-use products has raised concerns regarding their fate in organic waste treatment systems. Although anaerobic digestion (AD) represents an environmentally favorable end-of-life option due to energy recovery and low environmental impact, PLA degradation under anaerobic conditions is often incomplete for thicker products. This study investigated the effect of microbial acclimation on the degradation and kinetics of commercial PLA products with different thicknesses (50, 100, 450, and 750 um) under thermophilic AD conditions. Two sequential batch AD runs were conducted while maintaining a constant initial surface area across treatments to isolate the effect of thickness. In the non-acclimated run, biogas production decreased linearly with increasing thickness, confirming thickness as a key limiting factor for degradation. Microbial acclimation in the second run significantly enhanced degradation, increasing biogas production rates and reducing the time required to reach 90% of total biogas yield, particularly for thinner PLA products, which became fully compatible with hydraulic retention times of full-scale plants. For thicker PLA products, acclimation promoted surface erosion and fragmentation, resulting in substantial biogas production (up to 45% of theoretical yield within 45 days), although complete degradation was not achieved. Microbial analyses revealed shifts toward syntrophic fermentative bacteria and hydrogenotrophic methanogens associated with PLA-derived metabolite utilization. Overall, microbial acclimation proved to be an effective, low-cost strategy to enhance PLA degradation in thermophilic AD. The results support the integration of AD with post-treatment composting to manage thicker PLA products and enable efficient bioenergy recovery within a circular economy framework.