Role of thickness in enhanced polylactic acid (PLA) degradation under anaerobic digestion by microbial acclimation

The increasing use of Polylactic Acid (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, PLA degradation of thicker products under anaerobic conditions is often incomplete. This study investigated the effect of microbial acclimation on the degradation of commercial PLA products with different thicknesses (50, 100, 450, and 750 μm) 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. Run 2 showed enhanced degradation relative to the non-acclimated run, with higher biogas-production rates and shorter t90 values. This improvement was consistent with an effect of previous microbial exposure to PLA, although the contributions of inoculum mixing and carried-over material could not be separated from microbial acclimation. For the thinner PLA products, acclimation reduced the time required to achieve 90% of the total biogas yield to 42–45 days, approaching the upper range of the hydraulic retention times (HRTs) typically applied in full-scale AD plants. For the thicker PLA products, the improved performance observed in Run 2 was associated with visible fragmentation and biogas production corresponding to approximately 45% of the theoretical yield within 45 days, although complete degradation was not achieved. Microbial analyses revealed shifts in the relative abundance of taxa previously associated with syntrophic fermentative bacteria and hydrogenotrophic methanogens. These taxonomic patterns were consistent with the potential utilization of PLAderived intermediates, although microbial activity and direct involvement in PLA degradation were not assessed. Overall, results indicate that microbial acclimation may represent an effective, low-cost strategy to enhance PLA degradation in thermophilic AD, although other inoculum-related effects may also have contributed to the improved performance observed in Run 2. The results suggest the integration of AD with post-treatment composting to manage thicker PLA products and enable efficient bioenergy recovery within a circular economy framework.