IMPROVING THE CIRCULARITY OF BIODEGRADABLE BIOPLASTICS BY PRODUCING BIOGAS: A FULL-SCALE ASSESSMENT

Abstract Polylactic acid (PLA) is one of the commonly used bioplastics, promoted as a sustainable alternative to conventional plastics. Their anaerobic co-digestion with the organic fractions of municipal solid waste (OFMSW) is a promising end-of-life scenario, as it reduces pre-treatment and increases process efficiency and biogas production. However, PLA shows poor biodegradability under anaerobic digestion (AD) conditions, requiring long retention times (HRTs) compared to industrial OFMSW plants’ HRTs. This limits their integration into biogas plants and their potential for integration in the circular bioeconomy. To overcome HRT's limitations of PLA, Microbial acclimation has emerged as an eco-friendly strategy to enhance bioplastic degradation; however, its potential to degrade PLA in co-digestion with OFMSW remains unexplored. Moreover, PLA commercial products are present in the waste stream with varying thicknesses and shapes due to their diverse applications; however, the impact of product thickness on the anaerobic degradation performance of these products, when the same total surface area is maintained, is poorly understood. In this regard, this thesis addresses these gaps by investigating, in the first study, microbial acclimation as a strategy to enhance PLA biodegradability and biogas production under mono- and co-digestion, as well as the microbial communities enriched during PLA acclimation in different settings. Moreover, in the second study, the role of product thickness on their anaerobic degradation performance was studied. The first study results demonstrate that microbial acclimation significantly improves PLA biogas yield in mono-digestion by up to 152% (831 ± 11 NL kgVS⁻¹) and biogas production rate from 27 to 47 NL kgVS⁻¹ d⁻¹, with a reduction in the lag phase by 5 days. This enhancement was linked to the enrichment of the PLA-degrading bacteria Tepidanaerobacter. While in PLA + OFMSW co-digestion, biogas production increased by 69% (827 ± 69 NL kgVS⁻¹), the biogas production rate increased to 58 NL kgVS⁻¹ d⁻¹, accompanied by a 7-day reduction in the lag phase. In the second study, the effect of PLA product thickness on their anaerobic biodegradation performance was investigated using four commercial products (Cup50, Cup100, Spoon450, Fork750) while maintaining the same total surface area. The findings showed that, in run1, cumulative biogas production differed significantly (p ≤ 0.05) between all PLA products of varying thickness. The thinnest product, Cup50, produced the highest biogas yield (836 ± 81 NL kgVS⁻¹), which decreased with PLA thickness: Cup100 (731 ± 25 NL kgVS⁻¹ >Spoon450 (633 ± 23 NL kgVS⁻¹) > Fork750 (470 ± 15 NL kgVS⁻¹). after acclimation (run2), cumulative biogas production was similar among the products (p > 0.05). Nevertheless, the biogas production rate increased from run1 to run2. Kinetic analysis (modified Gompertz) showed that microbial acclimatation increased the degradation rate, resulting in a reduction in the time needed to produce 90% of the total producible biogas (t₉₀) from 108 to 42 days for Cup50 and from 98 to 45 days for Cup100, with these retention times being comparable to those that operate at full-scale plant. However, thicker products showed a different kinetic behavior (zero-order) after acclimation, with t₉₀ reduced from 112 to 92 days in Spoon450 and Fork750 from 146 to 86 days, which still exceeding the full-scale plant HRT. However, microbial acclimation recovered 45% of their biogas potential within 45 days. Additionally, incomplete PLA decomposition suggests that AD and composting can provide free bioplastic digestate. Moreover, combining microbial acclimation strategies with insights into product thickness paves the way for more efficient and sustainable PLA management and renewable energy recovery in the circular bioeconomy.