Digestibility and fate of biodegradable plastic mulch films in thermophilic anaerobic digestion

Recent developments in biodegradable mulches (BDMs) provide a sustainable and eco-friendly alternative to persistent polyethylene (PE) mulches, addressing plastic pollution and disposal challenges; however, BDM degradation requires an extended period for complete mineralization under natural in-situ conditions. This study investigates the potential of thermophilic anaerobic digestion (AD) as an alternative to the in-situ degradation of BDMs. Two commercially available BDMs were evaluated in batch AD, initial characterization, and post-AD analysis of digestate were carried out for remaining fragments and liquid. In the TGA, a major weight loss in both BDMs at 400 °C indicated a high polybutylene-adipate-co-terephthalate (PBAT) content (>70 %), along with biobased polymers. After 160 days of digestion, cumulative methane yields were 226 and 129 mL CH₄/g VS for BDM Samples 1 and 2, respectively. These values correspond to 36 % and 25% of their respective theoretical methane potentials (628 and 508 mL CH₄/g VS) due to the low biodegradability of PBAT. FTIR analysis of BDM fragments showed similar spectral features with some shifts and new peaks (OH group) due to hydrolysis. The biomethane potential (BMP) tests showed no accumulation of volatile fatty acids (VFA), but soluble chemical oxygen demand (sCOD) increased due to partial biodegradation. Further analysis detected different monomers of PBAT, such as 1,4-benzenedicarboxylic acid, in the liquid phase of the digestate. This suggests that while most of the particles disintegrated (>98 % size reduction), the thermophilic anaerobic microorganisms were not able to mineralize BDMs completely, thus requiring further treatment.