Case study: enhancing methane production and polylactic acid decomposition in mesophilic anaerobic digestion system with hydrogen enrichment

Anaerobic digestion under hydrogen (H₂)/carbon dioxide (CO₂)-versus nitrogen (N₂)-purged conditions was examined for the potential for biogas enhancement in the presence of polylactic acid. With or without polylactic acid, H₂/CO₂ purging demonstrated a 25% higher methane (CH₄) production, reaching approximately 160 NmL CH₄/g VS_add compared to N₂ purging. When H₂/CO₂ was purged with polylactic acid, there was reduced dominance of Spirochaetales, resulting in fewer intermediates that caused a similar amount of CH₄ yield. Despite similar CH₄ yield to conditions without polylactic acid, verification through Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) confirmed extensive polylactic acid decomposition. DSC revealed double melting peaks at 151.8 °C and 156.1 °C, indicating increased crystallinity and vigorous polylactic acid decomposition, particularly in the amorphous region, under H₂/CO₂ purging. This aligned with the decrease in the FTIR carbonyl index, visually confirmed using SEM. Metagenome sequencing highlighted the prevalence of hydrogenotrophic methanogens, Anaerolineales, Bacteroidales, and Thermoanaerobacterales under H₂/CO₂ purging, demonstrating higher polylactic acid degradation compared to N₂ purging conditions. This study revealed a potential for biogas upgrading with waste management of polylactic acid.