Temperature and particle size influence on polylactic acid anaerobic biodegradation

The anaerobic biodegradability of polylactic acid (PLA) in four different particle sizes ranging from 125 to 4000 µm was compared under both mesophilic and thermophilic conditions using biomethane potential tests. Under thermophilic conditions, the final methane yield was 489 ± 22 L CH₄ kg -1 VS, regardless of particle size. Under mesophilic conditions, the methane yield of the powder (125 - 750 µm) reached 464 L CH₄ kg -1 VS after 1400 days, while the methane yields for the other particle sizes had not yet converged and ranged between 306 and 368 L CH₄ kg -1 VS. The experimental results clearly showed that temperature has a greater influence on the rate of degradation than particle size. However, the smaller the particle size, the faster the degradation rate under both temperature conditions. The particle size of the crushed PLA pellets decreased from 3.3 to 2.8 mm after 60 days and to 1.7 mm after 90 days, whereas the percentage of crystallinity increased from 0.6% to 0.9% at 60 days and to 2.2% at 90 days. Infrared spectroscopy showed changes in the asymmetric vibrations of the ester group and the carbonyl stretching because of PLA degradation. Although PLA is inherently biodegradable, its slow degradation rate limits its conversion within the operational time frame of full-scale anaerobic digesters. • Temperature had a higher impact than particle size on degradation kinetics. • Thermophilic conditions showed faster degradation rates than mesophilic conditions. • Complete degradation of powdered PLA required approximately 1400 days at 35 °C. • Particle size did not affect the ultimate methane yield of PLA. • Ultimate methane yields were not affected by temperature.