Thermoalkaline pretreatment as a key step to poly-lactic acid (PLA) biodegradation in anaerobic digesters

Among various end-of-life options for polylactic acid (PLA), anaerobic digestion (AD) has emerged as a promising strategy for its valorisation. However, PLA exhibits limited biodegradability under standard AD conditions (35 ºC, 20-25 days). This study aims to increase the biodegradability of PLA through various pretreatments, thereby facilitating its effective co-digestion with wastewater treatment plant (WWTP) sludge. With this aim, different pretreatments (physical, chemical, and enzymatic) were evaluated at laboratory scale to determine the most effective. This study provides a systematic evaluation of operational parameters and a comprehensive comparison of pretreatment methods for PLA to achieve its anaerobic degradation in WWTPs. Chemical pretreatment (alkaline hydrolysis with NaOH) exhibited the highest efficiency, achieving complete hydrolysis and 75–80% biodegradability in biomethane potential assays. Twelve chemical pretreatment experiments were conducted at various temperatures ranging from 30 to 90 ºC and NaOH concentrations from 0.3 M to 1 M. The hydrolysis rates increased with both temperature and NaOH concentration. Alkaline hydrolysis with NaOH depolymerised PLA into lactate monomers, which are readily biodegradable under AD conditions. This process allows PLA to be integrated into AD systems at WWTPs, promoting renewable energy generation. A novel first-order kinetics model was developed to describe the PLA hydrolysis considering temperature and NaOH concentration. Reaction rates rose with both, from 6 L·mg −1 ·h −1 at 30°C to 48 L·mg −1 ·h −1 at 90°C. The model reproduced the temporal evolution of soluble chemical oxygen demand, with predicted values closely matching experimental data (R² = 0.983), demonstrating its robustness in capturing PLA hydrolysis kinetics. • PLA biomethanisation requires prior hydrolysis. • NaOH pretreatment achieves complete hydrolysis and 75–80% biodegradability. • PLA is converted into lactate, enabling WWTP integration and methane production. • Hydrolysis rate rises with temperature and NaOH concentration. • Mathematical model accurately predicts PLA hydrolysis.