Hydrothermal pre-treatments can make PLA and PBS bioplastics suitable for anaerobic digestion

Bioplastics production is constantly growing, and so is the bioplastic waste that is discarded together with the organic fraction of municipal solid waste (OFMSW). The optimal treatment for this waste category foresees an anaerobic digestion (AD) treatment followed by composting. In this scheme bioplastics, even if certified as compostable, do not degrade satisfactorily. They are found almost unaltered in the compost, lowering its quality, or are rather separated at the entrance of OFMSW treatment plants and sent to landfills. In this study, we assessed whether bioplastics degradation rate and biogas yield in AD can be enhanced by subjecting them to a mild hydrothermal pre-treatment. Three different disposable cutleries based on poly(lactic acid) (PLA), poly(butylene succinate) (PBS) and cellulose were subjected to a hydrothermal treatment for 1 h at 160, 180 and 200 °C. Then, they were fed to mesophilic and thermophilic biochemical methane potential (BMP) tests to evaluate the performance enhancement. At 180 and 200 °C, the treatment brings a nearly total hydrolysis of PLA and PBS, while its effect on cellulose is minor. In AD, the hydrolysed bioplastics show remarkably higher biogas yields and production rates, with very high degradation efficiencies, while the improvements for cellulose are more inoculum-dependent. This pre-treatment may hence tackle the management issues of bioplastics in OFMSW treatment plants, avoiding their separation and discard.