Improving mesophilic anaerobic digestion of compostable packaging through efficient and versatile thermo-alkaline pre-treatment

Abstract

In the current European regulatory context, which requires its Member States to collect biowaste separately with a view to its organic recovery, the co-processing of compostable food packaging with biowaste by mesophilic anaerobic digestion (AD) offers the dual opportunity of promoting its conversion into biogas while reducing plastic contamination of the environment. However, most packaging items on the market - particularly those made from poly(butylene adipate-co-terephthalate) (PBAT), polylactic acid (PLA) and polybutylene succinate (PBS) - show poor biodegradability under mesophilic AD conditions. To overcome this limitation, thermo-alkaline pre-treatments conditions were carried out to hydrolyze the most resistant polymers and improve their digestibility during AD. As PBAT is the most recalcitrant, the pre-treatment conditions were first optimized using pure PBAT, before extending the application to various compostable commercial items, including bags and coffee capsules. Optimal pre-treatment conditions (3 M KOH at 70 °C for 4 h) allowed for the effective solubilization of all the items tested. As a result, Biochemical Methane Potentials (BMP) were significantly increased with improvements ranging from 2 to 3 times for bags and 1.4 to 17 times for coffee capsules, even though they contained PBAT, PLA or PBS. By sufficiently reducing the molecular weight of recalcitrant polymers so that they can be assimilated by microorganisms, this pre-treatment has demonstrated its ability to enable their biodegradation in mesophilic AD. This versatile and universal pre-treatment approach offers a promising solution recovering biowaste with compostable packaging by converting them into methane while improving the quality of AD digestates.