Chemical pre-treatment for bioplastic anaerobic digestion

Abstract

The non-degradability of plastics and their extended use led to their accumulation in the environment, posing a risk to both ecosystems and human health while hindering the achievement of a circular economy. To address this issue, there is a growing interest in the production of biodegradable bioplastics, which favour their after-use disposal and circularisation. In Europe, the use of biodegradable poly(D,L-lactide) (PLA) in commercial packaging designed for single use has gained momentum due to growing consumer and industrial awareness, supported by initiatives like the European Green Deal. Despite being considered a promising solution, the introduction of bio-sourced materials in packaging generates a new waste stream that is challenging the existing composting plants and anaerobic digesters.

Hence, this thesis's primary focus is to optimize the bioplastic biodegradation process under anaerobic conditions, aiming to convert it into biomethane. To achieve efficient biodegradation within industrial timeframes, a thermo-alkaline pre-treatment is proposed as a viable solution for PLA and PHBV.

During the investigation of different thermo-alkaline conditions, it was observed that PLA shows a two-phase solubilization kinetics. The first phase involves a rapid increase in solubility, crystallinity and a decrease in pH during the initial 2 hours of pre-treatment. Thereafter, a second phase follows, characterized by lower rates for all these parameters, where pH reaches neutrality or slightly acidity. Towards the end of this second phase, a residual increase in crystallinity and solubility was observed.

In all pre-treatment cases, the final measured point in the kinetics showed that the remaining solid phase had a degree of polymerization below the biodegradation threshold of 10-20 kDa established in the literature. This could translate into good biodegradation potential once introduced into the anaerobic digester.