Biorefining of Thermoplastic Starch via Depolymerization and Methane Arrested Anaerobic Digestion

In the circular plastics economy, biodegradable plastic waste streams become a resource to be recycled with thoughtful integration of physiochemical and microbial processes. Anaerobic digestion as well as the carboxylate platform provide opportunities to convert complex biomass, including biodegradable plastic waste. Here, we study how a commercial thermoplastic starch (TPS) product, which typically displays poor digestibility into biogas, can be biorefined into various chemicals. First, abiotic depolymerization was studied under mesophilic (35 °C) and thermophilic conditions (55 and 70 °C) over 56 days. The results showed accelerated hydrolysis and microplastic formation at higher temperatures, impacting the TPS morphology and disintegration process. TPS material characterization revealed the presence of PBAT (polybutylene adipate-co-terephthalate) and PLA (polylactic acid) as copolymers. The highest hydrolysis efficiency was 36.3%, with glucose, lactic acid (LA), terephthalic acid (TPA), adipic acid (AA), and 1,4-butanediol (1,4-BDO) identified. Besides abiotic treatment, methane-arrested anaerobic digestion of solid TPS and/or hydrolysates was studied within 14 days. Hereby, up to 23.1% of the provided materials was converted into volatile fatty acids. Consumption of glucose and lactate suggests that anaerobic biological conversion including microbial chain elongation occurred, while 1,4-BDO, AA, and TPA were unconverted. With these findings, a biorefinery concept was developed to recover chemicals from TPS-containing waste streams.