Unveiling the impact of bioplastic bags on high-solids anaerobic digestion and subsequent hydrothermal liquefaction of source separated organics

The popularity of plastics stems from their versatility, but their extensive production and slow degradation contribute to environmental pollution. Bioplastics offer a promising alternative: these are made from renewable sources and are designed to break down naturally within a few years. With rapid urbanization and the increasing global population, there has been a substantial rise in both the generation of source-separated organics (SSO) and the utilization of bioplastics. However, bioplastics can be comingled with SSO in the waste management system, leading to concerns about their fate in high-solid anaerobic digestion (HSAD) digestate and their potential environmental consequences. This study investigated the co-digestion of bioplastics and SSO in an integrated process involving HSAD followed by hydrothermal liquefaction (HTL) under different operating conditions (280 °C, 330 °C, and 370 °C for 10 min). During the HSAD process, bioplastics broke down into smaller fragments, causing a significant drop in methane yield. After 45 days, the control reactor produced 182.39 ± 2 mL/g VS, while the bioplastic-loaded reactor yielded only 95.91 ± 1.7 mL/g VS. However, bioplastics were completely transformed by HTL, reducing the potential environmental risk of bioplastic pollution. The experimental observations demonstrated that as the temperature increased from 280 to 330 °C, the weight percentage of biocrude increased, reaching a maximum yield of 38 %. However, with a further temperature increase from 330 to 370°C, both the weight percentage of biocrude and hydrochar decreased, and there was a peak rise in the weight percentage of total gas. At 330 °C, the maximum higher heating value for bioplastics digestate and the highest crude production compared to the control were achieved. This study provides critical insights into the degradability and transformation of bioplastics and SSO in integrated HSAD and HTL processes. The findings advance our understanding of bioplastic behavior in waste management systems and demonstrate a sustainable pathway for mitigating bioplastic pollution while enhancing resource recovery. By integrating HSAD and HTL, this research offers a novel framework for bioplastic valorization and contributes to the development of circular economy strategies.