Emerging Directions in Sequential Hydrothermal Liquefaction and Anaerobic Digestion: Advancing Resource Recovery from Diverse Sludge Streams

This study investigates the sequential integration of hydrothermal liquefaction (HTL) and anaerobic digestion (AD) as a strategy for resource recovery from municipal wastewater (MW) and semiconductor packaging wastewater (SPW) sludges. The primary objective is to determine the influence of HTL pretreatment on conversion efficiencies, water quality metrics, and subsequent anaerobic biodegradability. Specifically, the research focuses on biogas generation, COD removal, and the potential to promote circular resource utilization. HTL was conducted under controlled temperature (150–374 °C) and pressure (10–25 MPa) conditions, followed by batch AD at 41 °C using hydrogen- and methane-producing inocula at various ratios, specifically 20%, 50%, and 80%. Key variables, including total solids (TS), suspended solids (SS), chemical oxygen demand (COD), pH, and electrical conductivity (EC), were monitored to assess degradation efficiency and resource recovery. Additionally, chemical modifications in HTL-processed sludge were characterized using Fourier Transform Infrared Spectroscopy (FTIR). Results indicate that MW sludge achieved significant reductions in TS (65.3%) and enhanced biogas production of 156.7 mL/g VS at 80% inoculum. These figures reflect high biodegradability and compatibility with AD. In contrast, SPW sludge demonstrated limited COD removal (26.6–85%) and lower biogas yields of 154.0 mL/g VS. These results are likely due to elevated salinity and compositional complexity. These findings suggest that while HTL pretreatment significantly improves MW sludge utilization, SPW sludge may require additional or alternative treatment strategies. Overall, this study clarifies key factors influencing the performance of integrated HTL-AD systems across distinct sludge types and lays a foundation for the further development of sustainable sludge management processes.