Hydrothermal Liquefaction of Sludge for Biocrude Production and Synergistic Utilization of Byproducts in Anaerobic Digestion

Hydrothermal liquefaction (HTL) is a promising thermochemical process for converting sludge into biocrude. However, the effective utilization of other HTL byproducts, such as hydrochar (HC) and an aqueous phase (HTL-AP), remains a significant challenge. This study investigates the synergistic utilization of HTL-AP and HC in anaerobic digestion (AD) for effective biogas production. HTL experiments were conducted at three temperatures (280 °C, 330 °C, and 370 °C). The higher HTL temperatures increased the biocrude yield while reducing HC and AP production. The methane production in AD was influenced by the HTL-AP composition and the presence of HC, with the highest cumulative methane production (327 ± 1.01 mL/g VS) observed for AP produced at 280 °C with HC addition. The addition of HC improved methane production by 25% at 280 °C, 29% at 330 °C, and 19% at 370 °C compared to reactors without HC. HC amendment increased the relative abundance of Firmicutes, Clostridium, and Methanobacterium, supporting improved microbial syntrophic interactions, particularly at 330 °C. The results further suggested that HC might also facilitate direct interspecies electron transfer (DIET), enhancing methane production by promoting electron exchange between syntrophic bacteria and methanogens. These findings highlight the potential of co-utilizing HTL byproducts in AD as a sustainable strategy for sludge management and bioenergy recovery.