Processing High-Solid Sludge Through Hydrothermal Liquefaction to Boost Anaerobic Fermentation and Bioresource Yield

The increasing need for effective sludge management has positioned hydrothermal liquefaction (HTL) as a viable solution, harnessing its capability to transform organic materials into renewable resources under elevated temperature and pressure conditions. This research seeks to assess the performance of HTL in processing high-solid organic sludge by examining the removal efficiencies of chemical oxygen demand (COD), total solids (TS), and suspended solids (SS), together with improvements in biogas potential (BGP) and hydrogen yield. Experimental procedures were carried out within a temperature range of 100–210 °C and pressure levels of 20–80 kg/cm2, using a hydrogen-producing microbiome (HMb) and anaerobically digested sludge as inoculants for anaerobic fermentation. Multivariate analysis was applied to investigate the influence of temperature and pressure on COD, TS, and SS removal rates as well as BGP, while a series of batch tests further confirmed the effects of these parameters on fermentation outcomes. Findings revealed that COD, SS, and TS removal efficiencies reached 90.6%, 91.5%, and 87.4%, respectively, under conditions of 100 °C and 60 kg/cm2. The maximum biogas potential (BGP) of approximately 500 mL was attained at 180 °C, whereas hydrogen production demonstrated substantial enhancement within the HTL pressure range of 40–60 kg/cm2, decreasing beyond this range. Additionally, total dissolved solids (TDS) reached a peak concentration of 389 g/L under conditions of 180 °C and 40 kg/cm2, emphasizing HTL’s positive impact on enhancing methane fermentation efficiency. These findings demonstrate that HTL pretreatment, when operated under optimized temperature and pressure conditions, offers a promising approach for enhancing both waste reduction and bioenergy recovery from high-solid organic sludge.