Hydrothermal liquefaction of digestate from the organic fraction of municipal solid waste: Optimization of operating parameters

The growing demand for clean energy and the increasing waste volumes have spurred interest in waste-to-energy processes. Anaerobic digestion (AD) is a well-established technology that converts the organic fraction of municipal solid waste (OFMSW) into biogas, with digestate as a challenging byproduct to manage. This study explored the potential for further valorizing digestate from OFMSW through hydrothermal liquefaction (HTL). A broad range of operating parameters - temperature (300–360 °C), residence time (10–60 min), and dry solid loading (5–30 wt%) - was systematically investigated using a rigorous statistical approach and complementary characterization techniques, leading to new insights on performance indicators. Higher temperatures resulted in a limited residual solid yield (as low as 7 wt%), increased biocrude production (up to 32 wt%) and altered its composition, with an increase in aromatics, a reduction in sterols, and a higher aromatic-to-aliphatic nitrogen ratio. The effect of an increase in dry solid loading on the HTL of digestate, a factor crucial to its economic feasibility, was investigated for the first time. This resulted in higher yields of residual solids (up to 24 wt%) at the expense of aqueous phase soluble compounds and altered the elemental ratios of biocrudes and solids. This study demonstrates that HTL can convert digestate from OFMSW into biocrude, capturing up to 48% of the feedstock’s embedded energy in a more concentrated form (~31 MJ/kg), making it a suitable substitute for fossil-derived fuel oil.