Valorization of Sewage Sludge by HTL after Deep Eutectic Solvent Pretreatment in an Experimental Unit

Municipal sewage sludge disposal is a major challenge for wastewater treatment plants due to its high moisture content and pollutant load, which increase handling costs and environmental risks. In this context, hydrothermal liquefaction (HTL) has attracted attention as an alternative for sewage sludge valorization because it enables the direct conversion of wet biomass into liquid products without prior drying. This study investigated the conversion of raw and deep eutectic solvent (DES)-pretreated sewage sludge in a continuous-flow HTL experimental unit, focusing on process performance and liquid fraction generation. The DES was prepared from choline chloride and lactic acid (1:1) and applied as a pretreatment prior to HTL. The sludge presented 33.20% cellulose, 24.12% hemicellulose, 21.80% lignin, and a higher heating value of 17.43 MJ kg⁻¹, indicating favorable characteristics for thermochemical conversion. HTL experiments were conducted at 300 °C and 100 bar, with a flow rate of 1 mL min⁻¹ and a reaction time of 100 min. Under these conditions, both raw and pretreated feedstocks were converted, generating a liquid fraction. FTIR analysis revealed the presence of oxygenated and aliphatic functional groups in the products. Compared with the raw sludge product, the fraction obtained from pretreated sludge showed stronger aliphatic C–H bands and lower relative O–H intensity, suggesting a shift toward a less oxygenated and more hydrocarbon-like composition, with greater potential for energy and chemical applications. Overall, the results indicate that combining DES pretreatment with continuous-flow HTL is an effective approach for sewage sludge valorization.