Integrated co-HTL and two-stage extraction of fecal sludge and rice husk toward high-yield middle-to-heavy distillate-rich biocrude

Hydrothermal liquefaction (HTL) is a promising technology for converting wet waste into biocrude; however, fecal sludge (FS)-derived biocrude is typically dominated by light fuel fractions, limiting its suitability for middle-distillate and heavy-fuel applications. This study presents a novel integrated co-hydrothermal liquefaction (co-HTL) and two-stage biocrude extraction (TSBE) approach using FS and rice husk (RH) to produce high-yield, middle-to-heavy distillate-rich biocrude. FS was used directly in its natural wet state, while RH served as both a lignocellulosic co-feedstock and a total solids (TS) regulator. Co-HTL was performed in a 12-L batch reactor at 320 °C with varying TS contents at different FS: RH blending ratios. The maximum total biocrude yield of 59.7 wt% was achieved at a 1:1 ratio with 15.5 wt% TS. TSBE using ethyl acetate and acetone recovered residual biocrude entrapped within biochar, increasing overall carbon recovery to 80.9%. Elemental analysis revealed reduced O/C and N/C ratios with an HHV of 36.9 MJ/kg. GC–MS and TGA analyses showed increased formation of long-chain hydrocarbons and middle- and heavy-fuel fractions, respectively. The proposed strategy demonstrates a scalable and energy-efficient pathway for transforming urban wet waste and agricultural residues into renewable biocrude suitable for diesel, marine fuel, and industrial energy applications.