Effects of municipal sludge composition on hydrothermal liquefaction products: Optimizing energy recovery via combination with anaerobic digestion

This study evaluated the performance of an optimized hydrothermal liquefaction (HTL) process for municipal sludge with downstream anaerobic digestion (AD) for aqueous by-product valorization. Mixed sludge with various primary and secondary sludge ratios and digested sludge from mesophilic and thermophilic AD from plants under seasonal and operational fluctuations was tested. Results showed shifts in product yield, biocrude and hydrochar composition, and energy recovery (ER), with higher secondary sludge leading to increased hydrochar heavy metals and phosphorus. However, biocrude maintained consistent C (72–75 %), H (9–10 %), and N (4–5 %) contents, higher heating value (35–37 MJ/kg), dry-ash-free yield (48 ± 3 %), and ER (69 ± 1 %), demonstrating adaptability of the optimized HTL condition for varying feedstock. Biocrude ER was driven by sludge composition, ranking lipids > proteins > carbohydrates. Mesophilic AD effectively treated HTL aqueous, achieving the highest overall ER (78–82 %), energy return on investment (10.8–11.1), and net energy yield (15.7–16.2 MJ/kg, dry basis) in HTL-AD system, outperforming AD and AD-HTL-AD configurations for sludge treatment. These findings demonstrate the robustness of optimized HTL condition across diverse sludge feedstocks and highlight the potential of HTL-AD integration to enhance ER and resource sustainability in wastewater treatment practice. • Municipal sludge composition significantly affects HTL product yields and contents • Fluctuations in mixed sludge have minimal impacts on biocrude energy recovery • Sludge macromolecules contribute to biocrude by lipids > proteins > carbohydrates • More secondary sludge in feedstock leads to more metals and phosphorus in hydrochar • HTL-mesophilic anaerobic digestion maximizes energy recovery from municipal sludge