A comprehensive assessment of Integrating anaerobic digestion and hydrothermal liquefaction Processes: Harnessing energy from sewage sludge

• Assessed energy recovery in two schemes: HTL-AD and AD-HTL in integrated systems. • Analyzed HTL operating conditions impacts’ on product yield in an integrated system. • Explored process efficiency at 250–350 °C and retention times of 30 & 60 min. • Achieved maximum energy recovery at 300 °C for 60 min with HTL-AD configuration. • Optimized operating conditions enhanced energy recovery in integrated systems. Integrating anaerobic digestion (AD) and hydrothermal liquefaction (HTL) offers a promising approach for efficient sewage sludge management and enhanced energy recovery. This study systematically evaluates the energy recovery efficiency of two sequencing configurations: HTL followed by AD and AD followed by HTL, at varying HTL operating conditions of 250, 300, and 350 °C for 30 and 60 min each. Our results demonstrate that the HTL-AD sequence yields higher energy recovery in the form of biocrude, with a significant concentration of fatty acids due to the high lipid content in primary sludge. Conversely, the AD-HTL sequence recovers more energy in the form of biomethane, attributed to the easily degradable nature of primary sludge with lower nitrogen content. Energy recovery for the HTL-AD sequence ranges from 47.2 % to 84.5 %, while the AD-HTL sequence ranges from 57.2 % to 77.3 %. The HTL-AD system recovers the highest energy at 300 °C for 60 min, whereas at other operating conditions, the AD-HTL system achieves higher energy recovery than the HTL-AD system. These findings provide valuable insights for optimizing sewage sludge valorization processes and advancing toward a sustainable circular bioeconomy. Future research should focus on long-term stability, economic feasibility, and scalability assessments of these integrated systems.