Hydrothermal liquefaction of sewage sludge: A comprehensive review of biocrude oil production, byproducts valorization, and future perspectives

Climate change is driving global efforts toward carbon neutrality and expanding renewable energy sources. Hydrothermal liquefaction (HTL) of sewage sludge offers a promising pathway for sustainable biocrude oil production. This review systematically analyzes 956 records from Web of Science and Scopus databases, with 179 articles selected for detailed analysis following PRISMA guidelines. It presents the first comprehensive and systematic analysis of biocrude oil production and byproducts valorization from the HTL of sewage sludge. Key findings highlight that mixed sludge, with a balanced organic matter composition, is ideal for biocrude oil production, achieving an average yield of 38.95 % (range: 35.3–42.6 %). Higher biocrude oil yields are more likely to be achieved under reaction conditions of approximately 350 °C and a holding time of 30 min, as indicated by 2D kernel density estimation of the collected literature. These optimal conditions are summarized as a reference point for future studies, although the exact operating conditions may need specific exploration depending on the sludge properties. The transformation of organic matter follows the order: lipids > proteins > carbohydrates > lignin/humic substances, with diverse complex reactions driving biocrude oil formation. The biocrude oil contains significant heteroatom content-nitrogen (5.5 %, range: 0.23–9.3 %), sulfur (0.9 %, range: 0–4.3 %), and oxygen (15.7 %, range: 6.7–62.8 %)-which necessitate upgrading for biocrude oil applications. Nitrogen primarily distributes into the aqueous phase, while phosphorus and metals accumulate in the solid phases, offering opportunities for resource recovery. HTL also generates byproducts in aqueous (36.67 %, range: 0.19–60.3 %), solid (22.03 %, range: 0.43–50.73 %), and gaseous (13.71 %, range: 0.2–64.68 %) phases, which can be effectively valorized through proper management, promoting both industrial applications of HTL and the development of a circular economy. This work serves as a valuable resource for researchers, policymakers, and industry stakeholders, providing insights into biocrude oil production and byproduct utilization, advancing sustainable sludge management toward global carbon neutrality goals.