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

Wang, Z; Li, X; Liu, H; Lin, CSK; Wang, Q

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

Wang, Z

Li, X

Liu, H

Lin, CSK Wang, Q

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Publisher:: PERGAMON-ELSEVIER SCIENCE LTD
Publication Type:: Journal Article
Citation:: Renewable and Sustainable Energy Reviews, 2025, 224
Issue Date:: 2025-12-01

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Field	Value	Language
dc.contributor.author	Wang, Z https://orcid.org/0000-0002-8227-2602
dc.contributor.author	Li, X https://orcid.org/0000-0003-1768-9556
dc.contributor.author	Liu, H https://orcid.org/0000-0003-4513-4988
dc.contributor.author	Lin, CSK
dc.contributor.author	Wang, Q https://orcid.org/0000-0002-5744-2331
dc.date.accessioned	2026-01-13T01:23:13Z
dc.date.available	2026-01-13T01:23:13Z
dc.date.issued	2025-12-01
dc.identifier.citation	Renewable and Sustainable Energy Reviews, 2025, 224
dc.identifier.issn	1364-0321
dc.identifier.issn	1879-0690
dc.identifier.uri	http://hdl.handle.net/10453/191709
dc.description.abstract	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.
dc.language	English
dc.publisher	PERGAMON-ELSEVIER SCIENCE LTD
dc.relation	http://purl.org/au-research/grants/arc/FT200100264
dc.relation.ispartof	Renewable and Sustainable Energy Reviews
dc.relation.isbasedon	10.1016/j.rser.2025.116086
dc.rights	info:eu-repo/semantics/openAccess
dc.subject	09 Engineering
dc.subject.classification	Energy
dc.subject.classification	40 Engineering
dc.title	Hydrothermal liquefaction of sewage sludge: A comprehensive review of biocrude oil production, byproducts valorization, and future perspectives
dc.type	Journal Article
utslib.citation.volume	224
utslib.for	09 Engineering
pubs.organisational-group	University of Technology Sydney
pubs.organisational-group	University of Technology Sydney/Faculty of Engineering and Information Technology
pubs.organisational-group	University of Technology Sydney/Faculty of Engineering and Information Technology/School of Civil and Environmental Engineering
pubs.organisational-group	University of Technology Sydney/UTS Groups
pubs.organisational-group	University of Technology Sydney/UTS Groups/Climate Change & Health Research Collaborative (CCHRC)
pubs.organisational-group	University of Technology Sydney/UTS Groups/Centre for Technology in Water and Wastewater (CTWW)
pubs.organisational-group	University of Technology Sydney/UTS Groups/Chancellor's Research Fellows
utslib.copyright.status	open_access	*
dc.rights.license	This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0). To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/
dc.date.updated	2026-01-13T01:23:11Z
pubs.publication-status	Published
pubs.volume	224

Abstract:

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.

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http://hdl.handle.net/10453/191709