Two-stage hydrothermal process turns wastewater sludge into cleaner biofuel
Biochar Editorial Office, Shenyang Agricultural University
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Product characteristics and nitrogen evolution pathways of two-stage hydrothermal liquefaction of municipal sludge
view moreCredit: Guanyu Jiang, Mingxin Xu, Mingyi Guo, Shiming Niu, Pan Wang, Zhiyong Duan, Chengming Li, Krzysztof Kapusta, Pavel Aleksandrovich Strizhak & Donghai Xu
Scientists have developed an improved method to convert municipal wastewater sludge into higher quality renewable fuel while significantly reducing harmful nitrogen compounds, offering a promising pathway for cleaner energy and sustainable waste management.
Municipal sludge is a by-product of wastewater treatment that is produced in massive quantities worldwide. Managing this material remains a growing environmental challenge. Traditional treatment methods often involve high costs, pollution risks, and limited resource recovery. A new study published in Energy & Environment Nexus demonstrates how a two-stage hydrothermal liquefaction approach can transform sludge into cleaner bio-oil with improved fuel properties.
The research shows that hydrothermal liquefaction, a process that converts wet biomass into oil-like fuel under moderate temperature and pressure, can be significantly enhanced by using a staged reaction strategy. While conventional single-step liquefaction produces bio-oil efficiently, it often generates fuel containing high levels of nitrogen, which can reduce fuel quality and cause emissions problems during combustion.
“Our work reveals a more effective way to control how nitrogen moves and transforms during sludge conversion,” said the study’s corresponding author. “By introducing a two-stage process, we can produce bio-oil with lower nitrogen content while still generating valuable fuel products.”
In the new method, sludge undergoes an initial low-temperature treatment followed by a higher-temperature conversion step. Researchers compared three processing routes: traditional direct liquefaction, consecutive two-stage processing, and separated two-stage processing. They found that although the separated two-stage method produced slightly less oil overall, it substantially improved oil quality.
The study revealed that the separated two-stage process reduced nitrogen levels in bio-oil by up to 37 percent. Lower nitrogen content is important because nitrogen-rich fuels can deactivate catalysts during refining and increase pollutant emissions. The improved process also increased the concentration of desirable fuel compounds such as hydrocarbons, alcohols, and esters.
Researchers discovered that the first stage of the separated process plays a critical role. During this stage, most nitrogen compounds move into the water phase instead of remaining in the oil. This shift significantly improves the chemical composition of the final bio-oil product and helps limit the formation of nitrogen-containing molecules that reduce fuel performance.
The team also analyzed how nitrogen changes chemically during the conversion process. They found that proteins and other nitrogen-rich materials break down into smaller compounds that either dissolve in water or transform into solid residues. By controlling reaction conditions, the process minimizes the amount of nitrogen that ends up in the final oil product, improving combustion characteristics and energy density.
Beyond improving fuel quality, the research highlights the potential of sludge as a valuable resource rather than waste. With global sludge production continuing to rise due to population growth and urbanization, technologies that recover energy and reduce environmental impact are increasingly important.
“Municipal sludge is often viewed as an environmental burden, but it also represents a major untapped energy source,” the researchers noted. “Our findings provide new insight into optimizing sludge conversion technologies and improving the sustainability of wastewater treatment systems.”
The authors suggest that further upgrading techniques, such as catalytic treatment, could enhance the fuel even more by removing remaining oxygen and nitrogen compounds. They believe the two-stage hydrothermal liquefaction approach could support future efforts to integrate waste treatment with renewable energy production.
The study provides a foundation for scaling up sludge-to-fuel technologies and advancing circular economy strategies that transform waste into clean energy resources.
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Journal reference: Jiang G, Xu M, Guo M, Niu S, Wang P, et al. 2026. Product characteristics and nitrogen evolution pathways of two-stage hydrothermal liquefaction of municipal sludge. Energy & Environment Nexus 2: e004 doi: 10.48130/een-0025-0017
https://www.maxapress.com/article/doi/10.48130/een-0025-0017
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About Energy & Environment Nexus:
Energy & Environment Nexus (e-ISSN 3070-0582) is an open-access journal publishing high-quality research on the interplay between energy systems and environmental sustainability, including renewable energy, carbon mitigation, and green technologies.
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Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Product characteristics and nitrogen evolution pathways of two-stage hydrothermal liquefaction of municipal sludge
