A growing body of research is redefining how scientists view a once-overlooked byproduct of biomass processing. A new review published in Biochar reveals that process water generated during hydrothermal carbonization, long treated as waste, could play a key role in sustainable agriculture and the circular bioeconomy.
Hydrothermal carbonization is an emerging technology that converts wet organic materials such as food waste, sewage sludge, and agricultural residues into a carbon-rich solid known as hydrochar. While much attention has focused on this solid product, the accompanying liquid fraction, called process water, has remained largely underutilized. According to the authors, this liquid can account for up to 70 percent of the original material and contains substantial amounts of nutrients and organic compounds.
“This process water is not just a byproduct. It is a nutrient-rich resource with significant potential for agriculture,” said the study’s corresponding author. “Our work highlights how it can be transformed from a disposal challenge into a valuable input for crop production and environmental management.”
The review synthesizes findings from recent studies on the composition and applications of this liquid. It shows that process water contains high levels of nitrogen, phosphorus, potassium, and organic carbon. These components are essential for plant growth and can support its use as a liquid fertilizer or soil amendment. In some cases, applying diluted process water has increased crop yields by up to nearly 30 percent and improved nutrient use efficiency by 15 to 30 percent in crops such as rice.
Beyond direct fertilization, the liquid can also be integrated into broader resource recovery systems. For example, nutrients can be extracted through processes such as struvite precipitation, which recovers phosphorus and nitrogen for reuse. The organic content can also be converted into energy through anaerobic digestion, producing methane-rich biogas. These approaches align with circular economy principles by reducing waste and maximizing resource efficiency.
However, the study also emphasizes that careful management is required before agricultural use. Process water can contain compounds such as organic acids, phenols, and salts that may inhibit plant growth if applied without treatment. Strategies such as dilution, co-application with other organic inputs, or pre-treatment methods like neutralization and filtration can significantly reduce these risks.
The authors note that operational conditions during hydrothermal carbonization strongly influence the quality of the process water. Factors such as temperature, reaction time, and feedstock type can be adjusted to tailor nutrient content and reduce harmful components. This opens the door to designing customized liquid fertilizers suited for specific crops and farming systems.
Importantly, the review highlights broader environmental benefits. Using process water as a fertilizer substitute can reduce reliance on synthetic fertilizers and lower greenhouse gas emissions. Life cycle assessments suggest that such substitution may cut global warming impacts by up to 50 percent under certain scenarios.
Despite these promising findings, the authors call for more long-term field studies and large-scale demonstrations. Questions remain about variability across feedstocks, long-term soil impacts, and regulatory frameworks for safe application.
Overall, the research signals a shift in perspective. What was once considered wastewater is now being reimagined as a multifunctional resource that supports sustainable agriculture, renewable energy, and waste reduction.
“This is a step toward closing nutrient loops and building more resilient agricultural systems,” the authors conclude.
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Journal Reference: Chu, Q., Liu, X., Feng, Y. et al. Process water from hydrothermal carbonization: from waste to liquid fertilizer and soil health amendment in circular bioeconomy. Biochar 8 , 96 (2026).
https://doi.org/10.1007/s42773-026-00614-y
About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
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Literature review
Process water from hydrothermal carbonization: from waste to liquid fertilizer and soil health amendment in circular bioeconomy
27-Apr-2026