Turning process water from hydrothermal carbonization into a resource for sustainable agriculture

A liquid byproduct from hydrothermal carbonization, long overlooked as wastewater, may hold significant promise as a liquid fertilizer-like amendment and resource for more sustainable agriculture, according to a new review published in Biochar.

Hydrothermal carbonization, or HTC, is a thermochemical process that converts wet biomass and organic wastes into hydrochar. Because the process works with water-rich feedstocks such as sewage sludge, food waste, manure, and microalgae, it avoids the need for energy-intensive drying. Yet while hydrochar has received major research attention, the liquid fraction generated during the process, known as HTC process water or HTC-PW, has often been treated mainly as a disposal problem.

The new review, led by Qingnan Chu, Xiangyu Liu, and colleagues, argues that this view is changing. HTC-PW can contain high levels of organic carbon and plant nutrients, including ammonium nitrogen, phosphorus, and potassium. Depending on the feedstock and reaction conditions, HTC-PW may contain ammonium nitrogen up to thousands of milligrams per liter, potassium above 5,000 milligrams per liter, and substantial dissolved organic matter.

“HTC process water should not simply be regarded as a waste stream,” said corresponding author Zhimin Sha. “With proper characterization, treatment, and application, it can become part of a circular system that recovers nutrients, reduces waste discharge, and supports soil and crop management.”

The review synthesizes recent studies showing several possible pathways for HTC-PW use. These include direct application after dilution, co-application with biogas slurry, nutrient recovery through conditioning methods such as struvite precipitation, and additional valorization routes such as anaerobic digestion for methane production or catalytic reforming for hydrogen.

Agronomic studies summarized in the review show that properly managed HTC-PW can improve soil dissolved organic matter, enhance nutrient retention, and support crop productivity. In rice systems, for example, HTC-PW applications have been associated with yield increases of 6.7 to 29.2 percent and improved nutrient use efficiency. Other studies suggest that HTC-PW can influence soil microbial communities in ways that favor nutrient cycling.

The authors also emphasize that HTC-PW is not a one-size-fits-all fertilizer. Its properties vary strongly with feedstock and processing conditions, including temperature, residence time, solids loading, pH, additives, and recirculation. Mild HTC conditions may preserve more soluble nutrients for fertilizer value, while more severe conditions may reduce some organics and shift nutrients into the solid hydrochar. This tunability means HTC-PW could be designed for different uses, from rice paddies to fertigation systems.

However, the review also highlights important safety and management challenges. HTC-PW may contain high salinity, phytotoxic organic compounds, heavy metals, and variable nitrogen forms that can influence nitrous oxide emissions. The authors recommend dilution, neutralization, bioassays, contaminant monitoring, and compliance with local regulations before agricultural use.

“The key is controlled use rather than direct, untested application,” Sha said. “Future work should focus on long-term field trials, standardized quality indicators, and predictive tools that help match HTC-PW properties with specific crops, soils, and environmental goals.”

The review further notes that life-cycle and techno-economic assessments suggest scenario-dependent benefits. When HTC-PW replaces mineral fertilizer or avoids wastewater treatment costs, systems may reduce global warming potential and improve economic performance. Still, the authors caution that more field-scale evidence is needed to confirm long-term benefits for soil physical structure, greenhouse gas mitigation, and farm-level feasibility.

By reframing HTC-PW from a troublesome effluent into a recoverable resource, the review points toward a broader shift in waste management: turning organic residues into integrated streams of carbon, nutrients, and energy for a circular bioeconomy.

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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   

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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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