Hydrothermal carbonization is a thermochemical technique that processes wet biomass, such as sewage sludge, food waste, manure, and microalgae, without requiring energy-intensive drying steps. While much research has focused on hydrochar, the solid carbonaceous product, the aqueous phase generated during the reaction has received far less attention. Traditionally, HTC-PW has been treated as a waste management challenge, often disposed of at environmental cost. However, the latest insights suggest that this liquid fraction is a reservoir of organic carbon, macro- and micronutrients, and bioactive compounds that can be harnessed to improve soil health and crop productivity.

Qingnan Chu, Xiangyu Liu, and their colleagues spearheaded this review, meticulously compiling findings from recent studies to highlight the multifaceted value embedded in HTC-PW. Their analysis reveals that this process water can contain exceedingly high concentrations of ammonium nitrogen, phosphorus, potassium, and dissolved organic matter, varying extensively depending on feedstock types and process conditions, such as temperature, residence time, and pH. For example, ammonium nitrogen levels may reach thousands of milligrams per liter, while potassium often exceeds 5,000 mg/L, positioning HTC-PW as a potent nutrient source.

Beyond its nutrient content, HTC-PW carries potential functional benefits for soils. Studies cited in the review demonstrate that when HTC-PW is prudently managed and applied, it promotes soil dissolved organic carbon, enhances nutrient retention capacity, and fosters beneficial shifts in soil microbial communities that facilitate nutrient cycling. Application trials in paddy rice fields have reported yield improvements of up to nearly 30%, alongside enhanced nutrient use efficiency, which could translate into reduced reliance on synthetic fertilizers and lower environmental footprints.

The versatility of HTC-PW offers exciting opportunities for tailored applications in diverse agricultural contexts. The review clarifies that its chemical composition and efficacy are strongly influenced by hydrothermal carbonization parameters. Milder HTC conditions tend to preserve more bioavailable nutrients, ideal for direct application as liquid fertilizer, while harsher treatments channel nutrients into the solid hydrochar fraction, modifying the residual aqueous phase accordingly. This tunability opens doors to customized formulations adapted to specific crops, soil types, and management objectives, from rice paddies in Asia to fertigation systems in greenhouse environments.

Nevertheless, the authors caution against unregulated or indiscriminate use of HTC-PW. Potential challenges include elevated salinity, phytotoxic organic compounds, heavy metal concentrations, and variable nitrogen forms—all of which may impact plant health and greenhouse gas emissions, particularly nitrous oxide. To mitigate these risks, the review recommends a series of control measures: dilution to reduce salinity, pH neutralization, comprehensive bioassays to assess toxicity, stringent contaminant monitoring, and compliance with local agricultural and environmental regulations.

In addition to direct soil amendment, the review presents innovative valorization approaches that expand HTC-PW’s utility beyond fertilization. Conditioning methods such as struvite precipitation enable recovery of high-purity nitrogen and phosphorus compounds, facilitating nutrient recycling and reducing environmental discharge. Meanwhile, integrating HTC-PW into anaerobic digestion or catalytic reforming processes offers pathways to generate methane or hydrogen fuel, respectively, merging waste valorization with renewable energy production in holistic resource recovery frameworks.

From a systems perspective, life-cycle assessments and techno-economic analyses reveal nuanced outcomes dependent on application scenarios. When HTC-PW replaces synthetic fertilizers or circumvents costly wastewater treatments, the total environmental impact often decreases, lowering global warming potential and improving economic feasibility for farmers and waste processors alike. However, the review underscores the critical need for long-term, large-scale field experiments to validate these preliminary findings and to understand the broader implications for soil structure, greenhouse gas fluxes, and circular economy viability.

This synthesis marks a pivotal turning point in how researchers and practitioners perceive HTC-PW—from a problematic effluent to a valuable bioresource integrated within sustainable agriculture. The findings align with global imperatives to enhance nutrient use efficiency, reduce agrochemical dependency, and close nutrient loops in agricultural landscapes. By refining characterization methods, developing predictive models, and instituting standardized quality metrics, future research can further optimize HTC-PW utilization tailored to diverse agroecosystems, enhancing soil fertility while mitigating environmental burdens.

“Controlled, monitored application is the key,” emphasizes corresponding author Zhimin Sha. “The challenge lies in unlocking HTC process water’s full potential while safeguarding environmental and crop health. With continued innovation and rigorous field validation, we can transform what was once considered waste into a cornerstone of regenerative farming.”

In the quest for resilient food systems amid climate pressures and resource constraints, HTC process water exemplifies how scientific ingenuity is redefining waste management. This liquid byproduct—rich in carbon and nutrients—may soon become indispensable in sustainable intensification strategies, turning organic residues into energy and nutrient streams that fuel productive soils and thriving crops, driving forward a circular bioeconomy.

Subject of Research:
Process water from hydrothermal carbonization as a liquid fertilizer and soil health amendment in agriculture.

Article Title:
Process water from hydrothermal carbonization: from waste to liquid fertilizer and soil health amendment in circular bioeconomy

News Publication Date:
27-Apr-2026

References:
Chu, Q., Liu, X., Feng, Y., Li, D., Yin, S., Chen, C., & Sha, Z. (2026). Process water from hydrothermal carbonization: from waste to liquid fertilizer and soil health amendment in circular bioeconomy. Biochar, 8, 96. https://doi.org/10.1007/s42773-026-00614-y

Image Credits:
Qingnan Chu, Xiangyu Liu, Yanfang Feng, Detian Li, Shuai Yin, Chengrong Chen & Zhimin Sha

Hydrothermal carbonization itself is an emerging thermochemical technology designed to transform wet biomass—including food waste, sewage sludge, and agricultural residues—into hydrochar, a carbon-dense solid material valued for its energy content and soil amendment properties. Much of the research to date has concentrated on optimizing hydrochar yield and characteristics; however, growing attention now focuses on the associated aqueous fraction produced during the reaction. Understanding and harnessing this process water is critical to realizing the full environmental and economic benefits of hydrothermal carbonization.

The chemical milieu of process water is intricate, containing elevated concentrations of macronutrients vital for plant growth as well as a considerable pool of dissolved organic carbon. These constituents have demonstrated agronomic benefits when employed as liquid fertilizers or soil health amendments. Experimental applications on crops like rice have yielded impressive results, including yield increments approaching 30% and enhancements in nutrient use efficiency by 15–30%. This dual benefit highlights the capacity of process water to contribute significantly to food production systems while mitigating the environmental footprint of synthetic fertilizers.

From a resource recovery perspective, the process water can serve as a valuable input in nutrient recycling frameworks. Advanced recovery techniques, such as struvite precipitation, enable the selective extraction of phosphorus and nitrogen, which can be recycled into fertilizers, maintaining nutrient cycles and decreasing dependency on finite mineral reserves. Moreover, the organic-rich fraction holds promise for bioenergy generation through anaerobic digestion, producing methane-rich biogas that can offset fossil fuel consumption within agricultural operations.

Despite these advantages, the application of process water in agriculture requires meticulous management due to the presence of potentially phytotoxic substances such as organic acids, phenols, and salts. Untreated or improperly handled process water may inhibit plant growth or result in soil degradation. Consequently, strategies including dilution, blending with other organic amendments, and pre-treatment methods like neutralization and filtration are often necessary to mitigate toxicity and optimize agronomic outcomes.

Critically, the physicochemical properties of process water are not static but are profoundly influenced by the operational parameters of hydrothermal carbonization. Variables such as reaction temperature, residence time, and feedstock characteristics modulate nutrient concentrations and the spectrum of inhibitory compounds present. By calibrating these conditions, it becomes feasible to tailor process water outputs, effectively customizing nutrient profiles and reducing harmful components to develop site- and crop-specific liquid fertilizers.

Ecological advantages associated with repurposing process water are noteworthy. Substitution of conventional synthetic fertilizers with nutrient-rich process water can substantially reduce greenhouse gas emissions linked to fertilizer manufacturing and application. Life cycle assessments reveal potential reductions in global warming potential by up to 50% under optimized scenarios, contributing meaningfully to climate mitigation efforts within the agricultural sector.

Yet, despite promising laboratory and small-scale field trial results, substantial uncertainties remain. The long-term impacts of repeated process water application on soil health parameters—including microbial communities, nutrient cycling, and salinity—require in-depth investigation. Additionally, variability in feedstock composition and process conditions necessitates thorough large-scale demonstrations to validate agronomic consistency and economic feasibility.

From a regulatory and safety standpoint, frameworks to govern the use of process water as a fertilizer are not yet fully established. Developing standardized guidelines to ensure safe application rates, treatment protocols, and environmental monitoring will be essential to facilitate the adoption of this technology by farmers and agribusinesses, and to assure public and environmental health.

Research to date signals a transformative opportunity: repositioning hydrothermal carbonization process water from a disposal problem into a valuable, multifunctional resource that integrates seamlessly with circular bioeconomy principles. Such an approach dovetails with the broader move towards regenerative agriculture and sustainable resource management, aiming to close nutrient loops and enhance agricultural system resilience.

Authors of the recent comprehensive review in Biochar emphasize the importance of continued interdisciplinary research efforts, advocating for long-term field studies and pilot projects that explore the scalability and integration of process water utilization within diverse agricultural landscapes. This research is pivotal to not only advancing scientific understanding but also fostering commercial application and policy development.

Ultimately, this paradigm shift of viewing process water as a nutrient-rich liquid fertilizer and soil amendment offers a promising conduit for advancing sustainable agriculture. It exemplifies innovative resource recovery from waste streams and contributes to reducing the environmental footprint of food production, aligning with global imperatives for climate-smart and circular bioeconomic strategies.

Subject of Research: Process water from hydrothermal carbonization and its applications as liquid fertilizer and soil health amendment in circular bioeconomy.

Article Title: Process water from hydrothermal carbonization: from waste to liquid fertilizer and soil health amendment in circular bioeconomy.

News Publication Date: 27-Apr-2026

Web References:
http://dx.doi.org/10.1007/s42773-026-00614-y

References:
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

Image Credits: Qingnan Chu, Xiangyu Liu, Yanfang Feng, Detian Li, Shuai Yin, Chengrong Chen & Zhimin Sha

Keywords: Hydrothermal carbonization, process water, liquid fertilizer, soil amendment, circular bioeconomy, sustainable agriculture, nutrient recycling, hydrochar, waste valorization, bioenergy, greenhouse gas reduction, plant nutrition.