In a comprehensive review published in the journal Biochar, authors Qingnan Chu, Xiangyu Liu, and their team examine the untapped potential of process water generated during hydrothermal carbonization. This thermochemical process, which converts various types of wet biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More into solid hydrochar, also produces a significant volume of nutrient-rich liquid often dismissed as waste. By shifting the perspective from waste disposal to resource recovery, the researchers demonstrate how this liquid can be utilized as a potent soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More and liquid fertilizer. This transition is essential for developing a circular bioeconomy that reduces our reliance on mineral fertilizers and minimizes environmental harm.

The chemical composition of this process water makes it an ideal candidate for agricultural use, as it contains extremely high levels of organic carbon and vital plant nutrients. Depending on the original material used, such as sewage sludge or food waste, the liquid can provide up to 4,400 milligrams per liter of ammonium nitrogen and over 6,300 milligrams per liter of potassium. These nutrients are readily available for plant uptake, which explains why direct application of the diluted liquid has shown such impressive results in crop trials. For instance, rice paddies treated with this process water saw yield increases ranging from 6.7 to 29.2 percent, highlighting its effectiveness as a high-performance bio-fertilizer.

One of the most promising application methods is the co-application of process water with biogas slurry. Biogas slurry is typically alkaline, which can lead to nitrogen loss through ammonia gas escaping into the atmosphere. Because the process water is often naturally acidic, mixing the two creates a buffered solution that stabilizes the nitrogen. Research shows that this strategy can reduce ammonia volatilization by 4.2 to 65.5 percent compared to using traditional urea fertilizer. This not only keeps more food-producing nutrients in the ground but also protects nearby aquatic ecosystems from the harmful effects of nutrient runoff.

Beyond its use as a fertilizer, the liquid byproduct can be processed to recover high-purity minerals or generate renewable energy. Using a method called struvite precipitation, engineers can recover up to 99 percent of the phosphorus and 88 percent of the nitrogen from the water as a solid, slow-release fertilizer. Additionally, the high organic content of the water supports the production of methane through anaerobic digestion, yielding 250 to 350 milliliters of methane per gram of chemical oxygen demand. This dual-purpose utility as both a fertilizer source and an energy feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More provides multiple pathways for industrial and agricultural sites to improve their sustainability and economic bottom line.

While the benefits are significant, the study also addresses the importance of managing potential risks such as high salt content and organic toxins. Raw process water has a high electrical conductivity, which can stress sensitive crops if it is not properly diluted. Scientists recommend a dilution ratio between 1:3 and 1:9 to ensure the liquid is safe for plants while still providing ample nutrition. Furthermore, treating the water with calcium can help neutralize acidity and remove heavy metals, making it suitable for modern irrigation and hydroponic systems. Routine monitoring of salt levels and organic compounds acts as a reliable control panel for transforming this waste stream into a precision agricultural input.

The life-cycle assessment of this technology suggests that substituting mineral fertilizers with hydrothermal process water can reduce a farm’s global warming potential by 20 to 50 percent. By avoiding the energy-intensive production of traditional fertilizers and diverting wastewater from expensive treatment plants, the system creates both environmental and economic value. As research moves toward long-term field trials and the use of machine learning to predict water quality, this innovative approach is set to become a cornerstone of sustainable waste management and food production systems worldwide.

Source: 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).