Large-scale anaerobic digestion generates millions of tons of wet biogas digestate annually, presenting disposal challenges due to its complex composition and high moisture content. Converting this organic waste into valuable humic-like substances offers a promising valorization pathway. The researchers investigated how different reaction media—specifically deionized water versus liquid biogas slurry—and varying reaction temperatures between 180 and 260 degrees Celsius affect the yield, selectivity, and chemical properties of the resulting artificial humic acid.

The findings indicate that employing biogas slurry as the reaction medium significantly improves nitrogen retention and increases the formation of desirable humic components. While lignin serves as the primary precursor for aromatic structural growth, the alkaline environment and abundant nitrogen species provided by the slurry promote accelerated decomposition and drive Maillard-type condensation reactions. At an optimal temperature of 220 degrees Celsius, the system achieved a maximum artificial humic acid selectivity of 40.16 percent in the hydrochar and successfully retained 26.02 milligrams of nitrogen within the humic product. Molecular and spectroscopic analyses confirmed that these reactions embed nitrogen into stable heterocyclic rings, resulting in materials with lower carbon-to-nitrogen ratios favorable for agricultural use.

This work establishes a practical, integrated approach for managing biogas plant co-products while generating nutrient-enriched soil amendments. By coupling solid digestate with liquid slurry under moderate subcritical conditions, the process bypasses the need for external chemical additives, offering an efficient strategy for closed-loop agricultural nutrient recycling and soil carbon enhancement.

Source: Zhao, Y., Liu, Z., Yao, Z., Wang, D., Su, J., Yang, J., Huo, L., Jia, J., Li, H., & Zhao, L. (2026). Valorization of biogas digestate through hydrothermal humification for nitrogen-enriched artificial humic acid production. BiocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More, 8, 145.