Hydrothermal Carbonization of Digestate from Lignocellulosic Biogas Power Plants for Sustainable Soil Improvement and Low Carbon Emissions

This research explored hydrothermal carbonization to enhance digestate from lignocellulosic biogas power plants as a soil amendment for low-carbon agriculture. Hydrochar was produced from real digestate via hydrothermal carbonization at 225–265 °C, hydrothermal carbonization at 265 °C demonstrating optimal properties for agricultural use. Key benefits include enhanced phosphorus retention, controlled-release nutrient behavior as indicated by Chlorella vulgaris cultivation in its water-soluble fraction, and safe application as a soil amendment, with heavy metal concentrations within regulatory limits and absent in the water-soluble fraction. Based on theoretical calculations, labile carbon—represented by the water-soluble organic carbon fraction in hydrochar—was reduced 15-fold at 265 °C compared to the digestate. This reduction could decrease greenhouse gas emissions from 441 to 29 tons CO₂-equivalent annually in a 1 MW biogas plant scenario, while sequestering 766 tons of carbon in cropland soils. Additionally, theoretical calculations suggest co-digestion with hydrothermal wastewater could enhance nutrient recovery and methane production, contributing to low-carbon emissions. These findings underscore hydrothermal carbonization’s potential for sustainable biogas power plant, agriculture, and climate change mitigation.