Evaluating direct use fertilizer potential of hydrothermal liquefaction solid mineral products: Integrating anaerobic digestion and hydrothermal liquefaction

Sustainable agriculture is vital for reducing reliance on harmful chemical fertilizers and ensuring environmental and economic stability. Integrating Hydrothermal Liquefaction (HTL) with anaerobic digestion (AD) can improve biomass efficiency and recovery of both energy (liquid fuels) and valuable nutrients, essential for achieving circular bioeconomy. Biogas digestate is rich in nutrients, and the HTL process can concentrate these nutrients in the solid phase, making the resulting product well-suited for agricultural applications. In this study, HTL solids were produced from biogas digestate using a continuous bench-scale HTL pilot plant to assess their potential as direct-use fertilizers. The optimal conditions for keeping contaminants within allowable limits in mineral products were identified by testing various reaction temperatures, with and without the use of a catalyst. The elemental composition of mineral products (2179 and 2181) showed high carbon varying from 51.61 and 48.41 wt%, phosphorus varying from 3.46 to 3.07 wt% and sufficient nitrogen contents varying from 1.19 to 1.13 wt%. Furthermore, carbon stability of 2179 and 2181 was measured to be 1.2 and 0.3, respectively, which indicated good quality of HTL mineral products. Organic contaminants and heavy metals in HTL mineral products obtained after biocrude recirculation cycles using the HTL pilot plant were found to be close to or below the allowed limits set by Danish legislation. Avoidance (EC50 1.62 %) and seed germination (EC50 21.37 %) tests of the mineral product indicated a moderate level of biotoxicity and a substantial plant growth stimulation potential. A maximum amount of 39 kg/ha for 2179 and 55 kg/ha for 2181 is suitable for field use without harming soil organisms. The phosphates recovery from mineral products into phosphorus-based fertilizers like struvite and hydroxyapatite using mild acids and bases as leaching agents showed around 90 % phosphates recovery and 70 % bioavailability. The synthesized fertilizers were characterized using X-ray diffraction (XRD) spectroscopy. Overall, HTL mineral products present a sustainable alternative for enhancing soil fertility, reducing reliance on harmful chemical fertilizers, and supporting both environmental and economic stability, thereby contributing to a circular economy.