A Comprehensive Experimental Investigation on Sustainable Nutrient Recovery from Food Waste via Hydrothermal Carbonization with the Addition of Deep Eutectic Solvents

Hydrothermal carbonization (HTC) has emerged as a promising technique for food waste treatment. However, food waste is composed of complex components, including refractory proteins and polysaccharides, which lead to low efficiency and high costs during the HTC process. Enhancing the decomposition of food waste while enabling efficient nutrient recovery remains a significant challenge for the widespread application of HTC in food waste management. This study introduces deep eutectic solvents (DESs) to enhance treatment efficiency during the HTC of food waste. A comprehensive characterization of the resulting hydrochar and aqueous phase was conducted, and the effect of DES addition on the migration and speciation of phosphorus and nitrogen species during HTC was investigated. The results indicated that the addition of DESs promoted the decomposition of food waste, reducing the hydrochar yield from 22.6% to 20.2% and decreasing the volatile matter content in the hydrochar from 86.63% to 71.60% at 200 °C. Additionally, DESs significantly lowered the nitrogen content in the hydrochar from 5.99% to 3.77%. By disrupting the hydrogen-bonding networks in proteins and polysaccharides, DESs facilitated the dissolution of organic matter into the aqueous phase. Furthermore, with DES addition, 5.06 mg of phosphorus species was enriched in the hydrochar, compared to only 1.78 mg in the control group without DESs. This study provides a sustainable strategy for the efficient treatment of food waste while simultaneously enabling the effective recovery of valuable nutrients.