Integration of Hydrothermal Carbonization into Food Waste Anaerobic Digestion Systems: A Techno-economic Analysis and Environmental Assessment

The increasing pressure to reduce greenhouse gas emissions and restore soil carbon necessitates integrated waste-to-resource strategies. This study evaluates the integration of hydrothermal carbonization (HTC) into a food-waste anaerobic digestion (AD) system to improve carbon efficiency, enhance biomethane production, and reduce environmental impacts. Digestate from a full-scale AD plant was treated via HTC at 160 °C and 220 °C, producing hydrochar and aqueous hydrothermal liquor (AHL). Experimental results from HTC, biomethane potential (BMP) tests, and storage emission measurements were combined with life cycle assessment (LCA) and techno-economic analysis (TEA) to evaluate system performance relative to conventional digestate sanitization. Results show that AHL recirculation reached cumulative methane yields of 343-345 NmL CH4 g.¹ VS after 50 days, while hydrochar significantly reduced methane emissions during storage, particularly at higher HTC temperatures. Compared with the reference system, HTC reduced climate impacts by 56% (160 °C) and 64% (220 °C), mainly due to improved carbon stabilization and reduced emissions from digestate handling. The TEA indicated internal rates of return of 18% (160 °C) and 9% (220 °C), demonstrating economic viability, particularly at lower HTC temperature.