Decoding the Valorization of Hydrothermal Carbonization Process Water: From Chemical Complexity to Circular Waste-Management Pathways

Hydrothermal carbonization (HTC) is considered a promising thermochemical process for converting organic wastes into high-quality char and process water (PW), a carbon- and nutrient-rich stream often considered wastewater. This review systematically analyzes the impacts of HTC operating conditions (i.e., feedstock, time, and temperature) on PW composition and its subsequent valorization routes, including PW recirculation, anaerobic digestion (AD), and soil amendment. We demonstrate that moderate HTC conditions (180–200 °C for 2 to 4 h) using nitrogen-rich feedstocks generate PW with high biodegradability and low toxicity, making it favorable for biological valorization such as AD. In contrast, high-severity conditions with lignocellulosic feedstocks are preferable for recirculation, which enhances hydrochar yield and fuel properties through in situ catalysis. The novelty of this work, which proposed an integrated engineering HTC-PW framework, systematically links process parameters, PW chemistry, and optimal downstream application, including detoxification strategies and techno-economic feasibility. This integrated approach moves HTC towards a circular loop by enhancing energy and nutrient recycling efficiency, mitigating environmental impact through direct discharge, and contributing to achieving multiple Sustainable Development Goals (SDGs 6, 7, 12, 13, and 15).