Unlocking sustainable opportunities for sludge-derived aqueous phase via hydrothermal carbonization

Hydrothermal carbonization (HTC) minimises the volume and environmental hazards of sludge while generating useful products. However, the substantial volumes of toxic HTC-aqueous phase (HTC-AP) generated by this technology are often overlooked, posing a significant barrier to its advancement. This review thoroughly examines the characteristics, transformation mechanisms, treatment, and recycling technologies associated with sludge-derived HTC-AP. Additionally, it suggests utilizing machine learning to forecast the properties of HTC-AP. The characteristics of HTC-AP are closely linked to the chemical properties of the sludge and parameters of the HTC process, with higher proportions of high-nitrogen sludge and higher temperatures correlating with increased toxicity. Detoxification is essential before HTC-AP recycling. Achieving the full treatment and recycling of HTC-AP in a single process is challenging and necessitates greater integration and synergy between systems. A strong connection between the upstream sludge HTC process and downstream HTC-AP value-added strategy is vital for ensuring process sustainability. Furthermore, this review proposes a novel strategy that combines a machine learning approach with multi-process integration using systems thinking to enhance the energy yield and economic performance and optimize the HTC process. This comprehensive review provides guidance for optimizing sludge-derived HTC-AP recycling and HTC process regulations, with the goal of increasing the sustainability of HTC for sludge management.