Synergistic Hydrothermal Treatment of Incineration Fly Ash and Sewage Sludge: Effect of Hydrothermal Solution Circulation

In our previous study, we demonstrated that the hydrothermal process is a feasible method for the synergistic treatment of incineration fly ash and sewage sludge. However, the hydrothermal solution inevitably produced during this process presents a challenging problem. In this article, we investigated the characteristics of the hydrothermal solution and discussed in detail the effects of its circulation on hydrothermal products (including both solution and solid phases). Results showed that pH and electrical conductivity in the hydrothermal solution increased with increasing cycle number. Total organic carbon (TOC) and total nitrogen in the liquid phase increased significantly with cycle number, although the proportion of organic acids in TOC remained small, not exceeding 30%. The process created a highly alkaline environment, which promoted heavy metal immobilization and improved the solid-phase yield through sedimentation or chemosynthesis. However, an ion “accumulation effect” occurred with increasing the cycle number of the hydrothermal solution. This suggests that pretreatment is necessary for solution reuse in practical applications.