# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### The Application and Disadvantages of HT in Sludge  
To achieve the goals of sludge reduction, harmlessness, and resource recovery, researchers have conducted extensive studies. Findings suggest that hydrothermal technology (HT), as an emerging sludge treatment method, has attracted significant attention in recent years. This technology operates under specific temperature and pressure conditions to treat sludge. Through HT reactions, the organic matter in sludge can be decomposed and transformed, thereby reducing its organic content and overall volume [23]. Additionally, HT technology promotes the stabilization of heavy metals in sludge, mitigating potential environmental hazards. However, literature reviews indicate that the effectiveness of standalone hydrothermal treatments is limited. For example, research by [PERSON] et al. found that the addition of tann acid (TA) could assist in HT treatment by lowering the effective HT temperature for deep dewatering of sludge proteins. Nonetheless, this approach faces significant economic challenges and has limited effectiveness in removing antibiotics and protein binding [24]. As the development of technology, HT technology has been continuously optimized, and the three commonly used methods at this stage mainly include hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL), and hydrothermal gasification (HTG). Despite their respective advantages, these technologies still face challenges in practical applications, as shown in Table 2. Among the three commonly used technologies, HTC is the most widely applied. It operates at temperatures between 180 \({}^{\circ}\)C and 250 \({}^{\circ}\)C, producing a solid byproduct that can be used as biochar or a soil amendment. However, the treated sludge may still contain high levels of organic matter, particularly large organic molecules, and requires further dewatering. In addition, HTC has limited effectiveness in removing heavy metals from the sludge, which constrains its potential for safe disposal and resource recovery. As noted by [PERSON] et al., the hydroc  
Figure 2: Source and characteristics of sludge. contains higher concentrations of heavy metals and has low energy value, limiting its use in agriculture and energy applications. To address this, their study optimized the process by using co-hydrothermal carbonization to improve the performance in these areas [25]. HTL, on the other hand, operates at higher temperatures (250 \({}^{\circ}\)C-350 \({}^{\circ}\)C) and produces liquid fuels such as bio-oil. However, the efficiency of HTL varies significantly depending on the types of organic materials present in the sludge, and the resulting liquid phase often requires further treatment to meet discharge standards. For example, [PERSON] et al. found that the liquid phase from HTL-treated sludge did not meet regulatory standards, necessitating additional treatment using wet oxidation technology [26]. Moreover, persistent organic pollutants and heavy metals may still remain in the treated sludge, affecting the quality and usability of the final product. Compared to HTC and HTL, HTG is less frequently applied. HTG converts sludge into gaseous products, such as methane, at higher temperatures (350 \({}^{\circ}\)C-600 \({}^{\circ}\)C), achieving significant reduction in sludge volume. However, this process is energy-intensive and requires complex equipment. As noted by [PERSON] et al., supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) still face technical challenges in achieving high efficiency. For instance, initiating and sustaining the reaction requires preheating the feedstock to supercritical temperatures, typically using direct heating via fuel combustion or electric heaters (EH). Furthermore, coking and salt precipitation during preheating reduce the efficiency of the EH and heat exchangers (HX), potentially leading to blockages and system shutdowns [27].