# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Issues and Optimization Strategies in HTO of Sludge
#### 2.5.2 Optimizing the Solution of HTO Technology
The research indicates that an appropriate number of additives can enhance COD and VSS removal rates and promote ammonia nitrogen generation. However, excessive additive amounts can lead to reduced dewatering performance or lower SCOD removal rates. Therefore, it is crucial to determine an optimal dosage to avoid secondary pollution caused by excessive additions. Overall, the addition of catalysts significantly enhances the effectiveness of HTO for sludge treatment, leading to improved pollutant removal, better dewatering, and greater resource recovery. In summary, CWO demonstrates unique advantages in sludge treatment. Compared to traditional hydrothermal oxidation, it achieves higher reaction efficiency, enabling deep oxidative decomposition of organic matter in sludge in a shorter time frame, significantly reducing processing time and energy consumption. Moreover, this technology is environmentally friendly, converting organic substances in sludge into harmless materials and facilitating resource recovery, thereby minimizing environmental pollution and treatment costs. As environmental protection standards continue to rise,  
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CWO has substantial development potential. It can be integrated with other methods to further enhance sludge treatment efficacy. In the future, ongoing optimization of catalysts and processes is expected to establish this technology as one of the mainstream approaches for sludge treatment, providing robust support for the harmless, reduced, and resource-efficient management of sludge. Moreover, oxidants and catalysts can work synergistically in HTO technology to achieve more significant treatment effects on sludge containing high levels of antibiotics. For instance, [PERSON] et al. studied the use of acidified red mud (ARM) in a catalytic wet peroxide oxidation (CWPO) system to degrade antibiotic-laden sludge and produce the hydrogen carrier formic acid. In their experiments, under conditions of 90 \({}^{\circ}\)C, a reaction time of 30 min, H\({}_{2}\)O\({}_{2}\) concentration of 20 mL/L, ARM dosage of 0.8 g/L, pH = 7, and stirring at 500 rpm, the yield of formic acid reached 792.38 mg/L [32]. Similarly, [PERSON] et al. investigated the use of Fe (II)-activated persulfate-assisted hydrothermal treatment (Fe (II)-PS-HT) to enhance nitrogen removal efficiency from sewage sludge (SS) under relatively mild conditions (150 \({}^{\circ}\)C, 20 min). Their findings revealed that the nitrogen removal rate from SS treated under these optimal conditions was 35.0% higher than that of SS subjected to conventional high-temperature treatment [64]. These studies demonstrate that the incorporation of oxidants and catalysts enables HTO technology to achieve excellent sludge treatment outcomes without the need for high temperatures. This approach offers new insights into the degradation and resource recovery of sludge. Subcritical water (100 \({}^{\circ}\)C \(<\) T \(\leq\) 373.946 \({}^{\circ}\)C), also known as superheated water, thermal pressurized water, or pressurized hot water (PHW), is a liquid with pressure above its saturated vapor pressure (SVP). [PERSON] et al. concluded that, in the presence of air, subcritical water at relatively high temperatures (200-325 \({}^{\circ}\)C) and pressures (5-15 MPa) can generate large amounts of highly reactive hydroxyl radicals with high oxidation potential. As a result, subcritical water oxidation has demonstrated excellent performance as an advanced oxidation process (AOP) for degrading refractory wastewater and sludge [65]. Based on this example and the above-mentioned content, it was found that the previous optimization efforts were all subcritical; although the treatment effect of sludge was enhanced, the reaction times were not fast, all of them being around 1 h or even longer. In order to speed up the reaction rate, researchers also discovered the supercritical water oxidation technology for sludge treatment.