# 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  
(1) Oxidizer assisted HTO technology to treat sludge  
As is well known, oxygen is the most commonly used oxidant in sludge HTO. However, controlling oxygen pressure and other parameters can be quite complex. To address this issue, researchers have found that H2O2 can serve as an alternative oxidant. Its addition can replace oxygen, eliminating the need for air separation equipment, reducing gas-liquid mass transfer resistance, and accelerating the reaction rate. Moreover, oxidants like H2O2 have the advantages of being non-toxic, harmless, and highly oxidative. Additionally, H2O2 can react with organic pollutants to generate -OH radicals, which possess strong oxidative power (with a redox potential of 2.8 eV) and high selectivity, making them effective in removing harmful substances. In addition to H2O2, persulfates (PMS), such as potassium peroxymonosulfate and potassium peroxylsulfate, have also garnered significant attention due to their higher oxidation potential. These oxidants can react with organic pollutants to generate -SO4\({}^{2-}\) radicals, which also positively impact sludge treatment [4]. By adjusting the dosage of the oxidant, the redox potential of the reaction can be controlled, subsequently influencing temperature and pressure. For municipal sludge, the addition of oxidants can improve dewatering performance. For example, [PERSON] et al. studied the effect of PMS on sludge dewatering in hydrothermal oxidation processes. As the amount of PMS increased, both the SCST and CWR initially increased and then decreased with increasing PMS dosage, while the MC first decreased and then increased [21]. For instance, [PERSON] et al. investigated the effects of HTO temperature and H2O2 concentration on the product distribution of municipal sludge, including solid-phase moisture content, heavy metal migration behavior,VFAs, NH\({}_{3}\)-N levels, and pH. The results indicated that increasing the amount of H\({}_{2}\)O\({}_{2}\) and HTO temperature significantly improved sludge dewaterability. However, the potential toxicity fraction of solid residues containing Pb and Cd increased with higher H\({}_{2}\)O\({}_{2}\) concentrations yet remained lower than that of the original material. Additionally, both VFAs and NH\({}_{3}\)-N concentrations showed an increasing trend, facilitating further resource recovery from sludge [44]. Moreover, sodium persulfate (SPS) can also serve as an oxidant to assist hydrothermal technology in treating municipal sludge. [PERSON] et al. found that the active persulfate oxidation method is an effective deep treatment technology for improving sludge dewaterability. Under optimal conditions--a hydrothermal temperature of 145 \({}^{\circ}\)C, a reaction time of 2 h, and a persulfate concentration of 150 mg/g DS--the CST reduction efficiency reached a maximum of 90.5%. Furthermore, over 90% of phosphorus and heavy metals remained in the sludge cake after hydrothermal treatment, and the addition of sodium persulfate can convert phosphorus into more stable forms while removing heavy metals, thereby reducing environmental risks [49]. In summary, the incorporation of oxidants into hydrothermal oxidation technology shows tremendous potential and advantages in sludge treatment. Their application not only enhances treatment efficiency, but also improves sludge dewaterability and the degree of harmlessness. Treatment of sludge with catalyst-assisted HTO  
In addition to the fact that the addition of oxidants can improve the treatment of sludge by HTO technology, the addition of catalysts can as well. In recent years, the development of CWO technology has aimed to achieve safe and efficient sludge treatment [46]. By incorporating catalysts into the reaction system, this method effectively lowers the reaction temperature and pressure, enhances the oxidation efficiency, and shortens the reaction time, thus accelerating the oxidative decomposition of organic substances in sludge.