# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Research on Influencing Factors of Sludge Treatment by HTO  
In order to improve the treatment efficiency of sludge by HTO technology, researchers have conducted a large number of explorations of process optimization. Summarizing the research in recent years, it is found that the factors affecting the treatment of sludge by HTO technology include reaction temperature, time, pressure, pH value, etc. Comparing the performances of various influencing factors in improving the efficiency of HTO technology, it is found that the change in reaction temperature has a more significant improvement on the treatment effect of various sludges. This is because high temperatures can accelerate the dissolution and mineralization of sludge and promote the reaction. In contrast, reaction time and pressure have relatively less influence. The following section will discuss these factors and their impacts on the treatment of various sludges in detail. (1) Reaction temperature  
The reaction temperature of HTO technology is the most important parameter affecting the HTO process. Studies have shown that reaction temperature directly affects the reaction rate and the distribution of products. Generally, a higher temperature can increase the reaction rate of various sludge treatments and promote the degradation of organic matter, thus affecting the removal efficiency of COD and VSS [38]. In addition, it also has a certain influence on the generation of volatile fatty acids. Next, this article will conduct relevant discussions and summaries on the impact of temperature changes on the results when HTO is used to treat two types of sludge, municipal sludge and industrial sludge. The study on municipal sludge treatment found that the change in the reaction temperature of HTO technology is crucial to sludge treatment, and the increase in reaction temperature is conducive to enhancing the removal ability of organic pollutants and the recovery of materials that can be used as carbon sources. For example, [PERSON] et al. studied the treatment effect of wet oxidation technology on sewage plant sludge after the addition of a catalyst and compared the primary and secondary relationships of three influencing factors: temperature > concentration of Ca(ClO)\({}_{2}\) > molar ratio of FeSO\({}_{4}\).7H\({}_{2}\)O/Ca(ClO)\({}_{2}\). The results prove that reaction temperature is an important factor affecting the sludge dewatering performance. The concentration of Ca(ClO)\({}_{2}\) and the molar ratio of FeSO\({}_{4}\).7H\({}_{2}\)O/Ca(ClO)\({}_{2}\) have less influence. And they simultaneously used SPSS software to analyze the Pearson correlation coefficients of 18 operating parameters on sludge dewaterability (CST, SRF, and W\({}_{c}\)). The results show that temperature is the main influencing factor [39]. [PERSON] et al. studied the effects of temperature and oxygen pressure on HTO reaction and the basic properties of dissolved organic matter (DOM). The results show that, at 260 \({}^{\circ}\)C and an oxygen pressure of 0.9 MPa, the removal rates of total chemical oxygen demand (TCOD) and VSS reach the maximum values of 72% and 99%, respectively. And it is concluded that the influence of reaction temperature on the removal of TCOD and VSS is usually more significant in the operation of hydrothermal oxidation technology. An increase in temperature leads to an increase in the removal rates of TCOD and VSS. In addition, formic acid is generally present at lower temperatures, while acetic acid accumulates with the increase in temperature and oxygen pressure [40]. [PERSON] et al. also found that an increase in the reaction temperature of HTO technology is beneficial to the formation of volatile fatty acids such as acetic acid after municipal sludge treatment. Among them, the concentration at 160 \({}^{\circ}\)C for 180 min is 1.28 \(\pm\) 0.07 g/L (g/L in this paper is equivalent to g/dm\({}^{3}\) in the SI standard), and the concentration at 180 \({}^{\circ}\)C for 180 min is 1.65 times.