# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Research on Influencing Factors of Sludge Treatment by HTO
However, the degradation rates of non-volatile fatty acids such as oxalic acid, pyruvate acid, and formic acid accelerate with the increase in temperature, while the degradation rate of formic acid is not so obvious. But the decrease in the maximum concentration of short-chain fatty acids (SCFAs) due to excessive temperature can be attributed to the greater mineralization of organic matter. At higher temperatures, the oxidation pathway leading to the formation of CO\({}_{2}\) is more favorable than the oxidation pathway leading to the formation of acetic acid [41]. [PERSON] et al. also found that, under optimal conditions, as the reaction temperature increases, the acetic acid concentration increases from 1220 mg/L to 3250 mg/L, and the contribution of protein decreases from 26.7% to 2.3%. The content of carbohydrates also decreases with the increase in temperature due to the oxidation of carbohydrates and the loss of carbonyls related to the \"caramel\" or Maillard reaction. This study proves that the change in temperature has an important influence on the content changes of some components in sludge [42]. In addition, [PERSON] et al. found that when persulfate and sludge are co-treated hydrothermally, higher temperatures and solid contents will also lead to the melting and deformation of microplastics (MPs), exacerbating the aging of polypropylene MPs, resulting in a rough surface, an increase in the carbonyl index, and a change in crystallinity [43]. For industrial sludge, the reaction temperature of HTO technology also has a significant impact on the removal of pollutants and resource recovery treatment. For example, [PERSON] et al. discussed the influence of reaction conditions on the wet oxidation of excess sludge in the treatment process of caprolactam wastewater. The results show that, as the reaction temperature increases from 180 to 260 \({}^{\circ}\)C, the removal of COD and VSS is accelerated because a higher reaction temperature means higher reaction energy, which is beneficial to the increase in COD and VSS removal rates. It is worth noting that when the reaction temperature is too high, the removal rate of COD will not increase significantly because it may induce corrosion and is not conducive to equipment maintenance [35]. [PERSON] et al. designed an improved CWO method for treating excess activated sludge in the process of coal chemical wastewater treatment. The results prove that, as the temperature increases, the solubility of oxygen increases, which increases the removal of organic pollutants in the solution. In addition to enhancing the removal efficiency of organic pollutants, an increase in reaction temperature can also increase the concentration of volatile fatty acids [38]. [PERSON] et al. 's study also found the same conclusion. As the reaction temperature increases, the removal rates of COD and VSS increase significantly, and an increase in reaction temperature can also promote the production of volatile fatty acids [15]. Combining all these results with the results of the impact of temperature changes on sludge treatment by hydrothermal oxidation technology sorted out in Table 3 shows that reaction temperature is a key reaction parameter in the reaction and can significantly affect the relevant treatment effects of sludge, mainly in the following aspects: (1) The higher the reaction temperature, the removal rate of organic pollutants in sludge, and the removal rates of COD and VSS are all significantly enhanced. However, too high a temperature may reduce or stabilize the removal rate. (2) An increase in reaction temperature is beneficial to enhancing the dewatering performance of sludge. This is because the sludge particles become smaller and the pores increase significantly after treatment by HTO technology, which is conducive to the release of intercellular water, so the dewatering performance is improved.