# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Research on Influencing Factors of Sludge Treatment by HTO
This is likely due to the increased production of hydroxyl radicals, which have strong oxidative capabilities. They concluded that the highest TCOD removal rate occurs when the oxygen addition is at 1.0 MPa [46]. [PERSON] et al. explored not only the effects of reaction temperature and time, but also the impact of oxygen supply on hydrothermal oxidation. Their results confirmed that higher oxygen supply effectively accelerates the oxidation reaction rate, thereby facilitating the removal of organic compounds, including COD and VSS [35]. [PERSON] et al. found that the COD removal rate is significantly influenced by the amount of oxygen added; as the initial oxygen pressure increases, the wet oxidation process becomes more favorable. In contrast, the VSS removal rate is only slightly affected by variations in oxygen supply. This may be attributed to the hydrolysis of the sludge, as VSS removal remains above 80% even when the initial oxygen pressure is at 0.2 MPa [38]. [PERSON] et al. also observed that COD removal significantly increases with changes in oxygen pressure, while VSS removal rates show little change, consistent with the findings of previous studies [15]. For municipal sludge, increasing the initial oxygen pressure affects the TOC removal rate and acetic acid generation during HTO. For instance, [PERSON] et al. investigated the effects of reaction temperature and oxygen pressure on dissolved organic matter in the hydrothermal treatment of urban sludge. Their results indicated that the optimal TCOD and VSS removal rates occur at an initial oxygen pressure of 0.9 MPa, with both rates increasing as the reaction temperature and oxygen pressure rise [40]. However, changes in oxygen pressure do not always influence sludge treatment. [PERSON] et al. studied the effects of HTO on sludge and found that, while the TOC dissolution rate rapidly increased in the initial reaction stage with rising temperatures, variations in oxygen pressure did not significantly enhance TOC dissolution rates. Notably, at high temperatures, increasing oxygen pressure leads to a significant rise in acetic acid concentration, while changes in other VFAs remain minimal [29]. In summary, based on the discussions from the studies and the results in Table 5, it is evident that changes in initial oxygen pressure have a greater impact on COD removal rates compared to VSS removal rates. Additionally, the effect of oxygen pressure becomes more pronounced with increasing temperature. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Sludge Type**} & **Treatment Process** & \multirow{2}{*}{**Optimal IOP**} & \multirow{2}{*}{**Effect of Oxygen Pressure (P) Change**} & \multicolumn{2}{c}{**Change in**} \\  & & & & & **(mg/L)** & **Reference** \\ \hline \multirow{4}{*}{Urban Sludge} & Thermal hydrolysis-HTO & & & P increases, the change in oxygen pressure does not significantly improve the solubilization rate of TOC but leads to a significant increase in acetic acid concentration & \multirow{2}{*}{[29]} \\  & & & & & **(mg/L)** & \\ \hline \multirow{4}{*}{Urban Sludge} & Thermal hydrolysis-HTO & & & P increases, the change in oxygen pressure does not significantly improve the solubilization rate of TOC but leads to a significant increase in acetic acid concentration & \multirow{2}{*}{[29]} \\  & & & & & **(mg/L)** & \\ \hline \multirow{4}{*}{Coal Chemical Sludge} & \multirow{4}{*}{HTO} & \multirow{4}{*}{1.3 MPa} & \multirow{4}{*}{P increases, the removal rate of COD increases significantly, while the removal rate of VSS increases gradually} & \multirow{4}{*}{[35]} \\  & & & & & **(mg/L)** & \\ \cline{1-1} \cline{4-6}  & & & & & **(mg/L)** & \\ \hline \end{tabular}
\end{table} 
Table 5: Influence of initial oxygen pressure (IOP) change on sludge treatment by HTO technology in recent years.