# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Research on Influencing Factors of Sludge Treatment by HTO
(4) The effect of pH  
In addition to factors such as reaction temperature, time, and initial oxygen pressure that directly affect reaction efficiency, pH also significantly influences the HTO process for various types of sludge or the treatment of industrial sludge, particularly pharmaceutical sludge. HTO under alkaline conditions enhances the potential for resource recovery. [PERSON] et al. found that, when the pH of the process was adjusted to 12.56, the concentration of volatile fatty acids (including acetic acid, propionic acid, isobutyric acid, and isovaleric acid) increased to 4819 mg/L. This suggests that adjusting the pH through the addition of reagents can improve the quality of HTO sludge solution as a potential carbon source [16]. Additionally, the addition of alkaline reagents promotes the removal of organic pollutants and the conversion of ammonia nitrogen during HTO. For example, [PERSON] et al. discovered that, after adding NaOH, the soluble chemical oxygen demand (SCOD) yield increased initially and then decreased as the NaOH concentration rose. The trends in NH\({}_{3}\)-N and NO\({}_{3}\)-N levels in the solution were consistent with the SCOD yield. This indicates that the addition of an appropriate amount of alkali promotes ammonia nitrogen conversion and organic pollutant removal from the sludge [14]. For municipal sludge, [PERSON] et al. studied Fe (II)-activated persulfate-assisted hydrothermal conversion of waste-activated sludge (WAS) and found that, without pH adjustment (initial pH is 6.5), Fe\({}^{2+}\)-activated PMS oxidation led to a 6.14-fold increase in standardized-capillary suction time (SCST) after 2 min of conditioning, a 58.3% increase in centrifuged weight reduction (CWR), and a reduction in moisture content (MC) to 57.5%, which was nearly equal to the value at pH 8. This result demonstrates that an excellent dewatering \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sludge Type**} & **Treatment Process** & **Optimal IOP** & **Effect of Oxygen Pressure (P) Change** &
\begin{tabular}{c} **Change in** \\ **Efficiency/Yield** \\ **(mg/L)** \\ \end{tabular} & **Reference** \\ \hline Coal Chemical & & & & P increases, the removal rate of COD & \\  & & & is greatly affected by the amount of & (P: 0.2–1 MPa) & \\  & & & oxygen added, and the removal rate & The removal rate of \\  & & & of VSS is only slightly affected by the & & COD: 43–78\%; and \\  & & & & possible reason for the removal of VSS & VSS: 82–93\% & \\  & & & is the hydrolysis of sludge & & \\ \hline Pharmaceutical & & & P increases, the removal rate of COD & & \\  & & & increases with increasing oxygen & (P: 0.2–1 MPa) & \\  & & & pressure (an increase in the & The removal rate of \\  & & & concentration of oxidants usually & COD: 30–60\%; and \\  & & & leads to an increase in the oxidation & VSS: 73–87.3\% & \\ \hline Pharmaceutical & & & & P increases, the removal rate of TCOD & (P: 0.2–1 MPa) & \\  & & & increases significantly, while the & The removal rate of \\  & & & increases significantly, while the & TCOD: 40–75\%; and \\  & & & removal rate of VSS increases slightly & VSS: 80–93.6\% & \\ \hline Pharmaceutical & & & & & (P: 0.2–1 MPa) & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._performance can be achieved under acidic conditions without pH adjustment [21]. Furthermore, [PERSON] et al. found that adding NaOH enhanced the removal rates of VSS and COD, indicating that alkaline conditions favor the removal of organic pollutants [47]. In summary, acidic conditions may accelerate reaction rates and facilitate the oxidation of some organic compounds, but they also increase the risk of equipment corrosion. Alkaline conditions reduce oxygen solubility and reactivity, slightly slowing reaction rates, but they promote the removal of organic pollutants from the sludge. A neutral pH allows for moderate reaction rates with less equipment corrosion.