# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Research on Influencing Factors of Sludge Treatment by HTO
(3) As the reaction temperature continues to increase, the concentration of volatile fatty acids in the treated sludge is higher than that of non-volatile fatty acids, and most research results show that the concentration of acetic acid is significantly increased because a higher temperature can promote the solubilization and mineralization of these organic substances. (4) In addition to the above three common conclusions, researchers have also found that as the reaction temperature increases, the generation of NH\({}_{3}\)-N will also increase significantly, gaseous carbon w\({}_{\text{CO}_{2}}\) will rise, and liquid carbon \(\omega_{\text{aqueous}}\) and \(\omega_{\text{solid}}\) will decrease, proving that an increase in temperature is conducive to the conversion of carbon from liquid to gaseous. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sludge Type** & **Treatment Process** & **Optimal Temperature (\({}^{\circ}\)C)** & **Impact of Temperature (T) Change** & **Change in Efficiency/Yield** & **References** \\ \hline Urban Sludge & HTO-Alkaline & 214 & T increases, SCOD yield significantly increases then stabilizes, NH\({}_{3}\)-N generation significantly increases & (T: 130–210–290 \({}^{\circ}\)C) & \\ Urban Sludge & HTO-Alkaline & 214 & T increases, SCOD yield significantly increases then stabilizes, NH\({}_{3}\)-N generation significantly increases & 25,000–35,000–30,000; & [11] \\ Urban Sludge & Thermal hydrolysis -HTO & 180 & T increases, TOC removal rate significantly improves, carbohydrates and proteins increase then decrease, the concentration of NH\({}_{3}\)-N and acetic acid significantly increases & [29] \\ \hline  & & T increases, potential toxicity of Pb increases, potential toxicity of Cd increases then decreases (still lower than raw material), the concentration of acetic acid & (T: 100–250 \({}^{\circ}\)C) & \\ Municipal Sludge & CWPO & 230 & T increases, and the concentration of propionic acid, isobutyric acid, and n-butyric acid decreased gradually, the concentration of ammonia nitrogen also increased first and then decreased & 25,000–35,000–30,000; & \\ Urban Sludge & HTO & 160 & T increases, and it is beneficial to the formation of acetic acid. Volatile fatty acids are produced more than non-volatile fatty acids (the concentration of propionic acid increases with increasing temperature, while oxal acid, pyruvate, and formic acid are degraded) & [44] \\ Urban Sludge & HTO & 160 & T increases, the dissolution of EPS and the release of heavy metals are accelerated; the removal rates of Cu decrease first and then increase, and the removal rate of Zn decrease with the increase in temperature & [41] \\ \hline  & & & T increases, the removal of SCOD decreases, the increase in temperature promotes the conversion of organic nitrogen to NH\({}_{3}\)-N and phosphorus from organophosphates and phosphates to non-hydroxyl phosphates, the concentration of humic acid, protein, and polysaccharide increases first and then gradually decreases & (T: 180–260 \({}^{\circ}\)C) & \\ Urban Sludge & HTO & 240 & T increases, the removal rates of COD and VSS are significantly increased & 25,000–30,000; & \\ Urban Sludge & HTO & 260 & T increases, the removal rates of COD and VSS are significantly increased & 20–72.6\%; and VSS: & \\ \hline  & & & T increases, the removal rates of & (T: 180–260 \({}^{\circ}\)C) The removal rate of COD: 20–72.6\%; and VSS: & \\ Pharmaceutical Sludge & CWO & 260 & T increases, the removal rates of & (T: 180–260 \({}^{\circ}\)C) The removal rate of COD: 20–72. It is worth noting that if the reaction temperature is too high, excessive acetic acid generated in the reaction is not easily oxidized, which will lead to no significant increase in the COD removal rate. On the other hand, excessive reaction temperature will cause more serious corrosion, which will increase the construction cost of the reaction system [35].