# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 1 Introduction
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Treatment** \\ **Method** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Investment,** \\ **Operating COST** \\ \end{tabular} } & \multirow{2}{*}{**Advantages**} & \multirow{2}{*}{**Disadvantages**} & \multirow{2}{*}{**China**} & \multirow{2}{*}{\begin{tabular}{c} **Development Trend** \\ **Counting** \\ \end{tabular} } \\ \cline{5-6}  & & & & & **China** & \begin{tabular}{c} **Other Developed** \\ **Counting** \\ \end{tabular} \\ \hline \multirow{4}{*}{\begin{tabular}{c} Landfilling \\ \end{tabular} } & \multirow{4}{*}{Low, Low} & Low treatment cost, simple operation & \begin{tabular}{c} Occupies large \\ land area, potential \\ groundwater \\ contamination, not \\ sustainable in the \\ long term \\ \end{tabular} & \begin{tabular}{c} Usage is high, \\ declining in recent \\ years, but still \\ common (small \\ urban and rural) \\ \end{tabular} & \begin{tabular}{c} Restricted by \\ environmental laws, \\ the use of the \\ European Union and \\ other countries has \\ gradually declined \\ \end{tabular} \\ \hline \multirow{4}{*}{\begin{tabular}{c} Aerobic \\ Composting \\ \end{tabular} } & \multirow{4}{*}{Medium, Medium} & Enables resource recovery, produces fertilizers requirements for sludge properties & \
\begin{tabular}{c} Requires large \\ space, long \\ treatment cycle, \\ specific \\ requirements for \\ sludge properties \\ \end{tabular} & Mostly used in urban sewage treatment plants &
\begin{tabular}{c} It is one of the main \\ treatment methods \\ used by sewage \\ treatment plants in \\ developed countries \\ such as Europe \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of technical investment, operating cost, advantages and disadvantages, and development trends of sludge treatment at this stage. To overcome these drawbacks, hydrothermal oxidation technology (HTO) has garnered increasing attention. As a novel sludge treatment method, as shown in Figure 1, HTO offers high treatment efficiency, effective detoxification, and potential resource recovery. It is well known that \"hydrothermal (HT)\" refers to homogeneous or heterogeneous chemical reactions occurring in aqueous solutions within closed systems at temperatures above 100 \({}^{\circ}\)C and pressures exceeding 1 bar (1 bar = 0.1 MPa). Hydrothermal oxidation, as a subset of hydrothermal technologies, involves thermochemical reactions in the presence of an oxidizing agent during the hydrothermal process. HTO can be classified based on the state of water into subcritical water oxidation (120-370 \({}^{\circ}\)C, 2-20 MPa) and supercritical water oxidation (above 574.2 \({}^{\circ}\)C and 22.1 MPa). Under subcritical conditions, such as in wet oxidation (WO) or catalytic wet oxidation (CWO), organic matter is partially oxidized, producing small organic molecules like acetic acid as the main byproducts. In contrast, during supercritical hydrothermal oxidation, organic matter is typically mineralized into carbon dioxide and water, with a mineralization efficiency that can reach as high as 99.999% [11; 14; 15].