# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Hydrothermal Oxidation Technology
#### 2.3.2 Principle of HTO Technology  
The principle of HTO in treating sludge primarily relies on the degradation or transformation of organic pollutants and harmful substances through oxidation reactions under high-temperature and high-pressure conditions. This technology takes advantage of the unique physical and chemical properties of water in subcritical or supercritical states, particularly its solubility and the accelerated rate of oxidation reactions, to achieve sludge reduction, detoxification, and resource recovery. Both subcritical (WO, CWO) and supercritical conditions in hydrothermal oxidation share common features, where the process involves two main steps: pyrolysis and oxidation. In the pyrolysis step, the floc structure of the sludge is disrupted, releasing macromolecular organic compounds into the liquid phase. Subsequently, during the oxidation step, these macromolecules are oxidized into smaller organic molecules, with organic nitrogen and phosphorus being converted into inorganic nitrogen (e.g., N\({}_{2}\), NH\({}_{3}\), NOx) and inorganic phosphorus (e.g., phosphates). The key difference between the two conditions lies \begin{table}
\begin{tabular}{c c c c c} \hline
**Comparison Item** & **Hydrothermal** & **Hydrothermal** & **Hydrothermal** & **Hydrothermal Oxidation** \\
**Carbonization (HTC)** & **Liquefaction (HTL)** & **Gasification (HTG)** & **(HTG)** \\ \hline Temperature & & & & \\ Conditions & 180 \({}^{\circ}\)C–250 \({}^{\circ}\)C & 250 \({}^{\circ}\)C–400 \({}^{\circ}\)C & 350 \({}^{\circ}\)C–600 \({}^{\circ}\)C & 125 \({}^{\circ}\)C–320 \({}^{\circ}\)C \\ \hline Pressure Conditions & 2-6 MPa & 10–25 MPa & 20–30 MPa & 0.5–10 MPa \\ \hline \multirow{4}{*}{Decomposition of Organic Matter} & Limited decomposition: large molecules may remain (the removal rate of COD: 68–75\%) & Moderate decomposition, mainly producing bio-oil (the removal rate of COD: 50–85\%) & High decomposition into gases, leaving little organic residue & Very efficient organic matter decomposition into small molecules (CO\({}_{2}\), H\({}_{2}\)O) (the removal rate of COD: more than 78\%) \\ \hline \multirow{4}{*}{Dehydration Effect} & Moderate dehydration, solid product still retains moisture (reduction around 50–70\%) & Produces liquid products, so dehydration is moderate (reduction around 60–80\%) & High dehydration, little solid residue & High dehydration efficiency, leading to effective sludge volume reduction (reduction around 80–90\%) \\ \hline \multirow{4}{*}{Resource Recovery Potential} & Produces solid & Produces bio-oil for energy, some nutrient recovery possible (HHV is 30–40 MJ/kg bio-oil) & Gaseous products like methane can be used as energy sources & High potential for heat recovery, metals and other resources are more easily recoverable \\ \hline \multirow{4}{*}{Advantages} & Low temperature, easy to operate, solid product for energy use & Produces valuable liquid fuels, higher energy recovery potential & High energy yield in the form of gases, efficient organic conversion & Significant reduction and harmless treatment effects, high pollutant decomposition efficiency, and great resource recovery potential \\ \hline \multirow{2}{*}{Disadvantages} & Large molecules remain, requires post-processing & Complex system, high pressure and temperature needed & Expensive setup, complex reaction control & Overall treatment efficiency: high, but improvements are needed in cost efficiency \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of commonly used hydrothermal technology and hydrothermal oxidation technology for sludge treatment [31; 32; 33]. in the oxidation mechanism. In subcritical conditions (WO, CWO), the oxidation process follows a traditional free-radical chain reaction, which may leave behind small molecular compounds like acetic acid in the final products [26; 28; 29]. In contrast, the SCWO process exhibits a higher efficiency in organic matter mineralization.