# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Hydrothermal Oxidation Technology
#### 2.3.2 Principle of HTO Technology
Due to the extremely high temperature and pressure, oxidation is not limited to the conventional free-radical chain reactions; instead, direct oxidation between oxygen molecules and organic compounds occurs in the supercritical state, leading to complete mineralization into carbon dioxide and water. Therefore, the reaction mechanisms involved in the oxidation process include the oxidation and mineralization of organic matter, the transformation of organic nitrogen, and the transformation of organic phosphorus, as shown in Figure 3.  
First is oxidation and mineralization of organic matter: Under subcritical conditions in HTO, the process mainly involves the traditional free-radical chain reactions, as shown in Figure 3a, which can be divided into three stages: chain initiation, chain propagation and development, and chain termination [34; 35]. (1) Chain initiation: the process by which reactant molecules generate free radicals. In this process, the organic matter in the sludge will produce HO- through thermal reaction with oxygen, as follows (1)-(3):  
\[(\mathrm{SS})\mathrm{RH}+\mathrm{O}_{2}\rightarrow\mathrm{R}\cdot+\mathrm{ HOO}\cdot\ ((\mathrm{SS})\mathrm{RH}\ \mathrm{is\ organic\ matter\ in\ sludge}), \tag{1}\]  
\[(\mathrm{SS})\mathrm{RH}+\mathrm{HOO}\cdot\ \mathrm{R}\cdot+\mathrm{H}_{2} \mathrm{O}_{2}, \tag{2}\]  
\[\mathrm{H}_{2}\mathrm{O}_{2}+\mathrm{M}\rightarrow\mathrm{2 OH}\cdot\ (\mathrm{M\ as\ catalyst}), \tag{3}\]  
(2) Chain development or transfer: Free radicals interact with molecules, alternately making the number of free radicals increase rapidly. As shown in the following Formulas (4)-(6),  
Figure 3: Reaction mechanism of sludge treatment by hydrothermal oxidation technology (including (**a**) free radical reaction, (**b**) organic nitrogen conversion, (**c**) phosphorus conversion. due to its high electron affinity (568 kJ), HO- can oxidize all hydrogen-containing organic compound RH in the sludge to generate organic free radical R-, while organic free radical R- reacts with O2 to form organic peroxyradical ROO-, which further removes hydrogen atoms from RH. The organic hydroperoxide ROOH and another organic radical R- are formed. \[(\mathrm{SS})\mathrm{RH}+\mathrm{HO}\cdot\rightarrow\mathrm{R}\cdot+\mathrm{H_{2}O}, \tag{4}\]  
\[(\mathrm{SS})\mathrm{RH}+\mathrm{O_{2}}\rightarrow\mathrm{ROO}\cdot, \tag{5}\]  
\[\mathrm{ROO}\cdot+\mathrm{(SS)RH}\rightarrow\mathrm{ROOH}+\mathrm{R}\cdot, \tag{6}\]  
(3) The termination of the chain (7)-(9): Free radicals collide with each other to generate stable molecules, and the growth process of the chain is interrupted. \[\mathrm{R}\cdot+\mathrm{R}\cdot\rightarrow\mathrm{R}-\mathrm{R}, \tag{7}\]  
Figure 3.0  
Figure 3.1  
From the above reaction mechanism, it can be seen that the concentration of free radicals (HO-, HOO-, R-, ROO-), as the intermediate transfer of the whole chain reaction, plays a decisive role in enhancing the efficiency of the hydrothermal oxidation technology. In order to improve the application of hydrothermal oxidation technology in practice, it is necessary to comprehensively consider and optimize according to the characteristics of sludge, treatment requirements, and economic costs. In the supercritical state, the mineralization process is a simple direct oxidation of oxygen molecules and organic matter to produce carbon dioxide and water [36]. Second is transformation of organic nitrogen [36]. A comparison between subcritical and supercritical conditions reveals differences in the transformation of organic nitrogen. In subcritical conditions, due to the relatively mild reaction environment, the conversion of organic nitrogen is often incomplete, and the main products include ammonia (NH3), nitrites (NO2-), and nitrates (NO3-). In contrast, under supercritical conditions, the conversion efficiency is significantly faster, and the residual nitrogen oxides (NOx) are minimal. Ammonia nitrogen (NH3-N) can be further oxidized to small molecules like N2 under appropriate conditions. As illustrated in Figure 3b, [PERSON] et al.