# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Research on Influencing Factors of Sludge Treatment by HTO

Table 3: _Cont._Reaction time  
Aside from reaction temperature, adjusting the reaction time also has a significant impact on the process. Reaction time refers to the duration for which the sludge undergoes oxidation under specific temperature and pressure conditions. This time directly affects the treatment efficiency. Generally, as reaction time increases, the removal rates of COD and VSS gradually improve. The main reason for this is that, during the hydrothermal oxidation process, the sludge undergoes hydrolysis, causing the VSS in the solid phase to quickly dissolve into the liquid phase. Over time, degradable organic matter accumulates in the liquid, and through the action of hydroxyl radicals and other reactive species, it is converted into intermediate products that are harder to oxidize further, such as carboxylic acids like acetic and formic acids [38]. The accumulation of these intermediates may adversely affect COD removal rates. Therefore, determining the optimal reaction time, considering both oxidation efficiency and economic cost, is crucial for enhancing sludge treatment performance. In current research on industrial sludge, notable examples include a study by [PERSON] et al., which optimized conditions at a reaction temperature of 260 \({}^{\circ}\)C, with a duration of 60 min, an initial oxygen pressure of 1.0 MPa, and a catalyst concentration of Cu/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) at 5.0 g/L. This setup achieved the highest removal rates for VSS at 93.6% and TCOD at 76.5%. The results indicate a linear relationship between the increase in removal rates and reaction time [46]. Similarly, [PERSON] et al. investigated the effects of reaction conditions on the hydrothermal oxidation of excess sludge during the treatment of caprolactam wastewater. They found that at 260 \({}^{\circ}\)C, with an initial oxygen pressure of 1.3 MPa and a duration of 60 min, the removal rates of COD and VSS were 78.6% and 89.3%, respectively. Additionally, the accumulation of carboxylic acids resulting from sludge degradation increased with extended reaction time. Both the removal rates of COD and VSS also exhibited gradual increases over time, showing a similar trend. Notably, the VSS removal rate was already high within a short timeframe, likely because the solid-phase organic matter in the sludge transfers to the liquid phase, thereby enhancing VSS removal [35]. For municipal sludge, the effects of varying reaction time in HTO technology are similar, highlighting the need to determine an optimal reaction time for effective sludge treatment. For example, [PERSON] et al. investigated the impact of various HTO conditions in the presence of NaOH on the treatment of urban sludge. They found that a reaction time of 30 min resulted in the highest SCOD yield at 21.67%. As reaction time increased, SCOD yield initially decreased before stabilizing. The concentrations of NH\({}_{3}\)-N and NO\({}_{3}\)-N gradually declined with extended reaction time. During the wet oxidation process, soluble proteins are degraded, leading to the formation of new organic compounds such as peptides and amino acids. Microbial cells also decompose over time, resulting in a gradual increase in the total phosphorus (TP) concentration. This study demonstrated that changes in reaction time significantly influence the harmlessness and resource recovery of sludge [11]. Additionally, [PERSON] et al. found that, at temperatures of 200 \({}^{\circ}\)C or higher, most organic matter dissolves within 10 min. With prolonged treatment time, the mineralization rate gradually increases due to sustained oxygen supply. The pattern of SCOD in sludge from 0 to 30 min shows an initial increase followed by a decrease, indicating rapid dissolution of organic matter from the solid phase, followed by gradual mineralization [42]. The current research on the effects of varying reaction time on the HTO of sludge reveals several key conclusions, which are summarized in Table 4: (1) the commonly used reaction time is typically around 60 min.