# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Issues and Optimization Strategies in HTO of Sludge
#### 2.5.2 Optimizing the Solution of HTO Technology
This approach allows for harmless treatment while reducing operational costs. The CWO technology is particularly useful for treating sludge contaminated with organic pollutants and heavy metals, offering advantages such as rapid reaction speed, effective treatment, and low energy consumption. In this technology, catalysts play a crucial role, and selecting an appropriate catalyst is essential for improving sludge treatment efficiency. Catalysts can be categorized into two types based on their phase: homogeneous and heterogeneous. Homogeneous catalysts, including soluble transition metal salts, are characterized by high activity and low cost. Among them, Cu-based catalysts are the most widely used; however, they exhibit biotoxicity and are challenging to recover. In contrast, heterogeneous catalysts can be reused and primarily include noble metal catalysts, transition metal catalysts, rare earth element catalysts, and carbon-based catalysts. The following section will introduce the catalysts used in CWO technology in recent years and their catalytic efficiencies, as shown in Table 6. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & **Original Sludge Type** & **Reaction Condition** & **Treatment Results** & **Reference** \\ Cu-Ce/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) & Antibiotic-laden sludge & 260 \({}^{\circ}\)C, 60 min, 1 MPa, 5 g/L (catalyst dosage) & The removal rate of COD: 81.2\% & [15] \\ Cu-loaded Al\({}_{2}\)O\({}_{3}\) & Maleic acid (500 mg/L) & 70 \({}^{\circ}\)C, 60 min, 5 g/L (catalyst dosage), H\({}_{2}\)O\({}_{2}\): 0.09 mol/L & The removal rate of TOC: 98\% & [50] \\ \hline Cu/Al\({}_{2}\)O\({}_{3}\) & Phenol (280 mg/L) & 70 \({}^{\circ}\)C, 60 min, 5 g/L (catalyst dosage), H\({}_{2}\)O\({}_{2}\): 0.09 mol/L & The removal rate of TOC: 96.8\% & [51] \\ \hline Cu-Ce/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) & Pharmaceutical sludge & 260 \({}^{\circ}\)C, 60 min, 1 MPa, 10 g/L (catalyst dosage) & The removal rates of COD and VSS: 62.5\%, 86.8\% & [22] \\ \hline Cu-Ce/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) & Coal chemical sludge & 260 \({}^{\circ}\)C, 60 min, 1 MPa, 7 g/L (catalyst dosage) & The removal rate of VSS: 93.2\% & [38] \\ \hline Cu/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) & Pharmaceutical sludge & 260 \({}^{\circ}\)C, 60 min, 1 MPa, 5 g/L (catalyst dosage) & The removal rate of COD and VSS: 76.5\%, 93.6\% & [46] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of catalysts used in CWO technology in recent years. Recent research indicates that non-homogeneous catalysts dominate current studies, reflecting their increasing importance. These catalysts serve diverse functions, enabling the treatment of sludge as well as the removal of organic pollutants from wastewater. Notably, catalysts composed of the same element can yield varying removal efficiencies for different types of sludge. For instance, when comparing the VSS removal rates of Cu-Ce/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) for antibiotic sludge, pharmaceutical sludge, and coal chemical sludge, it was found that the catalyst performed best with antibiotic sludge [15; 22; 38]. Therefore, selecting an appropriate catalyst is crucial for enhancing the efficiency of catalytic hydrothermal oxidation in sludge treatment. In addition, the related effects of the addition of catalysts on the treatment of sludge by HTO technology were summarized. The study found that the effects of the addition of catalysts on the treatment of sludge by HTO technology mainly included the following aspects: The first was enhanced removal of pollutants. For instance, [PERSON] et al. utilized a synthesized Cu-Ce/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) catalyst in a CWO process to treat excess activated sludge from coal chemical wastewater. The results showed that, under conditions of 7.0 g/L catalyst, 1.0 MPa initial oxygen pressure, 260 \({}^{\circ}\)C temperature, and a reaction time of 60 min, the removal rate of volatile suspended solids reached 93.2%, while the COD removal rate was 78.3% [38]. [PERSON] et al.