# 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
compared the efficiency of sludge treatment using HTO with and without catalysts, finding that the use of a homogeneous catalyst improved COD removal efficiency by over 10%. This enhancement is attributed to the ability of the catalyst to lower operational temperatures, increase reaction rates, and shorten retention times. Furthermore, when comparing different catalysts, it was observed that using Cu-Ce/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) at 260 \({}^{\circ}\)C, 1.0 MPa, and 10 g/L for 60 min yielded the highest removal rates for VSS and COD, reaching 87.3% and 72.6%, respectively [22]. The second was an improved dewatering performance: [PERSON] et al. studied the interaction between peroxydisulfate, zero-valent iron (ZVI), and reaction time on sludge dewatering. Under optimal experimental conditions, the moisture content of the sludge was reduced to 54%, enhancing the dewatering performance of the hydrothermal oxidation technology. Additionally, when using the SC-M catalyst, the removal rates for metacresol and TOC were 98.1% and 84.2%, respectively [62]. The \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & **Original Sludge Type** & **Reaction Condition** & **Treatment Results** & **Reference** \\ \hline Cu/g-C\({}_{3}\)N\({}_{4}\)@AC & Phenol & 120 \({}^{\circ}\)C, 60 min, 0.5 MPa & The removal rate of COD: 92.6\% & [52] \\ \hline FeOCl/Montmorillonite & Fuchsine (200 mg/L) & 60 \({}^{\circ}\)C, 210 min, 1 g/L (catalyst dosage), H\({}_{2}\)O\({}_{2}\): 0.38 mol/L & The removal rate of COD: 70.8\% & [53] \\ \hline CuO doped with Ce & Quinoline (100 mg/L) & 75 \({}^{\circ}\)C, 85 min, 1 g/L (catalyst dosage), H\({}_{2}\)O\({}_{2}\): 196 mmol/L, pH = 7.3 & The removal rate of Quinoline & \\  & & 140 \({}^{\circ}\)C, 120 min, 2 MPa, 10 g/L & The removal rate of COD: 98.1\%, 86.1\% & [54] \\ \hline Cu/TiO\({}_{2}\) & Cresol (1000 mg/L) & 140 \({}^{\circ}\)C, 120 min, 2 MPa, 10 g/L & The removal rate of COD: 79.1\%, the degradation of cresol: 100\% & [55] \\ \hline Ce-modified Cu-based & Phenol (1000 mg/L) & 160 \({}^{\circ}\)C, 330 min, 3 MPa, 40 mg (catalyst dosage), pH = 9 & The removal rate of COD: 89\% & [56] \\ \hline N-La\({}_{2}\)CuO\({}_{4}\) & Phenol (8000 mg/L) & 140 \({}^{\circ}\)C, 30 min, 1 MPa, 1 g/L (catalyst dosage), pH = 9 & The removal rate of COD: 87.1\% & [57] \\ \hline Fe\({}_{5}\)C@NCNT/PSSF & Phenol (1000 mg/L) & 80 \({}^{\circ}\)C, 7 h, H\({}_{2}\)O\({}_{2}\): 5.1 g/L & The removal rate of TOC: 41\%, phenol conversion: 90\% & [58] \\ \hline PrFe\({}_{5}\)Co\({}_{1}\)\({}_{\text{ex}}\)O\({}_{3}\)/Mt & 2-Hydroxybenzoic acid & 80 \({}^{\circ}\)C, 210 min, 0.5 g/L (catalyst dosage), H\({}_{2}\)O\({}_{2}\): 20.56 mmol/L, pH = 5.5 & Degradation rate 97.6\%, the removal rate of COD: 75.23\% & [59] \\ \hline CoFe\({}_{2}\)O\({}_{4}\)@SiO\({}_{2}\) & Chlorobenzene & 110 \({}^{\circ}\)C, 40 min, 0.3 g/L (catalyst dosage), PS: 15 mM & The removal rate of TOC: 41\% & [60] \\ \hline CuO/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) & Reactive Red X-3B & 80 \({}^{\circ}\)C, 150 min, 5.5 g/L (catalyst dosage), H\({}_{2}\)O\({}_{2}\): 0.39 mol/L, pH = 8 & Degradation rate 90.72\%, the removal rate of TOC: 45.26\% & [61] \\ \hline FeSO\({}_{4}\)/Ca(ClO)\({}_{2}\) & Mechanical processing wastewater sludge & 180 \({}^{\circ}\)C, FeSO\({}_{4}\).7H\({}_{2}\)O/Ca (ClO)\({}_{2}\): 1.25, Ca(ClO)\({}_{2}\): 0.04, pH: 5.9 – 7.4 & Moisture content: 51.72\% (initial sludge: 76.30\%) & [20] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: _Cont._third was increased resource utilization of sludge as a carbon source: [PERSON] et al. found that adjusting catalysts and temperature during hydrothermal oxidation promotes the production of VFA from sludge. These VFAs can serve as an organic carbon source, thus reducing the overall costs associated with hydrothermal oxidation technology [46]. It is important to note that the regulation of catalyst dosage also has a significant impact on sludge treatment. Table 7 illustrates the effect of varying additive dosages on the hydrothermal oxidation of sludge in current studies.