# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Resource Recycling
#### 2.7.2 Liquid Phase Products  
The liquid products consist of partially oxidized organic substances and dissolved inorganic salts, mainly volatile fatty acids (mainly acetic acid, formic acid, propionic acid, etc.), ammonia nitrogen, etc. These liquids can be further processed to recover energy and nutrients, with several main applications as follows:  
1. Soil Amendments  
The liquid products obtained from hydrothermal oxidation contain essential nutrients. After appropriate treatment and purification, they can be used as liquid fertilizers for agricultural irrigation, providing nutrients to crops and promoting growth while reducing reliance on freshwater resources. For instance, [PERSON] et al. proposed a resource recovery method, revealing that, after treatment at 240 \({}^{\circ}\)C, phosphorus, potassium, and heavy metals remained in the solid phase. The effluent contained ammonium nitrogen at 2800 mg/L, amino acids near 3000 mg/L, humic acid derivatives at 2400 mg/L, and acetic acid at 3900 mg/L. Acetic acid aids in seed germination, while the other components are crucial for root growth. Thus, the solid products from sludge treatment have potential as soil amendments. Applying these products to impoverished soils can enhance organic matter content and promote plant growth. Their study demonstrated that diluted effluent significantly improved germination and development of three plant species, especially scallions, with a germination index exceeding 200% [42]. It is important to strictly control pollutant levels during application, employing methods such as filtration, sedimentation, and biological treatment to remove harmful substances and ensure compliance with standards.
2. Carbon Source  
Under hydrothermal conditions, sludge undergoes hydrolysis and oxidation, generating small organic acids such as acetic acid, formic acid, and oxalic acid as primary products, which can serve as carbon sources in wastewater treatment. [PERSON] et al. noted that using a Cu-Ce/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\) catalyst at an initial oxygen pressure of 1.0 MPa and 10 g/L of catalyst allowed the exothermic wet oxidation reaction to sustain itself. The produced low molecular weight carboxylic acids have potential commercial applications as organic carbon sources in biological wastewater treatment [22]. Similarly, [PERSON] et al. found that hydrothermally treated coal chemical sludge yielded primarily acetic acid, formic acid, and oxalic acid, which can also serve as carbon sources in wastewater treatment. Therefore, wet oxidation can facilitate the resource recovery of organic carbon [35]. Additionally, research by [PERSON] et al. indicates that sludge treated via catalytic hydrothermal oxidation has potential as a carbon source, as the degradation products mainly include acetic acid and propionic acid. Biological treatment experiments showed that the degradation liquid after catalytic wet oxidation has the potential to meet the carbon source requirements for biological treatment, aiding in the removal of COD and TN [15]. This work confirms the effectiveness of the catalyst in treating antibiotic-laden sludge, providing new insights for the rational disposal of such sludge.