# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Resource Recycling
#### 2.7.1 Solid Products  
The solid products obtained from the hydrothermal oxidation of sludge typically possess the following characteristics: first, the pollutants, mostly inorganic substances, are effectively removed; second, they contain essential nutrients such as nitrogen, phosphorus, potassium, calcium, and magnesium. Based on these characteristics, the solid products can be utilized in several ways.  
1. Construction Materials  
After hydrothermal oxidation treatment, the sludge demonstrates not only an improved dewatering performance, but also significantly reduced organic pollutants. The inorganic content of the solid products is relatively high and stable. Therefore, these solid products can be further processed for use as construction materials, such as brick-making raw materials. Their particle size and composition characteristics can meet the strength requirements for bricks, facilitating resource recovery from sludge and reducing the need for natural raw materials. For example, [PERSON] et al. demonstrated that, after catalyst treatment, the COD and VSS of sludge were reduced to a certain extent, and the resulting inorganic residues could be used for the production of construction materials [22]. [PERSON] et al. found that, following hydrothermal oxidation, the sludge particles decreased in size and showed increased porosity, which aided in the release of intercellular moisture, thereby enhancing dewatering performance. The solids can be used to produce ceramic aggregates or permeable bricks. Additionally, these materials can serve as fillers for wastewater treatment, effectively utilizing waste [38].
2. Fertilizers and Soil Conditioners  
The treated solid products contain certain amounts of nutrients and minerals, such as nitrogen, phosphorus, potassium, calcium, and magnesium, allowing for their use as fertilizers. For instance, [PERSON] et al. studied phosphorus recovery from wastewater sludge treated by hydrothermal oxidation. Under high-temperature (up to 300 \({}^{\circ}\)C) and high-pressure (up to 200 bar) conditions, the process achieved high removal rates of organic matter (up to 85% COD) and total solids (up to 75%). Liquid and solid fractions recovered from the effluent contained significant amounts of recoverable phosphorus. In the liquid effluent, phosphorus was recovered as struvite (MgNH4 PO4.6H2O), a slow-release fertilizer, reaching concentrations of up to 90 mg P/L. The recovered struvite can be utilized as fertilizer [37]. Similarly, [PERSON] et al. found that high reaction temperatures converted organic phosphorus and phosphate into non-hydroxyl phosphorus. The sludge cake enriched with non-hydroxyl phosphorus post-hydrothermal oxidation can serve as a soil conditioner. Notably, researchers also validated this hypothesis using seed germination  
Figure 4: Application of sludge-treated products.  
tests, finding that the effluent from treated sludge promoted plant germination and growth, although different seeds required varying concentrations. The results indicated that the effluent had a beneficial effect on seed growth [45].