# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion  
### Introduction of Sludge  
As is shown in Figure 2, sludge, as an inevitable byproduct of domestic wastewater and industrial effluent treatment processes, is a viscous substance composed of solid impurities, suspended solids, and colloidal materials [17]. Essentially, it represents the solid fraction of wastewater. Depending on its source, sludge can mainly be categorized into two types:  
1. Municipal sludge: This type is derived from urban wastewater treatment plants and primarily generated from domestic sewage. It contains a certain amount of organic matter, nitrogen, phosphorus, and other nutrients, which can potentially be utilized as resources under specific conditions. However, municipal sludge also inevitably contains harmful substances such as pathogens, which, if not properly treated, can pose significant risks to the environment and human health [17; 18].
2. Industrial sludge: This sludge is produced during various industrial processes, including those in the chemical, pharmaceutical, dyeing, papermaking, and metallurgy sectors. Due to the diverse raw materials and processes used in different industries, industrial sludge has a highly complex composition. It may contain heavy metals and toxic substances, significantly increasing the challenges associated with its treatment [18].  
In-depth research reveals that sludge possesses several key characteristics [19; 20; 21; 22]: first, sludge typically has an extremely high moisture content, often exceeding 95%. Such a high water content significantly increases the sludge volume, posing substantial challenges for subsequent treatment and escalating transportation costs and complexity. The water in sludge exists in four forms: free water, surface water, interstitial water, and bound water. Among these, the non-free water (surface, interstitial, and bound water) is tightly bound to sludge particles, making it difficult to separate using conventional methods. This further contributes to the large volume of sludge, complicating treatment and dewatering efforts. Second, sludge is rich in organic matter, containing a large number of microorganisms,bacteria, parasite eggs, and heavy metal ions. The presence of these components makes sludge prone to decomposition and odor, which can negatively impact the surrounding environment if not properly managed. Third, the particles in sludge are very small, resulting in a large specific surface area and strong adsorptive properties. This characteristic complicates the treatment process, as sludge can easily adsorb other substances. Finally, sludge has a relatively low specific gravity, generally less than that of water, which makes it readily suspend in water, further complicating treatment efforts.  
Given the composition and characteristics of sludge, several key aspects should be emphasized during treatment. On one hand, research should focus on volume reduction, particularly addressing the significant volume caused by the high moisture content. On the other hand, it is essential to remove residual organic pollutants and heavy metals from the sludge to achieve environmentally friendly resource recovery. Therefore, the search for efficient and sustainable sludge treatment technologies is of critical practical significance.