# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 1 Introduction  
With the rapid development of industry, the number of wastewater treatment plants has increased significantly. Currently, the main wastewater treatment methods include biological treatment [1]; adsorption [2]; advanced oxidation processes such as UV/H\({}_{2}\)O\({}_{2}\), Fenton, and photo-Fenton [3]; UV/S\({}_{2}\)O\({}_{8}\)\({}^{2}\)- and UV/HSO\({}_{5}\)\({}^{-}\)[4]; plasma degradation processes [5,6]; photocatalysis [7]; coagulation-floculation [8]; electrochemical treatment [9];and membrane filtration [10]. Although these methods can effectively treat wastewater, another problem that arises after treatment is the residual sludge. Wastewater contains many suspended solids, and most of these solids form sludge through adsorption or sedimentation. Sludge is the main byproduct of wastewater treatment plants, and its production has been increasing year by year. According to the Ministry of Housing and Urban-Rural Development of China, the sludge produced by urban areas and wastewater treatment facilities exceeded 60 million tons in 2022 (based on an 80% moisture content), showing a rapid growth trend compared to 50 million tons in 2020. It is estimated that by 2025, the sludge discharge will reach 90 million tons. Therefore, sludge treatment and disposal have become the primary potential source of secondary pollution in the wastewater treatment process [11]. Research indicates that most sludge consists of microorganisms, organic and inorganic components, water-soluble substances, proteins, pathogens, polysaccharides, difficult-to-degrade organic pollutants, heavy metals, and salts [12]. Consequently, sludge generally has a high organic content, complex composition, and high moisture and ash content. Improper management of harmful substances in sludge can pose a serious environmental threat. To effectively manage sludge generated from wastewater treatment plants and other sources, researchers have made considerable efforts and conducted in-depth studies. Currently, there are four main traditional sludge treatment methods, as shown in Table 1: landfilling, aerobic composting, anaerobic digestion, and sludge drying and incineration [13]. The results in the Table 1 clearly show that the frequency of use of sludge treatment methods in China is landfill > anaerobic digestion > dry incineration > aerobic composting; The frequency of use in other developed countries is sorted as follows: anaerobic digestion > dry incineration > aerobic composting > landfill. It can be seen that the domestic method of landfill sludge treatment is still extensive, while foreign countries focus on anaerobic digestion due to environmental restrictions. A summary of the costs and advantages/disadvantages of these methods reveals that landfilling is simple and cost-effective, making it widely used in sludge management. However, it occupies substantial land and poses risks of leaching toxic substances due to high moisture content and heavy metal concentrations, potentially contaminating soil and groundwater. Biochemical techniques like anaerobic digestion and aerobic composting exhibit selectivity toward harmful substances but have limited applicability. On the other hand, drying and incineration face constraints due to lengthy processes and high costs, and they may generate secondary pollutants like dioxins [13; 14; 15]. In summary, traditional sludge treatment methods face multiple challenges, including land use, high costs, and the potential for secondary pollution.