# Recent Advances in Hydrothermal Oxidation Technology for Sludge Treatment
## 2 Results and Discussion
### Issues and Optimization Strategies in HTO of Sludge
#### 2.5.3 Optimization Measures for the Reactor  
In addition to the optimization of the hydrothermal oxidation process itself, improvements in reactor design and process flow are also crucial for enhancing sludge treatment efficiency. Traditional HTO technologies operate at high temperatures and pressures, leading to expensive equipment costs and safety concerns. Therefore, optimizing reactors and refining process designs can significantly increase treatment efficiency, reduce operational costs, and promote the widespread application of HTO in sludge treatment. WO technology is an early form of HTO. Initially, reactors were designed as high-pressure vessels where the organic matter in sludge reacts with oxygen under controlled temperatures (150-374 \({}^{\circ}\)C) and pressures (2-15 MPa). The Zimpro wet oxidation reactor is a classic example of this technology; it efficiently removed organic pollutants from sludge. However, its high energy consumption and equipment costs limited its widespread use [66]. As demand grew, reactor designs evolved from simple high-pressure vessels to catalytic wet oxidation reactors, where metal catalysts were introduced to lower reaction temperatures and pressures, improving efficiency. Later, SCWO technology marked a significant breakthrough. The unique properties of supercritical water enabled the effective decomposition of organic matter at higher temperatures and pressures, although issues such as equipment cost and material corrosion remained [36]. For instance, [PERSON] and colleagues proposed a novel energy recovery method based on injecting reactor effluents into gas turbines, utilizing ultrafiltration and evaporation units for front-end concentration while employing SCWO reactors as the main treatment unit. This method was found to be more efficient compared to simpler direct-use methods [67].  
In recent years, further advancements in HTO technology have included the development of tubular reactors, fixed-bed reactors, and continuous-flow reactor designs, all aimed at making the process more efficient. For example, the Vertech wet oxidation reactor significantly reduced energy consumption through heat recovery [68]. The introduction of multiphase reactors, which incorporate solid catalysts in liquid-phase reactions, has notably increased the reaction rate between oxygen and organic matter. The latest wet peroxide oxidation reactors (WPO) utilize hydrogen peroxide as a strong oxidizing agent, achieving high removal efficiencies for organic matter at lower temperatures and pressures [69]. Consequently, researchers have increasingly focused on reactor optimization. For instance, [PERSON] et al. proposed a hydrothermal catalytic plasma reactor to expand the treatment capacity for water-soluble volatile organic compounds (VOCs), such as toluene, acetaldehyde, acetic acid, and ammonia. They also installed a manganese dioxide catalytic unit downstream to remove ozone, further improving efficiency. [PERSON] et al. developed an energy recovery system utilizing gas turbines to optimize the entire process flow by recovering energy from reactor effluents [67; 70]. Additionally, [PERSON] et al. designed a dual-shell reactor using hydrothermal flames as an internal heat source, effectively avoiding corrosion, salt blockages, and overheating while demonstrating advantages in both economic and energy efficiency [27].  
Overall, reactors used in wet oxidation and HTO technologies have evolved from simple high-pressure vessels to more advanced, environmentally friendly designs that incorporate catalysts, supercritical conditions, and energy recovery innovations. These developments have greatly enhanced sludge treatment efficiency while reducing costs. As these optimizations continue, the potential for HTO technology in sludge treatment will expand significantly.