Wet oxidation technology
Wet oxidation, also known as wet combustion, is an effective method to treat high-concentration organic wastewater, its basic principle is to pass air under high temperature and high pressure conditions, so that the organic pollutants in the wastewater are oxidized, according to the treatment process can be divided into wet air oxidation and wet air catalytic oxidation two categories.
1 Wet air oxidation method
The first to develop Wet Air Oxidation (WAO) method and realize industrialization is Zimpro company in the United States, which has applied WAO process to the treatment of toxic and harmful industrial wastewater such as olefin production waste washing liquid, acrylonitrile production wastewater and pesticide production wastewater. WAO technology is to pass air under high temperature (125~320℃) and high pressure (0.5~20MPa), so that the polymer organic matter in the wastewater is directly oxidized and degraded into inorganic matter or small molecular organic matter.
The removal rate of organophosphorus and organic sulfur was as high as 95% and 90% respectively by using wet air oxidation technology to pretreat Dimethoate wastewater. Zimpro's WAO process has high efficiency and short reaction time, but because the technology requires high temperature and high pressure, the required equipment investment is large, and the operating conditions are harsh, it is difficult to be accepted by general enterprises, so the wet air catalytic oxidation method using catalysts to reduce the reaction temperature and pressure or shorten the reaction residence time has been widely paid attention to and studied in recent years.
2 Wet air catalytic oxidation method
The Catalytic Wet Air Oxidation (CWAO) process is to add appropriate catalysts in the traditional wet oxidation process to enable the oxidation reaction to be completed under milder conditions and in a shorter time. Thus, the temperature and pressure of the reaction can be reduced, the oxidation decomposition capacity can be improved, the reaction rate can be accelerated, the residence time can be shortened, and the corrosion of the equipment can be reduced, and the operating costs can be reduced.
The key problem of wet air catalytic oxidation is the catalyst with high activity and easy recovery. CWAO catalysts are generally divided into metal salts, oxides and compound oxides. According to the form of catalysts in the system, the wet air catalytic oxidation method can be divided into homogeneous wet catalytic oxidation method and heterogeneous wet catalytic oxidation method.
Homogeneous wet catalytic oxidation process. In the homogeneous wet catalytic oxidation method, because the catalyst (mostly metal ions) is a soluble transition metal salt, these salts exist in the form of ions in the wastewater, at the level of ions or molecules by triggering the free radical reaction of the oxidant and constantly regenerate and catalyze the oxidation of organic matter in the water. In the homogeneous wet catalytic oxidation method, because the catalyst acts independently at the molecular or ion level, the molecular activity is high, and the oxidation effect is better. However, because the catalyst in the homogeneous wet catalytic oxidation method exists in the form of ions, it is difficult to recover and reuse from wastewater, and it is easy to cause secondary pollution.
Heterogeneous wet catalytic oxidation method. Heterogeneous wet catalytic oxidation is the addition of insoluble solid catalyst to the reaction system, and its catalytic action is carried out on the surface of the catalyst. The size of the specific surface area of the catalyst has a great influence on the degradation rate of organic matter. The effect of wet catalytic oxidation is also different due to the different composition types of solid catalysts and the different properties of wastewater. In the multiphase wet catalytic oxidation method, because the solid catalyst does not dissolve, does not lose, the activation regeneration and recovery are easy, so its application prospect is very broad.
