Progress in the application of catalytic ozonation for industrial wastewater treatment

At present, catalytic ozone deep treatment processes are divided into two categories: one is a separate catalytic ozone oxidation process, such as single-stage/multi-stage catalytic ozone oxidation; Another type is the composite process of ozone catalytic oxidation and biochemistry, such as catalytic ozone oxidation followed by aeration biofilter or biological activated carbon. The catalytic ozone oxidation unit is often connected after the secondary sedimentation tank, before the filter or effluent. For the selection of catalysts, supported catalysts are generally used in engineering, utilizing carriers with larger specific surface areas to better adsorb ozone and pollutants, and improve the mass transfer efficiency between the two. The common form of structure for catalytic ozone units is a packing tank, and the upstream/downstream flow of wastewater does not significantly affect the actual operating effect. In addition, regular backwashing of the packing is required

In the application of catalytic ozonation technology for industrial wastewater, pH is a key factor determining catalytic efficiency. For ozone molecules, an increase in pH enhances the alkaline activation process, promoting the production of hydroxyl radicals through indirect oxidation pathways to degrade pollutants. At the same time, pH will determine the form of organic pollutants in wastewater, and there are differences in the reaction rates between protonated and deprotonated forms of organic matter and ozone and hydroxyl radicals. On the other hand, pH directly affects the surface charge of the heterogeneous catalyst used, thereby affecting the catalytic activity. It is widely believed that the catalytic activity is optimal when the pH of the wastewater is close to the zero point charge (pHpzc) of the catalyst. At this point, the ion exchange capacity of the catalyst surface is the weakest, making it difficult for the surface hydroxyl groups, which serve as active sites of the catalyst, to undergo coordination exchange with common coexisting ions such as SO42- and NO3- in industrial wastewater.

In the application research of catalytic ozonation for the treatment of various industrial wastewater, it has been found that the COD removal rate increases first and then decreases with the increase of pH. Therefore, for the deep treatment of specific industrial wastewater, there is an optimal pH. The gradual increase of pH is conducive to the decomposition of ozone by OH - in water to produce · OH, triggering a chain reaction of free radicals. When the pH rises to a certain value, the system will produce a high concentration of · OH, and the quenching reaction between free radicals will cause ineffective consumption of oxidants, weakening the mineralization ability of the system. Due to the limitation of gas-liquid mass transfer efficiency, increasing the amount of ozone added cannot significantly improve the mineralization effect of industrial wastewater in a sustained manner. The addition of catalysts can improve the mass transfer efficiency of ozone to a certain extent and promote the generation of free radicals. However, the self quenching phenomenon of free radicals caused by high ozone flux limits the effect of simply increasing the ozone dosage to improve mineralization rate. The common ozone dosage in engineering is 30-80 mg/L, and the common gas-liquid contact time is 0.5-1 hour. 1 g of O3 in the catalytic ozone unit can reduce approximately 0.6-1.5 g of COD.