基于臭氧与超声氧化降低页岩气压裂返排液COD

针对页岩气压裂返排液化学需氧量（COD）高，难以直接排放的问题，研究了臭氧氧化（O₃）、超声氧化（US）和臭氧与超声联用氧化（O₃+US）三种方式降低页岩气压裂返排液COD的效果。结果表明：O₃+US因能产生更多的自由基而具有更好的降低COD效果。O₃+US联用氧化返排液过程中，首先是臭氧直接氧化有机污染物生成醛酮等物质，然后再是自由基氧化降解，返排液颜色会出现特征变化。另外研究了水样pH、超声波功率、催化剂种类和加量、反应时间等因素对O₃+US联用氧化降低COD的影响，结果表明其降低COD的效率随pH的增大而减小，随超声波功率的增大先增大后减小，随作用时间的延长而增大。综合考虑，推荐降低页岩气压裂返排液COD的氧化条件为：臭氧质量浓度42mg/L、pH为2.5左右、超声波功率800W、催化剂MnO₂加量0.45g/L、反应时间100min，处理后COD降低68.17%。同时，降解动力学拟合分析显示MnO₂催化下O₃+US联用氧化降低页岩气压裂返排液COD的过程更符合二级动力学特征。

COD reduction treatment of flowback water from shale gas hydraulic fracturing based on oxidation with ozonation and ultrasound

In order to solve the problem of high COD and difficulty in direct discharge of flowback water from shale gas hydraulic fracturing, three methods including individual ozonation, individual ultrasound, and ozonation-ultrasound combination (O₃+US) were compared. O₃+US had the best performance because it could produce much more radicals than the other two methods. During the O₃+US treatment, firstly, the organic pollutants in flowback water could be oxidized directly to form aldehydes and ketones by O₃, and then oxidative degradation of free radicals happened. At the same time, the color of flowback had the characteristic change corresponding to the process. The effects of pH, US power, catalyst type, catalyst dosage, and reaction time on COD reduction of O₃+US were investigated. The COD reduction increased with decreasing pH and increasing reaction time, and increased firstly and then decreased with increasing US power. The recommended O₃+US condition was as follows: O₃ concentration 42mg/L, pH 2.5, US power 800W, catalyst MnO₂ 0.45g/L, and reaction time 100min. The COD reduction was 68.17% at the recommended condition. In addition, it was found that the degradation kinetics of organic pollutants in flowback water by O₃+US was likely to be second-order kinetics.