## 3 Results and Discussion
### Performance of AOP Pretreatment
Hydroxyl radical, on the other hand, is known to be less selective than ozone despite its higher reactivity [15]. In our ozone oxidation test, it seems that most MDEA was degraded by ozone, while its degradation by hydroxyl radical was restricted, considering the report that hydroxyl radical generated from ozone could be rapidly scavenged by other organic and inorganic constituents of wastewater resulting in a nearly diffusion-controlled reaction scheme [34]. Reminding that the aim of introducing the AOP is not the complete degradation of MDEA but its partial oxidation, the ozone method seems to be more suitable for the given AOP-combined BVTP scheme than the Fenton method. Although the ozone oxidation method was proved to have the advantage of selective MDEA degradation, it still had a problem of requiring a rather large amount of ozone supply. The peroxone method using H\({}_{2}\)O\({}_{2}\) together with ozone could be a solution to reduce the ozone requirement, so its performance was also investigated. As can be seen from Figure 3, the peroxone method exhibited similar MDEA and COD removal trends comparable to the ozone method. The final MDEA and COD removal efficiencies were also compatible, showing 93.3% (from 20,548 mg/L to 1382 mg/L) and 27.2% (from 44,721 mg/L to 32,575 mg/L). These final MDEA and COD removal efficiencies were achieved with a greatly reduced ozone supply amount. In the ozone method, the amount of ozone consumed for the MDEA degradation was estimated to be 0.499 g-O\({}_{3}\)/g-MDEA (see, Figure 4). In the peroxone method, on the other hand, it was reduced to 0.323 g-O\({}_{3}\)/g-MDEA, indicating that the peroxone method could significantly reduce the ozone requirement for the MDEA degradation. However, it should be noted that the use of H\({}_{2}\)O\({}_{2}\) in the peroxone method could negatively affect the selective degradation of MDEA because the diffusion-controlled reaction scheme described above could be deteriorated by the excessive generation of hydroxyl radical form H\({}_{2}\)O\({}_{2}\). Indeed, the formate concentration in the peroxone-pretreated wastewater was slightly lowered than ozone oxidation case (see Table 2). Hence, the amount of H\({}_{2}\)O\({}_{2}\) addition would be a critical operating condition for the peroxone method, which is indeed a trade-off problem between the benefit of saving the ozone requirement and the loss of the selectivity in the MDEA degradation. With the operating condition given in this study, the benefit was more significant and the selective oxidation of MDEA was still possible. In this study, the operating conditions for each pretreatment method were not optimized in detail since the main purpose was just confined to the screening of an adequate AOP method applicable to the AOP-combined BVTP scheme adopted for the treatment of the reclaimer wastewater. However, the performance results of the AOP pretreatment tests clearly supported that the peroxone method would be the most proper one. Moreover, the benefit of using peroxone method was more obvious if comparing the chemical reagent costs required for the MDEA degradation (see Table S1). Therefore, we finally selected it as the AOP method for our AOP-combined BVTP scheme.