## 3 Results and Discussion  
### Performance of AOP Pretreatment  
Figure 3 represents the MDEA and COD concentration profiles during the Fenton, ozone, and peroxone oxidation tests. Among various AOPs, we selected these three methods as candidates for our AOP-combined BWTP considering their ease of use in full-scale  
application. In Figure 3, one can easily notice that the Fenton method decomposed MDEA most rapidly compared to the others, but its final MDEA removal efficiency was relatively low compared to the other methods. In the Fenton oxidation, the hydroxyl radical generated from the reaction of Fenton reagents (Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\)) is the key oxidant responsible for the pollutant degradation, so the rapid MDEA degradation rate implies the strong reactivity of hydroxyl radical with MDEA. The low MDEA removal efficiency was the result of using rather mild conditions for the Fenton reaction (H\({}_{2}\)O\({}_{2}\)/MDEA = 13.8 mol/mol and H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\) = 58.33 mol/mol). We adopted this condition referring to the report of [PERSON] et al. [7] with the intention of improving the biodegradability of the reclaimer wastewater via partial oxidation of MDEA. In this condition, however, it seems that the added amount of Fenton reagents was not sufficient for the complete removal of MDEA. The final MDEA and COD removal efficiency of the Fenton method were 70.3% (from 20,548 mg/L to 6111 mg/L) and 47.0% (from 44,721 mg/L to 23,710 mg/L), respectively. The different extent of MDEA and COD reduction means that MDEA was not completely oxidized but just partially decomposed into other organic intermediates as we intended. [PERSON] et al. [4] reported the MDEA degradation mechanism by hydroxyl radical, where some biodegradable intermediates, such as formic acid (CH\({}_{2}\)O\({}_{2}\)), oxalic acid (C\({}_{2}\)H\({}_{2}\)O\({}_{4}\)), and acetic acid (CH\({}_{3}\)COOH), are generated during the MDEA degradation. Other researchers who investigated the alkanolamine degradation using Fenton method also reported similar results [2; 3; 7]. As can be seen in Table 2, a significant amount of formate was still present in the Fenton-pretreated wastewater. Other intermediates presumed to be DEA, MEA, and oxalate were also observed in the HPLC peaks obtained for the MDEA analysis, although their concentrations were not determined quantitatively (see, Figure S1). In the ozone oxidation test, the MDEA concentration was gradually decreased to 1179 mg/L, resulting in the final removal efficiency of 94.3%. COD was also reduced \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Fenton Pretreated** & **Ozone Pretreated** & **Peroxone Pretreated** \\ \hline COD (mg/L) & 23,710 & 34,487 & 32,575 \\ MDEA (mg/L) & 6111 & 1179 & 1382 \\ Formate (mg/L) & 5479 & 20,228 & 17,787 \\ Phenol (mg/L) & -1 & 10.57 & 10.71 \\ SCN\({}^{-}\) (mg/L) & -1 & -1 & 50.4 \\ \hline \hline \end{tabular}  
* Not determined.  
\end{table} 
Table 2: Characteristics of AOP-pretreated reclaimer wastewaters. Figure 3: MDEA and COD concentration profiles during the advanced oxidation pretreatment tests: (a) MDEA and (b) COD. accordingly, but its removal efficiency was only 22.9%. It should be noted that the difference between the MDEA and COD reduction extent was much more remarkable in the ozone oxidation compared to the Fenton oxidation, which implies that the ozone method is more suitable for the selective degradation of MDEA. This can be also confirmed from the fact that formate, one of the major intermediates of MDEA degradation, remained at a level comparable to its initial concentration even after the ozone pretreatment (see Table 2). In the ozone oxidation, not only hydroxyl radical formed from ozone decomposition, but also ozone itself can act as a strong oxidant. According to [PERSON] [15], ozone can react selectively with non-protonated amine. [PERSON] et al. [32] and [PERSON] et al. [33] also reported that ozone had a high selectivity for the degradation of varied amine compounds.