## 3 Results and Discussion
### UV/H\({}_{2}\)O\({}_{2}\) and UV/Cl\({}_{2}\) for T&O Removal, Lab-Scale  
This study investigated two different UV based AOPs, i.e., UV/H\({}_{2}\)O\({}_{2}\) and UV/Cl\({}_{2}\) processes, to enhance the geosmin and 2-MIB removal efficiencies after the PMC-based coagulation process. Both of the processes are known to generate hydroxyl radical (\(\bullet\)OH), which reacts non-selectively with a wide range of organic and inorganic compounds, by the photolytic decomposition of H\({}_{2}\)O\({}_{2}\), HOCl, and OCl\({}^{-}\)[35] as following equations:  
\[\mathrm{H_{2}O_{2}+\textit{h}v}\rightarrow\mathrm{2\bullet OH}, \tag{1}\]  
\[\mathrm{HOCl+\textit{h}v}\rightarrow\mathrm{\bullet OH+Cl\bullet}, \tag{2}\]  
\[\mathrm{OCl^{-}+\textit{h}v}\rightarrow\mathrm{O\bullet^{-}+Cl\bullet}, \tag{3}\]  
\[\mathrm{O\bullet^{-}+H_{2}\ O}\rightarrow\mathrm{\bullet OH+OH^{-}}. \tag{4}\]  
For the case of the UV/Cl\({}_{2}\) process, it is known that there are formation of reactive chlorine species (i.e., Cl\(\bullet\), Cl\({}_{2}\)\({}^{-}\)\(\bullet\), and ClO\(\bullet\)), but it was reported that OH\(\bullet\) is a primarily radical species to the degradation of taste-and-odor compounds as compared to reactive chlorine species which are formed in the UV/Cl\({}_{2}\) process [36].  
Treatment in the presence of algae species using oxidation processes has drawbacks such as undesirable toxins and taste and odor compounds [37; 38]; therefore, the AOPs were applied after the coagulation process with the optimum PMC dose, i.e., 20 mg/L. In addition, since AOPs performance is significantly affected by DOC and UV\({}_{254}\) level [39], higher performance of AOPs is expected after the coagulation with the PMC-based process.  
Figure 7 shows the geosmin and 2-MIB removal with the UV/Cl\({}_{2}\) and UV/H\({}_{2}\)O\({}_{2}\) processes. UV doses were varied as around 200, and 700 mJ/cm\({}^{2}\), and oxidant dose was 5 mg/L. The initial pH of the test water was approximately 6.9, but it was increased to 7.4 after the injection of 5 mg/L of Cl\({}_{2}\) because of the basic pH of sodium hydrochloric acid solution and low buffer system of tested water (alkalinity = 27.5 mg/L as CaCO\({}_{3}\)). No pH change was observed after the injection of H\({}_{2}\)O\({}_{2}\).  
In both processes, the removal efficiency of geosmin and 2-MIB increased as UV dose increased. The removal of geosmin and 2-MIB in the processes could be explained by the \(\bullet\)OH formation and reaction with those compounds. It is known that geosmin and 2-MIB are not photo-reactive compounds [12] but highly reactive with \(\bullet\)OH (k\({}_{\mathrm{OH,geosmin}}\) = 7.8 \(\times\) 10\({}^{9}\) M\({}^{-1}\cdot\)s\({}^{-1}\) and k\({}_{\mathrm{OH,2-MIB}}\) = 5.1 \(\times\) 10\({}^{9}\) M\({}^{-1}\cdot\)s\({}^{-1}\)) [40].  
At the same UV dose of 700 mJ/cm\({}^{2}\), the UV/H\({}_{2}\)O\({}_{2}\) process shows higher removal efficiency (87.2% of geosmin and 71.4% of 2-MIB) than UV/Cl\({}_{2}\) process (62% of geosmin and 38% of 2-MIB). This result could be explained by the speciation of free chlorine (HOC1 and OCl\({}^{-}\)) with pKa 7.5. At the condition of pH higher than 7.5, OCl\({}^{-}\) is the predominant species, and it is known that the \(\bullet\)OH scavenging rate by OCl\({}^{-}\) (k\({}_{\mathrm{OH,OCl_{-}}}\) = 8.80 \(\times\) 10\({}^{9}\) M\({}^{-1}\cdot\)s\({}^{-1}\)) is much higher than H\({}_{2}\)O\({}_{2}\) (k\({}_{\mathrm{OH,H2O2}}\) = 2.7 \(\times\) 10\({}^{7}\) M\({}^{-1}\cdot\)s\({}^{-1}\)) and HOCl (k\({}_{\mathrm{OH,HOCl}}\) = 8.46 \(\times\) 10\({}^{4}\) M\({}^{-1}\cdot\)s\({}^{-1}\)[41]; therefore, it is reported that the removal efficiency of geosmin and 2-MIB in the UV/H\({}_{2}\)O\({}_{2}\) process could be higher at pH above 7 than the UV/Cl\({}_{2}\) process [36].  
Between the two compounds, geosmin removal efficiency was higher than 2-MIB in both processes. This is because geosmin has higher reactivity with \(\bullet\)OH than 2-MIB.