## 3 Results and Discussion
### Total Residual Oxidant in Direct OCI\({}^{-}\)-Injection and Electrolysis
#### 3.1.2 Effects of Current Density during Electrolysis  
Effects of the current density (2.2, 9.4, 16.6, and 27.7 mA/cm\({}^{2}\)) on TRO during the in situ electro-chlorination of seawater were investigated. For both WS and ES, the TRO concentration increased as the applied current density and time increased (Figure 3). No significant difference in the TRO concentration was observed between WS and ES, probably because the salinities of WS and ES were almost the same. More than 230 mg/L of TRO could be produced via the in situ electrochlorination of seawater within 5 min under the current density of 27.7 mA/cm\({}^{2}\): 240 mg/L for  
Figure 2: Effect of initial OCI\({}^{-}\) concentration on formation of TRO in (**a**) WS and (**b**) ES, and (**c**) comparison of chlorine demands between WS and ES (volume: 0.4 L; temperature: 20 \(\pm\) 1 \({}^{\circ}\)C; salinity: 28.9 PSU for WS and 28.7 PSU for ES; pH: 8.3 for WS and 8.0 for ES). WS and 235 mg/L for ES. In other words, at the current density, TRO of 16.8-18 mg Cl\({}_{2}\)/cm\({}^{2}\)/min can be produced. If an electrode with a width of 0.5 m is installed along the inner surface of a pipe with a radius of 1 m, the surface area of the electrode will be 3.1 m\({}^{2}\) and produce TRO at 520-557 g Cl\({}_{2}\)/min. Considering the flow pattern developing along the inner surface of the pipe for a cooling system, the amount of TRO generated by seawater electrolysis will be enough to provide a desirable TRO concentration (e.g., \(>\)0.5 mg Cl\({}_{2}\)/L) for the cooling water flowing the pipe (confirmed by a simulation; data not shown). Formation of ClO\({}_{3}\)\({}^{-}\) and ClO\({}_{4}\)\({}^{-}\) in Electrolysis and Direct OCl\({}^{-}\)-Injection  
The kinds and concentrations of CBPs depend on several factors, such as oxidant contact time, organic and inorganic contents, reaction temperature, and solution pH [17; 28; 29; 30; 31; 32]. If chlorine is added to seawater via direct OCl\({}^{-}\)-injection or the electrochemical process, chlorite, chlorate, or perchlorate can be generated through the following reactions (Equations (8)-(20) in Table 2) (Figure 4) [11; 13; 33; 34; 35; 36]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Equation** & **Equivalent** & **Equations No.** \\ \hline
6 OCl\({}^{-}\) + 3 H\({}_{2}\)O \(\rightarrow\) 2 ClO\({}_{3}\)\({}^{-}\) + 4 Cl\({}^{-}\) + 6 H\({}^{+}\) + 3/2 O\({}_{2}\) + 6 e\({}^{-}\) & 0.46 V & (8) \\
6 HOCI + 3 H\({}_{2}\)O \(\rightarrow\) 2 ClO\({}_{3}\)\({}^{-}\) + 4 Cl\({}^{-}\) + 12 H\({}^{+}\) + 3/2 O\({}_{2}\) + 6 e\({}^{-}\) & 0.46 V & (9) \\ Cl\({}^{-}\) + 3 H\({}_{2}\)O \(\rightarrow\) ClO\({}_{3}\)\({}^{-}\) + 6 H\({}^{+}\) + 6 e\({}^{-}\) & \(-\)1.45 V & (10) \\ Cl\({}^{-}\) + 2 OH\({}^{-}\) \(\rightarrow\) OCl\({}^{-}\) + H\({}_{2}\)O + 2 e\({}^{-}\) & \(-\)0.94 V & (11) \\ Cl\({}^{-}\) + 4 OH\({}^{-}\)\(\rightarrow\) ClO\({}_{2}\)\({}^{-}\) + H\({}_{2}\)O + 4 e\({}^{-}\) & \(-\)0.76 V & (12) \\ OCl\({}^{-}\) + 2 OH\({}^{-}\)\(\rightarrow\) ClO\({}_{2}\)\({}^{-}\) + H\({}_{2}\)O + 2 e\({}^{-}\) & \(-\)0.59 V & (13) \\ ClO\({}_{2}\)\({}^{-}\) + 2 OH\({}^{-}\)\(\rightarrow\) ClO\({}_{3}\)\({}^{-}\) + H\({}_{2}\)O + 2 e\({}^{-}\) & \(-\)0.35 V & (14) \\ ClO\({}_{2}\)\({}^{-}\) + OCl\({}^{-}\)\(\rightarrow\) Cl\({}^{-}\) + ClO\({}_{3}\)\({}^{-}\) & & (15) \\ ClO\({}_{2}\)\({}^{-}\) + HOCl \(\rightarrow\) ClO\({}_{3}\)\({}^{-}\) + H\({}^{+}\) + Cl\({}^{-}\) & & (16) \\ HClO\({}_{2}\) + H\({}_{2}\)O \(\rightarrow\) ClO\({}_{3}\)\({}^{-}\) + 3 H\({}^{+}\) + 2 e\({}^{-}\) & & (17) \\ ClO\({}_{3}\)\({}^{-}\) + H\({}_{2}\)O \(\rightarrow\) ClO\({}_{4}\)\({}^{-}\) + 2 H\({}^{+}\) + 2e\({}^{-}\) & 0.95 V & (18) \\
2 ClO\({}_{3}\)\({}^{-}\) + 2 Cl\({}^{-}\) + 4 H\({}^{+}\)\(\rightarrow\) Cl\({}_{2}\) + 2 ClO\({}_{2}\) + 2 H\({}_{2}\)O & & (19) \\ ClO\({}_{3}\)\({}^{-}\) + Cl\({}^{-}\) + 2 H\({}^{+}\)\(\rightarrow\) HClO\({}_{2}\) + HOCl & + & (20) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Formation of ClO\({}_{3}\)\({}^{-}\) and ClO\({}_{4}\)\({}^{-}\) during chlorination.