## 3 Results and Discussion
### Determination of Optimal Conditions  
Verification of the normal distribution of the data using the KS test was positive (\(p>0.05\)), as was the verification of the equality of variances using the LA test (\(p>0.05\)) for the experimental data. ANOVA was performed using the design of a fractional orthogonal experiment to determine the statistically significant variables (\(p<0.05\)). For the EC process, the significant variables were current intensity (I) and agitation; however, pH was not statistically significant. The explanation for this result was that, when conducting the preliminary experimental tests, we chose to use pH values (6.0, 7.0, and 8.0) that favored the removal of COD. The significant variables for the EO process were the addition of an additional electrolyte (NaCl), distance between electrodes (D), and pH. Surface graphs of the EC and EO processes were generated (Figures 7 and 8, respectively). Recall that the EO process was conducted in the second stage of treatment, with the effluent generated under the maximum COD removal conditions in the EC. The experimental conditions for the maximum COD removal in the EC process were I: 4.3 A, agitation: 120 RPM, and pH: 7.0, with a COD removal of 63%. In the case of the EO process, the maximum COD removal conditions were NaCl: 1.0 g L\({}^{-1}\); distance between electrodes: 0.75 cm; I: 2 A, pH: 7; removal of 82% COD. The models indicated in Equations (5) and (6) were generated to obtain the percentage of COD removal (%R COD) by means of RSM. It was possible to explain 97% and 98% of the variability in the data for EC and EO, respectively, which guaranteed the statistical value of the proposed models.  
\[\%\text{R DQO}=219.7+19.72\text{ I}+0.3012\text{ RPM 62.91 pH - 4.728 I}^{2}-0.001217\text{ RPM}^{2}+3.628\text{ pH}^{2}+2.907\text{ I}^{*}\text{pH} \tag{5}\]  
\[\%\text{R DQO}=-149.7+16.04\text{ NaCl}+206.5\text{ D - 0.599 I}+34.01\text{ pH - 6.271 NaCl}^{2}-128.76\text{ D}^{2}-2.579\text{ pH}^{2} \tag{6}\]  
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Parameter**} & \multicolumn{2}{c}{**Electro-Coagulation**} & \multicolumn{2}{c}{**Electro-Oxidation**} \\ \cline{2-5}  & **R\({}^{2}\)** & **k (min\({}^{-1}\))** & **R\({}^{2}\)** & **k (min\({}^{-1}\))** \\ \hline COD & 0.98 & 0.1610 & 0.98 & 0.1844 \\ DOC & 0.98 & 0.2062 & 0.98 & 0.1390 \\ Color & 0.99 & 0.3699 & 0.98 & 0.5466 \\ NH\({}_{3}\)-N & 0.98 & 0.1095 & 0.97 & 0.2475 \\ Cl\({}^{-}\) & 0.97 & 0.1048 & 0.98 & 0.1715 \\ \hline \end{tabular} \({}^{\text{R}}\)—coefficient of determination; k—kinetic constant; COD—chemical oxygen demand—DOC: dissolved organic carbon; NH\({}_{3}\)-N—ammonical nitrogen; Cl\({}^{-}\)—chlorides.  
\end{table}
Table 5: Estimation of pseudo-first reaction kinetic constant for optimal conditions of electro-coagulation and electro-oxidation systems.  
Figure 8: Surface graph of the electro-oxidation process at an electrical current of 1 A. I—intensity of electrical current; D—distance between electrodes; R COD—remotion COD; COD—chemical oxygen demand.  
Figure 7: Surface graph of electro-coagulation process at pH 7. R COD—COD emotion; I—intensity of electrical current; RPM—revolutions per minute; COD—chemical oxygen demand.