The independent variables of reaction time (X1), current density (X2), pH (X3), H2O2 ml/l of PW (X4) and H2O2/Fe2+ molar
ratio (X5). They were coded with low and high levels in the CCD as shown in Table 1. COD and color removal efficiencies (Y1 and Y2, respectively) were considered as the dependent factors (response). The response was expressed as removal (%) which could be calculated by using the following equation.
Removal (%) = Ci − C0
Ci × 100 (4