3.1. Fitting the response surface models to significant
independent variables
Central composite Box–Behnken experimental design was chosen
for finding out the relationship between the response function
(Y) and variables (X). Values of the independent variables (X1–X3)
as well as their variation limits, and the experimental data obtained
for six responses (Y1–Y6) are presented in Table 1. In this table the
independent variables levels are presented in terms of the original
unit in addition to coded levels (in parentheses). The coded values
for initial pH (X1), hydrogen peroxide and ferrous ion dosages
(X2 and X3) were set at three levels: −1 (minimum), 0 (central)
and +1 (maximum). Experimental results are shown as overall COD
removal efficiency (Y1), COD removal by oxidation (Y2) and coagulation
(Y3), mineralization (Y4), HS removal (Y5), SVR (Y6).