testing the null hypothesis. If the P-value is less than the significance level, the null hypothesis is rejected, which means that the term is significant (Lee et al., 2010). Some of the insignificant termsof the model were not eliminated to maintain the model hierarchyand obtain a not significant lack of fit. Eliminating all insignificantterms causes that the adjusted R2(a measure of the amount ofvariation around the mean explained by the model, adjusted forthe number of terms in the model) decreases and the lack of fitbecomes significant (Chaibakhsh et al., 2010). The results showthat the coagulant dose and dye concentration significantly affectthe color removal performance of O. basilicum and the dye con-centration is the most significant parameter. However, pH (withinthe selected range) is not a significant factor in the color removalefficiency of the natural coagulant. The parameters also have nointeractive effects on the response. This means that the modifica-tion of one of the factors does not affect the other factors insidethe working region. Negative coefficient values of the individualfactors, A, B and C, indicate that the factors negatively affect the