Application of the central composite design and response surface methodology to the advanced treatment of olive oil processing wastewater using Fenton's peroxidation

Abstract

The central composite design (CCD) technique was used to study the effect of the Fenton's peroxidation on the removal of organic pollutants from olive oil mill wastewater (OMW). The ratio of hydrogen peroxide-to-Fe(II) (x₁) was between 1.67 and 8.33. Fe(II) concentration was constant at 0.03 M while the H₂O₂ concentration was set at three levels: 0.05, 0.15 and 0.25 M. Based on the molarity ratio, the selected ratio were in the low range of Fe(II)-to-H₂O₂ ratio (≪1). While based on the wt/wt ratio, the tested Fe(II)-to-H₂O₂ ratios were in the range of ≤1:5. pH (x₂) was between 3 and 5. The concentration of OMW (x₃) was varied between 40 and 100%. The influence of these three independent variables on the four dependent variables, i.e. COD, total phenolics (TP), color and aromatocity removal was evaluated using a second-order polynomial multiple regression model. Analysis of variance (ANOVA) showed a high coefficient of determination (R²) value of 0.902–0.998, thus ensuring a satisfactory adjustment of the second-order regression model with the experimental data. H₂O₂-to-Fe(II) ratio had significant effect on all the four dependent variables. The positive sign for the regression coefficient of this regressor variable indicated that the level of the pollutant removal increased with the increased levels of factor x₁ from 1.67 to 8.33 and this effect was the most pronounced for TP removal. pH had also significant effect on the pollutant removal and the effect was the most noticeable for TP reduction. The negative coefficient of this variable (pH) indicated that level of the pollutant removal decreased as the pH increased from 3 to 5. The negative coefficient of the interaction between variable x₁ and x₂ indicated that a simultaneous increase in H₂O₂-to-Fe(II) ratio with decrease in the pH of the reaction led to an increase in the COD, TP and color removal. Quadratic models were predicted for the response variable, i.e. pollutant removal, and the maximum model-predicted removals were 56, 100, 33 and 32% for COD, TP, color and aromatocity, respectively. Optimum conditions for this wastewater treatment was obtained based on the performance of the Fenton's peroxidation in the experiment where the H₂O₂-to-Fe(II) ratio was at its high level (8.33) and the pH and OMW concentration were 4 and 70%, respectively.