The effect of pH on the decolorization performance of O.basilicum is shown in Fig. 3a. The pH of the solution was adjustedto be in the range of 6.0–8.5. The selected pH range was within thelimits of effluent discharge standards, the refore there would be norequirement for pH adjustment after the treatment process. In the coagulation process, pH of the solution affects the surface chargeof both coagulant and dye (Patel and Vashi, 2013). As can be seenin the graph, the effect of pH on the color removal is not significantwithin the selected range. However, the COD reduction efficiencyof the coagulant is highly pH-dependent and maximum COD reduc-tion could be obtained at pH 6.5 (Fig. 3b). At this optimum pH, theparticles surface charge is reduced or neutralized and the suspen-sion is destabilized. CR is an anionic dye; therefore, the neutralizingof negatively charged CR molecules at acidic pH, leads to the irdestabilization and flocculation. Furthermore, at acidic pH, nega-tively charged groups on the polysaccharide are protonated andneutralized. However, in acidic pH lower than the optimal value,the solubility of freshly formed sludge could increase which leadsto the decrease in dye removal (Patel and Vashi, 2012). It seemsthat changes in the pH is more effective on the removal of resid-ual moieties of the dye molecules than the color producing part,therefore its effect on COD reduction is more significant than colorremoval.