In this study a series of jar test experiments, designed using Taguchi method, were carried out to determine the optimum conditions for tannery wastewater treatment, and the effects of type and dose of coagulant, type and dose of coagulant aid and pH on the efficiency of coagulation-flocculation process were evaluated. The goal parameters to find the optimum conditions were each of chemical oxygen demand (COD), chromium concentration, total dissolved solids (TDS) and turbidity of the wastewater, or all together, which were used to track the efficiency of the treatment process. Coagulant type in COD removal, pH in chromium removal, coagulant aid type in TDS removal and coagulant aid dose in turbidity removal were the most effective factors. The optimum conditions that were obtained for the treatment of the wastewater were: 800 ppm FeCl3 as coagulant, 600 ppm Na2CO3 as coagulant aid and pH 7.5 for COD removal; 1600 ppm FeCl3 as coagulant, 100 ppm CaO as coagulant aid and pH 7.5 for chromium removal; 1600 ppm FeCl3 as coagulant, pH 9 and 300 ppm Na2SiO3 as coagulant aid for TDS removal; and 800 ppm PAC (polyaluminum chloride) as coagulant, pH 7.5 and 600 ppm Na2CO3 as coagulant aid for turbidity removal. The optimum conditions found for COD removal, were selected as optimum conditions for the cases where the removal of aforementioned parameters, all together, is treatment goal. (C) 2010 Elsevier B.V. All rights reserved.