The objective of this work was to optimize the experimental conditions for adsorption of reactive azo dye using a waste-derived adsorbent, modified charcoal ash. With this aim, Box-Behnken and Central Composite Design models were applied to achieve maximum dye adsorption and minimum operation costs. In the models studied, independent variables were pH (1-12), ash dosage (0.02-0.1 g/50ml), dye concentration (10-200mg/L), and operation time (10-130 min). The quadratic models were developed for the predetermined responses (dye removal and operation cost), and it was clearly seen that the experimental data fit well to model predictions statistically (R-2 >= 0.89 and "Prob>F"<0.005). Experimental conditions for optimum dye removal of 90.2 % were determined as pH 2, 0.08 mg/50 ml ash dosage, 80.5 mg/L dye concentration, and 100 min agitation period. Operating cost which includes expenses for modification of adsorbent, arrangement of solution pH, and sample shaking was calculated as 1.17 (sic)/m(3) for optimized conditions.