Decolorization of textile dyestuff Everzol Turquoise Blue G by simultaneous biodegradation and adsorption was investigated using the white-rot fungus Coriolus versicolor and an activated sludge culture in a laboratory scale activated sludge unit. The fungus was used in the form of a biofilm on surfaces of inclined plates immersed in the aeration tank. Wood ash was used as adsorbent and environmental conditions were adjusted to T = 25 degreesC and pH = 5. Box-Wilson experimental design was employed to evaluate the effects of major process variables, dyestuff and adsorbent concentrations, and sludge retention time on decolorization efficiency. Response function coefficients were determined by regression analysis of experimental data and predictions were found to be in good agreement with the experimental results. The highest colour removal efficiency was found to be E = 82% at dyestuff and adsorbent concentrations of D-0 = 200 mg 1(-1), A = 150 mg 1(-1) and a sludge age of theta(c) = 20 days, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.