The aim of this research work was to decolorize anaerobically digested (UASB) and aerobically treated distillery wastewater of a process for ethanol production. An Aspergillus species isolated from the soil was used for decolorization. A sequential Plackett-Burman design in three stages was used for optimization of decolorization conditions. Both maximum decolorization and COD reduction under optimized conditions were obtained as 84%. By implementing the optimum values in a pilot scale of an activated sludge system a continuous process for decolorization was conducted. The decolorization efficiency of this system with 48 h retention time was about 75% in a limited period of 4 days. For extension of the decolorization period in a continuous process a sequencing batch reactor was used. This system with a cycle time of about 4.5 h and decolorization efficiency of 70% extended for 12 days which was more than previous reported works. The colour of undiluted wastewater reduced by only 40% with a cycle time of 17.5 h and the system was stable for about 4 days. (C) 2004 Elsevier Ltd. All rights reserved.