The effects of hydraulic retention time (theta(H) = 12-72 h), initial COD concentration (COD0 = 1000-8000 mg l(-1)) and initial dyestuff concentration (D-0 = 50-400 mg l(-1)) on COD removal and decolorization of vinylsulfone/monochlorotiazine bifunctional, monoazo type textile dyestuff, Reactive Red 195, Sumifix Supra Br Red 31317 150% gran in a sequential anaerobic-aerobic treatment system were investigated in this study. A facultative anaerobic PDW was used as dominant microbial culture in the anaerobic unit for decolorization. The system was operated continuously at room temperature (T = 19-22 degreesC). It consisted of an anaerobic packed column and a conventional activated sludge unit which was fed with effluent from an anaerobic unit. Metal mesh particles were used as microorganism support material in the anaerobic packed column. Experimental results indicated that the system could be operated at theta(H) = 18 h, 3000 mg l(-1) initial COD concentration and 100 mg l(-1) dyestuff concentration to obtain over 85% decolorization efficiency in an anaerobic reactor. No significant colour removal (15%) was observed under aerobic conditions while COD removal was mainly obtained in the aerobic unit with maximum 90% efficiency. (C) 2002 Elsevier Science Ltd. All rights reserved.