The combination of up-flow anaerobic sludge blanket (UASB) and sequencing batch reactor was chosen as an anaerobic and aerobic system, respectively, in the treatment of Orange 11-containing wastewater. The Orange 11 and COD removal efficiencies were improved in the UASB system when the operating temperature and hydraulic retention time were increased up to 30 degrees C and 48 h, respectively. Nearly complete decolorization (>95%) was accomplished in UASB system when working at 0.3 g/l d of Orange 11 loading rate at 30 degrees C with hydraulic retention time of 24 h. The electron-scanning microscope (SEM) photographs showed that there were discrepancies in the microorganism composition of the two systems, which was probably the main reason for different treatment performance in COD and Orange 11 removals in both systems. (C) 2004 Elsevier B.V. All rights reserved.