Nutrient removal from synthetic wastewater in a five-step sequencing batch reactor (SBR) was investigated at different hydraulic residence times (HRTs). The nutrient removal process consisted of anaerobic (An), anoxic (Ax), oxic (Ox), anoxic (Ax), oxic (Ox) and settling phases. The sludge age (SRT) was kept constant at 10 days, while the HRT of each phase was varied. Effects of hydraulic residence time of each phase on COD, nitrogen (NH4-N) and phosphate (PO4-P) removal were investigated. The hydraulic residence time of each phase was varied at four different levels and the most suitable residence time, resulting in maximum overall nutrient removal was determined. The highest observed COD, nitrogen (NH4-N) and phosphate (PO4-P) removal efficiencies were 96, 87 and 90%, respectively, which was obtained with An/Ax/Ox/Ax/Ox phase residence times of 2/1/4.5/1.5/1.5 h. (C) 2004 Elsevier Inc. All rights reserved.