Nutrient removal from synthetic wastewater was studied as a function of nutrient composition in a five-step sequencing batch reactor (SBR) in order to determine the most suitable nutrient ratios maximizing nutrient removal. The nutrient removal process consisted of anaerobic, anoxic, oxic, anoxic, oxic phases. Hydraulic residence times (HRT) of the aforementioned phases were kept constant at 2/1/4.5/1.5/1.5 h with 1/2 h of settling phase. Solids retention time (SRT) was constant at 10 days in all experiments. COD/N and COD/P ratios in the nutrient medium were considered as independent variables with a constant initial chemical oxygen demand (COD) of 1200 mg l(-1). Box-Wilson statistical experiment design was used to determine the effects of independent variables on COD, NH4-N, and PO4-P removal efficiencies. The results were correlated by a response function and the coefficients determined by regression analysis. COD/N/P ratio resulting in maximum COD, NH4-N and PO4-P removal efficiencies was found to be 100/2/0.54 yielding COD, NH4-N and PO4-P removal efficiencies of 95, 94 and 99%, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.