Nutrient removal from synthetic wastewater by sequencing batch operation was studied at different specific nutrient loading rates (SNLR). Nutrient removal in a sequencing batch reactor (SBR) was a five-step process consisting of anaerobic (An), anoxic (Ax), oxic (Ox), anoxic (An) and oxic (Ox) phases with hydraulic residence times (HRT) of 2/1/4.5/1.5/1.5 h, respectively. The settling step used at the end of the operation was 45 min for all experiments. The initial COD concentration was varied between 600 and 4800 mg l(-1) at eight different levels with constant COD/N/P ratio of 100/3.33/0.7. Effects of SNLRs on COD, NH4-N and PO4-P removal were investigated. Percent nutrient removals decreased and effluent nutrient levels increased with increasing nutrient loading rates. The highest COD (99%), NH4-N (99%) and PO4-P (97%) removal efficiencies were obtained with the initial COD concentration of 600 mg l(-1) at COD loading rate of nearly 40 mg COD (g biomass)(-1) h(-1). However, the sludge volume index (SVI) decreased with increasing COD loading rate resulting minimum SVI of 46 ml g(-1) at COD loading rate of nearly 86 mg COD (g biomass)(-1) h(-1). Biomass concentration increased with increasing SNLR resulting in biomass concentration of 3.84 g l(-1) at COD loading rate of 86 mg COD (g biomass)(-1) h(-1). (C) 2003 Elsevier Science Ltd. All rights reserved.