Nutrient removal from synthetic wastewater was investigated using a four-step sequencing batch reactor (SBR) at different salt (NaCl) concentrations in order to determine the salt inhibition effects on biological nutrient removal. The nutrient removal process consisted of anaerobic, oxic, anoxic and oxic phases with hydraulic residence times (HRT) of 1/3/1/1 h and a. settling phase of 3/4 h. Solid retention time (SRT) was kept constant at 10 days in all experiments. Salt content was varied between 0 and 6% (w/v) at eight different levels. Effects of salt inhibition on the rate and extent of COD, NH4-N and PO4-P removals were investigated. The rate and extent of COD, NH4-N and PO4-P removals decreased while the effluent nutrient concentrations and the sludge volume index (SVI) increased with increasing salt content. (C) 2003 Elsevier Inc. All rights reserved.