Nutrientremoval from synthetic wastewater was investigated using a four-step sequencing batch reactor (SBR) at different phenol (C6H5OH) concentrations in order to determine the inhibition effects of phenol on biological nutrient removal. The nutrient removal process consisted of anaerobic, oxic, anoxic, and oxic phases with hydraulic residence times (HRT) of 1 h/3 h/1 h/1 It and a settling phase of 3/4 h. Solids retention time (SRT) was kept constant at 10 days in all experiments. Initial phenol concentrations were varied between 0 and 600 mgl(-1) at seven different levels. The effects of phenol on COD, NH4-N, and PO4-P removals and effluent nutrient levels were investigated. Phenol was almost completely degraded up to 400 mg l(-1) phenol concentration resulting in almost negligible inhibition effects on COD, NH4-N, and PO4-P removals. Nutrient removals were adversely affected by phenol at concentrations above 400 mg l(-1). Above 95% COD, 90% NH4-N and 65% PO4-P removal was obtained for phenol concentrations below 400 mg l(-1). The sludge volume index (SVI) was almost constant around 45 ml g-' for phenol concentrations below 400 mg l(-1) but increased to 90 ml g(-1) at a phenol level of 600 mg l(-1). (C) 2003 Elsevier Ltd. All rights reserved.