COD, nitrogen, phosphate and para-chlorophenol (4-chlorophenol, 4-CP) removal from synthetic wastewater was investigated using a four-step sequencing batch reactor (SBR) at different sludge ages and initial para-chlorophenol (4-CP) concentrations. 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 0.75 h. A Box-Wilson statistical experiment design was used considering the sludge age (5-25 days) and 4-CP concentration (0-400 mg l(-1)) as independent variables. Variations of percent COD, NH4-N, PO4-P and 4-CP removals with sludge age and initial 4-CP concentration were investigated. Percent nutrient removals increased with increasing sludge age and decreasing 4-CP concentrations. Low nutrient removals were obtained at high initial 4-CP concentrations especially at low sludge ages. However, high sludge ages partially overcome the adverse effects of 4-CP and resulted in high nutrient removals. COD, NH4-N, PO4-P and 4-CP removals were 76%, 72%, 26% and 34% at a sludge age of 25 days and initial 4-CP concentration of 200 mg l(-1). Sludge volume index (SVI) also decreased with increasing sludge age and decreasing 4-CP concentrations. An SVI value of 104 ml g(-1), was obtained at a sludge age of 25 days and initial 4-CP of 200 mg l(-1). (c) 2005 Elsevier Ltd. All rights reserved.