In this study, the performance of a two stage rotating biological contactor (RBC) was evaluated for the treatment of synthetic wastewater containing peptone, 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) at 5 rpm. Also, the effect of biogenic substrate (peptone) concentration on the reactor performance was investigated. High chlorophenols (> 98%) and COD (> 94%) removals were achieved throughout the reactor operation in the first stage and the second stage behaved as a polishing step. The observed max [mum 4-CP and 2,4-DCP removal rates in the first stage were 2305 mg/L day (18.3 g/m(2) day) and 1202 mg/L day (9.5 g/m(2) day), respectively. Deacclimation of biomass was carried out for 3.5 months after 260 days of operation with the reactor feed turning on to contain peptone only. Upon addition of 4-CP (200 mg/L) and 2,4-DCP (100 mg/L), a rapid reacclimation of biomass was observed within 16 days. Shock loading experiments with 4-CP (822.7 +/- 1.4) and 2,4-DCP (424.6 +/- 1.9 mg/L) resulted in around four times higher effluent 4-CP concentrations than 2,4-DCP in both stages, which may be due to strong competitive inhibition of 2,4-DCP on 4-CP degradation. (c) 2006 Elsevier B.V. All fights reserved.