Para-chlorophenol (4-chlorophenol, 4-CP) containing synthetic wastewater was treated using a rotating perforated tubes biofilm reactor (RTBR). Box-Wilson statistical experiment design and response surface methodology (RSM) were used to investigate the effects of the feed COD, 4-CP contents and the A/Q (biofilm surface/flow rate) ratio on percent COD, 4-CP and toxicity removals. Increases in A/Q ratio and the feed COD contents improved COD, 4-CP and toxicity removals. High feed 4-CP contents adversely affected the system performance due to toxic effects of 4-CP on microorganisms. High A/Q ratio and feed COD contents were required for effective 4-CP and toxicity removals at high feed 4-CP contents. More than 95% 4-CP and toxicity removals were obtained with an A/Q ratio of 186 m(2) d m(-3), and feed COD of 6000 mg L(-1) with the feed 4-CP content of 1000 mg L(-1). The RTBR was proven to be more efficient than the RBBR used for the same purpose in our previous studies. (C) 2010 Elsevier Ltd. All rights reserved.