2,4,6-Trichlorophenol (TCP) containing synthetic wastewater was biologically treated using a novel rotating brush biofilm reactor for TCP, COD and toxicity removal. Effects of major operating variables such as the feed TCP and COD concentrations and A/Q (biofilm surface area/feed flow rate) ratio on the performance of the biofilm reactor were investigated. A Box-Belinken statistical experiment design method was used by considering the feed TCP (0-400 mg L-1), COD (1000-4000 mg L-1) and A/Q ratio (37-256 m 2 d m(-3)) as the independent variables while percent TCP, COD, and toxicity removals were the objective functions. The data were correlated with a quadratic model and the response function coefficients were determined by regression analysis. Percent TCP, COD and toxicity removals determined from the response functions were in good agreement with the experimental results. TCP, COD and toxicity removals increased with increasing A/Q ratio and decreasing feed TCP concentrations. Optimum conditions resulting in maximum COD, TCP and toxicity removals were found to be A/Q ratio of nearly 194 m(2) d m-3, feed COD of nearly 4000 mg L-1 and feed TCP of less than 400 mg L-1. (c) 2007 Elsevier Inc. All rights reserved.