A hybrid-loop bioreactor system consisting of a packed column biofilm and an aerated tank bioreactor with an effluent recycle was used for biological treatment of 2,4,6 tri-chlorophenol (TCP) containing synthetic wastewater. The effects of sludge age (solids retention time) on chemical oxygen demand (COD), TCP, and toxicity removal performance of the system were investigated for sludge ages between 5 and 30 days, while the feed COD (2600 +/- 100 mg L-1), TCP (370 +/- 10 mg L-1), and the hydraulic residence time (25 h) were constant. Percent TCP, COD, and toxicity removals increased with increasing sludge age resulting in nearly complete COD, TCP, and toxicity removal at sludge ages above 20 days. Biomass concentrations in the packed column and in the aeration tank increased with increasing sludge age resulting in low reactor TCP concentrations, and therefore, high TCP, COD, and toxicity removals. More than 95% of COD, TCP, and toxicity removal took place in the packed column reactor. Volumetric rates of TCP and COD removal increased due to increasing biomass and decreasing effluent TCP and COD concentrations with increasing sludge age. The specific rate of TCP removal was maximum (120 mg TCP gX(-1) day(-1)) at a sludge age of 20 days. TCP inhibition was eliminated by operation of the system at sludge age above 20 days to obtain nearly complete COD, TCP, and toxicity removal.