Biological treatment of 2,4,6-trichlorophenol (TCP) containing wastewater in a hybrid bioreactor system with effluent recycle

Due to the toxic effects of trichlorophenol (TCP) on microorganisms, biological treatment efficiencies of TCP containing wastewaters are usually low. Synthetic wastewater containing 2,4,6-TCP was biologically treated in a hybrid-loop bioreactor system consisting of a packed column biofilm and an aerated tank bioreactor with effluent recycle in order to improve COD and TCP removals. Effects of the feed TCP concentration on COD, TCP and toxicity removal performance of the system were investigated for the feed TCP between 50 and 450 mg L(-1) while the sludge age (solids retention time, SRT) and hydraulic residence time (HRT) were kept constant at 20 d and 25 h, respectively. Biomass concentrations in the packed column and in the aeration tank decreased with increasing feed TCP concentrations due to toxic effects of TCP on the organisms. Low biomass concentrations in the system at high feed TCP contents resulted in low COD, TCP and toxicity removals. Therefore, percent TCP, COD and toxicity removals decreased with increasing feed TCP concentrations especially above 400 mg L(-1). The effluent TCP concentrations were lower than 20 mg L(-1) for the feed TCP concentrations below 390 mg L(-1) resulting in TCP and COD removals above 90%. Specific rates of TCP and COD removals increased with the feed TCP due to low biomass concentrations at high TCP contents. The system should be operated at a feed TCP lower than 400 mg L(-1) in order to obtain more than 90% TCP, COD and toxicity removals under the specified experimental conditions.