Biological treatment of hazardous chemical substances emitted into the environment has been expected to be a desirable treatment method. In this study the biodegradation of an herbicide, S-ethyl dipropylthiocarbamate (EPTC), has been investigated in an airlift bioreactor in batch experiments using waste activated sludge, freely suspended and immobilized, under aerobic conditions. The influence of activated sludge loadings (MLSS = 2000-9000 ppm), EPTC concentrations (S-o = 2-11 ppm) and aeration rates (Q(g) = 0.4-3.51 min(-1)) on the degradation rate was discussed. The results show that for a constant activated sludge concentration of 4162 ppm, the biological degradation rate of EPTC increased with increasing aeration rate up to 2.5 1 min(-1). Although the increase of activated sludge concentration enhanced the biological degradation rate, the higher aeration rate was required to suspend activated sludge uniformly through the reactor and supply enough oxygen to the activated sludge. The maximum biodegradation rate was 0.16 h(-1) at an activated sludge loading of 6000 ppm and aeration rate of 3.51 min(-1). Also, it was observed that the biodegradation rates for immobilized activated sludge were higher than those for freely suspended activated sludge. Finally a mathematical model was proposed to describe the biodegradation of EPTC in the bioreactor. (c) 2005 Elsevier Ltd. All rights reserved.