Exhausted carbon which comes from the H2S adsorption process may be a hazardous waste. In this study, exhausted carbon was re-used in biofiltration for H2S removal. Two identical columns were used for exhausted carbon (Column A) and fresh carbon (Column B). They were operated in the same mode with 35 ppmv of H2S gas at an empty bed residence time (EBRT) of 10 s. The results show that the removal efficiency of H2S in the two columns was almost identical at 95-100%. The removal mechanisms of H2S was explored and explained by developing a mathematical model. The model incorporated mass transfer, biodegradation, adsorption, as well as biofilm growth. The developed model can predict the experimental results very well. The modeled results suggest that the removal of H2S in Column A was attributed to the adsorption mechanism much less than in Column B during the start-up stage, while the removal of H2S by the biodegradation in Column A was much higher. The removal of H2S by the adsorption was significantly affected by the biodegradation. The simulation results also suggest that column A achieved the steady-state biodegradation in a shorter time than in Column B. This could result from higher biomass concentration of biofilm in Column A. due to the extra sulfur source from pre-adsorbed sulfur on exhausted carbon besides H2S gas feeding. (C) 2010 Elsevier B.V. All rights reserved.