This study was aimed to identify optimal operating conditions for $H_2S$ removal using biofilter and to propose a mathematical model. The biofilter (8 cm φ × 110 cm of working height) used was packed with celite stones (0.5-1.5 cm φ). Firstly, the effects on $H_2S$ removal efficiency of $H_2S$ concentration and space velocity were investigated in the range of 13-500 ppm and 100-500 $h^{-1}$, respectively. It was found that the space velocity was more influencing factor on $H_2S$ removal than $H_2S$ concentration. When the space velocity was 100 $h^{-1}$, $H_2S$ was completely removed. Up to 300 $h^{-1}$ of space velocity, the $H_2S$ removal efficiency was over 98%. The maximum load at which the $H_2S$ concentration in the effluent satisfied the regulation (below 0.2 ppm) was found to be 102.72 g-S·$m^{-3}\cdot h^{-1}$ which was at least twice higher than previously reported data. Secondly, it was investigated whether if the cells could have sufficient $H_2S$ removal activity when the biofilter was to be run again after a shut-down for a certain period of time. Even after 3 weeks of starvation, the removal efficiency showed a similar value to that before starvation. In modeling, the biofilter was considered to consist of 3 parts: the gas phase, the biofilm, and the sorption volume. The effect of $H_2S$ adsorption to celite stones was also considered. The $H_2S$ concentration profiles estimated by the developed model for various space velocities were found to agree well with the experimental data when the $H_2S$ concentration was relatively low. However in the case of high $H_2S$ concentrations, significant deviations were observed.
