Biological treatment is an emerging and prevalent technology for treating off-gases from wastewater treatment plants. The most commonly reported odorous compound in off-gases is hydrogen sulfide (H2S), which has a very low odor threshold. A self-designed, bench-scale, cross-flow horizontal biotrickling filter (HBF) operated with bacteria immobilized activated carbon ( termed biological activated carbon - BAC), was applied for the treatment of H2S. A mixed culture of sulfide-oxidizing bacteria dominated by Acidithiobacillus thiooxidans acclimated from activated sludge was used as bacterial seed and the biofilm was developed by culturing the bacteria in the presence of carbon pellets in mineral medium. HBF performance was evaluated systematically over similar to 120 days, depending on a series of changing factors including inlet H2S concentration, gas retention time (GRT), pH of recirculation solution, upset and recovery, sulfate accumulation, pressure drop, gas-liquid ratio, and shock loading. The biotrickling filter system can operate at high efficiency from the first day of operation. At a volumetric loading of 900 m(3) m(-3) h(-1) ( at 92 ppmv H2S inlet concentration), the BAC exhibited maximum elimination capacity (113 g H2S/ m(-3) h(-1)) and a removal efficiency of 96% was observed. If the inlet concentration was kept at around 20 ppmv, high H2S removal ( over 98%) was achieved at a GRT of 4 s, a value comparable with those currently reported for biotrickling filters. The bacterial population in the acidic biofilter demonstrated capacity for removal of H2S over a broad pH range ( pH 1-7). A preliminary investigation into the different effects of bacterial biodegradation and carbon adsorption on system performance was also conducted. This study shows the HBF to be a feasible and economic alternative to physical and chemical treatments for the removal of H2S.