Hydrogen Sulfide (H2S) is an odorous and highly toxic gas commonly encountered in various commercial and municipal sectors. Three novel, laboratory-scale, Horizontal-Flow Biofilm Reactors (HFBRs) were tested for the removal of hydrogen sulfide (H2S) gas from air streams over a 178-day trial at 10°C. Removal rates of up to 15.1 g [H2S] m(-3) h(-1) were achieved during the trial, demonstrating that HFBRs are a suitable technology for the treatment of H2S-contaminated airstreams at low temperatures. Bio-oxidation of H2S in the reactors led to the production of H(+) and sulfate (SO4(2-)) ions resulting in acidification of the liquid phase. Reduced removal efficiency was observed when a loading rate of 15.1 g [H2S] m(-3)h(-1)was applied to the HFBRs. The addition of NaHCO3to the liquid nutrient feed (SWW) during this period resulted in improved H2S removal. The bacterial community in the HFBRs was investigated by sequencing and fingerprinting the 16S rRNA genes. Bacterial diversity was low, likely due to the harsh environmental conditions prevailing in the systems, and the HFBRs were dominated by two species from the genus Acidithiobacillus and Thiobacillus. Even so, there were significant differences in microbial community structure between distinct zones in the HFBRs due to the influence of alkalinity, pH and SO4 concentrations. Despite the low operating temperature, this study indicates that HFBRs have excellent potential to biologically treat H2S contaminated airstreams.