Biogas,which is anaerobically produced from various organic wastes,has greatlyattracted research and industrial attentions,because biogas can replace fossil fuels as an alternativeenergy source.However,biogas commonly contains undesirable impurities such as hydrogensulfide that must be pre-treated to avoid facility corrosion,unnecessary by-product production,and possible public exposure and complaints.Treatment methods to remove hydrogen sulfidefrom biogas are generally classified as physical,chemical and biological processes.Among them,the biological processes have been investigated because of their relatively low operating costs andminimal generation of undesirable by-products.In this study,a combined biochemical oxidationprocess using sulfur-oxidizing bacteria(SOB)was developed to remove high strength hydrogensulfide.The SOB requires dissolved oxygen for the biological oxidation of hydrogen sulfide in themicrobial/liquid phase,but the biogas itself should not be altered with respect to methane andoxygen contents.Therefore,the combined biochemical oxidation process developed in this studyconsisted of a hydrogen sulfide absorption column and a microbial oxidation column.Hydrogensulfide contained in the biogas was mass-transferred to the liquid medium in the absorptioncolumn,and the absorbed hydrogen sulfide was subsequently removed in the microbial oxidationcolumn in the presence of oxygen.The microbial/liquid medium was continuously recirculatedbetween both columns.The inlet loading of hydrogen sulfide was 28.5 g/m3/hr,and the overallremoval efficiency of 70%was maintained throughout the operational period of 25 days.The pHof the liquid medium rapidly declined to 1.2 from its initial pH of 3.2,and microbial density andactivity of the SOB were found to be high at this extremely low pH condition.In addition,themass balance analysis for sulfur was conducted to determine the extent of oxidation by the SOB,and it showed that 83%of hydrogen sulfide introduced to the combined reactor was transformedto the sulfate ion(SO42-),suggesting that the biochemical oxidation progressed actively.Consequently,a combined biochemical oxidation using the SOB showed advantages overconventional methods for the treatment of high strength hydrogen sulfide from biogas.