a biological process is described involving the conversion of so 2 to h 2 s using sulfate reducing bacteria in co - culture with facultatively anaerobic heterotrophs under anaerobic conditions in a maintenance medium which includes a substrate fermentable by the facultatively anaerobic heterotrophs to produce an end product which acts as the carbon and energy source for the sulfate reducing bacteria . glucose and sewage sludge are the two fermentable substrates described . the process is described as being used in conjunction with a process for removing so 2 from a flue gas and then converting the so 2 to h 2 s . the h 2 s may be reacted with some remaining so 2 in a claus reactor to produce elemental sulfur or the h 2 s may be used as a reducing gas to regenerate the sorbent being used to remove the so 2 from the flue gas .