Microbial reduction of sulfur dioxide with anaerobically digested municipal sewage biosolids as electron donors.

A concentrated stream of sulfur dioxide (SO2) is produced by regeneration of the sorbent in certain new regenerable processes for the desulfurization of flue gas. We have previously proposed that this SO2 can be converted to elemental sulfur for disposal or byproduct recovery using a microbial/Claus process. In this process, two-thirds of the SO2-reducing gas stream would be contacted with a mixed culture containing sulfate-reducing bacteria (SRB), where SO2 would act as an electron acceptor with reduction to hydrogen sulfide (H2S). This H2S could then be recombined with the remaining SO2 and sent to a Claus unit to produce elemental sulfur. The sulfate-reducing bacterium, Desulfovibrio desulfuricans, has been immobilized by coculture with flocforming heterotrophs from an anaerobic digester, resulting in a SO2-reducing floc that may be collected from the effluent of a continuous reactor for recycle by gravity sedimentation. The carbon and energy source for these cultures was anaerobically digested municipal sewage solids. The maximum specific activity for SO2 reduction in these cultures, in terms of dry weight of D. desulfuricans biomass, was 9.1 mmol of SO2/h.g. The stoichiometry with respect to the electron donor was 15.5 mg of soluble COD/mmol of SO2 reduced.