MICROBIAL REDUCTION OF SULFUR-DIOXIDE AND NITRIC-OXIDE

Two process concepts have been developed for a microbial contribution to the problem of flue gas desulfurization and NO(x) removal. We have demonstrated that the sulfate-reducing baterium Desulfovibrio desulfuricans can be grown in a mixed culture with fermentative heterotrophs in a medium in which glucose served as the only carbon source. Beneficial cross-feeding resulted in vigorous growth of D. desulfuricans, which used SO2(g) as a terminal electron acceptor, with complete reduction of SO2 to H2S in 1-2s of contact time. We have proposed that the concentrated SO2 stream, obtained from regeneration of the sorbent in regenerable processes for flue gas desulfurization, could be split with two-thirds of the SO2 reduced to H2S by contact with a culture of sulfate-reducing bacteria. The resulting H2S could then be combined with the remaining SO2 and used as feed to a Claus reactor to produce elemental sulfur. However, the use of glucose as an electron donor in microbial SO2 reducing cultures would be prohibitively expensive. Therefore, if microbial reduction Of SO2 is to be economically viable, less expensive electron donors must be found. Consequently, we have evaluated the use of municipal sewage sludge and elemental hydrogen as carbon and/or energy sources for SO2 reducing cultures. Heat and alkali pretreated sewage sludge has been successfully used as a carbon and energy source to support SO2 reduction in a continuous, anaerobic mixed culture containing D. desulfuricans. The culture operated for nine months with complete reduction Of SO2 and H2S. Another sulfate-reducing bacterium, Desulfotomaculum orientis, has also been grown in batch cultures on a feed of SO2, H-2 and CO2. CoMplete reduction Of SO2 to H2S was observed with gas-liquid contact times of 1-2s. We have also demonstrated that the facultative anaerobe and chemoautotroph, Thiobacillus denitrificans, can be cultured anoxically in batch reactors using NO(g) as a terminal electron acceptor with reduction to elemental nitrogen. We have proposed that the concentrated stream of NO(x), as obtained from certain regenerable processes for flue gas desulfurization and NO(x) removal, could be converted to elemental nitrogen for disposal by contact with a culture T denitrificans. Two heterotrophic bacteria have also been identified which may be grown in batch cultures with succinate or heat and alkali pretreated sewage sludge as carbon and energy sources and NO as a terminal electron acceptor. These are Paracoccus denitrificans and Pseudomonas denitrificans.