Microbial reduction of sulfur dioxide and nitric oxide

Abstract

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 bacterium 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 SO 2(g) as a terminal electron acceptor, with complete reduction of SO 2 to H 2S in 1–2 s of contact time. We have proposed that the concentrated SO 2 stream, obtained from regeneration of the sorbent in regenerable processes for flue gas desulfurization, could be split with two-thirds of the SO 2 reduced to H 2S by contact with a culture of sulfate-reducing bacteria. The resulting H 2S could then be combined with the remaining SO 2 and used as feed to a Claus reactor to produce elemental sulfur. However, the use of glucose as an electron donor in microbial SO 2 reducing cultures would be prohibitively expensive. Therefore, if microbial reduction of SO 2 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 SO 2 reducing cultures. Heat and alkali pretreated sewage sludge has been successfully used as a carbon and energy source to support SO 2 reduction in a continuous, anaerobic mixed culture containing D. desulfuricans. The culture operated for nine months with complete reduction of SO 2 and H 2S. Another sulfate-reducing bacterium, Desulfotomaculum orientis, has also been grown in batch cultures on a feed of SO 2, H 2 and CO 2. Complete reduction of SO 2 to H 2S was observed with gas-liquid contact times of 1–2 s. 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.