Microbial reduction of sulfur dioxide in immobilized, mixed cultures of sulfate-reducing bacteria with sewage digest as carbon and energy source

Conclusions

A considerable increase in soluble COD and organic acid concentrations was achieved in a continuous process for AD-MSS medium preparation. This reactor utilized less DAF solids and chloroform, and used no additional salts compared with previous investigations. It resulted in a COD of 5000 mg/L, double that of previous studies. This increase in available carbon source for SO₂-reducing bioreactors, together with the corresponding decrease in required hydraulic loading to the bioreactors, serves to decrease the capital and operating cost for the proposed microbial SO₂-reduction process. Our initial investigation of the immobilization of mixed SRB cultures with alginate or κ-carrageenan gel demonstrated that biocatalyst immobilization did not impair biocatalyst performance. Sulfur dioixde throughputs of 0.19 and 0.17 mmol/(h·L) were achieved when the same amount of biomass was used in free- and immobilized-cell forms, respectively. The κ-carrageenan-cell beads were found to be more stable than the alginate-cell beads under the operating conditions. Summarizing the results of the experiments, it can be concluded that the immobilized-cell bioreactors gave equal conversion rates of SO₂ as free cells on a per-biomass basis and have the potential to increase greatly the volumetric productivity of the reactor. Owing to the limited reactor volumes available, maximal biomass loading in the immobilized-cell reactors was not realized in the present study. Future work will increase the biomass concentration in packed and fluidized beds of immobilized biocatalyst in order to increase reactor throughput and further improve the economic viability of the proposed microbial SO₂-reduction process.