Biological sulfate reduction using molasses as a carbon source.

The feasibility of using a laboratory-scale upflow anaerobic sludge blanket process for sulfate reduction with molasses as a carbon source was demonstrated. Competition between methane-producing bacteria (MPB) and sulfate-reducing bacteria (SRB) was influenced by the chemical oxygen demand-to-sulfur (COD:S) ratio in the feed. Sulfate removal greater than 80% could be achieved at COD:S greater than 10 when MPB predominated. Activity of MPB and SRB was inhibited at a dissolved sulfide concentration of approximately 200 mg/L. Competition between MPB and SRB was intense as the COD:S was reduced from 5 to 2. Further reduction in the COD:S to 0.7 led to the formation of sulfidogenic granules. The COD removal decreased to approximately 30% at a COD:S less than 2 because of accumulation of sulfurous precipitates and the nonbiodegradable portion of molasses in the sludge. Reduced gas production rates further imposed limitations on diffusion of the organic substrate into granules. Sulfidogenic process operation yielded sulfate removal as great as 70% at a COD:S of approximately 3.5.