Sulfatereducing Bacteria

Sulfate-reducing bacteria also are found in anaerobic digesters along with acetate-forming bacteria and methane-forming bacteria. If sulfates are present, sulfate-reducing bacteria such as Desulfovibrio desulfuricans multiply. Their multiplication or reproduction often requires the use of hydrogen and acetate—the same substrates used by methane-forming bacteria (Figure 2.2).

When sulfate is used to degrade an organic compound, sulfate is reduced to hydrogen sulfide. Hydrogen is needed to reduce sulfate to hydrogen sulfide. The need for hydrogen results in competition for hydrogen between two bacterial groups, sulfate-reducing bacteria and methane-producing bacteria.

When sulfate-reducing bacteria and methane-producing bacteria compete for hydrogen and acetate, sulfate-reducing bacteria obtain hydrogen and acetate more easily than methane-forming bacteria under low-acetate concentrations. At substrate-to-sulfate ratios <2, sulfate-reducing bacteria out-compete methane-forming bacteria for acetate. At substrate-to-sulfate ratios between 2 and 3, competition is very intense between the two bacterial groups. At substrate-to-sulfate ratios >3, methane-forming bacteria are favored.

The hydrogen sulfide produced by sulfate-reducing bacteria has a greater inhibitory effect at low concentrations on methane-forming bacteria and acetate-forming bacteria than on acid-forming bacteria.

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