Sulfate and Nitrate Reduction

Sulfate-reducing bacteria of the genera Desulfovibrio and Desulfotomaculum grow in anaerobically treated wastewater that contains sulfate (Zeikus 1979). Several organic compounds can be mineralized or partly used with lactate, i.e. Desulfovibrio de-sulfuricans (Gottschalk 1986; Yoo 2000):

2 CH3CHOHCOO- + SO2- ^ 2 CH3COO- + 2 HCO- + 2 H+ + S2- (8.13)

and acetate by Desulfobacter postgateii (Widdel and Pfennig 1981):

Beside these chemoorganoheterotrophic sulfate reducers even chemolitho-auto-trophics have been isolated (Brandis and Thauer 1981; Nethe-Jaenchen and Thauer 1984):

Depending on the pH, the sulfur exists mainly in the form of S2 (high pH) or H2S (low pH):

The formation of H2S is a serious problem because of its corrosive effects and its smell. Therefore, it must be separated from biogas, e.g. by reaction with iron sharings. Another disadvantage of sulfate reduction is the consumption of acetate (Eq. 8.14) and hydrogen (Eq. 8.15). Via these reactions, sulfate-reducing and some denitrifying bacteria compete with the methanogenic bacteria for hydrogen:

and the CH4 content of the biogas is reduced.

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