ACETATE-FORMING BACTERIA
Acetate-forming (acetogenic) bacteria grow in a symbiotic relationship with methane-forming bacteria. Acetate serves as a substrate for methane-forming bacteria. For example, when ethanol (CH3CH2OH) is converted to acetate, carbon dioxide is used and acetate and hydrogen are produced (Equation 2.3).
CH3CH2OH + CO2 Æ CH3COOH+ 2H2 (2.3)
When acetate-forming bacteria produce acetate, hydrogen also is produced. If the hydrogen accumulates and significant hydrogen pressure occurs, the pressure results in termination of activity of acetate-forming bacteria and lost of acetate production. However, methane-forming bacteria utilize hydrogen in the production
of methane (Equation 2.4) and significant hydrogen pressure does not occur. CO2+ 4H2 Æ CH4+ 2H2 O (2.4)
Acetate-forming bacteria are obligate hydrogen producers and survive only at very low concentrations of hydrogen in the environment. They can only survive if their metabolic waste—hydrogen—is continuously removed. This is achieved in their symbiotic relationship with hydrogen-utilizing bacteria or methane-forming
bacteria. Acetogenic bacteria reproduce very slowly. Generation time for these organisms is usually greater than 3 days.
SULFATE-REDUCING 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.
Biogas Plant
Acetate-forming (acetogenic) bacteria grow in a symbiotic relationship with methane-forming bacteria. Acetate serves as a substrate for methane-forming bacteria. For example, when ethanol (CH3CH2OH) is converted to acetate, carbon dioxide is used and acetate and hydrogen are produced (Equation 2.3).
CH3CH2OH + CO2 Æ CH3COOH+ 2H2 (2.3)
When acetate-forming bacteria produce acetate, hydrogen also is produced. If the hydrogen accumulates and significant hydrogen pressure occurs, the pressure results in termination of activity of acetate-forming bacteria and lost of acetate production. However, methane-forming bacteria utilize hydrogen in the production
of methane (Equation 2.4) and significant hydrogen pressure does not occur. CO2+ 4H2 Æ CH4+ 2H2 O (2.4)
Acetate-forming bacteria are obligate hydrogen producers and survive only at very low concentrations of hydrogen in the environment. They can only survive if their metabolic waste—hydrogen—is continuously removed. This is achieved in their symbiotic relationship with hydrogen-utilizing bacteria or methane-forming
bacteria. Acetogenic bacteria reproduce very slowly. Generation time for these organisms is usually greater than 3 days.
SULFATE-REDUCING 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.
Biogas Plant
