Methane-forming bacteria or methanogens are a specialized group of Archaea that utilize a limited number of substrates (Table 17.1), principally acetate (CH3COOH), carbon dioxide, and hydrogen for methane production or methanogenesis. These substrates are the end products of more complex substrates that were degraded by fermentative bacteria.
Methane-forming bacteria are some of the oldest bacteria and are grouped in the domain Archaebacteria. The term "arachae" means ancient. Methane-forming bacteria have many shapes (bacillus, coccus, and spirillum), sizes (0.1 to 15|im), and growth patterns (individual cells, filamentous chains, cubes, and sarcina). Methane-forming bacteria are oxygen-sensitive anaerobes and are found in habitats that are rich in degradable organic compounds. In these habitats oxygen is rapidly removed by bacterial degradation of the organic compounds. Some methane-forming bacteria live as symbionts in animal digestive tracts.
Most methane-forming bacteria are active in two temperature ranges, the mesophilic range from 30°C to 35°C and the thermophilic range from 50°C to 60°C. At temperatures between 40°C and 50°C, nearly all methane-forming bacteria are inhibited. Fluctuations in temperature within the mesophilic and thermophilic ranges are not directly harmful to methane-forming bacteria. However, fluctuations in temperature may change the dominant fermentative bacteria that produce the substrates that are used by methane-forming bacteria. If the dominant fermentative bacteria produce substrates that cannot be used by methane-forming bacteria, then methanogenesis is inhibited. Therefore, fluctuations in the operating temperature of anaerobic digesters should be minimal.
Methane-forming bacteria are active within the pH range of 6.8 to 7.2. Methane-forming bacteria are sensitive to pH values <6.8 and >7.2. With decreasing pH, methane-forming bacteria become less active, while fermentative bacteria remain
TABLE 17.1 Substrates Used by Methane-Forming Bacteria
Substrate Chemical Formula
Carbon dioxide CO2
Carbon monoxide CO
active and continue to produce fatty acids.These acids destroy alkalinity and depress pH resulting in inhibition of methane-forming bacteria. Also, with decreasing pH, increases in the quantities of hydrogen sulfide (H2S) and hydrogen cyanide (HCN) occur. These two inorganic compounds are highly toxic to methane-forming bacteria. With increasing pH, an increase in the quantity of ammonia (NH3) occurs. Ammonia also is toxic to methane-forming bacteria. Therefore, anaerobic digesters should be operated at a near neutral pH value and should be monitored as needed to ensure an acceptable pH value and alkalinity residual.
Sufficient alkalinity is necessary for proper pH control. Alkalinity serves as a buffer that prevents rapid change in pH. Enzymatic activity of methane-forming bacteria is adversely affected by pH values <6.8 and >7.2. Adequate alkalinity in an anaerobic digester can be maintained by providing an acceptable volatile acid-to-alkalinity ratio. The range of acceptable volatile acid-to-alkalinity ratios is 0.1 to 0.2.
Because methane-forming bacteria reproduce very slowly (generation times of 3-30 days) and produce very few offspring (sludge) from the degradation of substrates (approximately 0.02 pounds of sludge per pound of substrate degraded), methane-forming bacteria require smaller quantities of most nutrients. However, there are a few nutrients that are required by methane-forming bacteria in quantities two to five times greater than most other bacteria. These nutrients are cobalt, iron, nickel, and sulfur.
Methanogenesis occurs through three basic biochemical reactions that are mediated by three different groups of methane-forming bacteria (acteoclastic methanogens, hydrotrophic methanogens, and methyltrophic methanogens). Ace-toclastic methanogens produce methane by "splitting' acetate (Equation 17.1). Hydrogenotrophic methanogens produce methane by combining hydrogen and carbon dioxide (Equation 17.2), and methyltrophic methanogens produce methane by removing methyl (-CH3) groups from simple substrates (Equation 17.3). In anaerobic digesters, acetoclastic methane-forming bacteria produce most of the methane, while hydrotrophic methane-forming bacteria produce approximately 30% of all methane. Methyltrophic methane-forming bacteria produce a relatively small quantity of methane in anaerobic digesters.
Acetate acetoclastic meaane-fomorng tecterm } CH4 + CO2 (17.1)
H + CO h ydro gen otrophi c methane-fo rming b acte ria ^ CI I + 2H O (17 2)
Methanol methyltr°phic methane-forming bacteria } + 2H2O
Insoluble, complex organic substrates
HYDROLYSIS ^_ PHASE 1
Soluble, simple organic substrates
FERMENTATION, ACID AND ALCOHOL PRODUCTION
Organic acids and alcohols
Acetate, carbon dioxide, hydrogen
METHANOGENESIS ^_ PHASE 3
FIGURE 17.1 Basic phases of anaerobic degradation of substrate.
There are three basic biological events or phases that occur in municipal anaerobic digesters with respect to methane production. These phases are (1) hydrolysis, (2) fermentation and acetate production, and (3) methanogenesis (Figure 17.1). During hydrolysis, hydrolytic bacteria solubilize large and complex compounds to small and simple compounds. During fermentation, fermentative bacteria convert the newly formed soluble compounds to organic acids, alcohols, CO2, and H2. As part of fermentation, many of the acids and alcohols then are converted to acetate. During methanogenesis, methane-forming bacteria convert CO2, H2, acetate, and several other limited substrates to methane. A comprehensive review of methane-forming bacteria is provided in The Microbiology of Anaerobic Digesters in the Wastewater Microbiology Series.
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