Advances in Biogas Technology - Biotechnology in China: Biofuels and Bioenergy - page 120

1 Methanogens Involved in Biogas Fermentation
1.1 Methanogens
Methanogens involved in biogas fermentation are very diverse. Taxonomically,
methanogens belong to the Euryarchaeota, and have been identified as Genera 25
of Class 3. The latest taxonomic system is shown in Figure 1 [ 1 ].
In recent years, psychrophilic methanogens and acidophilic methanogens with
specific environmental adaptability have also been discovered. Psychrophilic
methanogens that grow below 10 degrees celsius were not isolated until the 1990s
[ 2 ]; by the 2000s, 84 species of methanogens had been isolated and named [ 3 ].
Most acidophilic methanogens grow in specific acidic condition. These acidic
environments are mainly geothermal environments and mining areas that generally
contain contaminants. The main species of methanogen are Thiobacillus thiooxi-
dans, Thiobacillus ferrooxidans etc. [ 4 ].
In biogas fermentation systems, the non-methanogens and methanogens usually
coexist. These bacterias may benefit or inhibit each other, and thus a balanced
community structure can be maintained. Non-methanogens can (1) provide sub-
strates for the growth and methane production of methanogens, (2) maintain
appropriate oxidation-reduction potential (ORP) for methanogens, (3) remove
toxic substances that may inhibit methanogens, and (4) eliminate feedback inhi-
bition for methanogens. Meanwhile, non-methanogens and methanogens together
maintain an appropriate pH during biogas fermentation.
1.2 Conditions for Predominance of Methanogens
Temperature: The optimal temperature range for methanogen growth and methane
production is 25-30 C [ 5 ]. Trace metal elements: Addition of trace metal elements
can change the dominant species of methanogen, and facilitate increased acid
utilization and toxic resistance [ 6 , 7 ]. Salts: Sulfate-reducing bacterias (SRB)
compete with methanogens for substrate. Additionally, the sulfide produced has a
significant toxic effect on methanogens [ 8 ]. pH: The optimal pH range for meth-
anogen growth is 6.8-7.5. The medium pH affects the cell surface charge, the
ionization of organic compounds, and the microbial resistance to high temperature.
In addition, pH also significantly influences enzyme activity [ 9 ]. ORP: ORP is a
more accurate indicator of the system's anaerobic status than DO [ 10 ]. The
optimal ORP range for methanogens is below -350 mV. Anaerobic environments
are characterized by low ORP.