● Biogas is mostly composed of methane (CH4) and carbon dioxide (CO2).
● Biogas production is a multistep anaerobic process in which originally complex
organic (liquid or solid) wastes are progressively transformed into low
molecular weight products by different bacteria strains.
● Biogas generation using the anaerobic digestion process involves the biological
degradation of organic matter. The sequence of reactions in biomass to biogas
conversion is as follows: (1) hydrolysis, (2) acidogenesis, (3) acetogenesis, and (4)
Step I: Hydrolysis:
Within the first stage, complex organic compounds such as proteins, carbohydrates,
and lipids are decomposed into simple soluble products by extracellular enzymes,
such as cellulases, amylases, proteases, and lipases, excreted by fermentative bacteria.
The main hydrolytic reactions at this stage are as follows:
Step II: Acidogenesis:
Acidogenesis is the bioconversion through which simple monomers produced by the
hydrolysis are further converted into volatile fatty acids, such as acetic acid, propionic
acid, and butyric acid and trace alcohols, ketones, CO2, NH3, H2S, and H2.
Step III: Acetogenesis.
The hydrogen-producing acetogenic bacteria decompose and convert propionic acid,
butyric acid, and alcohols into acetic acid, hydrogen, and carbon dioxide, while the
homoacetogenic bacteria convert hydrogen and carbon dioxide into acetic acid.
Step IV: Methanogenesis
The acetic acid, hydrogen, and C1 compounds (formic acid, methanol, and methylamine)
are converted into the end product, methane by methanogens.
Two-thirds of the methane produced comes from acetic acid, while the rest comes from
hydrogen and C1 compounds. The energy available in organic substances, except that used
for cellular metabolism, is recovered by means of methane.The major reactions at this
● Methanogens are characterized by high physiological specialization and extremely
strict anaerobiosis. They belong to the Euryarchaeota of Archeobacteria, which
convert inorganic organic compounds into methane and carbon dioxide.
● Methanogens can be divided into two groups, acetate-consuming and hydrogen-
● These microorganisms exist widely not only in natural ecosystems, such as swamps,
lakes, marine sediments, and gastric juices of ruminant animals, but also in non-
natural ecosystems such as wastewater treatment, compost, and sludge digestion
● Methanogens with various morphologies like, rod, cocci, and spiral. Methanogens
share the following characteristics:
1. Extremely low growth rate – for example, the doubling time of Methanosaeta is 4–9
2. Strict anaerobiosis – they are sensitive to oxygen and oxidants, and thus cannot survive
with exposure to oxygen or air
3. Limited simple compounds as their nutrition sources
4. Living in neutral or weak alkaline environment with suitable temperature (30-40
5. Biogas is their major end product, which mainly consists of methane and carbon
Advantages of biogas
● Biogas is a green energy source in form of electricity and heat for the local grid.
● Environmentally friendly: recirculation of organic waste from industry and
● Less odour inconveniences when spreading slurry on the fields - fermented
slurry smells considerably less than normal slurry and the smell decreases faster.
● Plants absorb fermented slurry better, increasing the yield on the fields.
● Protection of subsoil water - improved nitrogen exploitation reduces leaching
and thereby drinking water contamination.
● Reduced costs for artifical fertilizer.
Biogas, X Liu and Z Yan, Chengdu Institute of Biology, Chengdu, China Z-B Yue, Hefei
University of Technology, Hefei, China