MP Board · Class 12 · Biology · Microbes in Human WelfareDiscuss the application of microbes in the production of biogas. Describe the structure of a biogas plant and the biochemical processes involved in biogas generation.

Introduction to Biogas and Microbes\nBiogas is a gaseous fuel consisting primarily of methane ($CH_4$), carbon dioxide ($CO_2$), hydrogen ($H_2$), and hydrogen sulfide ($H_2S$). It is produced by the anaerobic microbial degradation of biomass, mainly cattle dung (gobar), agricultural waste, and organic municipal refuse. Because cattle dung contains abundant anaerobic bacteria (methanogens), it is widely utilized for biogas production, often referred to as 'gobar gas'.

Structure of a Biogas Plant\nA standard biogas plant (such as the Khadi and Village Industries Commission design) typically consists of the following structural components:

• Digester Tank: An underground, air-tight brick and cement tank where organic slurry undergoes anaerobic digestion.
• Inlet Tank: A masonry tank where cattle dung is mixed with water to form a uniform slurry before being fed into the digester.
• Gas Holder / Dome: An inverted dome situated above the digester tank to collect the generated biogas.
• Outlet Pipe / Sludge Chamber: A chamber connected to the digester for the removal of spent slurry, which is subsequently dried and used as high-quality manure.

Biochemical Processes in Biogas Generation\nAnaerobic digestion occurs through three sequential biochemical stages mediated by distinct groups of microorganisms:

1. Hydrolysis / Solubilization: Complex organic polymers (cellulose, hemicellulose, proteins, and lipids) present in the biomass are broken down into simpler soluble monomers (sugars, amino acids, and fatty acids) by extracellular hydrolytic enzymes secreted by fermentative bacteria.

2. Acidogenesis (Acid Fermentation): Facultative and obligate anaerobic acid-forming bacteria (acidogens) convert the soluble monomers into volatile fatty acids (such as acetic acid, propionic acid, and butyric acid), along with carbon dioxide and hydrogen gas.

3. Methanogenesis: Strict anaerobic bacteria known as methanogens (e.g., Methanobacterium) act upon the volatile fatty acids and carbon dioxide-hydrogen mixtures produced in the previous stages.

   • Reactions involved: $CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O$ $CH_3COOH \rightarrow CH_4 + CO_2$
   • This process yields methane gas as the primary constituent of biogas.

Advantages of Biogas

• Provides clean, smokeless fuel for cooking and lighting.
• Reduces the incidence of waterborne diseases through safe disposal of organic waste.
• Yields nutrient-rich organic manure as a byproduct, minimizing dependence on chemical fertilisers.
• Helps mitigate global warming by capturing methane emissions from livestock waste.