NCERT-Biogas is a mixture of gases (containing predominantly methane) producedby the microbial activity and which may be used as fuel.

Biogas Plant Flow Diagram with Explanations

Process Steps:

1. Feedstock Preparation:
   • Input: Organic matter like manure, agricultural waste, food scraps, etc.
   • Physics/Chemistry: Size reduction through grinding or chopping increases surface area for efficient microbial digestion. High moisture content (60-80%) is optimal for microbial activity.
   • Biology: No specific biological activity occurs at this stage.
2. Hydrolysis:
   • Chamber: Hydrolysis Tank
   • Input: Pre-treated organic matter
   • Process: Complex organic molecules (proteins, carbohydrates, fats) are broken down into simpler forms like sugars, amino acids, and fatty acids by extracellular enzymes secreted by bacteria.
   • Biology: Primarily involves hydrolytic bacteria like Clostridium, Cellulomonas, etc.
   • Chemistry: Hydrolysis reactions involve breaking covalent bonds in organic molecules using water.
3. Acidogenesis:
   • Chamber: Acidogenesis Tank (may be combined with hydrolysis tank)
   • Input: Products of hydrolysis (sugars, amino acids, fatty acids)
   • Process: Acidogenic bacteria convert the simpler organic molecules into volatile fatty acids (VFAs) like acetic acid, butyric acid, and propionic acid, along with hydrogen, carbon dioxide, and ammonia.
   • Biology: Acidogenic bacteria like Clostridium, Enterobacter, etc. are involved.
   • Chemistry: Fermentation reactions are responsible for the conversion of organic molecules to VFAs, gases, and other products.
4. Acetogenesis:
   • Chamber: Acetogenesis Tank (may be combined with acidogenesis tank)
   • Input: VFAs, hydrogen, and carbon dioxide from acidogenesis
   • Process: Acetogenic bacteria convert VFAs, primarily acetic acid, into acetate, hydrogen, and carbon dioxide.
   • Biology: Acetogenic bacteria like Acetobacterium, Moorella, etc. are involved.
   • Chemistry: Fermentation reactions convert VFAs to acetate, hydrogen, and carbon dioxide.
5. Methanogenesis:
   • Chamber: Methanogenesis Tank
   • Input: Acetate, hydrogen, and carbon dioxide from acetogenesis
   • Process: Methanogenic archaea convert acetate and hydrogen into methane (CH4), the primary component of biogas, and carbon dioxide (CO2).
   • Biology: Methanogenic archaea like Methanosarcina, Methanobacterium, etc. are responsible.
   • Chemistry: Methanogenesis involves complex biochemical reactions utilizing enzymes like hydrogenotrophic methanogenesis or acetoclastic methanogenesis.
6. Gas Separation and Storage:
   • Process: Biogas is separated from the digestate (remaining liquid effluent) and stored in a gas holder for use as fuel or energy generation.
   • Chemistry: Biogas composition typically includes 50-70% methane, 20-30% carbon dioxide, 1-5% hydrogen sulfide, and other trace gases.

Additional Notes:

• The chambers may be combined or configured differently depending on specific plant design and desired biogas composition.
• Maintaining optimal temperature (35-55°C), pH (6.5-7.5), and nutrient balance is crucial for efficient microbial activity and biogas production.
• Biogas utilization offers environmental benefits by reducing greenhouse gas emissions and providing a renewable energy source.

+--------------------+
|     Feedstock     |
| (Organic Matter)  |
+--------------------+
         |
         v
+--------------------+
|  Pre-treatment    |
| (Grinding, Chipping)|
+--------------------+
         |
         v
+--------------------+
|  Hydrolysis Tank  |
| (Bacteria breakdown |
| complex molecules)|
+--------------------+
         |
         v
+--------------------+
|  Acidogenesis Tank |
| (Bacteria produce  |
| VFAs, H2, CO2)    |
+--------------------+
         |
         v
+--------------------+
| Acetogenesis Tank |
| (Bacteria convert |
| VFAs to acetate) |
+--------------------+
         |
         v
+--------------------+
| Methanogenesis Tank |
| (Archaea produce   |
| CH4, CO2)          |
+--------------------+
         |
         v
+--------------------+
|   Gas Separation   |
|     and Storage    |
+--------------------+
         |
         v
+--------------------+
|   Biogas (CH4, CO2, |
|   H2S, etc.)       |
+--------------------+
         |
         v
+--------------------+
| Energy Generation  |
| or Fuel Application|
+--------------------+

Additional Notes:
1. Pre-treatment involves size reduction through grinding or chopping to increase surface area for efficient microbial digestion. It also optimizes moisture content (60-80%) for microbial activity.
2. Hydrolysis: Complex organic molecules are broken down into simpler forms like sugars, amino acids, and fatty acids by extracellular enzymes secreted by bacteria.
3. Acidogenesis: Acidogenic bacteria convert the simpler organic molecules into volatile fatty acids (VFAs) like acetic acid, butyric acid, and propionic acid, along with hydrogen, carbon dioxide, and ammonia.
4. Acetogenesis: Acetogenic bacteria convert VFAs, primarily acetic acid, into acetate, hydrogen, and carbon dioxide.
5. Methanogenesis: Methanogenic archaea convert acetate and hydrogen into methane (CH4), the primary component of biogas, and carbon dioxide (CO2).
6. Gas Separation and Storage: Biogas is separated from the digestate (remaining liquid effluent) and stored in a gas holder for use as fuel or energy generation.

Chemical Reactions for Biogas Production

Biogas production involves a series of biological and chemical processes that break down organic matter into methane (CH₄), the primary component of biogas.

Stages and Reactions

• Hydrolysis:
• Process: Complex organic molecules (proteins, carbohydrates, fats) are broken down into simpler forms like sugars, amino acids, and fatty acids by extracellular enzymes secreted by bacteria.
• Example reactions:
  • Proteins + H₂O -> Amino acids
  • Carbohydrates + H₂O -> Sugars
  • Fats + H₂O -> Fatty acids
• Acidogenesis:
• Process: Acidogenic bacteria convert the simpler organic molecules into volatile fatty acids (VFAs) like acetic acid, butyric acid, and propionic acid, along with hydrogen, carbon dioxide, and ammonia.
• Example reaction: Glucose -> Lactic acid + H₂ + CO₂
• Acetogenesis:
• Process: Acetogenic bacteria convert VFAs, primarily acetic acid, into acetate, hydrogen, and carbon dioxide.
• Example reaction: 2 CH₃COOH (Acetic Acid) -> 2 CH₃COO^– (Acetate) + 2 H⁺ + CO₂
• Methanogenesis:
• Process: Methanogenic archaea convert acetate and hydrogen into methane (CH₄) and carbon dioxide (CO₂).
• Example reactions:
  • 4 H₂ + CO₂ -> CH₄ + 2 H₂O (Hydrogenotrophic)
  • CH₃COO^– + H⁺ -> CH₄ + CO₂ (Acetoclastic)

Overall Reaction (Simplified): Organic Matter + H₂O -> CH₄ + CO₂ + NH₃ + Other Byproducts

Note: This is a simplified representation, and the actual process involves multiple steps and diverse microbial interactions.