Meaning and Process
Anaerobic decomposition is the breakdown of organic matter by microorganisms in the absence of oxygen.
It commonly occurs in:
- waterlogged soils;
- flooded rice fields;
- wetlands;
- landfills;
- sewage-treatment systems;
- animal manure pits;
- biogas digesters.
During the process, microorganisms convert complex organic material such as carbohydrates, proteins and fats into simpler compounds.
The major stages are:
- Hydrolysis: Complex organic matter is broken into simpler molecules.
- Acidogenesis: These molecules are converted into organic acids, alcohols, hydrogen and carbon dioxide.
- Acetogenesis: Organic acids are converted mainly into acetic acid, hydrogen and carbon dioxide.
- Methanogenesis: Methanogenic microorganisms produce methane and carbon dioxide.
Products
The main products of anaerobic decomposition are:
- methane;
- carbon dioxide;
- hydrogen sulphide;
- ammonia;
- organic acids;
- nutrient-rich digestate.
The mixture of methane and carbon dioxide produced in a controlled digester is known as biogas.
Methane can be used as a fuel for cooking, heating and electricity generation. The remaining digestate can be used as manure after appropriate treatment.
Agricultural Importance
Anaerobic decomposition is important in agriculture because it occurs naturally in flooded paddy fields and poorly managed animal manure.
In continuously flooded rice fields, oxygen becomes limited. Microorganisms then decompose organic matter anaerobically and release methane.
Manure stored in liquid or oxygen-deficient conditions also produces methane and nitrous oxide.
Controlled anaerobic digestion can convert:
- cattle dung;
- crop residue;
- food waste;
- poultry waste;
- sewage sludge
into useful biogas and organic fertiliser.
Environmental Effects
Uncontrolled anaerobic decomposition can cause:
- methane emissions;
- foul odour;
- contamination of soil and groundwater;
- release of hydrogen sulphide;
- growth of disease-causing organisms;
- fire and explosion risks.
Methane is a powerful greenhouse gas. Its release from landfills, livestock waste and flooded fields contributes to climate change.
However, when methane is captured and used as energy, anaerobic digestion can reduce dependence on fossil fuels and improve waste management.
Anaerobic and Aerobic Decomposition
| Feature | Anaerobic decomposition | Aerobic decomposition |
| Oxygen | Occurs without oxygen | Requires oxygen |
| Main products | Methane, carbon dioxide and organic acids | Carbon dioxide, water and heat |
| Odour | Often produces strong odour | Usually produces less odour |
| Energy release | Slower and lower | Faster and higher |
| Common examples | Biogas plants and flooded soils | Composting and exposed soil |
| Climate impact | Can release methane | Mainly releases carbon dioxide |
Conclusion
Anaerobic decomposition is a natural microbial process that can either create environmental problems or provide useful energy. Its proper management through biogas plants, methane capture and scientific waste treatment can reduce pollution while generating renewable fuel and organic manure.
