Short note: Biomass to biogas

Biogas is a combustible gas produced when organic matter (biomass) – cattle dung, human excreta, kitchen waste, crop residue – is decomposed by bacteria in the absence of oxygen (anaerobic digestion). It contains about 50–70% methane (CH₄) and 30–40% CO₂, with small amounts of H₂S and water vapour.

Process (anaerobic digestion)

1. Hydrolysis – complex organics (carbohydrates, proteins, fats) broken into simple sugars, amino acids and fatty acids.
2. Acidogenesis – converted into volatile fatty acids, alcohol, CO₂ and H₂.
3. Acetogenesis – converted into acetic acid, H₂ and CO₂.
4. Methanogenesis – methanogenic bacteria produce CH₄ and CO₂.

 Dung + water -> [Inlet] -> [Digester] -> gas -> stove
                              |
                          [Outlet] -> slurry -> fertiliser

Conditions: temperature about 30–40 °C (mesophilic), pH about 6.8–7.5, airtight digester, proper dung–water mix (about 1:1).

Benefits

• Clean cooking and lighting fuel; replaces firewood and kerosene.
• Reduces indoor air pollution and eye/lung disease.
• Slurry is a rich organic fertiliser.
• Reduces deforestation and methane release from open dung; earns carbon credits (Nepal's biogas programme was registered under the CDM).
• Improves sanitation when toilets are connected.
• Saves women's time spent collecting firewood.

In Nepal: the Biogas Support Programme and AEPC have helped install several hundred thousand household plants, mostly the GGC-2047 fixed-dome model.

Limitations: needs cattle and water, lower gas output in cold hills in winter, initial cost.