---
title: "Short note: Biomass to biogas"
subject: "Technology, Environment and Society"
chapter: 6
key: "semester_7/technology_environment_and_society:biomass-to-biogas"
source: https://www.bctengineer.com/q/semester_7/technology_environment_and_society/biomass-to-biogas
site: BCT Engineering
---

# Short note: Biomass to biogas

Technology, Environment and Society · Chapter 6: Climate Change

- Asked in: 2080 Bhadra, 4 marks
- Video: [Methane production | Biogas plant | Working | Mechanism | Methanogens | Biogas production](https://www.youtube.com/watch?v=MYj13FB_Iac) — Bio science
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/technology_environment_and_society/biomass-to-biogas
- Whole chapter: https://www.bctengineer.com/md/semester_7/technology_environment_and_society/chapter-6

## Question

Write a short note on biomass to biogas.

## Short answer

Biogas is a combustible gas from anaerobic digestion of biomass, mainly 50-70% methane and 30-40% carbon dioxide. The process has hydrolysis, acidogenesis, acetogenesis and methanogenesis. Benefits include clean fuel, reduced indoor pollution, organic slurry fertiliser, deforestation reduction, sanitation and carbon credits; in Nepal many household plants use the GGC-2047 fixed-dome model.

## Answer

**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.

```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Biogas contains mainly:
   A. 50-70% methane and 30-40% CO2
   B. 90% oxygen
   C. Pure hydrogen
   D. 50% nitrogen
   Answer: A. Biogas is mainly methane and CO2 with traces of H2S. (Key point: Composition)

2. Biogas is produced by decomposition:
   A. In the presence of oxygen
   B. In the absence of oxygen by bacteria
   C. By burning
   D. By sunlight alone
   Answer: B. Anaerobic bacteria digest biomass without oxygen. (Key point: Anaerobic digestion)

3. Which is the correct order of anaerobic digestion?
   A. Methanogenesis, acetogenesis, acidogenesis, hydrolysis
   B. Hydrolysis, acidogenesis, acetogenesis, methanogenesis
   C. Acidogenesis, hydrolysis, methanogenesis, acetogenesis
   D. Hydrolysis, methanogenesis, acetogenesis, acidogenesis
   Answer: B. Complex organics are broken down step by step, ending with methanogenesis. (Key point: Stages)

4. Which are suitable digester conditions?
   A. 30-40 °C, pH 6.8-7.5, airtight
   B. 80 °C, pH 2
   C. Open to air, pH 11
   D. 0 °C, pH 4
   Answer: A. Mesophilic temperature, near-neutral pH and airtight conditions are needed. (Key point: Conditions)

## Related in other subjects

- [Biogas routes and climate change](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Abiogas-routes-climate-change) — Energy Environment and Society, chapter 3
- [Biomass and conversion routes](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Abiomass-conversion-routes) — Energy Environment and Society, chapter 3, asked 3×