---
title: "Biogas generation stages and affecting factors"
subject: "Energy Resources"
chapter: 3
key: "semester_6/energy_resources:biogas-generation-stages-factors"
source: https://www.bctengineer.com/q/semester_6/energy_resources/biogas-generation-stages-factors
site: BCT Engineering
---

# Biogas generation stages and affecting factors

Energy Resources · Chapter 3: Bio energy

- Asked in: year not given, 8 marks
- Video: [Biogas Production Explained | Anaerobic Fermentation of Organic Waste | BME515D Energy Engineering](https://www.youtube.com/watch?v=P0Dg6CL845c) — Mech Simplified
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/energy_resources/biogas-generation-stages-factors
- Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-3

## Question

Explain the process of biogas generation by anaerobic digestion. Discuss the factors that affect the production of biogas in a digester.

## Short answer

Anaerobic digestion occurs in four stages: hydrolysis, acidogenesis, acetogenesis and methonogenesis, producing methane (55-65%) and CO₂. Key factors affecting biogas production are temperature (mesophilic 30-40°C), pH (6.8-7.5), C/N ratio (20:1-30:1), retention time (30-60 days), total solids (7-10%), agitation, and absence of toxic materials.

## Answer

**Biogas** is a mixture of about 55–65 % methane, 35–45 % carbon dioxide and traces of H₂S, H₂ and N₂, produced when organic matter decomposes without oxygen. Its calorific value is about 20–24 MJ/m³.

#### Stages of anaerobic digestion
1. **Hydrolysis:** extracellular enzymes of bacteria break complex polymers (cellulose, proteins, fats) into soluble sugars, amino acids and fatty acids.
2. **Acidogenesis (fermentation):** acid-forming bacteria convert these into volatile fatty acids, alcohols, CO₂ and H₂.
3. **Acetogenesis:** the acids are converted into acetic acid, H₂ and CO₂.
4. **Methanogenesis:** methane-forming bacteria produce methane:
   $CH_3COOH \rightarrow CH_4 + CO_2$ and $CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O$.

The first two stages are done by acid formers; the methanogens are slow growing and sensitive.

#### Factors affecting gas production

| Factor | Effect and usual value |
|---|---|
| Temperature | Mesophilic range 30–40 °C is best; gas output drops sharply below 15 °C. Thermophilic 50–60 °C needs heating. |
| pH | 6.8–7.5; below 6.5 methanogens are inhibited. |
| Retention time | 30–60 days for cattle dung (depends on temperature). |
| C/N ratio | 20:1 to 30:1. Too high gives little gas, too low gives ammonia toxicity. |
| Total solids | 7–10 % in the slurry (dung:water about 1:1). |
| Nutrients and toxic materials | Needs N, P, K; detergents, antibiotics, heavy metals, high ammonia inhibit bacteria. |
| Agitation | Prevents scum and gives contact of bacteria with feed. |
| Loading rate | Overloading causes acid build-up (digester sours). |
| Seeding | Adding digested slurry with active bacteria speeds starting. |
| Particle size | Fine material digests faster. |

## Check yourself

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

1. What is the correct order of anaerobic digestion?
   A. Methanogenesis, hydrolysis, acidogenesis, acetogenesis
   B. Acidogenesis, hydrolysis, methanogenesis, acetogenesis
   C. Hydrolysis, acidogenesis, acetogenesis, methanogenesis
   D. Hydrolysis, methanogenesis, acetogenesis, acidogenesis
   Answer: C. Polymers break down, then acids form, then acetic acid, then methane. (Key point: Stages order)

2. Which reaction represents methane formation from acetic acid?
   A. CH3COOH → 2CH4
   B. CH3COOH → CH4 + CO2
   C. C6H12O6 → 2C2H5OH + 2CO2
   D. CH4 + 2O2 → CO2 + 2H2O
   Answer: B. Methanogens also make methane from CO2 + 4H2. (Key point: Methanogenesis)

3. What C/N ratio is best, and what happens if it is too low?
   A. 20:1 to 30:1; ammonia toxicity
   B. 5:1 to 10:1; too little gas
   C. 50:1 to 80:1; scum
   D. 20:1 to 30:1; too much gas
   Answer: A. Too high gives little gas; too low gives ammonia that inhibits bacteria. (Key point: C/N ratio)

4. Which conditions favour biogas production?
   A. Below 15 °C and pH 4
   B. 60-80 °C and pH 9
   C. 0 °C and pH 6
   D. Mesophilic 30-40 °C and pH 6.8-7.5
   Answer: D. Output drops sharply below 15 °C and methanogens are inhibited below pH 6.5. (Key point: Temperature and pH)

## Related in Energy Resources

- [Sizing a family biogas plant](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Abiogas-plant-sizing-cattle-dung) — Energy Resources, chapter 3

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