Skip to main content

Biogas generation stages and affecting factors

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.

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

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: CH3COOH→CH4+CO2CH_3COOH \rightarrow CH_4 + CO_2 and CO2+4H2→CH4+2H2OCO_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

FactorEffect and usual value
TemperatureMesophilic range 30–40 °C is best; gas output drops sharply below 15 °C. Thermophilic 50–60 °C needs heating.
pH6.8–7.5; below 6.5 methanogens are inhibited.
Retention time30–60 days for cattle dung (depends on temperature).
C/N ratio20:1 to 30:1. Too high gives little gas, too low gives ammonia toxicity.
Total solids7–10 % in the slurry (dung:water about 1:1).
Nutrients and toxic materialsNeeds N, P, K; detergents, antibiotics, heavy metals, high ammonia inhibit bacteria.
AgitationPrevents scum and gives contact of bacteria with feed.
Loading rateOverloading causes acid build-up (digester sours).
SeedingAdding digested slurry with active bacteria speeds starting.
Particle sizeFine material digests faster.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Energy Resources chapter 3 questions →