Methanogenesis (methanogenic stage)

Methanogenesis is the fourth and final stage of anaerobic digestion, in which methanogenic archaea convert acetate, hydrogen, and carbon dioxide — the end products of acetogenesis — into methane (CH4) and additional CO2. Methane is the primary energy-carrying constituent of biogas and the entire economic basis of a CBG plant, so methanogenesis is the stage that ultimately determines yield, gas quality, and revenue.

Methanogens are not bacteria — they are archaea, an entirely separate domain of life, and are among the most environmentally fragile microorganisms cultivated industrially. They follow two metabolic pathways:

• Acetoclastic methanogenesis: CH3COOH -> CH4 + CO2. Accounts for 65-70% of methane production. Dominated by genera Methanosaeta (low-acetate environments) and Methanosarcina (high-acetate, more robust).
• Hydrogenotrophic methanogenesis: 4H2 + CO2 -> CH4 + 2H2O. Accounts for 30-35% of methane production. Dominated by Methanobacterium, Methanobrevibacter, and Methanoculleus.

Operating constraints for methanogens are narrow:

• pH: 6.8-7.5 (death below 6.0).
• Temperature: 35-40 degC mesophilic or 50-55 degC thermophilic; sensitivity to drift exceeds 2-3 degC per day.
• Ammonia (NH3 free): tolerated up to 200-400 mg/L, inhibition above.
• VFA: best below 1,000 mg/L; impairment above 3,000 mg/L.
• Sulfide: free H2S above 100 mg/L is toxic.
• Heavy metals, antibiotics, chlorinated compounds: lethal at trace concentrations.

Methanogen doubling times are 2-12 days (versus minutes for many bacteria), so recovery from a major upset takes weeks. Indian CBG plants protect methanogenic populations through strict feedstock acceptance protocols (refusing pesticide-treated material, antibiotics-laced poultry litter, or industrial effluents), gradual changes in feeding regimes, and redundant temperature control. The trade-off in plant design is between thermophilic operation (faster methanogenesis, 25-35% higher volumetric productivity, better pathogen kill) and mesophilic operation (more stable, more forgiving of feedstock variability, lower heating energy). Mesophilic dominates Indian deployments precisely because feedstock streams are heterogeneous and methanogen stability matters more than peak productivity.