Anaerobic Digestion (AD) (Anaerobic Digestion)

Anaerobic Digestion (AD) is a four-stage biological process in which a consortium of microorganisms breaks down organic matter — agricultural residue, food waste, animal dung, sewage sludge, distillery effluent — in the complete absence of oxygen, yielding two products: biogas (a mixture of roughly 55-65% methane, 35-45% carbon dioxide, and trace H₂S and moisture) and digestate (a nutrient-rich slurry usable as fertiliser).

The four stages occur simultaneously in different microbial niches within the same reactor. Hydrolysis uses extracellular enzymes from facultative anaerobes to break long-chain polymers — cellulose, starch, proteins, fats — into soluble sugars, amino acids and long-chain fatty acids. Acidogenesis converts these soluble compounds into volatile fatty acids (mainly acetic, propionic, butyric), alcohols, hydrogen and CO₂. Acetogenesis converts longer VFAs and alcohols into acetic acid, hydrogen and CO₂ — the substrates that methanogens can finally use. Methanogenesis, performed by archaea (not bacteria), produces methane through two routes: the acetoclastic pathway splits acetic acid (responsible for around 70% of methane) and the hydrogenotrophic pathway combines hydrogen with CO₂.

Operating windows are tight. Mesophilic AD runs at 35-40°C and is the dominant Indian choice for stability and modest heating demand; thermophilic AD at 50-55°C gives 30-40% faster kinetics but is more sensitive to feedstock shocks. pH must stay 6.8-7.6, VFA below 2,000 mg/L, alkalinity above 3,000 mg CaCO₃/L, and the VFA-to-alkalinity ratio under 0.3-0.4. C:N ratio of feedstock should sit at 20:1 to 30:1 — below 15:1 ammonia accumulates and inhibits methanogens; above 35:1 nitrogen becomes limiting and gas yield drops.

Typical Indian yields run 80-120 m³ biogas per tonne of fresh cow dung, 200-300 m³/tonne for press mud, 150-250 m³/tonne for food waste, and 250-380 m³/tonne for paddy straw after pretreatment. The trade-off across the four stages is that hydrolysis is the slowest step for lignocellulose (requiring pretreatment), methanogenesis is the most fragile (requiring stable pH and temperature), and the chain is only as robust as its weakest link — which is why monitoring pH, VFA and alkalinity weekly is the operational discipline that separates well-run plants from underperforming ones.