AD (Anaerobic Digestion)

AD — Anaerobic Digestion — is the biological process by which a consortium of microorganisms breaks down organic matter without oxygen, producing biogas (55-65% methane, 35-45% CO₂, traces of H₂S and water vapour) and a nutrient-rich digestate residue. It is the core conversion technology behind every CBG plant in India and the same chemistry that occurs spontaneously in landfills, rice paddies and ruminant stomachs.

The process has four sequential biochemical stages performed by overlapping microbial communities. Hydrolysis breaks polymers (cellulose, starch, proteins, lipids) into monomers using extracellular enzymes from facultative anaerobes. Acidogenesis converts monomers into volatile fatty acids, alcohols, hydrogen and CO₂. Acetogenesis converts longer VFAs into acetic acid, hydrogen and CO₂. Methanogenesis — performed by methanogenic archaea, not bacteria — converts acetic acid (acetoclastic route, around 70% of methane) and H₂-plus-CO₂ (hydrogenotrophic route, around 30%) into methane.

Operating conditions are tightly bounded. Mesophilic AD at 35-40°C is the Indian default; thermophilic AD at 50-55°C gives faster kinetics but is more shock-sensitive. pH must stay 6.8-7.6, alkalinity above 3,000 mg CaCO₃/L, and the VFA-to-alkalinity ratio under 0.3-0.4 for stable operation. Hydraulic Retention Time (HRT) typically runs 20-30 days for wet AD on agricultural residue, 30-50 days for press mud, and 14-22 days for food waste. Organic Loading Rate (OLR) sits at 2.0-4.5 kg VS/m³/day for stable mesophilic operation, with thermophilic systems tolerating up to 5-7 kg VS/m³/day.

AD's commercial significance in India is that it is the only large-scale renewable-gas pathway proven at the scale SATAT requires. Competing technologies — gasification, hydrothermal carbonisation, pyrolysis-to-gas — are either pre-commercial at biogas scale or higher-cost per unit of energy. The trade-offs of AD are familiar: it is slow (HRT measured in weeks), requires careful feedstock and process management, depends on stable biology that can take 3-6 months to fully ramp up after commissioning, and produces large volumes of digestate that need handling. Against these, AD's robustness, ability to absorb varied feedstocks, and proven 30-year operating track record at commercial scale make it the foundation technology for the entire CBG sector.