anaerobic digestion (AD)
Also known as: AD process · anaerobic digestion process · biomethanation
Anaerobic Digestion (AD) is a biological process where microorganisms break down organic matter in the absence of oxygen, producing biogas (primarily methane) and digestate as by-products.
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What is anaerobic digestion?
Anaerobic Digestion (AD) is a microbial biochemical process in which a consortium of bacteria and archaea convert organic matter into biogas (a mixture of methane and carbon dioxide) and a nutrient-rich liquid-solid residue called digestate, all in the absence of free oxygen. It is the foundational technology of every compressed biogas (CBG) plant under India's SATAT (Sustainable Alternative Towards Affordable Transportation) initiative and the dominant pathway for converting wet organic waste into renewable energy.
The conversion happens in four sequential biological stages, each carried out by different microbial groups:
- Hydrolysis: enzymes break complex polymers (carbohydrates, proteins, fats) into soluble monomers (sugars, amino acids, fatty acids).
- Acidogenesis: fermentative bacteria convert monomers into volatile fatty acids, alcohols, hydrogen, and CO2.
- Acetogenesis: syntrophic bacteria convert VFAs into acetate, hydrogen, and CO2.
- Methanogenesis: archaea produce methane from acetate (acetoclastic) or from CO2 plus hydrogen (hydrogenotrophic).
The slowest stage governs throughput. Hydrolysis is rate-limiting for lignocellulosic feedstocks (paddy straw, Napier grass), while methanogenesis is rate-limiting for easily soluble feedstocks (food waste, molasses).
Indian commercial digesters operate primarily in the mesophilic temperature range (35-40 degC) at HRT 25-40 days, OLR 1-4 kg VS/m3/day, and pH 6.8-7.5. Typical biogas composition is 55-65% methane, 35-45% CO2, with 1,000-5,000 ppm H2S and trace siloxanes. AD is preferred over composting for wet feedstocks because it generates a marketable energy product and shrinks waste volume by 30-50%. The trade-offs are slower processing (compared to incineration), high capital cost (3-7 crore INR per TPD installed), and biological fragility — a poorly managed digester can fail within days, while a thermal plant simply trips and restarts. AD scales from 1 m3 household units up to 20,000 m3 industrial plants, making it the most flexible bio-energy technology in the Indian renewable energy mix.
Common questions about anaerobic digestion
Plain-English answers to what people most often ask.
What is the full form of AD?
What are the 4 stages of anaerobic digestion?
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