Anaerobic digestion
The microbial breakdown of organic matter in the absence of oxygen, producing biogas and a nutrient-rich digestate usable as fertiliser.
Anaerobic digestion (AD) proceeds through four biological stages: hydrolysis of complex polymers, acidogenesis to volatile fatty acids, acetogenesis to acetate and hydrogen, and methanogenesis, in which archaea convert these intermediates into methane and CO2.
Key design parameters:
- Temperature: mesophilic 35–42°C (most common, stable) or thermophilic 50–57°C (faster, better pathogen kill, more sensitive)
- Hydraulic retention time: 20–40 days mesophilic, 12–20 days thermophilic
- Organic loading rate: 2–5 kg volatile solids per m3 per day for wet continuously stirred tank reactors
- pH 6.8–7.5 and a balanced carbon to nitrogen ratio of 20–30:1
Specific methane yield ranges from about 0.2 m3 CH4 per kg of volatile solids for cattle manure to 0.45–0.5 for food waste and fats. Co-digestion of manure with higher-yield substrates is standard practice. Process failure typically results from ammonia inhibition or volatile fatty acid accumulation after overloading. Digestate retains most of the nitrogen, phosphorus and potassium of the feedstock and replaces mineral fertiliser, though open storage can emit residual methane and ammonia. Wastewater treatment plants use AD to stabilise sewage sludge, often covering 50–80% of plant electricity.
Related terms
All terms →- Biogas
A gas mixture of roughly 50–70% methane and 30–50% CO2 produced by anaerobic digestion of organic matter such as manure, food waste, sewage sludge and energy crops.
- Biomethane (RNG)
Biogas upgraded to more than 95% methane so that it meets natural gas quality standards and can be injected into the gas grid or used as vehicle fuel.
- CHP (Combined Heat and Power)
Simultaneous generation of electricity and useful heat from one fuel input, reaching total efficiencies of 75–90% versus about 35–60% for power-only plants.
- Methane leakage
Unintended release of methane from oil, gas and coal supply chains through leaks, venting and incomplete flaring, often expressed as a percentage of gas produced.