Anaerobic Digestion (AD) is a natural process where plant and animal materials (biomass) are broken down by micro-organisms in the absence of air.
- The AD process begins when biomass is put inside a sealed tank or digester.
- Naturally occurring micro-organisms digest the biomass, which releases a methane-rich gas (biogas) that can be used to generate renewable heat and power; this helps cut fossil fuel use and reduce greenhouse gas emissions.
- The remaining material (digestate) is rich in nutrients, so it can be used as a fertiliser.
Many forms of biomass are suitable for AD; including food waste, slurry and manure, as well as crops and crop residues. However, woody biomass cannot be used in AD because the micro-organisms can't breakdown the lignin, the compound that gives wood its strength.
AD is not a new technology, it has been used in the UK since the late 1800s, but now an increasing number of AD plants are being built in the UK to generate clean renewable energy. AD is also used to treat the waste produced in homes, farms, supermarkets and industries across the UK. This helps divert waste from landfill.
The products of AD are referred to as biogas and digestate. Biogas is a mixture of 60% methane, 40% carbon dioxide and traces of other contaminant gases. The exact composition of biogas depends on the type of feedstock being digested.
Biogas can be combusted to provide heat, electricity or both. Alternatively, the biogas can be 'upgraded' to pure methane, often called biomethane, by removing other gases. This pure stream of biomethane can then be injected it into the mains gas grid or used as a road fuel. For more information on biogas please use the links on the right hand side of this page.
Digestate is made from left over indigestible material and dead micro-organisms. It contains valuable plant nutrients like nitrogen and potassium. It can be used as a fertiliser and soil conditioner. For more information on digestate please use the links on the right hand side of this page.
If you want to know more about AD, read this brochure from Severn Wye Energy. You can also download a factsheet on anaerobic digestion from the NNFCC website.
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