The Official Information Portal on Anaerobic Digestion

FAQs

Q?

1. What is anaerobic digestion?

A.

Anaerobic digestion (AD) processes plant materials (biomass) into gas for heating and power. The gas is called methane or biogas. It is produced by bacteria, which digest biomass and produce methane as a by-product.

Biomass includes anything that is plant-derived: municipal solid waste, manure, crop residues, compost, food waste, paper and waste water. Crops can be grown specifically for use in AD, as a supplementary feedstock or a stabilising material. Biogas has been used in the UK since 1895, when gas from sewage was used in street lamps across the city of Exeter.

Q?

2. What does the UK produce that can be used in anaerobic digestion?

A.

The UK produces over 100 million tonnes of organic material that is suitable for treatment by AD. This includes:

  • 90-100 million tonnes of agricultural by-products like manure and slurry
  • 16-18 million tonnes of food waste (from households and industry)
  • 1.7 million tonnes of dry sewage sludge.

Q?

3. How much energy can you get from waste?

A.

The amount of energy produced by AD will vary depending on the material that goes into it and the particular type of digester that is used. Digesting 1 tonne of food waste can generate about 300 kWh of energy; slurry is lower yielding and purpose grown crops higher. According to the Renewable Energy Association, if all the UK's domestic food waste was processed by AD, it would generate enough electricity for 350 000 households.

Q?

4. How much energy could anaerobic digestion generate in the UK?

A.

AD could generate 10-20 TWh of heat and power per year by 2020. To put this in context, the UK's largest power station Drax sold 27.1 TWh of electricity in 2012. AD could represent 3.8-7.5% of the renewable energy we estimate will be required in 2020.

Q?

5. How many anaerobic digestion plants are there in the UK?

A.

AD has been used for many years in the UK by the water industry. It currently treats 66% of the UK's sewage sludge in AD plants. Beyond the water industry AD in the UK is in its infancy, but growing rapidly. There are currently around 100 non-water industry anaerobic digesters in the UK producing bioenergy. You can see the locations of operational AD plants on the Biogas Map. There are many more digesters that are currently in the 'planning' stage of development.

Q?

6. Is digestate the same as compost?

A.

No. Digestate is not compost, although they have some similar properties.  Compost is produced by aerobic (with air) decomposition of biological material and digestate is produced by anaerobic (without air) decomposition of biological material. They can both be used as fertiliser under specific regulations.

Q?

7. Does AD smell?

A.

There is some odour associated with the organic material that goes into a digester. However, AD can actually reduce nuisance odours as waste is delivered in closed vessels and vehicles, received in a closed reception area, and the digestion process takes place in a sealed tank. The digestion of slurry, for example, is significantly less odorous than the common practice of storing slurry in pits.

Q?

8. Is AD right for me?

A.

This website is a good place to start.  There is an AD cost calculator to look at the economics and there are lots of links to useful information and organisations.

The key questions for a potential developer are:

  • Do you have access to sufficient feedstock?
  • Do you have a market for the digestate?
  • Do you have good access, storing and handling facilities?
  • Are you willing to take on high capital project with capital rich initial period (i.e. can delayed returns be absorbed in your cash flow model)?

Q?

9. What are the benefits of AD?

A.

  • It turns waste into a resource. Instead of sending waste to landfill, we can use it to produce energy and fertiliser.
  • It produces fuel. Biogas can be used instead of fossil fuels.
  • It produces fertiliser. Fertilisers are made from fossil fuels. The digestate from this can replace some synthetic fertilisers.
  • It reduces our carbon footprint. The methane produced during AD is burned as fuel, and therefore releases CO2 into the atmosphere.  Because it comes from biomass, this does not contribute to climate change. However, if the same waste was left to degrade in a landfill breitling replica site, the methane produced could escape into the atmosphere: methane has a global warming potential 23 times larger than that of CO2. Therefore, harvesting and using methane from biomass can help to prevent climate change.
  • It can benefit many different people.  AD potentially benefits the local community, the environment, industry, farmers and energy entrepreneurs and government.

 

Q?

10. What are the drawbacks of AD?

A.

  • AD plants are 24-hour operations and as such they need to be fed regularly. Pumps and other machinery also need to be maintained to ensure production is not interrupted.
  • There can be noise, dust, and if there are leaks the potential of smells and environmental contamination. However, these issues are strictly controlled by environmental regulations, so should not occur. The liquid part of the digestate contains nitrates and other chemicals which should not be released to water but which can safely be spread to land or processed for wider use.
  • The use of biogas also releases CO2, which is a greenhouse gas. However, this is offset because the biogas produced in AD replaces fossil fuels when it is used for heat, power or transport. If the waste were landfilled it would naturally rot and release methane, a potent greenhouse gas.