What is anaerobic digestion?

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.

What materials can anaerobic digestors process?

Manure, feed spills, crop residues, offal and most domestic and industrial organic wastes can be used as feed materials for digesters. However, care must be taken to make sure that pathogens that cause diseases in livestock, such as mad cow disease or avian influenza, are prevented from entering the anaerobic digesters.

How much energy can you get from waste?

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 ton 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.

How much energy could anaerobic digestion generate in the UK?

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.

Does the AD process produce a smell?

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.

What are energy crops?

Agricultural producers can grow some crops for the sole purpose of producing energy using renewable energy technologies such as anaerobic digestion, combustion, ethanol or biodiesel production, these crops are known as energy crops.

Are there opportunities to integrate anaerobic digestion with other renewable energy technologies?

Investors, governments and researchers are still exploring the opportunities for integrating ethanol, biodiesel and anaerobic digestion plants. At least a couple of such plants are being built in North America, one in Ontario and another in Nebraska.

What is biogas?

Microorganisms convert a fraction of the organic waste matter into methane (CH4) and carbon dioxide (CO2) gases during the anaerobic digestion process. This mixture of gases is known as biogas. The composition of biogas is 50 to 75 percent CH4 and 25 to 50 percent CO2. Power generators, engines, boilers and burners can use biogas as a fuel like natural gas.

What is digestate?

Digestate is the effluent coming from the digester at the completion of the digestion process. Digestate has nutrient value, so producers can apply digestate to land, much like manure.

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

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

How many anaerobic digestion plants are there in the UK?

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 130 non-water industry anaerobic digesters in the UK producing bioenergy, however, there are many more digesters that are currently in the ‘planning’ stage of development.

Is digestate the same as compost?

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.

What is meant by cogeneration?

Most of the anaerobic digesters in the agriculture industry process a single type of waste known as substrate. An example of this material is cattle feedlot manure. The term co-digestion means processing different types of agricultural wastes in an anaerobic digestion facility. Additional organic feed materials, other than the primary one, are known as co-substrates.

Is it expensive to install an anaerobic digestion plant and what is the payback period?

The capital costs of anaerobic digester plants are very high and may range from a few hundred thousand to tens of million pounds, depending on the size of the operation. Some of the feasibility studies in the UK concluded that when operated under either the optimum or the worst conditions the payback period can range from 2 to 10 years respectively.