Anaerobic digestion (AD)

Anaerobic digestion (AD) may seem complex but it has a lot of potential, including improving the value of slurry. It is currently a favourite of UK Government policy.

AD is an established technology that is familiar to many farmers and producers. The process digests crops, agricultural and other biological matter (wastes, manures, plant residues etc), to produce biogas which may be burned in an engine to produce electricity and heat, or refined for injection into the national gas network as biomethane.

The Green Gas Support Scheme

Government policy favours Anaerobic Digestion (AD) plants, specifically where it can be used to produce biomethane for injection into the gas grid to form part of the decarbonisation of the heat network in the UK. Announcements in the March 2020 budget, and subsequent launch of a consultation (Green Gas Support Scheme), demonstrate this.

The Green Gas Support Scheme is very similar in structure to the Renewable Heat Incentive (RHI) but is there to support gas production for the grid. It has been designed to increase the amount of biomethane that is put into the natural gas network as part of a decarbonisation programme. This will, in turn, include other non-carbon-based gases such as hydrogen. In creating biomethane, the government will undoubtedly require that the feedstock used for AD plants is predominantly from waste, including slurries such as dairy slurry.

The opportunities for slurry

Slurry itself rarely produces enough gas to be a feedstock on its own and is usually part of a feedstock mix. However, it is clear there is opportunity for improving the value that slurry has where other wastes become limited in supply. This could be by:

• Providing feedstocks to very large plants
• Feeding much smaller, even micro-AD plants

Being experienced in producing grass and maize silage, dairy farming is well set up for producing crops for feeding to AD plants. Opportunities exist within arable farms to introduce energy crops as part of the rotation. It is no coincidence that some of the best run AD plants are based in diary as people in the sector understand the value of high-quality, high-yielding feedstocks.

While large-scale AD provides the best balance between install and operational cost against benefits, many producers are interested in micro-AD plants that can manage their wastes and deliver energy mainly for their local use. Such systems have been developed many times but are rarely that successful. Often, the most successful systems are developed and driven by enthusiasts requiring time and dedication

What does the future hold for AD?

Some industry representatives believe that uptake of AD has been slow because of perceived complexity and unreliability. A plug-and-play approach, or self-contained installations owned and operated by others, would be of more interest, and improvements are needed for this.

It has also been pointed out that if slurry is intended as the feedstock, cows must be kept in sheds as outdoor grazing prevents slurry from being collected. In the UK, the lack of success in micro-AD is in contrast to some of our European partners. Therefore, as an industry, we may look to other countries and sectors to provide and showcase their expertise so that we can capitalise on the technology where it fits within the UK Dairy sector.