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Why bigger isn’t always better – the opportunities for farm scale AD

Date Published: 17/04/2024

Anaerobic digestion (AD) can make a significant contribution to farm energy requirements, by producing heat, electricity or fuel as biomethane to power tractors. AD also enables farmers to improve slurry and manure management while reducing greenhouse gas emissions.

Despite the multitude of benefits, AD at the farm level has been neglected by the current UK government and there is a need to reinvigorate the approach to on-farm biogas production.

In a recent Farm of the Future webinar, Richard Gueterbock chair was joined by Lucy Hopwood from NNFCC, Chris Mann from Bennamann, Chris Morris from Fre-Energy and Eoin Sharkey from Biofactory, to discuss how to make AD stack up from a farm business perspective.

Anaerobic digestion in the UK   

Speaking in the webinar, Lucy Hopwood from NNFCC shared that there are currently more than 650 operational AD sites in the UK. About 15% of these sites are biomethane injection facilities, which are generally larger scale facilities taking feedstock from multiple parties, which could be waste, crop or slurry. The remaining 85% are combined heat and power (CHP) facilities, which are generally smaller scale, using feedstock generated on site to deliver energy for use on the site and potentially for exporting too.

Support for anaerobic digestion

In recent years the focus of policy has been on the development of bigger projects.

Looking back, Lucy explained that from 2012 to 2019 there was a significant period of growth stimulated by the Feed in Tariff and the Renewable Heat Incentive (RHI). These schemes provided mechanisms which supported all types and scales of AD.

However, since they came to an end, the only government support available for AD has been through the Renewable Transport Fuels Obligation (RTFO), which only AD plants producing biomethane can be eligible for, or the Green Gas Support Scheme, which is narrow in its scope and only supports new biomethane injection facilities.

Lucy explained that there is currently no mechanism in place to drive growth in areas where there is a lot of potential.  

Where is the potential for anaerobic digestion in the UK?

AD is reliant on feedstock, so secure feedstock supply is essential to the successful running of an AD facility. Speaking in the webinar, Lucy highlighted several types of feedstock available in the UK, which have different levels of untapped opportunity:

  • Over 6 million tonnes of processing residues are generated each year, of which is 70% currently used
  • Over 10 million tonnes of food waste are generated each year, of which 60% is currently used
  • Over 2 million tonnes of agricultural residues are generated each year, 50% of which is used
  • Over 90 million tonnes of livestock waste (slurry and manure) are generated each year, of which just 3% is currently used

Livestock waste therefore represents the greatest opportunity to process waste via AD to generate renewable energy.

Energy from manure and slurry management

Also speaking in the webinar, Chris Mann from Bennamann shared one solution for reducing emissions and garnering energy from slurry, without the need to even build an AD facility.

He explained that lagoons release biogas continually, emitting carbon and losing a potential energy source. “For a long time, there was a misunderstanding of how much methane was coming off slurry lagoons, which is probably why nobody bothered to do anything about it, as it was grossly underestimated,” he said.

It has since been shown that roughly 2 to 2.5 times more emissions come from manure and slurry than from enteric (breath and flatulence) dairy emissions, which makes manure and slurry management the biggest opportunity to reduce emissions from dairy.  

A farm case study - Trenance Farm

Chris shared that a carbon audit undertaken at Trenance Farm, near Saltash, measured emissions at 734 tCO2 e.

In 2022, a retrofit lagoon cover was installed, which holds 1500 to 200 cubic metres of gas, and also diverts rainwater away from the lagoon, which has the added benefit of it not needing to be emptied so often.

Once biogas was being captured from the lagoon, the farm’s carbon footprint reduced to 87.53 tCO2 e. The biogas was then filtered and converted to compressed biomethane, which can then be used to power a generator on the farm or alternatively for vehicles. Chris added that the remaining digestate is displacing around 50-60% of the nitrogen fertiliser required on farm, while any need for P and K is completely eradicated.

In addition to that, Chris shared that the farmer has been operating regenerative practices, putting in hedges and trees, so the milk’s carbon footprint has dropped from 1kg CO2 per litre of milk to just 0.13kg.

Anaerobic digestion for dairy farms

Also focusing on the dairy sector, Eoin Sharkey from BioFactory spoke about how a different approach could enable dairy farmers to reap the benefits of using AD technology for slurry management, without some of the challenges of the past.

He shared that using AD for slurry management can increase farm energy independence, reduce slurry handling costs, improve resource use by capturing and retaining nutrient content and reduce emissions from slurry.

He highlighted that to date, AD plants have been operating with thin margins, which is why they have been growing larger over time, while relying on subsidies to turn a profit. He recognised that this model has not worked well for farmers, as it has required both high operating costs and high capital costs, there has been a management burden for maintenance, acquiring permits has been a barrier and a lot of farms simply do not have the space for a large AD plant in the farmyard.

Fully autonomous modular AD systems can use slurry as the feedstock to provide energy and heat to offset farm demand, paying for themselves typically within three to seven years, according to Eoin.

Benefits of on-farm anaerobic digestion

Although this blog has focused on dairy, AD can benefit many types of farm businesses. Speaking in the webinar, Chris Morris from Fre-energy summarised the key benefits of using AD technology on farm:

  • Co-digestion of farm generated wastes
  • Slurry provides essential bacteria for a stable AD process, supporting digestion of other feedstock materials
  • Farmers can replace bought-in fertiliser with digestate
  • It provides soil conditioner with added value, not just N, P and K
  • Reduces impact on the environment
  • Generates heat, electricity or gas, helping farm businesses to increase energy self-sufficiency, needs to be at appropriate scale for the farm

He suggested that AD is fundamentally a waste management process and that scale should be matched to locally available inputs.

To find out more from the speakers, watch the full webinar in the members’ resources section of the website. Not yet a member? Find out about membership options available to you today.