Michael Horton, associate director at Gravis, explains what anaerobic digestion is and how the biogas it produces can be used for the electricity and gas grid, for transport fuel and for natural fertiliser.

He also covers the advantages and disadvantages of anaerobic digestion, looks at its future potential, and highlights Gravis's experience in what is a core investment sector for the firm. 

You can watch the full video below.

Anaerobic digestion, or 'AD' as it is commonly referred to, is the process by which organic matter such as animal or food waste is broken down in a sealed, oxygen-free tank called an anaerobic digester. The microorganisms and bacteria contained within the organic matter produce a methane-rich biogas that is used as a renewal fuel, with the remaining solid and liquid matter - known as digestate - used as a biofertiliser. 

Looking at the diagram below*, it starts at the top with the feedstock supplier, who is also the landlord for on-farm AD plants. Each day, they deliver the feedstock such as energy crops, livestock manure, and other farm waste to the AD plant. The feedstock is then rolled and compressed before being covered by a waterproof membrane, which begins the fermentation process. The feedstock is then fed into the digester, where it stays for up to 40 days, allowing sufficient time for full digestion and biogas yield.

Looking at the right-hand side of the diagram, you can see that the AD plant is powered by combined heat and power engines (CHPs) which are fuelled with biogas. So, the first use for the biogas created by the AD process, is to provide more heat to the digester, and so the process continues. For CHP-to-grid plants, any excess power is then exported to the national grid as renewable electricity.

But that's not all it can be used for. Looking at the left-hand side of the diagram, you can see that biogas is diverted to a biogas upgrader where it is cooled and passed through a variety of membranes at high pressure, separating the methane and the CO2.

The methane is then blended with propane, prior to injection into the gas grid as biomethane, or compressed or liquified, so it can be used as a transport fuel.

The CO2 can also be captured and upgraded for use, typically in the food and beverage industry, but also be used in the healthcare industry. There are also considerable opportunities for sequestration allowing for the permanent storage and removal of the CO2 from the atmosphere.

Moving back to the centre of the image, the digestate (both in liquid and solid form) is extracted from the digester. The farmer can then spread it to land as a biofertiliser. This eliminates the use of synthetic fertilisers and completes the cyclical AD process.

The first advantage is AD baseload generation. It's reliable. It's not subject to the constraints of weather-dependent technologies such as wind and solar. Unless the plant is offline for maintenance, renewable electricity and biomethane will continue to be produced.

Another notable advantage is the improvements in technology such as micronisation and nutrient recovery, which together with the development of the UK's sequestration initiatives, particularly the North Sea cluster, provide AD with extensive opportunities for life extension well beyond the Renewable Heat incentive, Renewables Obligation and Green Gas Support subsidy schemes.

Then, there's the decarbonisation of highly carbon-intensive industry. For example, the production of nitrogen-based fertiliser emits more than five tonnes of carbon dioxide for each tonne of nitrogen fertiliser produced. Using digestate from AD eliminates these factory-produced emissions.

A further advantage is eliminating methane from the atmosphere. Methane produced by rotting manure and slurry is one of the hardest sources of UK and global emissions. AD goes a long way to ensuring this major source of agricultural emissions is mitigated. 

Water management is perhaps the biggest issue to mitigate given the long-term impact on a plant. All rainwater that falls within the boundary of an AD plant is deemed to be contaminated by the environmental agencies. While this water can be spread to land, there are seasonal and local restrictions to consider. Where the water cannot be spread to land, it must be treated accordingly prior to reintroduction to local waterways.

Given the large area that an AD plant occupies, the volume of water that requires management is often considerable and costly. Successful integration and operation of sustainable drainage systems (or SUDS ponds, as they are known), and other water treatment systems and lagoons are vital to AD water management.

And while AD power generation is reliable, it isn't immune to grid curtailment during periods of low demand. Sadly for biomethane plants, curtailments often lead to gas being flared into the atmosphere.

The AD industry also needs more government support to ensure its long-term potential is achieved. The Green Gas Support Scheme was introduced in November 2021 for four years with 37 application for registration received by March 2024. The scheme has since been extended to March 2028. There are no other subsidy schemes available at present that can support the wider development of AD in the UK and Northern Ireland.

Finally, not a disadvantage as such, but a barrier. There is considerable greenfield risk associated with delivering projects which contain a high level of technical complexity, at significant capital cost, whilst also ensuring that the accreditations that underpin the investment are obtained swiftly.

Typically, an AD plant will have high operating costs, particularly with feedstock and maintenance requirements. A suitably skilled and experienced operations and maintenance team is vital to providing a stable operating platform to deliver a reliable lifecycle and maintenance programme, whilst managing a wide network of suppliers and contractors.

AD is well established in the UK and is a ready-to-use technology. It is contributing to the decarbonisation of industry, particularly farming and waste management. AD has huge potential, with the biogas industry at the forefront of reducing greenhouse gas emissions by decarbonising energy production via sustainable, renewable energy technology.

The capture of CO2 and its sequestration will see AD plants with that technology likely become carbon neutral and potentially carbon negative given the savings in greenhouse gas emissions, whilst adding a reliable long-term secondary revenue stream.

Biomethane is an excellent source of renewable fuel that can continue to decarbonise the transport industry. Significant technological advancement in heavy goods vehicles, buses and even tractors, coupled with the RTFO (Renewable Transport Fuel Obligations) subsidies available, provide long-term upside for AD.

AD is a core asset class for Gravis. We have invested in on-farm biogas and biomethane plants throughout the UK and Northern Ireland since 2013 and currently have approximately £83m invested across 19 operational assets.

While it is a mature sector in the UK and Northern Ireland, AD still presents a significant opportunity to further contribute to the UK's transition to net zero, and Gravis will continue to support the industry moving forward.

Our first investment in AD was a portfolio of on-farm biogas plants in Northern Ireland which generate up to 3.5MW of renewable electricity, enough to power almost 3,000 houses annually.

Establishing trusted relationships with our farmer operators has been key to the ongoing success of the portfolio. As well as having long-term operating and feedstock contracts in place, the farmers will soon start taking ownership of their plants, and so will derive the full benefit of the remaining Renewables Obligation subsidy.