What Is Anaerobic Digestion and How Does It Work on a Farm?

What is anaerobic digestion?

Anaerobic digestion is a natural biological process. Microorganisms break down organic material inside a controlled environment where oxygen is absent.

The idea is relatively straightforward. Instead of simply storing organic material such as cattle slurry before spreading it to land, suitable material can first pass through an anaerobic digestion plant. During that process, microorganisms consume biodegradable material and produce a gas mixture known as biogas. A second material, known as digestate, leaves the process after digestion.

Defra describes anaerobic digestion as a process that can improve the capture and treatment of nutrients and greenhouse gas emissions from animal slurry and manure, compared with directly spreading untreated material.

How does a farm AD plant work?

Although individual AD systems vary, the overall process can be understood as a sequence of stages, from collecting the raw material through to using the energy it produces.

1. 01Organic material collected (e.g. slurry from housing)
2. 02Transferred via pumps, tanks and pipework
3. 03Material enters the sealed digester
4. 04Biogas is produced as material is digested
5. 05Biogas is used — typically in a CHP unit for electricity and heat

Why collection and transfer matter as much as the digester itself

The way slurry is collected matters. A farm where cattle are housed for significant periods may have a very different quantity of collectable slurry from a grazing-based system where much of the manure is deposited directly onto fields.

Getting material from the farm's existing slurry infrastructure to the digester also isn't just a question of the shortest route on paper. The shortest theoretical route between two pieces of equipment is not necessarily the best route once existing buildings, roads, underground services, drainage, access requirements and future maintenance are taken into account — which is why this stage of planning matters as much as the digester itself.

Inside the digester, temperature, mixing, loading and retention time are controlled according to the particular process and plant design, to create suitable conditions for the anaerobic microorganisms doing the work.

How is the biogas actually used?

Depending on the system, biogas may be used in equipment such as a combined heat and power unit, commonly called a CHP. A CHP engine can use the gas to produce both electricity and useful heat — electricity can potentially be consumed on the farm, while useful heat may also be recovered depending on the installation.

The Environment Agency's T24 farm AD exemption specifically recognises the production of biogas that can be burned to provide power on the farm or export electricity to the National Grid, where the installation meets the relevant exemption conditions.

What can go into an anaerobic digester?

The answer depends on the type of plant and the regulatory arrangements that apply. Farm-scale AD commonly involves the following kinds of material, though not every digester can legally or practically accept every type.

• ·Cattle slurry
• ·Farmyard manure
• ·Suitable plant material
• ·Purpose-grown crops, on some systems
• ·Other appropriate biodegradable feedstocks

What the T24 exemption permits

The Environment Agency's T24 exemption in England permits specified agricultural wastes including slurry, farmyard manure, certain plant tissue wastes and fully biodegradable animal bedding, subject to its conditions. Importing or processing other types of material can change the regulatory requirements considerably.

This is why the feedstock strategy should be established early, rather than decided after the plant has already been designed.

What is biogas?

Biogas is the gaseous product of anaerobic digestion. Its energy value comes primarily from methane. Instead of allowing organic material to decompose without making deliberate use of the resulting gas, AD captures the gas within a controlled system so that it can be used as an energy source.

On many smaller farm systems the most obvious use is generating electricity and heat through CHP. Larger and different types of AD facilities may also upgrade biogas into biomethane, but that is a different proposition from the smaller farm-scale installations that are the main focus of this guide.

What is digestate?

Digestate is the material remaining after anaerobic digestion. Importantly, the process does not simply make the slurry disappear — much of the nutrient content remains in the digestate, which can potentially be used agriculturally.

Where slurry or farmyard manure is the only waste feedstock under the T24 exemption, the Environment Agency states that the resulting digestate can be spread on agricultural land as fertiliser or soil conditioner without being regulated as waste, although other agricultural rules still apply.

Digestate storage therefore needs to form part of the overall farm infrastructure plan. A farmer considering AD should think beyond where the digester will go, and also ask where everything entering and leaving the digester will go.

What equipment does a farm AD installation need?

An anaerobic digester does not operate in isolation. Depending on the farm and the system, an installation may involve a considerable amount of supporting infrastructure.

• ·Slurry collection infrastructure
• ·Reception or transfer tanks, pumps and mixers
• ·Pipework, valves and digestate storage
• ·Concrete bases, trenches and access for maintenance
• ·Electrical cabling, ducts, service routes and controls
• ·CHP and, where used, separation equipment

Why two similar farms can need very different projects

This supporting infrastructure is one of the reasons two farms installing similar AD technology can still require very different projects. One farm may already have useful tanks, concrete and slurry infrastructure. Another may require substantial enabling works before the AD equipment can be installed.

Where should an AD plant be located on a farm?

There is no universal answer. The best location needs to be considered as part of the whole farm, rather than simply finding an empty patch of ground.

• ·Location of cattle housing and existing slurry storage
• ·Distance slurry needs to travel, and farm access
• ·Electrical infrastructure and available space
• ·Ground conditions, drainage and existing buildings
• ·Maintenance access and digestate storage
• ·Future farm development plans

Is anaerobic digestion becoming more common?

Defra's Farm Practices Survey provides useful context on how widely farm-scale AD has actually been adopted across England, rather than relying on anecdote.

What that trend does and doesn't tell you

That doesn't mean AD is right for every farm. It does show that anaerobic digestion is an established part of the agricultural energy and nutrient management landscape, rather than an experimental technology.

Is anaerobic digestion suitable for every farm?

No. Suitability depends on considerably more than simply owning cattle. A good AD project starts by understanding the farm, rather than selecting equipment first and trying to make the farm fit around it afterwards.

• ·How much slurry is actually collectable, and how consistently?
• ·How much electricity does the farm use, and when?
• ·Is there a useful heat demand and existing slurry infrastructure?
• ·Is there enough space, and what additional storage is required?
• ·What groundworks, planning and environmental requirements apply?
• ·What level of investment is justified?

What is involved in installing an AD plant?

The physical AD unit is only one part of the project. A farm-scale installation typically spans site assessment and layout, agreeing project scope, groundworks and concrete, tanks and associated infrastructure, trenches, pumps, mixers and pipework, electrical services, plant positioning and mechanical connection, and finally testing and commissioning support.

Charlie Bell Building specialises in this practical installation side of farm-scale AD projects, bringing together the farm, equipment and site infrastructure required to make the installation work as a whole — covered in more depth in our guide on what AD installation actually involves.