Using anaerobic digestion to reduce emissions

Anaerobic digestion (AD) plants let cattle and pig producers turn slurry and manure into renewable energy and heat, helping reduce the greenhouse gases (GHGs) released during storage.

How anaerobic digestion works

AD is a natural process that involves micro-organisms breaking down the organic matter in animal manures and other feedstocks.

This generates biogas, a combination of methane and carbon dioxide, which can be burned in a gas boiler to generate heat or in a combined heat and power (CHP) unit to produce heat and electricity, either for use on the farm or sold into the national grid.

If the carbon dioxide and other substances are removed from this it can produce biomethane which can be used in the same way as natural gas, either through the national gas grid or as a vehicle fuel.

Benefits for the environment and your farm

Processing livestock slurry or manure in an AD plant has multiple benefits for livestock farmers.

UK farms produce an estimated 100 million tonnes of slurry a year but only around three million tonnes of that is processed in AD plants. These vary in size, in system and in approach.

Some of the larger plants can generate substantial amounts of biogas, while micro–AD options exist for smaller-scale operations.

Reduces fugitive methane emissions

As the gases from AD are captured and utilised it reduces levels of fugitive methane emissions associated with storing slurry and manure.

Provides renewable energy

From an economic perspective, there is value in the energy captured, providing those businesses with an alternative income stream if the energy is sold, or reducing input costs if it is utilised on-farm.

If the energy and heat is used by the source farm, it improves the carbon footprint as it reduces the reliance of the business on GHG-emitting fossil fuels.

Produces digestate, which can displace artificial fertilisers

After those gases have been removed, the residual material (digestate) becomes a fertiliser for your grassland and crops.

The AD process converts the N in slurry and manure into a more readily available form and a proportion of the phosphate and potash too. This digestate can displace artificial fertiliser.

Nutrients in the digestate are more available to grassland and crops immediately after it is spread and, as it is in a more homogenous form, it allows for greater precision when it is applied to the fields.

However, digestate can generate more ammonia emissions than untreated slurry and manure when applied and careful nutrient planning is also needed at application to avoid nitrate leaching.

Considerations for establishing an anaerobic digestion system

Establishing an AD system comes with many considerations, from the feasibility of the system itself and its cost to planning and whether the intention is to operate it in-house or to outsource to a specialist firm.

Availability of consistent feedstock

The most important question to consider is whether the farm has sufficient year-round supply of slurry and manure as feedstock, as well as the required plant matter. If other products are required to make up any shortfalls, this could have possible environmental permitting implications.

Matching plant size to supply and grid capacity

The size of the plant should match the estimated supply of feedstock and local national grid capacity.

Technical requirements and staffing needs

Operating an AD plant requires technical know-how and time. Although, there are companies this can be outsourced to.

Intended use of energy and digestate

The end goal of an AD plant is to produce energy and heat and digestate.

If the plan is to supply the grid, it will require grid availability and there is also extra cost associated with an AD plant that exports its energy.

If the farm intends to use the digestate to fertilise its own land, the digestate and soils must be routinely analysed to ensure applications meet the soil and crop requirements.

The nutrient availability in digestate will also be dictated by conditions at spreading, soil type and the time of year it is applied.

For example, according to our Nutrient Management Guide (RB209), the crop-available nitrogen from whole digestate or separated liquor a spring application is 55% of the total N applied. This falls to 15% if applied to cereals in the autumn. For autumn applications to grass or oilseed rape it is 35%.

Planning and regulatory considerations

Location of the AD plant and storage tanks needs careful thought and planning.