Anaerobic digestion on a farm: the evidence base

Whether an anaerobic digestion plant on a farm is treated as agricultural development or as a waste installation depends almost entirely on where the feedstock comes from. A digester fed by slurry and crops grown on the holding, producing digestate spread on the same land, is arguably ancillary to the farming operation. One that imports food waste from twenty miles away is an industrial process that happens to sit in a field, and the planning evidence base for AD on a farm has to be built for whichever of those the scheme really is.

Scale and whether the proposal stays agricultural

The test is the character of the use, not the ownership of the land.

Case officers look at the proportion of feedstock originating on the holding, whether the gas or electricity is used on the farm or exported, where the digestate goes, and the traffic the operation generates. A plant consuming the farm's own slurry and maize, running a combined heat and power unit that supplies the dairy, sits comfortably as a farm operation. Once a majority of the feedstock arrives by lorry and the output is sold into the grid, the honest description is a renewable energy or waste installation in the countryside.

That description matters because it changes the policy tests. An agricultural operation engages farm diversification policy in the development plan. An industrial installation engages the countryside protection policies, the landscape tests, and generally the Green Belt tests where relevant.

Do not try to have it both ways in the application description. Officers notice, and a description that understates the use creates enforcement exposure later even if permission is granted.

Feedstock sourcing and the traffic it generates

Set out the annual tonnage by feedstock type, the origin of each, the vehicle type and the seasonal profile.

Maize and wholecrop silage arrive in a few intense weeks. Food waste and slurry arrive all year. A plant that averages six loads a day may see forty in harvest week, and the highway authority will assess the peak rather than the average. Present both.

The route from the public highway to the plant is usually the sharper issue on a rural site. Visibility splays at a farm access designed for tractors rarely satisfy a highway authority for articulated deliveries, and improving them can mean hedgerow removal that the landscape officer then objects to. Resolve that tension in the design rather than in the committee report.

A transport statement covering access geometry, swept paths, trip generation and the seasonal peak is the usual submission. Where the scheme is larger or the network sensitive, the authority may ask for a full transport assessment instead.

Digestate storage and spreading land

Digestate is the part of an AD scheme that most often unravels late.

You need storage sized for the period when spreading is not permitted or not possible, and you need land with the capacity to receive it. The Water Resources (Control of Pollution) (Silage, Slurry and Agricultural Fuel Oil) (England) Regulations 2010 govern the construction of storage, and the Farming Rules for Water constrain when and where the material can be applied.

Show the calculation: annual digestate volume, the closed period for the relevant nutrient, the resulting storage requirement in cubic metres, and the tank or lagoon capacity proposed. Then show the spreading land, with hectares, cropping and the nutrient budget. Where the land is not in your control, say so and explain the contractual arrangement.

Storage tanks are often the tallest structures on the site. Their height and their colour drive the landscape case more than the digester does, so fix them early and carry them through the landscape and visual impact assessment.

Odour and bioaerosol assessment

Odour is the objection that arrives from neighbours and the one environmental health will press hardest.

The sources are the feedstock reception area, the digestate store, the pasteurisation plant where one is needed, and any spillage on the yard. Enclosed reception with negative pressure and odour abatement is the standard answer for food waste plants, and a scheme that proposes open reception for imported waste will struggle.

An odour assessment should identify receptors, describe sources and controls, and either screen out the risk on separation distance or model it. Where sensitive receptors sit within a few hundred metres, the Environment Agency also expects bioaerosol risk to be considered for biowaste facilities, so check the current guidance for the separation distance it applies and what it requires inside it.

Separation is the most reliable mitigation available. Moving the reception area to the far side of the yard is worth more than any amount of written commitment to good housekeeping.

Noise from the combined heat and power unit

A CHP engine runs continuously, and continuous plant noise at night is assessed against background sound levels that are at their lowest then.

The method is BS 4142:2014+A1:2019, comparing the rating level of the specific sound with the representative background. Rural background levels can be very low, which makes an acceptable rating level low as well, and an engine in an uninsulated container will not meet it. Acoustic housing, attenuated ventilation openings and a considered orientation are normal rather than exceptional.

Commission the noise impact assessment early enough that the acoustician can influence the layout. A suitably qualified author is expected, and the survey needs to capture genuine night-time background, not an evening proxy.

Permitting alongside planning

Planning permission is one of at least three consents. The others run in parallel and answer different questions.

• An environmental permit under the Environmental Permitting (England and Wales) Regulations 2016 for the treatment of waste, with standard rules permits available for some AD configurations.
• Animal by-products approval where the feedstock includes category 3 material, with pasteurisation requirements that shape the plant layout.
• A connection agreement for export of electricity, or a gas to grid entry agreement, each with its own programme.

Screening under the Town and Country Planning (Environmental Impact Assessment) Regulations 2017 should be considered rather than assumed away. Installations for the disposal of waste appear in Schedule 2, and the applicable threshold and the column 2 selection criteria need checking against the proposal. A formal screening opinion from the authority removes the argument.

An illustrative example

A 400 hectare arable and dairy holding proposes a digester taking its own slurry plus 8,000 tonnes a year of maize grown on the farm, exporting biomethane to the grid. No waste is imported, so no environmental permit for waste treatment is needed and the traffic is internal except at harvest. The application is presented as farm diversification, with the transport case built around three weeks of silage carting. This example is illustrative.

The document bundle

For a farm scale plant with no imported waste, expect:

1. A planning statement setting out the agricultural link, the feedstock origin and the development plan assessment.
2. A design and access statement with layout, heights, materials and colours.
3. A transport statement with the seasonal profile and swept path drawings.
4. Noise and odour assessments by suitably qualified authors.
5. A landscape and visual appraisal, scaled to the sensitivity of the setting.
6. A digestate management plan with storage calculations and the spreading land.
7. Ecology survey work and the biodiversity net gain assessment, which the statutory metric requires a competent person to complete.

Where waste is imported, add the permitting position, the bioaerosol assessment and a fuller transport assessment. Where the site is in the Green Belt or a National Landscape, add the specific policy case rather than relying on renewable energy benefits alone.

Two things decide most of these applications: the honesty of the feedstock description and the quality of the digestate plan. Get those right and the rest is drafting.

Enter your postcode on PlanScope to see which reports the site is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.