Consenting an anaerobic digestion plant on a farm

An on-farm anaerobic digestion plant needs full planning permission and, in almost every case, an environmental permit as well. The planning question that decides everything else is whether the plant is agricultural development or an industrial waste installation, and the answer turns on where the feedstock comes from and where the gas or electricity goes. Anaerobic digestion plant planning applications that treat the use as a straightforward farm diversification, with a short statement and a block plan, are the ones that stall.

Budget for two consent processes running in parallel on different timetables, and do not let the planning application get ahead of the permitting advice.

Whether the plant is agricultural or industrial in planning terms

A digester fed entirely with slurry and crops from the holding, producing gas burned on the holding, has a reasonable argument for being ancillary to the agricultural use. Once third-party feedstock is imported, or the output is exported to the grid as a commercial enterprise, the use looks industrial and the agricultural exemption falls away.

That matters in three ways. The permitted development rights in Part 6 of Schedule 2 to the General Permitted Development Order apply only to buildings reasonably necessary for the purposes of agriculture within the unit, so a commercial plant cannot use them. In the Green Belt or a National Landscape, an industrial characterisation makes the policy test much harder. And a change of use element has to be applied for alongside the operational development, rather than assumed.

Say plainly in the Planning Statement what proportion of feedstock comes from the holding, what the imported fraction is, and where the energy goes. Officers who find the honest answer in the statement are easier to deal with than officers who deduce it from the transport chapter.

Feedstock sourcing and the transport consequence

Feedstock volume drives vehicle movements, and vehicle movements drive the highways case. A plant taking maize silage from the holding in a short harvest window has a very different movement profile from one taking food waste deliveries five days a week all year.

Set out, by feedstock type, the annual tonnage, the vehicle type, the number of loads, the delivery window and the seasonality. Then do the same for digestate leaving the site and for any imported digestate. The highway authority will want the peak week rather than the annual average, because the harvest peak is what tests the access.

Where food waste or other animal by-product feedstock is involved, the operation needs approval under the animal by-products regime from the Animal and Plant Health Agency, with pasteurisation and biosecurity requirements that shape the building layout. Design for that before the planning drawings are fixed, because retrofitting a reception hall with the right dirty and clean separation is expensive.

Odour from reception, storage and digestate

Odour is the most common ground of objection and the most common reason these applications go to committee. The sources are the feedstock reception area, open silage clamps, the digestate store and the spreading operation, and only the first two are within the application site boundary in most schemes.

An Odour Assessment should identify the sources, characterise them, model dispersion to the nearest sensitive receptors and set out the mitigation. Where food waste is involved, reception and pre-treatment need to be enclosed in a building under negative pressure with air drawn to a treatment system, usually a biofilter or a chemical scrubber. Open-air tipping of food waste is not acceptable near housing and should not be proposed.

The reference points are the Institute of Air Quality Management guidance on odour assessment and the Environment Agency's own odour management guidance, with BS EN 13725 for olfactometry where measurement is required. An odour management plan secured by condition and mirrored in the environmental permit is the normal outcome.

Digestate storage volumes and winter capacity

This is the technical point most often got wrong, and it is a planning point as well as a permitting one, because the tanks and lagoons take space and have a landscape effect.

Digestate cannot be spread to land through the closed periods, so the plant needs enough storage to hold output through the months when spreading is not permitted. Where the site is in a nitrate vulnerable zone, the storage capacity requirements and closed spreading periods are set by the current nutrient management rules, and the figures are revised, so check the current requirements on gov.uk rather than carrying a number across from an earlier scheme. The farming rules for water apply in all areas and restrict spreading where there is a risk of pollution regardless of zone status.

Undersized storage is the failure that shows up two years after commissioning, when the operator has nowhere to put digestate in February. Size it for the worst case, show the calculation in the application, and include any off-site satellite stores in the submission because they need their own permission.

Environmental permitting alongside permission

The plant will need an environmental permit from the Environment Agency under the Environmental Permitting (England and Wales) Regulations 2016. Standard rules permits exist for anaerobic digestion within defined limits, and a bespoke permit is needed where the operation falls outside them, for example on throughput, feedstock type or emissions to air.

Permitting and planning ask overlapping but different questions, and a planning authority is entitled to rely on the permitting regime for the control of emissions rather than duplicating it. That is a useful argument and it has limits: the authority still has to decide whether the use is acceptable in that location, and odour and amenity remain planning considerations.

Two practical sequencing points. Pre-application discussion with both the local planning authority and the Environment Agency is worth paying for on a scheme of this complexity. And the permit application needs process detail that the planning drawings should match, so run the two sets of documents past each other before either is submitted.

Environmental impact assessment screening

Anaerobic digestion plants frequently fall within Schedule 2 to the Town and Country Planning (Environmental Impact Assessment) Regulations 2017, through the categories covering waste disposal installations and industrial installations for the production of energy, where the development exceeds the applicable area threshold. Screening is therefore a live question rather than a formality.

Request a screening opinion from the authority early. A negative screening opinion, obtained before the main application, is a document worth having, and a positive one changes the programme by six months or more.

Where an environmental statement is required, the Regulations require it to be prepared by competent experts, and a Environmental Impact Assessment is a report type where a licensed or competent professional is legally required. An AI-assisted draft is a starting point for that professional and not a substitute for them.

Flare, combined heat and power, and gas-to-grid connection works

Decide the energy route early, because it changes the built form. A combined heat and power engine means an acoustic housing, an electricity connection and a heat use. Gas-to-grid means an upgrading plant, a propane injection facility, a grid entry unit and a pipeline to the network, and the connection point may be several hundred metres from the plant.

An emergency flare is normally required and it is visible. A flare stack of 8 to 10 metres with an occasional visible flame reads differently in a landscape from a cluster of tanks, and it should be shown on the elevations and in the photomontages rather than left off the drawings.

Pipeline and cable routes to a connection point outside the holding need their own consents, including possibly a separate planning application, and wayleaves or easements from intervening landowners. Those negotiations are often the critical path.

Landscape effect of tanks and domes

A digester tank with a gas-holding dome is a large curved form in a landscape of rectilinear farm buildings, and it does not read as agricultural. Clustering the plant with existing farm buildings, setting it against rising ground, and using dark recessive colours on tanks and ancillary buildings all help. Planting takes years to do anything, so do not rely on it for the early years of the assessment.

Where the site is in or near a National Landscape, a National Park or a designated landscape with a local plan policy, a Landscape and Visual Impact Assessment to GLVIA3 is proportionate. Elsewhere, a landscape appraisal with photomontages from the public rights of way and roads with views of the site usually answers the question.

Expect conditions on feedstock type and origin, annual throughput, vehicle movement numbers and hours, the odour management plan, the noise rating level at the boundary, flare operation, external colours and a landscape management scheme. Read the feedstock condition especially carefully, because a condition limiting imported feedstock will constrain the business model for the life of the permission.

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. Enter your postcode on PlanScope to see which reports a farm diversification scheme is likely to need.