Odour from farms and waste sites: distance and modelling

Odour from a livestock building, slurry store, anaerobic digester or composting operation is assessed in two places at once, and the two use different tools. The planning system asks whether the development is an acceptable use of land, including its effect on the amenity of nearby residents. The environmental permitting regime asks whether the operator can control emissions to an acceptable standard, through an odour management plan enforced by the Environment Agency.

Separation distance is the first thing everyone reaches for, and it is a screening tool rather than a standard. There is no distance at which odour becomes lawful, and no distance at which it becomes unacceptable. What matters is the source, the pathway, the receptor and the management.

Which regime applies to what

Intensive pig and poultry installations need a permit above defined capacity thresholds, expressed in bird places and production pig or sow numbers. Below those thresholds the planning application is where the odour question is settled, which means the applicant carries more of the assessment burden rather than less.

National guidance says the planning system should work on the basis that the pollution control regime will operate effectively. In practice that means a planning authority should not attempt to duplicate the permit conditions, and an applicant should not use the permit as an answer to an amenity objection. Both happen anyway.

Qualitative assessment, and the five factors

For smaller proposals a risk-based qualitative assessment is proportionate. It works through source, pathway and receptor, and then characterises the effect using five factors:

• Frequency. How often odour is likely to be detectable at the receptor.
• Intensity. How strong it is when present.
• Duration. How long each episode lasts.
• Offensiveness. What kind of odour it is, since decaying material is judged far more harshly than brewing or baking.
• Location. What the receptor is, and what people there are doing.

Those five carry real weight in a planning judgement, because they describe the experience rather than a concentration. An assessment that concludes there is a low risk of infrequent, low intensity, short duration odour of moderate offensiveness at a dwelling 300 m downwind, with the reasoning shown, is more useful to an officer than a bare distance.

Offensiveness is where operations divide. Regulatory practice groups odours into most offensive, moderately offensive and less offensive categories, with decaying animal material and septic effluent at the worst end, intensive livestock and sewage treatment in the middle, and food processing at the least offensive end. The category sets the benchmark concentration used in modelling.

Dispersion modelling, and the benchmarks

Where the operation is large, or receptors are close, or the permitting regime requires it, the assessment moves to dispersion modelling.

The chain runs like this. Odour emission rates are established for each source, usually from olfactometry measurements taken to the European standard for dynamic olfactometry and expressed in European odour units per second. Those rates go into a dispersion model with several years of hourly meteorological data from a representative station, terrain, and building effects where downwash matters. The model predicts odour concentration at receptors, reported as the 98th percentile of hourly mean concentrations over a year.

That percentile is then compared with benchmark concentrations by offensiveness category. Regulatory guidance uses benchmarks of around 1.5 European odour units per cubic metre for the most offensive odours, 3.0 for moderately offensive, and 6.0 for less offensive, at that 98th percentile. Take the current values from the regulator's published guidance, because they are the numbers a reviewer will check.

Source characterisation is where modelling goes wrong. A slurry lagoon is an area source with an emission rate per square metre. A naturally ventilated livestock building is a volume source with openings on more than one elevation. A fan-ventilated poultry house is closer to a set of point sources. Modelling a building as a single stack because it is easier will produce a result nobody can defend.

Mitigation that actually reduces emissions

Screening with trees is popular and does very little for odour, though it helps with the visual effect and can encourage mixing. The measures that work reduce emission at source or contain it.

1. Cover slurry and digestate stores, with a fixed or floating cover, and keep the crust intact where a crust is the containment method.
2. Enclose reception, tipping and mixing operations in a building held under negative pressure, with air drawn to treatment.
3. Treat extracted air through a biofilter, acid scrubber or combination unit, sized on airflow with a stated residence time and a maintenance regime.
4. Reduce the exposed surface area of any open lagoon, and avoid agitation in still, warm conditions.
5. Incorporate manures and digestate into soil quickly after spreading, and choose weather windows using a documented decision process.
6. Keep housing dry, since wet bedding and wet litter raise emissions more than stocking density does.

Every one of those has to be written as a commitment with a person responsible and a frequency, because that is what goes into the odour management plan and, in planning terms, into a condition.

Ammonia is a separate question

Odour and ammonia are often confused, and they behave differently. Ammonia is assessed against critical levels for vegetation and critical loads for nutrient nitrogen deposition on designated habitats, and it screens at much greater distances than odour does.

For an intensive livestock proposal, the ecological screening distances to protected sites reach several kilometres, and the assessment relies on modelled deposition rather than on odour concentration. A Preliminary Ecological Appraisal identifies the designated sites in range, and the ammonia modelling then supports the habitats assessment. This is a common reason for refusal on poultry proposals near heathland or ancient woodland, and it has nothing to do with smell.

Where the reports fit

An Odour Assessment sets out the sources, the assessment method, the receptors, the results and the mitigation, and it should state clearly whether it is a qualitative risk assessment or a modelled study. Mixing the two produces a document that satisfies neither reviewer.

Where the wider emissions picture matters, including traffic and combustion sources on a digester site, an Air Quality Assessment covers that ground.

Larger intensive livestock installations and waste installations can fall within the Environmental Impact Assessment Regulations 2017, either as Schedule 1 development above the capacity thresholds or as Schedule 2 development requiring screening. Where an Environmental Statement is required, the regulations require it to be prepared by competent experts, so an Environmental Impact Assessment draft is a starting point for those specialists rather than a substitute for them.

Councils usually expect a suitably qualified author for odour modelling, and professional review is strongly advised before submission.

Illustrative example. A proposal for two poultry houses sits 350 m from the nearest dwelling and 2 km from a heathland site. The odour risk assessment concludes a low risk with covered manure handling and dry litter management. The ammonia modelling shows deposition above a threshold at the heathland, and the scheme is amended with air scrubbing to reduce emissions. Odour was never the constraint. This example is illustrative only.

What to do first

Establish whether a permit is needed, because that determines who assesses what. Then identify every receptor and every designated habitat within the relevant screening distances. Then decide, with your consultant, whether a qualitative assessment is proportionate or whether modelling is needed, and agree that with the council and the regulator before the work starts.

Agreeing the method first saves the argument later.

This is general guidance, not legal advice. Check the position with your local planning authority and the Environment Agency before relying on it.

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