When odour modelling is needed and what it involves

Quantitative odour modelling is warranted when a qualitative screening shows a medium or high risk of an odour effect that mitigation cannot obviously resolve, when the proposal is one of the source types regulators expect to see modelled, or when the environmental health officer or the Environment Agency asks for it. It is not the default. Most odour cases in planning are settled on a risk-based screening supported by a specification for extraction and filtration, and commissioning dispersion modelling for a takeaway or a small bakery wastes money that would be better spent on the abatement.

Knowing which side of that line you are on is the useful skill. So this is about the scoping decision rather than the modelling method.

Start with the risk-based screening

The Institute of Air Quality Management's guidance on assessing odour for planning sets out a structure that officers recognise: characterise the source odour potential, assess how effective the pathway is between source and receptor, and assess the sensitivity of the receptors. Combining the three gives a risk of odour effect, which is then judged against the policy test of whether unacceptable harm to amenity would result.

Each element is evidenced rather than asserted. Source potential comes from the process, the throughput, whether material is handled in the open and how odorous it is. Pathway depends on distance, prevailing wind, topography, shelter and the height and velocity of any discharge. Receptor sensitivity distinguishes dwellings and schools from an industrial estate.

Do that work first, always. A screening that concludes low risk with a properly specified abatement system closes many applications without anything further.

Source types where modelling is usually expected

Some proposals attract a request for quantitative assessment almost automatically, because the consequences of getting it wrong are large and the sources are well characterised in the literature:

1. Anaerobic digestion, composting and waste transfer or treatment facilities.
2. Wastewater treatment works, sludge handling and pumping stations with wet wells.
3. Intensive livestock units, particularly poultry and pig housing, and slurry stores.
4. Rendering, pet food, fish processing and other high-odour food manufacture.
5. Landfill gas and leachate treatment.
6. Any of the above proposed within a few hundred metres of dwellings, or where dwellings are proposed near an existing one.

The last of those is the case practitioners miss. A residential scheme next to an existing sewage works or livestock unit needs the odour case made by the applicant for the housing, and the agent of change principle in national policy does not remove the need to show the new occupiers would have acceptable living conditions.

By contrast, commercial kitchen extraction is handled through the national kitchen ventilation guidance and a risk-based approach to filtration, termination height and separation, as our note on restaurant extraction ducts: routes and termination sets out. A Ventilation and Extraction Statement is the right document there.

What the quantitative route involves

Three pieces of work sit behind a modelled result, and each carries cost and programme.

Source characterisation and sampling comes first. Ducted sources are sampled at a defined point with the process running representatively. Area sources such as open windrows, lagoons or slurry surfaces need flux chamber or wind tunnel sampling, which is considerably more involved and more variable. For a proposed facility that does not yet exist, emission rates come from measurements at comparable operating plant or from published emission factors, and the choice has to be justified.

Then olfactometry. Samples are analysed by dynamic dilution olfactometry to BS EN 13725 using a screened panel, producing a concentration in European odour units per cubic metre, which combines with the volumetric flow to give an emission rate in odour units per second. Our note on when dynamic olfactometry is needed for an odour case covers the sampling and panel requirements in more detail.

Then dispersion modelling, using a recognised model with several years of hourly meteorological data from a representative station, surface roughness, terrain where relevant, building downwash and a receptor grid covering the sensitive locations. The output is normally a contour plot of the 98th percentile of hourly mean concentrations across the year, which is what the benchmarks are expressed against. The mechanics of that step are covered in odour dispersion modelling in an odour assessment.

Programme is the reason to make this decision early. Sampling has to happen when the plant is running representatively, which on a seasonal process can mean waiting for the right month, and olfactometry laboratories work to their own turnaround. Add model set-up, review and a likely round of regulator questions, and a quantitative odour case is a three to four month piece of work on a straightforward site. Scoping it at pre-application, with the environmental health officer's agreement on what will be assessed and against which benchmarks, removes most of that risk.

Benchmarks and what the numbers mean

The Environment Agency's odour guidance sets indicative benchmarks for the 98th percentile of hourly mean concentrations, graded by how offensive the odour is: the most offensive odour types are held to the lowest concentration, moderately offensive types to an intermediate value, and less offensive types to the highest. Check the current published figures rather than quoting a number from an old report, because the guidance is periodically revised.

Benchmarks are not statutory limits. They are a decision aid, and an exceedance at a single receptor with a well-founded management plan is a different case from widespread exceedance across a residential area.

Uncertainty, and reporting it honestly

Odour assessment carries more uncertainty than most technical disciplines in planning. Olfactometry has known precision limits, emission rates vary with process, weather and season, area source sampling is sensitive to method, and the choice of meteorological year moves results.

Report it. Give the range across the meteorological years modelled rather than one year, identify the dominant source of uncertainty, and say whether the conclusion holds across the range. A reviewer who sees uncertainty acknowledged trusts the rest of the report more, and an assessment presented as precise invites challenge it cannot survive.

Mitigation that changes the output

The measures that shift a modelled result are physical: enclosing the process and drawing it to a single treated discharge, increasing stack height and efflux velocity, biofilters or carbon abatement sized for the airflow with a specified media replacement regime, negative pressure in buildings, covering lagoons, and process changes that reduce the odour at source.

Measures that do not shift it: a complaints procedure, staff training alone, or a promise to review. Those belong in the odour management plan that supports operation, and an Odour Assessment should contain both, clearly separated.

Where the facility needs an environmental permit under the Environmental Permitting (England and Wales) Regulations 2016, the permit and the planning permission are determined separately and the odour evidence usually serves both. Sequence the work so the same sampling and modelling supports each, and say in the planning submission which controls will sit in the permit.

For specialist technical reports, qualified professional input may be required. 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.