Odour risk assessment for a waste or composting facility

An odour assessment for a waste facility answers a different question from the one an environmental permit answers, and confusing the two is why composting and waste applications stall. The permit asks whether the operator can control emissions to an acceptable standard. Planning asks whether the site is the right place for the activity at all, given who lives downwind. Both processes run in parallel, and each has its own decision-maker.

For an operator scoping a site, the practical sequence is: check separation distances first, characterise the sources second, model only if the first two leave the answer in doubt.

Why waste odour is assessed differently

Waste odours are biological, variable and offensive in character. A bakery emits an odour most people rate as pleasant; an in-vessel composting plant emits ammonia, amines and sulphur compounds that people rate as strongly unpleasant even at low concentrations.

Assessment methods reflect that. The recognised approach for planning uses the exposure factors together: how often an odour occurs, how intense it is when it occurs, how long each episode lasts, how offensive the odour is in character, and how sensitive the location is. Offensiveness carries real weight, and waste process odours sit at the offensive end of the scale.

Variability is the second difference. A waste site's emissions depend on feedstock, weather, stage of the process and whether something has gone wrong. An assessment based only on normal operation understates the position, and consultees know it.

Separation distances and their limits

The regulator's standard rules for open windrow composting require a separation of 250 metres from any sensitive receptor, and that figure has become the working benchmark for open composting in planning too.

Treat it as a screening tool rather than as a standard. A site more than 250 metres from the nearest dwelling is not automatically acceptable, and a site closer than that is not automatically refused if the process is enclosed with treated air.

What separation distance does not capture:

• Topography that channels air down a valley towards a village.
• Prevailing wind that puts a receptor downwind for most of the year.
• Receptors that are sensitive in ways a dwelling is not, such as a school playground, a hospital, a care home or a campsite.
• Cumulative effect where another odorous operation already exists nearby.
• Future receptors in an allocated housing site that has planning permission but is not yet built.

That last one matters more than operators expect. A permitted allocation downwind of your site is a material consideration, and it can be the difference between consent and refusal.

Source characterisation and emission estimates

Before any modelling, list the sources and describe them honestly. For a composting or waste transfer operation these normally include:

1. Reception and tipping, usually the largest single source, and the one most affected by how quickly material is moved inside.
2. Shredding and mixing.
3. Active composting, whether in vessel, in aerated bays or in open windrows.
4. Turning operations, which release a short, intense plume.
5. Maturation and storage of finished product.
6. Leachate collection, lagoons and dirty water systems.
7. Biofilter or scrubber exhaust, where enclosed air is treated.
8. Vehicle movements carrying odour off site on wheels and loads.

For each, record the emission area or volume flow, the expected odour concentration, and the hours of operation. Where measured data exists from a comparable operation, use it and say where it came from. Where it does not, use published emission factors and state the source and its uncertainty.

Odour concentration is measured by dynamic olfactometry to the European standard method, using a trained panel. It is the only defensible way to put a number on an odour source, and sampling has to be done properly, with the right bag material and analysis within the specified holding time.

Dispersion modelling and when it is needed

Model when separation is marginal, when receptors are sensitive, when topography is complicated, or when the authority asks.

A dispersion model takes the source terms, several years of local meteorological data and the terrain, and predicts odour concentration at receptor locations as a 98th percentile of hourly means over the year. The regulator's benchmarks sit at different values depending on how offensive the odour is, with the most offensive categories, which include waste processes, held to the lowest concentration.

Three things determine whether the model will be believed:

• The source terms. Garbage in, garbage out applies with unusual force here, and a model built on optimistic emission rates is worthless.
• The meteorological data. Use a representative station, a recent multi-year dataset, and state which years.
• The abnormal case. Run a scenario in which the biofilter is past its service life or a vessel door is open, because that is when complaints happen.

Report both the compliant normal case and the sensitivity cases. An assessment showing only the best case invites a consultee to assume the worst case is unacceptable.

The overlap with the environmental permit

National policy is clear that the planning system should assume the pollution control regime will operate effectively, and that planning should not duplicate controls that are the permitting authority's responsibility.

In practice that gives a division of labour:

The overlap is real, and the argument that everything is a matter for the permit will not succeed where the assessment shows unacceptable residual odour at existing homes. Location is a planning question and always has been.

Apply for both in parallel. A permit determination that arrives after a planning refusal is wasted money, and a planning consent that arrives before the permit position is understood can leave an operator with consent for something they cannot lawfully run.

Odour management plans

The odour management plan is the operational document, required under the permit, and it is worth submitting a draft with the planning application even though planning does not approve it.

A useful plan contains the source inventory, the control measures for each source, the operating procedures that keep odour down, the monitoring regime including sniff testing routes and meteorological recording, the complaint procedure with response times, the contingency arrangements for equipment failure, and the review triggers.

Submitting the draft does two things. It shows the authority that the operational detail has been thought through. And it gives the environmental health officer something specific to respond to rather than a general anxiety.

Where odour from agricultural and livestock sources is the comparison point, the modelling conventions differ somewhat, and those are covered in poultry unit odour modelling and distance.

Conditions the authority imposes

Expect conditions dealing with the matters the permit does not reach:

• Hours of operation, including restrictions on tipping and turning at weekends and in the evening.
• A requirement that all reception and processing takes place within the building with doors closed except for vehicle access.
• Maximum throughput, expressed in tonnes per annum, which is a planning matter because it controls scale and traffic.
• Landscape bunding and planting, which help perception more than dispersion but are frequently required.
• A restriction on outdoor storage of feedstock or finished product.
• Vehicle sheeting and wheel washing.

The throughput condition is the one operators should engage with most carefully at application stage, because increasing it later needs a further application.

An Odour Assessment usually travels with an Air Quality Assessment where bioaerosols or dust are also in issue, and the two should share the same source inventory. Note also that a planning permission does not prevent a later statutory nuisance complaint, a distinction explained in statutory nuisance after planning permission is granted.

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.