The source characterisation section of an Odour Assessment

Odour assessment source characterisation is the part of the report that decides everything downstream. The dispersion model can only redistribute what you put into it, so an emission rate taken from an unrelated study will produce a confident looking contour plot that means nothing. Characterisation means identifying each source, describing its physical release conditions and deriving an emission rate with a stated provenance, and it is the section a reviewing officer or an objector's consultant will attack first.

Identifying and describing each source

Start with an inventory rather than a narrative. Walk the process from delivery to despatch and list every point at which odour can leave the site, including the ones nobody wants to talk about.

Sources fall into recognisable types, and the type determines how the release is described.

• Point sources, such as a stack, a vent or a biofilter outlet, described by discharge height, internal diameter, efflux velocity and temperature.
• Area sources, such as an open tank, a lagoon, a windrow or an uncovered yard, described by their plan dimensions and the release conditions across the surface.
• Volume sources, typically a building releasing through ridge vents, louvres and doorways, described by the building envelope and the ventilation rate.
• Fugitive releases, such as a roller shutter left open during deliveries or a leaking duct joint, which are the hardest to quantify and the most common cause of complaint.

For each one, record the operating pattern. A source that runs for two hours on weekday mornings has a different exposure profile from one that runs continuously, and an assessment that treats everything as constant will either overstate or understate the result depending on the weather correlation.

Describe the abatement in place at each source as well, with its design removal efficiency and its actual condition. A carbon filter beyond its service life is an unabated source in practice.

Emission rates and where the figures come from

An emission rate is the odour concentration multiplied by the volumetric flow, and each term needs a source. Three routes exist and they are not equivalent.

Direct measurement at the site is the strongest. Samples are collected from the source and analysed by dynamic olfactometry under the European standard for that method, using a trained and screened panel, with the analysis carried out within the time limit the standard sets after sampling. Report the sampling date, the operating conditions at the time, the number of samples and the laboratory.

Measurement at an analogous site is the common compromise for a proposed development that does not yet exist. It is defensible provided the analogy is argued: same process, similar throughput, comparable abatement, and a statement of what differs.

Published emission factors are the weakest and sometimes the only option. Specific odour emission rates for agricultural buildings, waste facilities and wastewater assets appear in the literature, usually expressed per unit area or per animal. Where you rely on them, name the source, give the date, and say why the figure applies here.

Never blend the three without saying so. A model fed by one measured stack, two analogous area sources and a literature figure for the yard is perfectly respectable, and it has to be presented that way in a table rather than buried in a paragraph.

Odour units and how they are expressed

Odour concentration is expressed in European odour units per cubic metre, where one unit is the concentration at which half a panel can just detect the odour. It is a sensory quantity determined by people, not a chemical measurement, which is why the panel screening and the method matter so much.

Emission rate follows from that, in odour units per second for a point source. For area and volume sources it is commonly normalised, as units per second per square metre of emitting surface, or per animal place for livestock buildings, so that the figure can be scaled to the proposal.

Set the units out explicitly at the first use and keep them consistent. Mixing a rate expressed per square metre with a total for another source, without showing the arithmetic, is the error that invites a request for the model files.

Field observation and sniff testing

Field assessment complements the laboratory work and is the part that makes the report credible to a case officer who has visited the site.

A structured field survey uses trained assessors at fixed positions or along transects, recording odour presence, intensity on a defined scale, hedonic tone and the character of the odour, together with wind speed, direction and temperature at the time. Repeat visits across different conditions matter more than a long single visit, because odour travels on particular combinations of wind and stability.

Record the character in words that mean something. Sour, rancid, sweet, earthy, ammoniacal, solvent. Character drives offensiveness, and offensiveness drives the benchmark that the assessment is judged against.

Complaint history is evidence too, where it exists. Approach it carefully, because the absence of complaints is weak evidence of absence of odour, and a cluster of complaints may reflect one determined household rather than a widespread problem. Present it as context, with dates and locations, and say what it does and does not show.

Receptor sensitivity and exposure

Receptors are graded by sensitivity to odour exposure. Housing, schools, hospitals and care homes sit at the high end. Workplaces, offices and commercial premises are in the middle. Industrial areas, footpaths and open land are at the low end, because people there are transient or the setting already carries an expectation of odour.

Identify every receptor within the study area on a plan, with its use, and pick the worst affected of each grade for reporting. Future receptors count as well, so an allocated housing site or a committed permission nearby belongs on that plan.

Exposure is normally expressed as the 98th percentile of hourly mean concentrations over a full year of meteorological data, which captures the occasional bad hour rather than the average. That statistic is what the benchmarks are set against, and it comes out of the dispersion modelling, a stage examined in the piece on odour dispersion modelling.

Benchmark levels used in assessment

Professional guidance on odour for planning sets indicative benchmarks that vary with how offensive the odour is, expressed as the 98th percentile of hourly means at a high sensitivity receptor: in the order of 1.5 odour units per cubic metre for the most offensive odours, 3 for moderately offensive, and 6 for less offensive ones.

Those figures are benchmarks rather than standards. The assessment has to reach a judgement, taking in the frequency, intensity, duration, offensiveness and location of the exposure, and a result slightly above a benchmark for a less offensive odour is a different proposition from the same margin on a putrescible waste facility.

Classify the offensiveness explicitly and justify it. That single judgement moves the acceptability threshold by a factor of four, and it is the one an objector will contest if it is left unexplained.

Where the development is a wastewater or drainage asset, the odour question sits alongside capacity and connection issues, which is why the sewer undertaker correspondence described in the note on the sewer capacity enquiry in a foul sewage assessment often arrives in the same file.

Reporting uncertainty in the source terms

Uncertainty in odour work is dominated by the emission rate, not by the meteorology or the model. Say so, and quantify it as far as the evidence allows.

Present the emission inventory as a table with a central estimate and a range for each source, stating the basis of each. Then run the model on the upper end of the range for the sources that dominate the result, and report both outcomes. An assessment that shows the conclusion holding across the plausible range is far more persuasive than one that reports a single number.

Be candid about what has not been measured. Fugitive emissions from doors and deliveries are frequently omitted because they are hard to quantify, and an assessment that acknowledges the omission and explains its likely scale is more credible than one that pretends the site has none.

An Odour Assessment is one of the report types where councils expect a suitably qualified author and where professional review before submission is strongly advised. PlanScope drafts the structure and the policy narrative from your site details, flags every input that has to come from measurement, and the specialist then completes and signs the assessment. Browse the report types index to see where it fits, or enter your postcode to check what your 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.