Southwest Environmental Limited
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Environmental Consultancy Services to Industry, Business and Individuals

Odour Impact Assessment

Southwest Environmental Limited can undertake Odour Impact Statements and Odour Impact Assessment for you Planning Application. We use modern computer modeling methods, with atmospheric data to provide reports on a quick turn around.

Odour Impact Assessment Methods

Smell is a very subjective thing, the nicest of smells such as baked bread or roses, would hardly be considered a problem, however the worst of smells such as rotten flesh, or rotten eggs are a real burden on the nostrils. In order to assess odour a unit has been developed, an Odour Unit (ou), 1 ou/m3 is the limit at which humans can smell an odour, dependent on its character (hedonic score) am upper limit of acceptability is then set. For example livestock (chickens or pigs) have a limit set at 3 ou/m3, whilst something more unpleasant such as an abattoir would have an odour limit set at 1.5 ou/m3. The odour limit for AD Plants (Anaerobic Digestion) is set at 3 ou/m3.

Using a computer model (or a calculator if you are particularly patient!), you can work out odour levels (concentrations) at set points away from the source (chicken shed or factory), these data points can then be used to create contour, and so gauge odour concentration at a particular point away from the source.

The calculation usually employed by computer models is the "gaussian distribution" this is named after the famous mathematician Johann Carl Friedrich, who was an inspired mathematician even as a young child. This distribution model is also used in Ammonia Emissions Assessments.
odour_impact_assessment From this we end up with a modeled odour plume, which can be used for the basis of an odour impact assessment.

Of course these are modeled results, and all their aim to represent real world conditions, they will not.

Even the most complex models available and relatively simplistic in the method they use to determine dispersion of odour plumes. More complex models will use actual weather data taken from the study area to deuce likely conditions, presenting concentrations as a probabilistic concentration rather than a worst case scenario. This method is "above board" but is likely to under represent concentrations.

With sensible siting and implementation of odour management plans specific to the activity on site, odour concentration can be kept below thresholds, based on worst case scenarios.

Odour Impact from Various Sources

A model not real. You place numbers in to the model, press "go" and it gives you another number. The model developers have done there best to provide an accurate representation of real world scenarios.

There are some interesting factors that are observable when using the model. In that "area" type sources such as waste heaps or farm yards typically fair worse than "point" sources such as chimney or extract vents.

By example a farm yard which is covered in a hypothetical manure will fair worse than a cattle shed of the same area with point source ridge fans. This is strange, and unlikely to represent real world  scenarios, but the model is the best method at present so what ever the out put may be it is usually accepted.

Odour Mitigation (Reduction)

If it is shown that odur levels are too high then mitigative measures can be applied. This can include a variety of solutions which we recommend depending on the situation. For example a proposed slurry store on some design drawings, could be moved away a sensitive receptor such as a dwelling, to mitigate against impacts. However, an existing slurry store is not easy to move, in this instance a cover or croffles may be recommended to reduce exposed surface area.

Objecting Against a Scheme on the Grounds of Odour Impact

Most consultants use the same method for calculating odour. If the applicant has submitted an odour impact assessment with their application that points to favorable conditions at nearby receptors it is likely that our report would indicate the same. A typical method is to use a 95% or 99% certainty value concentration, this provide an "average" across the year depending on the wind direction and temperature etc.

The problem with this method is that if on occasion the odour level which be much higher than reported. If we consider just 10 days worth of data where the wind was blowing away from the receptor for 8 days . . . . we might have source odour levels of 20 ou/m3, but the reported figure would be perhaps 4 ou/m3 because of the "averaging". Now for 8 of those days the receptor location (house or school) would receive background levels of odour but for the 2 days down wind life might be unpleasant. However, this is how odour is assessed at present.

Even in cases where we have prepared reports for residents groups or parish councils against developments, they are not normally considered.  These political consideration are more important than the report itself, you may need the ear of your local councilors in order to make any adversary effective.