Odour dispersion calculation by MilieuPartners, presented on a map of the local area, clearly showing the applicable odour contours.

Percentile lines are used to describe the odour concentrations on a map of the surroundings of the plant. Percentile lines (or iso-concentration lines) connect places in a contour plot at which a certain threshold concentration (immission value) is exceeded during a certain percentage of the year. The dispersion calculations give hourly averaged immission concentrations.

Equipment for odour analysis

To dilute odour samples and present the dilutions to the assessors, an olfactometer is employed. The odorous air sample is diluted to a certain extent, using a dilution unit and odourless air. The diluted odorous air is presented for assessment to a panel of eight persons. There are three sniffing beakers, two for odourless air and one for (diluted) odorous air. The panel members have to indicate which of the three beakers releases odorous air. They are "forced" to make a choice (a 'forced triangle test'). The panel members are presented with flows that increase by a factor of 2 in odour concentration every time. This is done at least twice per measurement.


Odour measurement and atmospheric dispersion

Odour is quantified in terms of odour units (OU).

1 OUE/m3 is defined at that concentration of an odour in an air sample that can be perceived by 50% of the observers.
1 OUE/m3 equals the odour perception of 40 ppb n-butanol according to the CEN standard which is expected to be implemented soon in the EC.

A solution to an odour nuisance problem is usually a combination of two methods: odour abatement and atmospheric dilution. Every odour abatement system delivers only limited efficiency in terms of odour units. The remaining odour load in the cleaned gases should be reduced to such a level that, after dilution with ambient air, the odour cannot be perceived in a residential or other sensitive area. Calculations using official mathematical models are necessary to predict the dispersion. Main inputs are emission load, chimney height and atmospheric conditions.

The calculated concentrations are considered in relation to the duration and/or frequency with which a certain concentration level is exceeded.

Principle of odour concentration measurements

Odour concentrations are determined sensorically. The determinations are carried out by diluting an odorous air sample so many times that half the panel members can no longer distinguish the diluted odorous air from odourless air. For example, if it is necessary to dilute the original sample 2,000 times, its odour concentration is 2,000 odour units per m3 of air (in short: 2,000 ou/m3 ) at air conditions of 20 °C and normal humidity.

The average dilution factor is calculated using a computer programme. Panel members are selected on the basis of their sensitivity for smelling butanol, in accordance with standards developed for this purpose.

Related topics:
Odour terms and expressions
Chemical scrubber for odour control
Biofilter
Odour control for baking or frying
References