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E&A Environmental Consultants, Inc.
Air Dispersion Modeling

Air dispersion modeling can be used to evaluate the movement of odor from a source and determine the extent and frequency of odor impacts on a surrounding community. Models are often used to evaluate the potential for odor impacts from proposed facilities as part of the siting process or to compare different mitigation efforts in order to determine the best odor mitigation solution for existing facilities. Using modeling, many scenarios can be considered at low cost, and the modeling effort may save a facility the expense of constructing an inappropriately sited or designed facility and the hardship associated with public opposition.

The model that is typically used for this work is the EPA recommended ISCST3 model. This model takes local topographical and meteorological data into account and combines this information with emissions concentrations, site layout, operational parameters, and source dimensions to determine the movement of odors from the site. Modeling results are expressed as a series of isopleths, or concentric circles, which show the maximum concentration of odor that is projected to occur at any given point over the modeled time period.

The model can also be run to determine the frequency of impacts that are projected to occur at a particular receptor point within a set time period as well as to indicate the conditions under which impacts take place. For example, in some cases, the model may find that impacts all occur between 2 AM and 5 AM during winter months. This would be less of a concern than modeling results that project impacts throughout the day or impacts during summer months, when residents are more likely to be outdoors. The results might indicate that impacts occur only when wind blows towards a particular direction; in this case, site operations could be altered to avoid performing odor-releasing activities when wind is in that direction.

The following is a simple example of modeling and how it can be used to determine site design. This model was run for a proposed facility that was sited in a very developed area. The proposed facility was to be entirely enclosed so that the only source of odor would be the biofilter, but the biofilter was very large and located close to the property line.

The model was run using 25 dilutions to threshold (D/T) as a typical biofilter exhaust; the results of the model are shown in Figure 1.   Based on data collected at numerous composting facilities, 5 D/T was considered the odor nuisance threshold, meaning that odor concentrations above 5 D/T would be likely to cause an odor impact.  The red line shown on Figure 1 is the 5 D/T line, indicating where impacts are likely to take place.  As shown, impacts extend far beyond the property line, impacting the commercial district to the northeast of the site.

The model was then run for an enclosed biofilter with roof vents. Although the same number of odor units would be emitted, enclosing the biofilter improves dispersion because of several factors. The make-up air added to the biofilter exhaust dilutes the odor exiting the vents, the dispersion height from the roof is much higher than from the surface of the biofilter, and the velocity of the odor exiting from the fans enhances mixing with ambient air. As shown in Figure 2, enclosing the biofilter greatly reduced the range of odor impacts, but there were still some impacts off-site.

The biofilter scenario was run again, this time with four roof vents and an additional 10 percent make-up air added to the exhaust, and the result, as shown in Figure 3, was a complete elimination of off-site odor impacts.

This is an example of how odor modeling can prevent odor impacts and help the facility owners avoid a contentious situation. Once there is an odor problem, it is much more difficult to convince the community that the problem can be solved. To many, the presence of odors indicates a badly run facility, and once the public opposes a facility, the complaints may spread to other aspects of the operation. The best solution, therefore, is to prevent odors from the beginning. For existing facilities, odor models can be used as a tool to show the public that you are carefully considering mitigation measures and that decisions about facility design are taking their concerns about odor into account.
Figures are produced using Surfer Graphics software and do not translate well to *.jpg format.  For paper copies of the figures, please contact us at EAEnviron@aol.com.
Figure 1 - The odor source, a biofilter, is shown at the center of the figure, surrounded by the property line.  Numerical labels along the property line show the maximum odor concentration projected to occur at those points.  As shown, odor impacts, indicated by the red line, are projected to occur far off-site.  
Figure 2 - Enclosing the biofilter reduced the number of off-site odor impacts, but as shown, off site impacts were still projected.
Figure 3 - Enclosing the biofilter and adding 10 percent makeup air eliminated the projected off-site odor impacts.  The additional air diluted the odor concentration in the exhaust and increased the exit velocity from the biofilter vent, enhancing dispersion.