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Improvements in Odor Modeling Using Non-Steady-State Models and More Realistic Source Physics

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Odor modeling has been a routine aspect of the permitting process for odorous industries in Australia and Europe for many years. In the US, it appears to be less widely entrenched, but increasing in prevalence. Where specific odor criteria are set by regulators for project approval, odor assessments rely on predicting impacts on affected communities under critical combinations of meteorological conditions and emissions, which may be co-related.

For many odor sources, particularly non-buoyant or weakly buoyant near-surface sources, the most critical conditions affecting neighbors involve light wind, stable flows which typically occur between early evening and the first hour or two after sunrise. The widely used Gaussian plume or steady-state dispersion models such as AERMOD (US), AUSPLUME (Australia) and ADMS (UK/Europe) have major flaws which lead to potentially large model errors under these most critical conditions.

Furthermore, area and volume odor sources under conditions of relatively low ambient temperature (at night and early morning) and very light wind may exhibit slight positive thermal buoyancy and plume rise that is usually ignored in modeling studies. Plumes that rise even modestly - e.g. from wastewater ponds - under these conditions can lead to significantly different ground level concentrations, in terms of both magnitude and location, from plumes that are assumed to have no plume rise and to travel in straight lines.

In Australia and in parts of Europe, odor assessments increasingly employ non-steady-state models such as CALPUFF and deal with buoyancy effects, however subtle. CALPUFF is widely used for such purposes, particularly in Australia, and has the advantage of being able to deal much more realistically with calms and very light winds, unlike the steady-state models which must ignore the same light wind conditions that are most critical to most odor impacts. Data required for such models can be relatively easily generated by user-friendly meteorological models such as the Australian model, TAPM.

Buoyancy effects from sources such as poultry and animal housing and wastewater ponds can be treated in models such as CALPUFF and can significantly affect assessment outcomes, especially in areas where there is some terrain complexity.

The paper shows examples that compare non-steady-state modeling in combination with treatment of weak source buoyancy to more commonly applied techniques, and discusses implications for the permitting of odorous activities.
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Keywords: Australia; CFD; Calpuff; Dispersion Model; Emissions; Odor; Odor Assessment

Document Type: Research Article

Publication date: 2010-01-01

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