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Dispersion is the phenomenon where odor concentrations are reduced to below recognition or detection thresholds. It is influenced by meteorological conditions and topographic effects. Part VI
NO matter how well planned, sited or operated, composting facilities are always at risk for an off-site odor episode that can create challenges such as negative publicity, regulatory intervention and lawsuits. This article series, which began in 2012, has been dedicated to helping composters understand the intricacies of odor management. Part VI looks at the fundamentals of how odors behave in the environment.
As discussed in Part I (BioCycle, April 2012, p. 25), an odor is an air pollutant. It is a chemical emitted in the gaseous vapor phase and behaves in the atmosphere like other gasses. Air pollutant emissions can be characterized by type, source and elevation. Types of emissions include point sources (e.g., a smokestack), line sources (e.g., cars on a highway), area sources (e.g., from a forest fire), or volume sources (e.g., from a paint shop with multiple roof vents). Emissions from composting facilities are usually considered area or volume sources. Emissions can be further characterized as mobile versus stationary and urban versus rural, as the urban heat island effect makes the atmosphere above cities more turbulent, which affects dispersion. The elevation of an emission is another discriminant, with categories of surface, or ground level, near surface and elevated surface.
Gaseous or particulate emissions from a continuous source are emitted as a "plume" of material, so-called because the flow of emissions resembles a feather in shape and appearance. The emissions of smoke or steam from an elevated source are a visible example of a plume. Emissions from an intermittent source are considered "puffs" where a cloud of material is released and moves downwind.
Emission plumes and puffs are considered buoyant, dense or passive. A buoyant plume rises with distance and time from the source (Figure 1). They are lighter than air because they are at a higher temperature and lower molecular weight (density) than the ambient air around them. For example, the molecular weight of ammonia is 17.02, while the molecular weight of air is 28.97, so an ammonia emissions plume will rise. Conversely, a dense plume sinks with distance and time as it is...