Illinois State Water Survey
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This work addresses the question of how far from a hog facility odor will disperse and yet remain above the odor threshold, which is the concentration below which we can no longer smell it. Computer models are used to simulate the odor transport to determine the downwind odor strength. Work is underway to assess the effect of elevated sources, such as stack release of odor, on downwind odor strength. Another area of focus is to estimate the impact of trees near a swine facility on the downwind odor strength.
An important source of variation in odor transport is atmospheric stability. A plume in an unstable atmosphere experiences more vertical and horizontal diffusion than in a stable atmosphere and tends to disperse more rapidly, leading to lower downwind odor levels. Typically, the atmosphere is much more stable at night, suggesting the odor can travel longer distances from the source at night and still remain above the detection threshold. The figures above show results of computer simulations of the diurnal variation of odor dispersion based on meteorological observations over an eight-year period. The calculations indicate there is a wide diurnal variation of odor transport distances.
Beyond the immediate vicinity of the source odor, levels exceeding the detection threshold occur predominately at night. For the odor source considered, threshold exceedances beyond 500 m (1,640 ft) from the source occur over 15 times as often at nighttime as during the daylight hours. Over 94 percent of the threshold exceedances occurring beyond 500 m from the source do so during the nighttime. This implies that management activities associated with high odor emissions should be scheduled during daylight hours, which is typically the case anyway. There are times when stable atmospheric conditions occur during the daytime, and avoiding high-emission activities during those periods could reduce the odor impact.
The release of odor from a hog facility at an elevated height potentially has an important impact on reducing downwind odor levels. Here, the essential aspect is that the odor can disperse significantly before diffusing to the ground. In cases where the odor is below the detection threshold before reaching the ground, the odor impact is eliminated. Generally, an elevated source a few tens of feet high will have a much more pronounced effect on odor levels within a few hundred feet from the source and less effect at longer distances.