Odor Setback Model
Odor Sources Orientation and Shape

Odor emission sources may include an array of points (exhaust fans), lines (building side with curtain or doors opening) and areas (outdoor lagoons or other storage). A swine production facility may have several buildings and waste storage or treatment facilities that emit odors. All of the odor emission sources at a given production site should be grouped and encompassed spatially with a rectangle. A length to width ratio (L/W) is used to describe the shape of the encompassing rectangle and a direction (north-south or east-west) is used to give the orientation of the rectangle.

The orientation, shape (L/W) and distance between the odor emission source and the neighbors influence the exposure angle factor. For a long and narrow odor emission source that is oriented north-south, the likelihood of receiving odor are greater for a neighbor to the north than a neighbor to the east of the facility because of a larger exposure angle. However, the downwind odor concentration will be greater when the wind blows along the length of the facility, but the exposure angle is smaller.

A sensitivity test was conducted to study the interactive effects of the L/W ratio and the wind exposure angle on downwind concentrations. The line source Gaussian dispersion model was used to predict downwind gas concentrations with all combinations of wind speeds and stability. The results showed that percent exceedance increased with L/W ratio for directions parallel and perpendicular to the odor source. However, orientation and shape did not have a significant effect in the NW, NE, SE and SW directions. The following odor source orientation and shape factor (V) were assigned: V = 1.00 for L/W < 2, V=1.05 for L/W > 2 and < 4, V=1.10 for L/W > 4 and < 8 and V=1.15 for L/W >8.