Odor Dispersion Modeling and the Solution to Pollution is Dilution Dilemma

The well-worn saying 'the solution to pollution is dilution' has been thrown out for years as the approach to minimize and control air and odor pollutants through the use of stacks and vents. And while there is validity in diluting the strength of exhaust gases with ambient air, how successful that dilution is, along with how effective it is at reducing the impact of those odors and gases on the surroundings depends on a number of parameters that should be considered when designing exhaust systems. This is especially true for wastewater and collection processes, which due to the character of their exhausts, and their typical placement within the communities they serve, have the genuine potential to effect the quality of life and health in those communities. This paper will discuss the factors that affect the dispersion and dilution of exhaust gases from wastewater processes, and the methods used to determine the most effective odor exhaust design. These parameters include not only the strength and volume of odorous emissions, but also the size of the structures enclosing the processes and the proximity of surrounding buildings to the process systems (for downwash and reentry consideration), local meteorology (for the effects of wind direction, speed, and stability), the location of nearby sensitive receptors (residential areas, schools, parks, hospitals) and nearby terrain features to assess the potential for high impacts. The paper will discuss the relevant effects that each of these factors has on the dispersion and dilution of odor, and also the techniques that can be used to assist in selecting the best options for facilities that take these factors into account. Frequently, considerations other than the potential for high impacts result in stack designs that can actually lead to higher offsite (and onsite) impacts. When stack design is selectively based on operational concerns about drainage during heavy rainfall events with a rain cap, for example, it can result in the need for expensive redesign. The paper will provide real world examples of such designs, and their negative effects on the goal of dispersing odors. Also described are alternative designs and the odor dispersion modeling methods that can be used to determine the optimal design for exhaust systems. The paper will discuss: the principles important to designing appropriate exhaust systems at wastewater and collection facilities in order to prevent high odor impacts, how to evaluate the effectiveness of a proposed exhaust design using odor dispersion modeling, and how that modeling can be used not only to assist with design, but also with community outreach, and with regulatory compliance and permitting requirements. Odor dispersion modeling of real-world examples of factors relevant to effective dilution and dispersion in the design of process exhaust systems at wastewater and collection facilities will be shown to provide an important and cost-effective method for the design of exhaust stacks and vents.