ODOR MODELING FOR OVERALL ODOR CONTROL PLANNING

The Orange County Sanitation District (the Sanitation District or OCSD) has been proactive in developing overall odor control strategies to minimize nuisance odors beyond their facility fence lines. The Sanitation District's two wastewater treatment plants share boundaries with residents, the Santa Ana River Trail, and other sensitive receptors.In a continued effort to reduce off-site impacts from the two wastewater treatment plants, the Sanitation District initiated a comprehensive evaluation of the odors from both plants and modeled the odors from each process and potential odor source in terms of overall odor units in dilutions tothreshold (D/T). Historically, the Sanitation District based the evaluation of plant odors and their control solely on hydrogen sulfide (H2S) equivalents. The Sanitation District now recognizes that compounds other than H2S contribute to odors and basing control solely onH2S equivalents may result in inadequate treatment of emissions from processes where H2S is not the primary odor compound.The goal of the project was to develop atmospheric dispersion models that could be used to enhancethe understanding of how odors generated at both plants are transported to the fence line and offsite receptor locations. Based on meteorological data, geographical data, and estimates of odor emissions, the model objective was to predict the intensity, frequency, and spatial extent of nuisance odors generated at the plants.Various control options were developed to achieve fence line odor levels of 5, 10, 15 and 25 D/T, as determined by European Standard prEN 13725. Preliminary planning capital and O&M costs and siteconsiderations were developed for the various options required to meet the various fence line goals. The data was then used by the Sanitation District for a Triple Bottom Line Assessment (TBL)that considered the economic, social and environmental impacts of each option. As a result of these analyses, a fence line goal of 10 D/T was selected as a guideline for current and future designs.The subject of this paper is the approach to the comprehensive model effort and the results presented to the Sanitation District to assist with the development of their overall odor control plan. This paper focuses on model assumptions, control selection and planning phase costs. The use of the analysis for the Sanitation District's Triple Bottom Line Assessment will also be discussed.