Modeling Odors in the Age of Aermod

For over 25 years, the ISCST3 (Industrial Source Complex-Short Term) model was the workhorse regulatory dispersion model used in permitting, risk assessment, and odor assessment. However, while significant advances were being made in the fields of atmospheric diffusion and modeling over that period, these advances were not incorporated into ISCST3. In an effort to incorporate these scientific advancements into regulatory modeling, the United State Environmental Protection Agency (EPA) collaborated with the American Meteorological Society (AMS) to develop a model that could better characterize plume dispersion. The result of this collaboration was the AERMOD modeling system. It was developed to incorporate state-of-the-practice planetary boundary layer (PBL) principles into a plume dispersion model.After many years of development and enhancement, the AERMOD modeling system officially replaced the ISCST3 model in December 2006 as the preferred and approved regulatory model for simulating the impacts of emissions from a variety of sources. These include but are not limited to power plants, industrial facilities, landfills, hazardous waste facilities, and wastewater treatment plants. Although AERMOD has gone through numerous model evaluation studies over the course of its development and was used by some individual air permitting agencies for a number of years prior to its official promulgation, the application of AERMOD has proved challenging in a number of applications.Malcolm Pirnie has been modeling the odor emissions (using H2S) from a number of large, municipal wastewater treatment plants using the ISCST3 model in order to evaluate cost-effective odor control. Wastewater treatment plants generally combine a variety of process source types onsite, including tanks (“area sources”); odor control stacks and building vents (“point sources”); and pressure relief valves, open doors, and roof monitors (“volume sources”). Determining which sources may require control is an extremely important task. With impending plant upgrades, many of these same plants are now undergoing a similar odor control evaluation; however, the evaluations are now being performed with AERMOD. Using examples from the evaluations of two municipal wastewater treatment plants located in a large metropolitan area, this paper discusses a number of relevant issues that emerged with the use of AERMOD to assess offsite odor impacts, including:representative meteorological datacharacterization of surface characteristicsurban/rural optionsmodel processing time