MODELING OF ODORS AND AIR TOXICS: A COMPARISON OF THE ISCST3, AERMOD, AND CALPUFF MODELS

The modeling of the dispersion of contaminants in the atmosphere is a routine method for air quality management, both in the design and in the regulatory phases for environmental assessment and permitting. Major air emission sources in the United States must demonstrate compliance with the ambient air quality standards (AAQS) for regulated pollutants (“criteria pollutants”), and a number of other air quality criteria utilizing atmospheric dispersion modeling. As a result of the widespread utility of models, EPA has established a library of models that have undergone field validation and public and environmental agency review, along with formal guidance on modeling issues.With respect to odors, atmospheric dispersion modeling offers the same promise. Wastewater treatment plants, landfills, feedlots, and other sources of odors and air toxics are under increasing pressure to control emissions from nearby communities as the buffer zones around these sources shrink, and health and quality of life issues sometimes outweigh the regulatory standards in the eyes of the public. Various models, steady-state and puff, Gaussian and Lagrangian, have been used for odor assessment, and they are being used increasingly to determine not only source culpability, but to evaluate and rank control measures. As regulatory modeling continues to move towards in the direction of a single powerful and multipurpose model, it makes sense to model odors with the same model, if at all possible. This common approach to overall air quality assessment reduces the burden on a facility to defend a methodology outside standard modeling practice, expedites the review required by the regulatory agencies, and produces more consistent results, especially when a facility emits both odors and air toxics.This paper builds on previous work (Diosey, Hess and Pope, 2000) and discusses a preliminary evaluation of a wastewater treatment plant using three dispersion models that have been used in air permitting and regulatory analyses: Industrial Source Complex – Short Term (ISCST3), the AERMOD modeling system (AMS/EPA Regulatory Air Modeling System), and the CALPUFF modeling system (formerly California Puff Model). The ISCST3 model is currently the model of choice for most regulatory modeling for projects with multiple sources and source types in simple and complex terrain. AERMOD has recently been proposed to replace ISCST3 as the preferred model for these types of projects. CALPUFF is most extensively used for long-range transport modeling for PSD Class I analyses. The emissions of hydrogen sulfide from sources at the wastewater treatment plant will be analyzed using each of these models, and 1-hour impacts of H2S will be determined.