A Comparative Study of AERMOD Low_Wind Options on WWTP H2S Impacts

When used for the purpose of regulatory compliance with odor standards and criteria, odor modeling has traditionally used atmospheric dispersion models developed for and approved by federal, state, and local regulatory agencies for air permitting and formal environmental review studies. A model input that plays a critical role in determining the short-term impacts often used to characterize odor impacts is the meteorological data applied by the dispersion models. In the case of the AERMOD model, it has been found in recent years that there is a tendency for AERMOD to over-predict concentrations under very low wind speeds by underestimating the wind speed, turbulence, and mixing heights especially near ground level under these low wind conditions. In order to account for this over-prediction of impacts, several options have been incorporated into the AERMOD program. These non-default/beta options include:LOWWIND1 - increases minimum horizontal turbulence and turns off horizontal plume meander.LOWWIND2 - increases minimum horizontal turbulence and incorporates a modified meander component.LOWWIND3 - increases the minimum lateral turbulence intensity (sigma-v) and adjusts the dispersion coefficient to account for the effects of horizontal plume meander.While the options have specific conditions under which they may be applied, they nonetheless were designed to address problems with the AERMOD concentration estimates that were especially problematic for near-ground level and area sources, both of which are characteristic of typical sources at wastewater treatment plants (WWTPs). In order to investigate the effect of these Low_Wind options, three different municipal wastewater treatment plants ‒ one in a densely populated urban area, one in a suburban setting, and the third in a relatively sparsely populated rural area, were modeled in AERMOD (version 15181). The three facilities include a variety of source types, including point, area, and volume sources, representative of the different processes at such facilities. In an earlier paper, these three facilities were studied to determine the effect of using 1-minute ASOS data as opposed to using standard hourly meteorological data (Diosey, Farrell, and Jalbert, 2012). The 1-minute ASOS data was found to increase the number of valid data hours that were previously classified as calm and thus eliminated from the model calculations. In the current study, all three plants will be modeled for H2S using the three Low_Wind options. The results of the study compared the H2S impacts from the various WWTP sources under the Low_Wind options. The effects of the Low_Wind options on the offsite H2S impacts from the three WWTPs were described, and the implications of the options with respect to potential compliance with odor standards and criteria discussed.