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ODOR CONTROL STUDY 4 . 1 Model Limitations The results of this model should be considered an illustrative tool and a relative indicator of off-site odor impacts . Various assumptions were not included in this model , including building effects , terrain of the surrounding area , etc. In addition , the model was developed for worst case conditions and does not account for days without detectable wind speed . Therefore , the results should not be used as an absolute indication of off. site sulfide concentrations . Instead , the model results offer evidence of the potential effectiveness of alternative control measures . 4.2 Odor Detection Thresholds The thresholds for odorous sulfide compounds vary depending on the compound , the source , and the sensitivity of the receptors . Although much of the data is subjective , receptor recognition of H2S and organic sulfur compounds is generally about 1 ppbv. Significant research has been performed to estimate detection and complaint thresholds for H2S . Figure 13 depicts the odor threshold sensitivity for H2S . This figure shows the " Odor Impact Model " developed by the Ontario Ministry of the Environment and published in the WEF Manual of Practice No . 22 , Odor Control in Wastewater Treatment Facilities . The three curves shown in the figure show human responses to certain concentrations of H2S . The detection curve indicates the percentage of the panelists that could detect an odor that is faint, but unrecognizable . The complaint and annoyance curves reflect the percentage of panelists that might register a complaint or become annoyed by the specific concentration of H2-S . Other references -cite odor recognition threshold values , which are in between the detection and complaint curves . Odor recognition is when someone can detect an odor and recognize the character of the odor. Even though the H2S model may be more qualitative than quantitative , it shows that almost no panel members detected H2S at- a concentration below 1 ppbv. Similar response levels are expected for organic sulfur compounds such as dimethyl sulfide , mercaptans , and other organic sulfur compounds . Thus , 1 ppbv of H2S is considered a reasonable worse case odor detection threshold and 5 ppbv is considered a reasonable odor recognition threshold . The dispersion model was therefore calibrated to identify the location of these concentrations under worse case meteorological conditions . To visualize and prioritize potential odor control improvements , separate models were developed for several different improvement options . These options correspond to the improvements discussed above . Conservative control efficiencies were assumed for the major odor sources based on efficiencies generally achieved with installation of odor control measures. The conceptual improvements proposed and the potential reduction of odor emissions for each of the options are provided below: Existing Conditions with Filtrate Equalization Basin Emissions (Figure 14) and Aerobic Digester Emissions (April 2002) •J 24 Indian River County, Central Wastewater Treatment Facility 1 PBS&J #071230, February 2003