WASTEWATER ODOR MODELING TO PRIORITIZE PROCESS ODOR CONTROL AND SUPPORT DEVELOPMENT BUFFER ZONE

The residential and commercial development in the vicinity of the Drake and Mulberry Water Reclamation Facilities (DWRF, MWRF, Facilities) in Fort Collins, Colorado warranted an evaluation of odor impacts on the surrounding communities. The City of Fort Collins recently adopted a 1,000-foot development buffer zone in the local land use zoning for the area surrounding the wastewater treatment Facilities. The inclusion of buffer zone in the zoning laws allows for a more reasonable approach to control odors (as opposed to treating the majority of the odor sources). Air dispersion modeling was performed utilizing the United States Environmental Protection Agency approved Industrial Source Complex Short Term (ISCST3) model to determine the overall odor impact of the Facilities with the current odor control measures. The modeling results were utilized to rank the individual plant processes' contribution to the odor impact for an odor control mitigation plan and to justify the 1,000-foot development buffer zone. The modeling results aid in prioritizing which plant process odors should be mitigated to result in reducing the overall Facility impacts on the surrounding community beyond the buffer zone. The plant processes were ranked according to odor magnitude, extent, and frequency.An extensive sampling and testing program was performed to provide the odor emission data necessary for the dispersion modeling. To account for seasonal variations in wastewater generation and characteristics, winter and summer odor sampling of Facility processes was conducted. Odor threshold testing was performed by St. Croix Sensory, Inc., using an AC'CENT Dynamic Dilution Triangle Olfactomer and the EN13725 odor testing standard. The odors were quantified utilizing units of Detection Threshold (DT), which is an estimate of the number of dilutions necessary to make the odor source detectable by a receptor (i.e. individual in the community). The DT's assigned to each Facility process from the sampling were combined with known or calculated air flow rates to determine process emission rates for the ISCST3 model.The ISCST3 model incorporated the facility odor sampling results, plant physical configuration, local meteorological conditions, and the local topography. Isopleth maps were generated to display the extent of the odor impact area for all of the Facility process emissions. The Colorado Air Quality Control Commission does not have an odor regulation that specifically addresses municipal wastewater treatment facilities; however, for the purposes of this analysis, a 1-hour result greater than or equal to a 2 DT impact represents the odor nuisance level. Therefore, the 2 DT threshold line for each significant odor source was generated to determine an odor control strategy to reduce plant impacts to 2 DT or less at the 1,000-foot Facility buffer zone.The frequency of nuisance-level receptor odor impacts (odor concentrations greater than or equal to 2 DT) was also evaluated as an element of the odor source ranking process. A “MAX” output file was generated from the ISCST3 model and imported into a Microsoft Access© database to count the number of hours a receptor experienced nuisance-level odors. Additional isopleth maps were generated to display the number of hours the surrounding community was experiencing nuisance-level odors.