ODOR MODELING: CASE STUDY SHOWS COST SAVINGS AND FIELD VERIFIED RESULTS

Project Overview In response to odor complaints received from residents near the 19.5 million gallon per day (MGD) West County Wastewater Treatment Plant (WCWTP), the Louisville and Jefferson County Metropolitan Sewer District (MSD) initiated a program to cost effectively control odors at this plant. The program included air dispersion modeling to predict the impacts of potential odor control improvements, and an extensive community participation program to involve the affected residents in the problem identification, model evaluation, and selection of the control approach. Using air dispersion models calibrated by field observations, MSD identified a 2- phased approach to resolving odor issues. Field verification after completion of the first phase of improvements validated the model predictions, confirming that off-site odors were virtually eliminated with the Phase 1 improvements. This phased approach was successful in resolving community concerns regarding off-site odors and saved MSD nearly 4 million in the process, by eliminating the need for the second phase of improvements. This project is unique in its use of air dispersion modeling to select the odor control system and to verify the results afterwards with performance testing and community odor surveys. Goals and Objectives The goals established for the project were based on MSD's Facilities Buffer Zone Policy, which requires MSD to evaluate reasonable technical approaches to minimize detrimental off-site impacts of their facilities. The specific objectives of this project were to identify all the odor sources and to select the minimum-cost odor control systems that could reliably eliminate offensive off-site odors. To help ensure community acceptance, the project also included extensive public participation in the process, particularly in the area of identifying the problem through community odor surveys and plant tours. Identifying the Problem A comprehensive sampling and testing program was conducted to determine odor emission rates for the plant. In addition, community odor surveys (with residents' participation) were conducted to record odor character, to measure intensity and to identify the source of the odor. Odor testing revealed that the influent screen building (80%) and the activated sludge process (10%) were the highest sources of odor emissions from the plant site. The original intent, therefore, was to cover all the aeration tanks and treat all the air from the plant processes, at a cost of about 6 million. It was then decided to perform atmospheric dispersion odor modeling to predict the downwind odor concentrations measured in dilutions-to-threshold (D/T) from controlled and uncontrolled odor sources. The investigation of the alternative odor control strategies also included an examination of the frequency of any odor concentration greater than the established odor detection threshold DT of 7. Contour plots were prepared to view the number of times during the year the odors are above the 7 DT for short time periods. The Solution A modified 2-phased plan was prepared for odor control, which included covering the aerated influent channels to the activated sludge process and treating this air in a modular synthetic media biofilter and converting the aeration basins to fine bubble diffusers (which was already in the budget for energy saving reasons). The influent channels were found to be the most likely source of activated sludge odors detected off-site due to the mixing of raw sewage and return activated sludge. Other improvements included the installation of a 14,000 cfm carbon adsorber for the influent screen room, covers on the grit tanks and use of activated sludge diffusion to treat the collected air. The modeling results showed that with this modified plan the closest neighbors might occasionally notice a faint odor (between 7 and 24 DT), as compared to previous conditions when the model predicted as many as 500 occurrences per year with DT's as high as 80. On this basis, MSD decided to pursue this phased approach and test the odor control systems and conduct additional community surveys after the first phase was completed to see if the odor control systems then in-place were sufficient or if further odor control was required. The Benefits All the odor control systems were tested after start-up and the results showed an 87% reduction in overall odor emissions from the plant and a 97% reduction in hydrogen sulfide emissions. Treatment plant odors were never detected in the community surveys conducted following the odor control system startup and residents commented that they no longer detected the odor from the plant. The paper presents the odor levels tested prior to odor control, the odor levels assumed for the modeling and the odor levels actually tested in the performance testing of the equipment. The project clearly demonstrates the value of odor testing and modeling and the importance of follow up testing and monitoring to verify that the investment resulted in effective odor control. In this case, MSD saved 4 million and developed community acceptance with this approach.