Advanced Monitoring and Sewer Process Modeling for Odor Control

At the outset, Hite Creek Water Quality Treatment Center (WQTC) serving the Louisville Metropolitan Sewer District faced significant uncertainty regarding the sources of persistent odor complaints. Potential contributors included emissions from on-site treatment processes, fugitive releases at force main and gravity interceptor discharge points along the plant fenceline, and other unidentified sources. WQTC receives flow from five force mains and one 30' gravity trunk sewer. Two force mains with long detention times were suspected to be major contributors to elevated sulfide levels and associated odor at their discharge locations. This study was undertaken to identify the root causes of odor generation using advanced, continuous monitoring technologies to quantify sulfide in the collection system and at the plant influent. The field findings were used to develop a calibrated Mega-WATS sewer process model to optimize chemical dosing strategies, minimizing sulfide loads entering the plant and reducing fugitive emissions. The observed benefits and challenges associated with deploying advanced SulfiLogger technology for continuous measurement of dissolved sulfide at ten monitoring locations across the system are also discussed. Fieldwork included grab sampling and continuous measurements for key liquid and vapor phase parameters at selected locations in the collection system. Dissolved sulfide field measurements were collected using a combination of grab sampling field test kits (LaMotte and Gastec) and advanced continuous monitoring technology (SulfiLogger). At several locations when multiple sampling methods were used, average and maximum dissolved sulfide data recorded by the SulfiLogger differed significantly from measurements obtained through grab sampling. Grab samples capture sulfide concentrations only at a single moment in time, whereas the SulfiLogger continuously records data and reveals the full diurnal pattern. Grab sampling measurements also require manual handling of samples, which can introduce additional uncertainty to dissolved sulfide measurements that are already highly sensitive to rapid oxidation and turbulence. In addition, colorimetric nature of analysis kits relies on visual interpretation, making the results subject to the eyesight of the operator. While the SulfiLogger overcomes many limitations of grab sampling by providing continuous, high-frequency measurements that capture diurnal sulfide patterns, a few challenges were encountered during the study. The SulfiLogger system requires stainless steel accessories, which increases mobilization, transport, storage, and assembly time compared to grab sampling. The stainless-steel accessories were not always compatible with manhole covers or wet well hatches, limiting the ability to properly stabilize the unit. Poor stabilization allowed wastewater forces to buffet the device during deployment, degrading data quality and resulting in equipment damage. At the Glen Oaks and Chamberlain force main discharges (Figure 1) near the plant fenceline, stabilization was not possible, and turbulent drop conditions caused rapid sulfide volatilization. Repeated impacts due to wastewater force caused extensive wear to devices, rendering two sensors nonfunctional. Also, SulfiLogger data is usable only when the sensor remains fully submerged. At deep or cyclic discharge locations, periods of air exposure required removal using engineering judgment based on temperature trends. Additionally, the SulfiLogger measures aqueous H₂S. Total dissolved sulfide, bisulfide (HS⁻) and sulfide ion (S²⁻) are derived using simultaneous pH measurements, Henry's law, and acid dissociation constants for hydrogen sulfide. Post-processing to convert raw H₂S data to total dissolved sulfide was performed using the licensed pHSulfi tool operating on the Mega-WATS platform. Depending on pH variability within the system, the calculated total dissolved sulfide concentrations were observed to be substantially higher than the measured dissolved H₂S alone. Because H₂S is the dominant sulfide species mainly at pH values below 7, studies that rely on base addition to shift sulfide speciation and mitigate odors are limited when evaluated using SulfiLogger data alone. Even without base chemical dosing, when pH exceeds 8 there is very little dissolved H₂S; therefore, total dissolved sulfide estimates derived from SulfiLogger measurements are subject to reduced accuracy. The paper evaluates fugitive emissions from gravity sewer manholes and describes development of calibrated Mega-WATS sewer process models for five force mains using field data. The evaluation also includes multiple chemical dosing strategies and what-if scenarios to identify an optimized dosing approach to mitigate sulfide formation within the force mains and at subsequent discharge locations along the plant fenceline to reduce odors in the vicinity of the plant. Results from the collection system field work and sewer process modeling conducted in this study were used in conjunction with a separate, parallel air dispersion modeling effort to develop odor management recommendations aimed at reducing impacts to the surrounding neighborhood.