The focus on this technical session will be on a newly constructed biotrickling filter odor control system at a combined (storm + sanitary) 160 million gallons per day (MGD) pump station. The newly constructed odor control system replaces an existing carbon adsorber odor control system and provides treatment of odorous air from multiple locations, including a flow splitting structure where water falls approximately 10 feet over a weir. The new odor control system capacity was designed for influent hydrogen sulfide (H2S) loadings obtained through multiple sampling efforts during the design phase. Following startup in the spring season, changes were made in the collection system to divert flow from this pump station to alleviate capacity issues downstream due to another ongoing project. Approximately 20 MGD was diverted from this pump station and was treated by another wastewater treatment plant. Average inlet H2S concentrations during the design phase were calculated to be around 10 parts per million (ppm) and in the summer following the flow diversion, the average inlet H2S concentrations were recorded consistently above 100 ppm. The presentation will include discussion on the troubleshooting steps taken and the evaluation process to determine the mitigation measures aimed at reducing the liquid and vapor phase H2S influent to the pump station. Notably, the inverted egg-shaped influent combined sewer is the largest in the collection system, with a carrying capacity of nearly 3,500 cubic feet per second (CFS) at the pump station, or approximately 2,270 MGD. Typical dry weather flows, when H2S concentrations are generally greatest, are around 40 MGD. The higher H2S levels were thought to be due to lower flow velocities and increased travel time in the sewer caused by the flow diversion, which promoted greater H2S production within the wastewater. While the empty bed residence time was not an issue with the higher H2S loadings, system performance lagged. The team coordinated with the equipment manufacturer to fine-tune water cycle and recirculation cycle frequencies and durations and also re-seeded the vessel with return activated sludge (RAS) in an effort to “reset” the biology with the higher inlet concentrations. Despite these efforts, achieving the required 99% H2S removal remained elusive. To support our hypotheses regarding the higher than expected H2S levels, the team conducted process modeling of the upstream collection system to identify solutions for reducing H2S concentrations at the pump station. The modeling considered the increased wastewater travel time due to the flow diversion and evaluated the impact of industrial dischargers on sulfide generation within the sewer. Following the modeling effort, mitigation alternatives were assessed, and an optimized approach was developed to integrate with the existing odor control system, thereby improving biotower performance.