Sewer Force Main Odor Control and Surge Analysis: A Case Study for Optimum Design without Compromises

Gwinnett County Department of Public Utilities (GCDPU) retained Hazen and Sawyer (H&S) to conduct an odor control study along the 36-inch force main from the Alcovy River Pump Station to the F. Wayne Hill Water Resources Center (WRC). Upon the completion of the initial investigation, it was concluded that the odors released from the sewer force main were significantly influenced by the force main hydraulics; specifically surging. Surging was attributed to the current operation of the Alcovy River Pump Station. A surge analysis was required to minimize or eliminate foul air release while continuing to provide surge protection for the sewer infrastructure. The issue at hand was how to mitigate foul air being released while continuing to provide surge protection for the sewer infrastructure. Odor control and surge analysis are traditionally evaluated independently because sewer collection systems are generally characterized as gravity flows with limited pressure flows. However, in this case, the force main was approximately 10 miles long.The challenge here was how to concurrently attenuate surge pressure and reduce odor through air release valves that were originally part of the surge protection system. The goal was to optimize surge protection without requiring construction of costly new odor control facilities to treat foul air. This paper will discuss the modeling simulations, recommended improvements, and present both capital and Operation and Maintenance (O&M) cost resulting in significant savings to GCDPU.A surge model was developed by taking an existing hydraulic model from WaterCAD™ and converting it to KYPIPE SURGE2006™ (University of Kentucky). Different alternatives were evaluated with the focus on maintaining a full pressure pipe while the pumps were not in operation, thereby minimizing air pockets and foul air being released through the air release valves when the pumps start or stop. Based on the model results it was recommended that a 5,300-gallon sewer-service type of bladder tank and two control valves would be needed to mitigate the foul air volume, and hence alleviate the odor problem. The bladder tank dissipated the surge within the system, while the control valves maintained a full pipe when the pumps cycled off. The project goal was achieved – the elimination of chemical treatment for odor control and the protection of the pump station and force main systems from surge.