A Multi-Faceted Approach to Odor Control for a 28.5-Mile Long Force Main

Upon startup of the 16 km (10-mile) long, 91.4 cm (36 inch) diameter force main between the Alcovy River Pump Station (PS) and the F. Wayne Hill Water Reclamation Facility (FWHWRC), Gwinnett County began receiving odor complaints along the pipeline. A preliminary odor investigation of the system indicated that while sulfide concentrations were increasing through the length of the force main and the existing odor control chemical feed was ineffective, there were indications that significant surges were occurring within the pipeline. The following hydraulic and surge analysis showed that keeping the pipeline full during all operating scenarios could eliminate the foul air discharges at the problem air release valves (ARVs) and alleviate the odor complaints. A motor-operated control valve was installed at the end of the force main to modulate to induce additional head at low flows with the goal of keeping the pipeline full. A surge tank was also installed at the Alcovy River PS to minimize surging along the force main. This solution was extremely effective until the conditions of the system changed.In 2008 and 2009, Gwinnett County added flow from two additional constant speed pumping stations (Lower Big Haynes PS and Brooks Road PS) and two variable speed booster pumping stations (Brooks Road Booster PS and Alcovy River Booster PS) into the same force main. The entire length of the new system, known as the Eastside Conveyance System, was 45.9 km (28.5 miles) and included 153 possible operating scenarios. Within a few months of startup, odor complaints began to increase again. It was determined that the motor operated valves on the force main discharge were not able to keep up with the complexity of the system and could no longer prevent odor emissions from the air release valves. Several aspects of this study, combined together, were able to finally solve the odor problems on this force main:• Hydraulic/Surge Improvements – No feasible solution was determined through the surge modeling that eliminated odorous air discharges from the ARV locations. However, surge modeling identified one option as the most feasible and cost-effective solution: an elevated loop was installed at the FWHWRC to raise the discharge elevation of the force main by about 11.3 m (37 feet). This elevation minimized the amount of draining within the force main, but allowed the existing pumps to still be used. The elevated loop at the FWHWRC significantly cut down on air release volumes and pressures, but did not eliminate them entirely.• Liquid Phase Chemical Feed – Due to the long retention times in the system, a switch from Bioxide to magnesium hydroxide significantly reduced the H2S concentrations in the system. Determining the appropriate odor control chemical and chemical feed rates allowed the County to utilize activated carbon adsorption at the remote ARV sites, which would not have been practical at the prior H2S concentrations.• Foul Air Capture and Treatment - A vapor phase odor control system was provided at each pump station wet well to minimize odor complaints near the stations. Prior to construction being completed, two emergency odor control systems were also required: one at a pump station and one at an ARV along the force main. One emergency odor control system was fabricated by an on-call Contractor, under the Engineer’s direction, on a Friday afternoon, using materials at hand, to provide relief to the homeowners for the weekend.Vapor phase treatment was not initially feasible at several ARV locations. For instance, at one remote ARV location, the air release volume and pressure was so high, it caused the carbon to be blown out of the vessel on multiple occasions. In addition, the County had to change out the carbon on a weekly basis to prevent breakthrough, which was not sustainable. The surge improvements reduced the airflow and pressure from the ARV locations, while the chemical feed improvements reduced the H2S concentrations. These combined improvements allowed activated carbon to be used for odor treatment at the remote ARV locations.Effective and sustainable odor control along the Eastside Conveyance System would not have been feasible without the implementation of all facets of this project. Addressing the odor issues in this system required a truly multi-faceted approach.