A comprehensive capacity assessment was conducted of the liquid treatment train at the Western Lake Superior Sanitary District’s (District) wastewater treatment plant. The plant incorporates a high purity oxygen activated sludge system. The assessment included detailed wastewater characterization to calibrate activated sludge simulators, mixed liquor settling and flocculation testing, and secondary clarifier stress testing. The capacity assessment identified simple, low cost upgrades that would increase plant capacity, reduce power consumption, and greatly increase operational flexibility of taking aeration tanks and secondary clarifiers off-line. The recommended modifications to the HPO aeration system were implemented to eliminate the plant capacity constraint that existed in the secondary system. The surface aerator drive horsepower in the first stage of each of the four HPO trains was increased from 100 to 125 hp and the pitched blade design of the surface aerator was replaced with an offset vertical blade impeller design. The result of this retrofit was to increase the oxygen transferred into the first stage such that it would operate with a higher dissolved oxygen concentration than before. The level of oxygen transfer improvement also allowed the plant staff to take down one of the four trains for maintenance and repair, an event that was impossible to implement in the past. Lastly, plant staff were afforded a level of flexibility that allowed them to optimize operational control and energy savings.