Many wastewater treatment plants are being retrofitted to meet more stringent effluent requirements. One of the process tools being implemented is selector technology, which can improve settling characteristics, energy consumption and the overall robustness of the process. When the City of El Paso was required to upgrade its Haskell R. Street Wastewater Treatment Plant, the decision was made to construct new aerobic tankage while retrofitting existing pure oxygen basins into an anoxic selector. Although this arrangement maximized use of existing tankage, splitting the process presented both engineering and operational challenges. Plant staff has responded to those challenges by operating the plant in several different modes as the plant is optimized. This has resulted in the plant being operated with parameters that differ from the original design values. This paper reviews how plant operations have responded to the challenges of the split anoxic/aerobic system. In addition, the paper reviews the operational implications of the various process modes and in particular, operational cost related to energy consumption and dewatered sludge hauling.