A wastewater treatment process simulator was used as a design and planning tool at the 950,000 m3/d Metropolitan Wastewater Treatment Plant (Metro Plant) serving the Twin Cities of Minneapolis and St. Paul to prepare for the imposition of treated effluent phosphorus limits. The simulator was calibrated and verified on the existing 14-point step feed activated sludge system at the Metro Plant. The calibrated simulator was used to design a biological phosphorus removal system that meets a 1.0 mg/L total phosphorus limit, achieves seasonal nitrification, and also satisfies the plant operating staff’s preference to maintain nitrifying organisms in the system year-round. Use of the simulator enabled the design of a phosphorus removal and nitrification process within the existing tankage that meets future flow and load scenarios until the year 2020 but avoids the need for a major capital expansion of the plant that would encroach on the adjacent wetlands.