The City of Boulder converted their existing TF/SC process to a 25 mgd BNR activated sludge plant. Initial construction required removing 31-percent of the trickling filter capacity while still meeting effluent permit limits during construction. The TF/SC process was operated to develop nitrifying organisms to serve as seed for startup of the new activated sludge system. This made importation of seed sludge unnecessary. Monitoring of the secondary clarifiers was important to avoid excessive solids loading rates. Full nitrification was achieved within approximately one week after startup. A calibrated Biowin™ simulator was used for design as well as to closely predict startup and operating conditions. Dynamic simulations were used to define stable operating SRTs at each mixed liquor temperature. The value of using the operational tools developed for this project was demonstrated by the results during startup and the first year of operation.