This paper demonstrates how the effective capacity of each unit process in a wastewater treatment plant can be determined using appropriate process simulation models. One of the most important prerequisites for reliable simulation of plant performance is accurate calibration of the models using historical plant performance data and by judicious sampling for selected parameters. The plant capacity analysis was conducted at the Central Valley WRF in Utah. Liquid stream unit processes modeled included primary clarifiers, trickling filters, activated sludge tanks, and secondary clarifiers. Models used included the Trifil2 and TFO2 trickling filter models, the Brown and Caldwell NTF-BCTM for nitrification in trickling filters, the BioWinTM activated sludge simulation model, and the Brown and Caldwell State Point Analysis program for secondary clarifiers.