The Integrated Film Activated Sludge Process (IFAS) was developed to reduce the cost of additional facilities required to complete year round nitrification in the design of new or retrofit wastewater treatment plants. The purpose of this project was to develop a computer-based mechanistic model, called IFAS, which can be used as a tool by scientists and engineers to optimize their designs and to troubleshoot a full-scale treatment plant. The program also can be employed to assist researchers conducting their studies of IFAS wastewater treatment processes. IFAS enables the steady-state simulation of nitrification-denitrification processes as well as carbonaceous removal in systems utilizing integrated media, but this current version supports only sponge type media. The IFAS program was developed by incorporating empirical equations for integrated biofilm carbonaceous uptake and nitrification developed by Sen and Randall (1995) into the general activated sludge model, developed by the International Association on Water Quality (IAWQ, previously known as IAWRC), plus the biological phosphorus removal model of Wentzel et al (1989). The calibration and evaluation of the IFAS model was performed using existing data from both an IFAS system and a conventional activated sludge bench-scale plant operated over a wide range of Aerobic Mean Cell Residence Times (Aerobic MCRT's). The model developed provides a good fit and a reasonable prediction of the experimental data for both the IFAS and the conventional pilot-scale systems. The phosphorus removal component of the model has not yet been calibrated because of insufficient data and the lack of adequately defined parameters. / Master of Science