Computer simulation of activated sludge systems can be an invaluable tool for the design and optimization of single sludge biological nutrient removal processes for wastewater treatment. BIOSIM (Envirosim, undated) is a computer model designed to be run in a DOS environment on the personal computer. It provides the ability to model activated sludge process performance with various reactor configurations, modes of operation and wastewater characteristics, and includes the ability to model biological excess phosphorus removal. When applied to the City of Atlanta’s Volatile Fatty Acid Induced Phosphorus Removal (VIPR) Pilot Plant data, BIOSIM provided good predictions of system performance. Calibration was achieved with relatively minor changes to a small number of parameters based on ordinary operating data. A few key parameters should be evaluated for site specific application. Use of the flocCODsol test made determination of readily biodegradable substrate relatively simple and far less resource intensive that more sophisticated biological methods.