When tasked with meeting very low nutrient limits, it is often the variable nature of the effluent ammonia that causes difficulties in meeting effluent total nitrogen goals. The specific growth rate of the nitrifying organisms is a key parameter for determining the required solids retention time and consequent reactor sizing for meeting effluent ammonia goals. One key parameter influencing the specific growth rate is the bulk dissolved oxygen (DO) in the liquid. The specific growth rate is adjusted for DO by using half saturation oxygen coefficients for both the ammonia oxidizing and the nitrite oxidizing bacteria. Because of the influence of the effects of mass-transport resistance and the competition between heterotrophic and autotrophic bacteria within the activated sludge floc, operating parameters such as SRT influence the relationship between DO and nitrification rate. This study had the objective of determining the proper values of the half saturation oxygen coefficients for the Greeley WPCF, using a specific stress testing campaign and the Biowin 3™ simulator. It was found that the Biowin 3™ default values are too low, and that these parameters need to be increased for accurate predictions of effluent ammonia, nitrate and nitrite concentrations. Assignment of the proper values is essential for accurate determinations of the required operating DO concentrations, air quantities and the operating SRT. Applying the parameters found in this study to another plant showed that the adjusted parameters also provided excellent closeness of fit to measured plant data.