Simultaneous biological nutrient removal (SBNR) is referred to as the biological process where nitrogen and phosphorus in excess of that required for biomass synthesis are removed in a biological wastewater treatment system without defined anaerobic or anoxic zones. Although SBNR systems offer several potential benefits, these cannot be fully realized until the factors controlling SBNR are better understood. A single reactor activated sludge pilot plant was operated with acetate as the only influent organic in synthetic wastewater. The system was operated at 20°C and an SRT of approximately 10 days, and DO was maintained in the range of 0.3 to 0.5 mg/L. Once the system stabilized to a steady state, influent ammonia was increased step-wise during the 90 day study period from 20 to 60 mg N/L after which influent COD was doubled from 300 to 600 mg/L. The system exhibited SND, essentially with complete nitrification and a uniform amount of denitrification. Near-complete denitrification was observed for an influent rbCOD:N ratio of greater than 10:1. Biological phosphorus removal also occurred in the system; effluent P was approximately 0.25 mg P/L with an influent concentration of 8 mg P/L. Batch tests were conducted with mixed liquor from the pilot plant spiked with initial COD, ammonia and phosphate concentrations approximately equal to the pilot plant influent concentrations. Results provided further insights into the factors controlling SBNR. Model growth rate equations for two-step nitrification and denitrification were useful for discussing the phenomena, together with calibrating a model to the experimental results.