The objective of this study was to investigate what effect the concentration of microorganisms would have on substrate removal, microbial substrate storage, and oxygen utilization at a constant food-to-microorganism ratio. Batch experiments were conducted, under aerated and completely mixed conditions, using a domestic wastewater, a paper mill wastewater, and a food processing wastewater. A series of three batch experiments were run for each of these wastes. The food-to-microorganism ratio for each series was kept constant while the mixed liquor suspended solids concentration was varied for each of the experiments within the series. The following analyses were conducted on samples that were withdrawn at specified time intervals: filtered and settled COD, oxygen uptake, mixed liquor volatile suspended solids (MLVSS), pH, protein concentration and carbohydrate concentration. No significant uptake and subsequent release of organic substrate was observed for any of the wastewaters studied. For the same F/M ratio, the rate of removal of organic substrate and the degree to which it was removed in the activated sludge system was found to be a direct function of the MLVSS concentration. The change in the cellular carbohydrate to cellular protein ratio in the activated sludge during substrate metabolism was a function of the MLVSS concentration. As the MLVSS concentration increased, the carbohydrate to protein ratio, which is an indicator of substrate storage, also increased, even though the F/M ratio was held constant. / Master of Science