Effect of Aerator Size on the Performance of Activated Sludge Process at Outdoor Temperatures

Three activated sludge reactors of different sizes and designs, one in pilot-plant-scale of 7,000 gallon liquid volume, and two in benchscales of 7.5 gallon and 1.2 gallon liquid volumes, respectively, were used to evaluate the scale-up effects on the process at outdoor temperatures. The reactors were operated under equivalent conditions by regulating the mean cell residence time, mean hydraulic detention time, and biomass concentrations in their reactors so that these operating factors were respectively of the same values. The reactors were, installed outdoors, so that they were exposed to the same ambient temperatures ranging from 5 C to 13 C through the winter months, 1976-1977. In Phase I of the study, the flow regimes prevailing in the reactors were not of the same degree of mixing. While Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) removals were practically the same in the two bench-scale reactors, there was a small but statistically significant difference in BOO and COD removals between the bench-scale and pilot-scale reactors. However, the difference could be attributed to other than scaleup effects. As for NH--N removal, nitrification and total-phosphorus removal, there were no differences among the reactors. The data were further analyzed by using current kinetic models, and again there were practically no differences in the performance of the three reactors. Therefore, a bench-scale reactor, as small as 1.2 gallon, could be applied to generate treatability information as reliable as a pilot-scale reactor having a liquid volume of 7,000 gallons. In Phase II of the study, the results indicate that the modifications in operation intended to bring about the complete-mixing flow regime in all reactors did not alter the similarity in performance of the reactors as depicted in Phase I. This phase of the study also revealed the fact that there were neither biochemical reactions involving nitrification and total-phosphorus removal, nor significant carbonaceous bio-oxidation taking place in the final clarifier. Seemingly the assumption of no substrate removal by biochemical reactions in the final clarifiers, commonly made in developing the kinetic models for activated sludge processes, is reasonable. TABLE OF CONTENTS Page Title Page i Acknowledgements " Abstract iii Table of