The objective of this research was to characterize the performance of hypolimnetic aerators with respect to oxygen transfer and water flow rate to allow the development of two comprehensive process models. The oxygen transfer model is the first model that applies discrete-bubble principles to a hypolimnetic aerator, and the water flow rate model is the first that applies an energy balance to this particular type of lake aeration device. Both models use fundamental principles to predict hypolimnetic aerator performance, as opposed to empirical correlations. The models were verified with data collected from a full-scale hypolimnetic aerator installed in Lake Prince, which is a water supply reservoir for the City of Norfolk, Virginia. Water flow rate, gas-phase holdup and dissolved oxygen profiles were measured as a function of air flow rate. The initial bubble size was calculated by the oxygen transfer model using field data. The range of bubble diameters obtained using the model was 2.3-3.1 mm. The Sauter mean diameters of bubbles measured in a laboratory system ranged from 2.7-3.9 mm. The riser and downcomer DO profiles and gas holdups predicted by the model are in close agreement with experimental results. The water flow rate model was fitted to the experimental water velocity by varying the frictional loss coefficient for the air-water separator. An empirical correlation that predicts the loss coefficient as a function of superficial water velocity was obtained. The results of the correlation were similar to those predicted by literature equations developed for external airlift bioreactors. / Master of Science

The purpose of this research was to apply a discrete-bubble model to predict the performance of several hypolimnetic oxygenators. The model is used to predict the oxygen transfer rate in a hypolimnetic oxygenator based on the initial bubble size formed at the diffuser. The discrete-bubble model is based on fundamental principles, and therefore could also be applied to other mass transfer applications involving the injection of bubbles into a fluid. The discrete-bubble model has been applied to a linear bubble-plume diffuser, a full-lift hypolimnetic aerator and the Speece Cone with promising results. The first step in this research was to investigate the principals of bubble formation at a submerged orifice, bubble rise velocity and bubble mass transfer. The discrete-bubble model is then presented. The model traces a single bubble rising through a fluid, accounting for changes in bubble size due to mass transfer, temperature and hydrostatic pressure. The bubble rise velocity and mass transfer coefficients are given by empirical correlations that depend on the bubble size. Bubble size is therefore recalculated at every increment and the values for the bubble rise velocity and mass transfer coefficients are continually updated. The discrete-bubble model is verified by comparison to experimental data collected in large-scale oxygen transfer tests. Finally, the discrete-bubble model is applied to the three most common hypolimnetic oxygenation systems: the Speece Cone, the bubble-plume diffuser, and the full-lift hypolimnetic oxygenation systems. The latter being presented by Vickie Burris in her thesis, <i>Hypolimnetic Aerators: Predicting Oxygen Transfer and Water Flow Rate</i>. / Master of Science