Hypolimnetic Aerators: Predicting Oxygen Transfer and Water Flow Rate

Abstract

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.