When properly designed, hypolimnetic aeration and oxygenation systems can replenish dissolved oxygen in water bodies while preserving stratification. The three primary devices are the airlift aerator, Speece Cone, and bubble-plume diffuser. In each device, gas bubbles in contact with water facilitate interfacial transfer of oxygen, nitrogen, and other soluble gases. However, early design procedures for airlift aerators were empirical, while most bubble-plume models did not account for stratification or gas transfer. Using fundamental principles, a discrete-bubble model was first developed to predict plume dynamics and gas transfer for a circular bubble-plume diffuser. The discrete-bubble approach has subsequently been validated using oxygen transfer tests in a large vertical tank and applied successfully at full-scale to an airlift aerator as well as to both circular and linear bubble-plume diffusers. The performance of each of the four completely different full-scale systems (on a scale of 10 m or more) was predicted based on the behavior of individual bubbles (on a scale of about 1 mm). The combined results suggest thatthe models can be used with some confidence to predict system performance based on applied air or oxygen flow rate, initial bubble size, and, in the case of bubble plume diffusers, near-field boundary conditions. The discrete-bubble approach has also been extended to the Speece Cone, but the model has not yet been validated due to a lack of suitable data. The unified suite of models, all based on simple discrete-bubble dynamics, represents the current state-of-the-art for designing systems to add oxygen to stratified lakes and reservoirs.

[1] An existing linear bubble plume model was improved, and data collected from a full-scale diffuser installed in Spring Hollow Reservoir, Virginia, were used to validate the model. The depth of maximum plume rise was simulated well for two of the three diffuser tests. Temperature predictions deviated from measured profiles near the maximum plume rise height, but predicted dissolved oxygen profiles compared very well with observations. A sensitivity analysis was performed. The gas flow rate had the greatest effect on predicted plume rise height and induced water flow rate, both of which were directly proportional to gas flow rate. Oxygen transfer within the hypolimnion was independent of all parameters except initial bubble radius and was inversely proportional for radii greater than approximately 1 mm. The results of this work suggest that plume dynamics and oxygen transfer can successfully be predicted for linear bubble plumes using the discrete-bubble approach.Citation: Singleton, V. L., P. Gantzer, and J. C. Little (2007), Linear bubble plume model for hypolimnetic oxygenation: Full-scale validation and sensitivity analysis, Water Resour. Res., 43, W02405,

[1] A model for a linear bubble plume used for hypolimnetic oxygenation was coupled with a three-dimensional hydrodynamic model to simulate the complex interaction between bubble plumes and the large-scale processes of transport and mixing. The coupled model accurately simulated the evolution of dissolved oxygen (DO) and temperature fields that occurred during two full-scale diffuser tests in a water supply reservoir. The prediction of asymmetric circulation cells laterally and longitudinally on both sides of the linear diffuser was due to the uneven reservoir bathymetry. Simulation of diffuser operation resulted in baroclinic pressure gradients, which caused vertical oscillations above the hypolimnion and contributed to distribution of plume detrainment upstream and downstream of the diffuser. On the basis of a first-order variance analysis, the largest source of uncertainty for both predicted DO and temperature was the model bathymetry, which accounted for about 90% of the overall uncertainty. Because the oxygen addition rate was 4 times the sediment oxygen uptake (SOU) rate, DO predictions were not sensitive to SOU. In addition to bathymetry, the momentum assigned to plume entrainment and detrainment is a significant source of uncertainty in the coupled model structure and appreciably affects the predicted intensity of mixing and lake circulation. For baseline runs, the entrainment and detrainment velocities were assumed to be half of the velocities through the flux face of the grid cells. Additional research on appropriate values of the plume detrainment momentum for the coupled model is required.

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