Improved analysis and modeling of the oxygen mass transfer process in aeration systems

This study has developed two new separate and distinct oxygen mass transfer models for the evaluation and design of diffused/subsurface aeration systems and mechanical surface aerators. In contrast to currently used models, these models are based on a rigorous fundamental theoretical analysis of the oxygen mass transfer processes which occur in these very different types of aeration systems. The new diffused/subsurface aeration model separately accounts for subsurface mass transfer from a dispersed gas to a bulk liquid, as well as mass transfer occurring at a turbulent liquid surface in contact with a continuous gas phase. This model incorporates a very important surface reaeration oxygen mass transfer coefficient term into the overall oxygen mass balance. This new term does not appear in any previously used models. The new surface aerator model quantitatively separates the oxygen transfer process into a "liquid spray" mass transfer zone and a "surface reaeration" mass transfer zone, which has never previously been accomplished. Both of these models are able to fit experimental unsteady-state performance data as well as currently used models, even though the new models are more stringent and rigorously defined. The new models also provide for the first time the necessary theoretical framework for the prediction of the impact of changing environmental or process design conditions on the performance of these aeration systems. These new models represent a significant advancement and improvement in the evaluation, analysis, and design of diffused/subsurface and mechanical surface aeration systems.; This study also includes an experimental and theoretical evaluation of the performance of diffused/subsurface aeration systems under simulated high microbial solids levels conditions, such as those encountered in thermophilic aerobic digestion systems, which commonly employ high-purity oxygen as the aeration gas. A bentonite clay synthetic sludge mixture was used to simulate the hydrodynamic conditions that would be expected in a thermophilic aerobic digestion system operating at 50,000 mg/L of suspended solids. The effect of the high solids levels on aeration system performance was experimentally measured over a range of aeration system sizes, including a 13-liter bench-top unit, a 160-gallon pilot scale unit, and a large-scale 3600-gallon unit. It was found that the high solids level synthetic sludge mixture reduced the oxygen transfer rate 50-60 percent, compared to clean tap water under a wide range of operating conditions.