CFD Predictions in Large Mechanically Aerated Lagoons

Optimization of the design and operation of large (>500,000 m3) mechanically aerated wastewater treatment lagoons (known as Aerated Stabilization Basins or ASBs) requires a thorough understanding of both the hydraulic and biological processes within the treatment volume. Complex three-dimensional flows existing within these lagoons are difficult to quantify by measurement or simplified analyses, but are well suited to three-dimensional Computational Fluid Dynamics (CFD) modeling. Coupling a set of biological reaction equations to the CFD flow solver allows prediction of the three-dimensional evolution of biological processes throughout the lagoon.This paper describes a comprehensive numerical framework for predicting the flow and performance in large lagoons equipped with high-speed mechanical surface aerators. The method is based on three-dimensional CFD and can account for operational characteristics such as ASB geometry, number/location/power of mechanical aerators, sludge accumulation, internal baffles, temperature, influent flow rate, and influent concentrations of pollutants and nutrients. Prediction of ASB hydraulics includes the development of Residence Time Distribution (RTD) curves directly from the predicted three-dimensional flow field, supplanting the need for resource-intensive field dye studies. A simplified biological reaction model, based on an adaptation of the DYLAMO program developed at Weyerhaeuser for determination of ASB performance, demonstrates the complete methodology. Results for application to a full-scale industrial ASB show much promise for the method. The methodology can be readily applied to other wastewater processes, and the results allow wastewater engineers to combine their existing knowledge and expertise with the established power of CFD.