CFD predicitons 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 mperformance 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.

Wastewater treatment in industries such as Pulp and Paper is commonly performed in large (>500,000 m3) aerated lagoons known as Aerated Stabilization Basins (ASBs). Treatment in ASBs involves removing dissolved organic matter (quantified as Biological Oxygen Demand or BOD) through biological conversion to solids by synthesis of bacterial cells. Oxygen demand for the conversion process is met by introducing a number of high-speed mechanical aerators. The aerators also provide hydraulic mixing, necessary to maintain partial suspension of bacterial solids and ensure adequate contact with organic pollutants. In addition to organic matter and dissolved oxygen, microbiological growth requires nutrients such as nitrogen and phosphorus. Untreated wastewater from bleached Kraft pulping processes is often deficient in these nutrients, therefore supplements are added to ensure BOD removal meets target levels. Mahmood and Paice (2006) give a good overview of ASB design and operating practices in the Canadian Pulp and Paper Industry.

The discharge concentrations of BOD, bacterial solids, and nutrients are complex functions of the biological processes of growth, death, settling, digestion, and feedback that occur within the ASB. The magnitudes of these processes are dependent on local concentrations of BOD, bacteria, and nutrients within the ASB, which are in turn governed by the local flow velocities.

These flow velocities vary widely throughout the ASB from several meters per second near the aerators to near quiescence in other areas. The local flow velocities are functions of the ASB basin design, aerator numbers, positions, and horsepower, sludge accumulation, and flow baffling. To increase treatment performance aerators and baffles should be strategically placed to ensure a partially mixed flow regime that supports both biological reactions and staged solids settling. Both a locally detailed and globally holistic understanding of the flow and reactions in the ASB is key to reducing pollutant levels in existing systems and to designing new and more efficient pollution control systems.

Traditional analysis of ASB performance starts with the hydraulics, notably the development of Residence Time Distribution (RTD) curves. These curves provide general flow mixing and retention characteristics that are useful in analysis of the biological reactions. RTD curves are obtained experimentally in the field, a process that is intensive and time-consuming. Simple one-dimensional hydraulic models such as extended tank-in-series models (Levenspiel, 1999) are developed and parameters adjusted to best replicate the experimental RTD curve. Reaction rate equations are coupled to the hydraulic model to analyze/predict treatment performance (Sackellares et al., 1987, Barkley & Sackellares, 1987, and NCASI, 1985).

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