CFD Modeling of Primary Clarifiers: The State-of-the-Art
- A. G. Griborio - Hazen and Sawyer
- J. A. McCorquodale - Dept. of Civil Engineering, University of New Orleans
- J. A. Rodriguez - Dept. of Environmental Engineering, Universidad del Valle, Cali, Colombia
In recent years the use of Computational Fluid Dynamics (CFD) for the evaluation and optimization of clarifiers has become a common practice among sanitary engineers in the wastewater field. However, most applications, including research and engineering practice, focus on the evaluation of secondary settling tanks (SSTs). When properly applied, CFD models can also be a powerful tool for the design and retrofit of primary settling tanks (PSTs). Understanding the similarities and differences between primaries and secondary tanks is key to success as is proper calibration and field characterization. The model setups are similar in secondary and primary tanks; however, the order of magnitude of some of the processes may differ greatly. For example, the removal process in primaries is dominated by discrete settling and/or floating particles with a very high fraction of non-settleable particles which limits the achievable TSS and BOD removals. In PSTs, this fraction can exceed 40% of the total influent concentration while in SSTs this fraction is very small. The determination of this fraction is critical to modeling PSTs.
There is very little zone settling in primaries but the compression parameters are very important. While the typical equations applied to simulate zone settling and compressibility in SSTs can also be applied to PSTs, the field determination of the settling coefficients is very important as these coefficients may significantly differ from typical values in secondary tanks. Flocculation occurs in both primary and secondary settling tanks. Typically, the flocculation is greater in secondary clarifiers because of the higher concentrations and the presence of exocelluar polymers in activated sludges. Nevertheless, flocculation does occur in primary clarifiers and should be included in the model setup. Density currents are another aspect of the flow that is different in primary and secondary clarifiers. SSTs, typically, have strong bottom density currents induced by the high MLSS in the secondary influent while PSTs have weak solids induced density currents. Other important differences to consider are the sludge blanket concentrations, the sludge rheologies, sludge withdrawal methods and temperature effects.
This paper will discuss the modeling adaptations that may be required in applying clarifier models to primary tanks and will illustrate the calibration and application of CFD modeling to two different case studies, including the recommended field procedures for gathering adequate data for model verification.
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