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Understanding the hydraulic behavior of the anaerobic compartment of a BNR system using Computational Fluid Dynamics

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Flow characteristics of the anaerobic section of a biological phosphorus removal (BPR) pilot plant is investigated using Computational Fluid Dynamics (CFD) modeling tool. This includes the set-up and validation of an appropriate model and its application for detailed hydraulic analysis. The anaerobic tank investigated, consists of three compartments, identical in size and each equipped with an agitator. Validation of the CFD model is accomplished by: a) comparing experimentally measured Residence Time Distributions (RTD) to the CFD results and b) by using the CFD results to calculate the impeller dissipated power and comparing it to values found in literature. Hydraulic analysis illustrates good agreement between experimental and numerical RTD for various operational conditions. Comparing numerical with theoretical Hydraulic Residence Time (HRT = volume/flowrate) exhibits consistent results with regard to the whole anaerobic unit. Analysis of the separate compartments, however, reveals a high deviation of the Residence Time (RT) from one section to the other. A strong recirculation is found within the first compartment along with short-cuts in the following two sections at different conditions (flow and impeller speed) tested. Furthermore, analyzing velocity vectors and fluid particle trajectories, the performance of the second agitator is found to be insufficient to achieve the intended up-pumping effect and hence favors the “short cut behavior”.

Results of phosphate and COD (chemical oxygen demand) measurements during BPR operation reflect the impact of these hydraulics on the biological response.

The results of this study are used to point out the applicability of the model for detailed diagnostic and future design applications.

Document Type: Research Article


Publication date: 2002-01-01

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