Virtual Tracer Study For a Water Reclamation Contactor

Abstract

A capacity increase at the West Basin water recycling plant in California was necessary in order to handle an increasing population and ensuing demand. The disinfection process included a chlorine contactor, operating at 5 million gallons per day. It was believed that an additional train would be necessary to accommodate the increased load of 6.9 million gallons per day. Goals and ObjectivesRather than initially performing a costly physical tracer test at the higher flow volume, a computational fluid dynamics (CFD) model of the existing contactor was created followed by a virtual tracer study to determine the residence time, the objective being to verify the need for an additional train. Inadequate residence time results would indicate the need to move forward in contactor design and construction, whereas a result showing sufficient residence time would be followed by a physical tracer test confirmation. MethodologyAfter creating CAD geometry to represent the contactor and meshing the volume with the GAMBIT [1] preprocessing tool, FLUENT version 6.1 [2] was used to calculate steady state flow through the contactor. Uniform flow was assumed at the inlet, and a standard k-epsilon turbulence model was used to account for the effect of unsteady turbulent fluctuations on the average flow field. Figure 1 shows flow velocity contours on a horizontal slice through the contactor volume. A residence time calculation was performed using the Discrete Phase Model (DPM), in which massless particles were released at the contactor inlet and tracked throughout the domain. The time at which each particle left the contactor volume was recorded. A standard method for evaluating residence time in a contactor is the t 10 /T calculation, where t 10 is the time at which ten percent of a tracer passes through the tank, and T is the theoretical time it takes for all of the tracer to pass through the tank assuming uniform, or "plug" flow. Thus, for the virtual tracer study, t 10 is the time at which ten percent of the particles have exited. Generally, a value of 0.6 or greater for t 10 /T indicates an effective contactor design. Findings and SignificanceThe model results suggested that there was sufficient residence time to handle the increased flow rate in the existing contactor. West Basin then performed a physical tracer study, which confirmed the CFD results and reaffirmed that the residence time met the Department of Health Services' requirements. Through saving the utility an estimated $3 7874