Dilution of Desalination Discharge is Predictable

Discharges from desalination plants are emptied into natural bodies of water, and the differences in pH, temperature, and chemical composition can have detrimental effects on the organisms living there. While it is not an optimal disposal method, sufficient dilution can minimize any detrimental effects. In this study, Marti et al. performed tests on a desalination plant with dense discharge to verify the results of Roberts et al. (1997) in which an empirical scaling was produced to correlate Froude number with dilution. The dimensionless Froude number is used to characterize flows in relation to a carrying velocity. In the near field directly from the discharge nozzle, flows are governed by the jet pump and thus will have F>1, while in the far field, the discharge flow matches the speed of the surrounding water and should have F<1. To achieve better mixing and therefore dilution, it is necessary to have turbulent flow, which corresponds to higher Froude numbers. Any mixing that occurs in the far field is a result of background turbulence as the discharge will have settled into layered flow by that time, so therefore, it is necessary to ensure that proper mixing is accomplished in the near field. Data for the dilution at various points correlated results from the scaling of Roberts et al., however as the scaling was not created for F<20, the authors recommend further testing as they found the dilution to be higher than expected if the results from Roberts et al. are extrapolated. While the authors do not make a statement on the sufficiency of the dilution, if there is more dilution than expected as the velocity of the flow slows, this could mean better conditions for the surrounding aquatic environment.—Erin Partlan
Marti, C., Antenucci, J., Luketina, D., Okely, P., Imberger, J., 2011. Near-Field Dilution Characteristics of a Negatively Buoyant Hypersaline Jet Generated by a Desalination Plant. Journal of Hydraulic Engineering, 137, 57—65.
Marti et al. performed tests on a desalination plant during its regular maintenance period in order to test discharges of variable flows. Using its shut down and ramp up periods, data were taken for flows at full velocity, two-thirds velocity and one-third velocity. To determine dilution, measurements of salinity were taken at various heights in the water column; a total of 207 measurements were taken for the three flow rates. Measurements were also made of the background water currents and general meteorological data such as temperature. Over the three testing periods, the temperature shifted only slightly, but the water currents varied in magnitude and direction as the second day of testing coincided with an incoming storm. The study found that layered flow still occurred, and that the salinity column off the ocean floor varied for the three flow rates—four to eight meters for one-third flow, five meters for two-thirds flow, and nine meters for full flow.
The study of Roberts et al. resulted in empirical correlations for the thickness of the bottom layer and the minimum dilution achieved at the mixing zone edge with Froude number.  For the bottom layer thickness, data collected showed the height of the discharge flow to be less than two meters for reduced flow and between two and three meters at full flow. These data correspond roughly to the predicted values. The dilution achieved by the time the water reached the edge of the mixing zone was 50—55 times the initial discharge for reduced flows and 50—65 times for full flow. Again, these correlate with the predicted values. Marti et al. also found that dilution was higher than expected as flows dropped in velocity. This may be explained using the water flow data, which show that the discharge is entering a current going in the same direction and thus dilution should be increased. Further research is recommended to determine the applicability of Roberts et al. at these low flow rates.

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