Numerical study of brine plumes discharged from a desalination plant at different depths in the coastal waters of the Caspian Sea

In Reverse Osmosis (RO) desalination plants, the most important problem is the increase of salinity near the outfall. In this study, different scenarios of brine waste dispersion discharge of a desalination plant in terms of the depths of outlet system positions and physical properties of discharge system that meet the standard criteria of the Department of Environment of Iran are investigated.
In this work, the effect of the desalination effluent discharge site in terms of the depth of discharge location and emission of pollutants on a Caspian Sea coastal area (Neka) has been investigated and various scenarios have been implemented and proposed. Here the effectiveness of desalination effluent discharge depths of different scenarios, using a numerical model, have been considered. The model simulates unsteady 3D flows, by taking into account density variations, currents and other hydrographic conditions. The model has a dynamical nesting facility which gives a possibility of making an increase in resolution in areas of special interest. For increasing numerical efficiency, structured nested grids with three sizes of 90, 30, and 10 meters and uniform vertical mesh size equals to 0.5 meters have been used. In comparison with other common works, in this research, using a 3D non-hydrostatic (fully hydrodynamic) mathematical model to simulate the dispersion of saline water effluent, is an important feature. The effective density variation between the effluent and the receiving environment and generation of vertical flows resulting from this density variation, cannot be simulated using simplified mathematical models as they may face serious errors. Lack of the rapid diffusion and ideal conditions for plume development, illustrates that the worst condition for brine dispersion is a calm sea with minimum currents in coastal areas. So, the effects of the sea waves have been neglected and longshore wind induced current has been assumed to be a minimum of approximate value of 0.03 m/s.
The mean salinity in in the southern Caspian Sea is about 12.5 gr/lit and the desalination brine salinity has been considered as 25 gr/lit and the rate of fresh water and brine waste water production is about 6 m³/s. With these assumptions for rate of effluent discharge and sea conditions, different scenarios have been investigated using a 3D numerical model including different velocities and directions of a pair of jet fluxes in outlet system and outlet installation Reverse Osmosis (RO) desalination plants salinity near the outfall. In this study, depths of outlet system positions and physical properties of discharge system are mainly investigated.
The results show that in acceptable scenarios (with higher jet discharge speed and vertical direction of 30˚ to the vertical axis), the receiving environment has high brine concentrated area with a radius less than 200 meters. The results of different scenarios of discharge depths show that regarding the depths of discharge studied in this work (5, 10 and 15 meters), when the jet injection is closer to the horizontal direction, there is no significant difference between the results of different depths. But, in selected conditions, i.e. conditions where the angle of the effluent discharge jet is closer to the vertical axis (vertical direction of the jets is 30˚ to the vertical axis), deeper dischages create better conditions in terms of salinity propagation horizontally in the environment.