Minimum return dilution method to regulate discharge of brine from desalination plants

The mixing zone approach in regulating the discharge of brine and other toxic dense discharges has many limitations when applied in environmentally sensitive areas. A well-defined minimum return dilution is advocated in this study as an alternative method to regulate the disposal of brine and other toxic dense discharges. This study examined experimentally the development and dilution of turbulent vertical dense jets (or fountains) at small Froude numbers. The study complements an earlier larger Froude number investigation. The mean and fluctuating temperature fields were measured with fast responding thermocouples, and an emphasis was given to the minimum return dilution, which occurred just outside the edge of the discharge pipe. The study has revealed that at small Froude numbers (Fr < 5) the normalized minimum dilution, μmin/Fr, decreased linearly with the Froude number and it became constant only at larger Froude numbers (Fr > 7). Simple design equations for the calculations of minimum return dilution and maximum excess temperature and salinity at the level of the source are provided for small and large Froude number regimes. This study also recognized the advantage of using a vertical discharge configuration (inclination θ = 90o with horizontal) as opposed to an inclined configuration (0o ≤ θ < 90o) when discharging brine into water environments. The inclined discharge configuration has the potential of producing higher concentrations of brine and temperature near the source when ambient currents are in a direction opposite to the discharge.