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Brine Discharges from Desalination Plants: Mixing Processes, Modelling Techniques and Environmental Impact Regulations

Author(s): Tobias Bleninger

Linked Author(s): Tobias Bleninger

Keywords: Outfall; Jet; Plume; Emission; Imission; Standards

Abstract: The worldwide desalination capacity is increasing at a rapid pace. Although the desalination of seawater offers a wide range of public health, socio-economic, and environmental benefits, many concerns are raised over the potential negative impacts on the environment. The concerns mainly revolve around the concentrate and chemical discharges into the sea and the air pollutant emissions attributed to the energy demand of the desalination process. The impacts of a desalination plant discharge on the marine environment depend on the physical and chemical properties of the desalination plant reject streams, and the susceptibility of coastal ecosystems to these discharges depending on their hydrographical and biological features. The brine flows are considerably large, generally up to 40% (for membrane based technologies, like reverse osmosis, RO) and up to 90% (for thermal technologies, like multi-stage-flash, MSF, including cooling water) of the intake flow rate. Salinity and temperature directly influence the density of the effluent. The various density differences between the brine and the receiving water represented by the buoyancy flux causes different flow characteristics of the discharge. The dense RO effluent flow has the tendency to fall as negatively buoyant plume and spread as a density current on the sea-floor. The effluent from thermal desalination plants is distinguished by a neutral to positive buoyant flux causing the plume to rise and to spread on the sea-surface. Mixing processes for the case of submerged discharges are described, usually consisting of an initial high velocity jet flow with strong entrainment and mixing. In the near-field where jet-mixing is the dominating process, jet integral models are the standard tools for prediction. Subsequent advection by coastal currents and turbulent diffusion control the transport and mixing in the far-field. The modelling of bottom density-driven currents or surface buoyant spreading motions requires special mathematical formulation describing the particular transport and mixing attributes of these flows. This includes a strong damping of vertical entrainment combined with a significant density-induced spreading of these flows. Concepts regarding the design and assessment of brine discharge structures for the optimization of the initial dilution are presented within a framework of existing environmental regulations for such discharges. Recommendations include both discharge and ambient standards for salt excess or other chemical concentrations as well as spatially defined limitations, in form of mixing zones.


Year: 2010

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