Modelling and environmentally sound management of brine discharges from desalination plants

Sea water desalination plants discharge a concentrated brine effluent into coastal waters. Modern, large capacity plants require submerged discharges, in form of a negatively buoyant jet, that ensure a high dilution in order to minimize harmful impacts on the marine environment. Existing design practice favors a steep discharge angle of 60° above horizontal that is based on very limited laboratory data on dilutions at the level of maximum rise. However, examination of more recent laboratory data and the parametric application of CorJet, a jet integral model within the CORMIX expert system suggest that flatter discharge angles of about 30° to 45° above horizontal may have considerable design advantages. These relate to better dilution levels at the impingement location, especially if bottom slope on port height are taken into account, better offshore transport of the mixed effluent during weak ambient current conditions, and the ability to locate in more shallow water near-shore.

Sea water desalination plants discharge a concentrated brine effluent into coastal waters. Modern, large capacity plants require submerged discharges, in form of a negatively buoyant jet, that ensure a high dilution in order to minimize harmful impacts on the marine environment. Existing design practice favors a steep discharge angle of 60° above horizontal that is based on very limited laboratory data on dilutions at the level of maximum rise. However, examination of more recent laboratory data and the parametric application of CorJet, a jet integral model within the CORMIX expert system, suggest that flatter discharge angles of about 30° to 45° above horizontal may have considerable design advantages. These relate to better dilution levels at the impingement location, especially if bottom slope on port height are taken into account, better offshore transport of the mixed effluent during weak ambient current conditions, and the ability to locate in more shallow water near-shore. However given the paucity of reliable experimental data (notably dilution measurements) for the entire negatively buoyant jet including sloping bottom interaction, the above recommendations are considered preliminary. The summary of state of the art design methodologies for brine discharges shows that there is a further need for more experiments that describe the jet evolution and mixing in better resolution. To further corroborate them, a vigorous program of experimental studies using modern field-revolving techniques, such as LIF and PIV, supported by detailed CFD modeling, is called for in several laboratories. This appears crucial in view of ongoing design and siting activities for numerous new desalination plants all around the globe. Furthermore a detailed study of the impingement zone and the far-field mixing is necessary. Even after strong initial jet mixing the heavy effluent generally sinks on the seabed and develops a density current. Depending on topographical features (i.e. channels, submarine valleys or depressions) the density current is strongly limited in its spatial extents resulting in weak mixing and strong benthic impacts. In addition velocities at the sea-floor are small and vertical mixing inhibited by the strong density gradient. The extension of existing models promises better capabilities for future brine discharge assessments. In addition alternative concentrate management technologies (e.g. recycling or substitution of substances, or treatment of effluents) need to be examined. A summary of experiments, design alternatives and technologies could be given in a design manual for the standardization of concentrate management technologies including optimized outfall-intake designs for desalination plants to mitigate the image of desalination technologies being non-sustainable and expensive technologies regarding the natural sources.