Brine discharges from desalination plants spread over large spatial areas, affecting the benthic communities present in the sea bottom. The mitigation of its impact requires the development of new technical solutions able to improve the dilution processes at acceptable costs for new and existing plants.
The feasibility study of applying Venturi effect ejectors in brine discharges as a mechanism to improve the dilution versus conventional diffusers was made at the Maspalomas desalination plant in Gran Canaria.
For the purpose of developing this project, a joint venture was formed led by the Technological Institute of Canarias (ITC). As part of this joint venture, DHI´s contribution focused on the application of numerical models for the study of the brine dispersion:
- A MIKE 3 numerical model was used to simulate the brine plume far field, and the model was calibrated using measurements from several monitoring surveys.
- A CFD (Computational Fluid Dynamics) numerical model was used to reproduce the salinity near field generated by the Venturi ejector, based on scaled physical experiments.
The results showed that the Venturi ejector demanded high exit velocities at the nozzle entrance, in the order of 11m/s, to allow the required pressure difference to generate the draw effect. At this velocity range a dilution of 39 was generated, versus 27 reached by conventional diffusers. The high exit velocities at the entrance of the ejector decrease significantly once the jet is inside the nozzle, resulting in no impact on the receiving waters.
The higher efficiency of the Venturi ejector made it possible to reduce the impact on the Cymodocea nodosa marine pastures (a species of sea grass) existing in the outfall surroundings. The benefits of using numerical models for this purpose included fast and accurate knowledge about the behaviour of the plume of brine, improved physical understanding of the phenomenon reproduced in laboratory tests, and optimisation of the time and costs spent.