Saudi Water Authority (SWA) Strategies for raising the total dissolved solids (TDS) concentration of water produced and transported from desalination plants: Advantages and challenges

Maintaining the total dissolved solids content of desalinated seawater within regulated levels is critical for meeting regulatory limits and maintaining public confidence in the quality of the water supply. A number of strategies for maintaining total dissolved solids above the target value of 100 ppm while maintaining all other water quality parameters are described as employed in case studies in seawater desalination plants in the Kingdom of Saudi Arabia: These are reduction of second-stage Reverse Osmosis, addition of seawater, and increasing the extent of remineralisation. The main operational challenge found in increasing total dissolved solids is avoiding corrosion by maintaining the Larson-Skold index below acceptable levels. While acceptable results above 100 ppm can be achieved with relative ease, values above 150 ppm are invariably accompanied by Larson-Skold index levels above 1.2, which cannot easily be remedied without significant capital investment. As the legacy thermal desalination plants are retired, it will no longer be necessary to take specific action to maintain total dissolved solids above minimum levels, and maintaining system stability at higher levels will become the more significant challenge. Finding means to address the high Larson-Skold Index levels is thus a priority in the medium-term to avoid potential corrosion of the existing transmission systems, which were designed for conveyance of product water from thermal desalination plants.