Dolomite for Supplementation of Desalinated Drinking Water in Saudi Arabia with Magnesium, Calcium, and Hydrogen Carbonate Ions Part 2

Maintaining the concentration of magnesium in potable water above minimum levels has been suggested to have public health benefits. A twelve-month trial of attempting this goal by partial replacement of limestone with dolomite in eight out of twenty-six post-treatment contactors at the Ras al Khair seawater desalination plant, the largest such plant in Saudi Arabia with a daily production of over 1,000,000 m³ of desalinated water. Over the course of the trial increases in Mg concentration in the range 1 to 2 ppm were achieved without necessitating increases in carbon dioxide utilization or any reduction in production volume. Alkalinity, calcium, and total dissolved solids remained within acceptable parameters. Calculated supersaturation values suggest strongly that it will not be possible to increase concentrations significantly further at the pH and temperature conditions of the study. Thus, while use of dolomite to this extent is a very low-cost strategy for magnesium supplementation, its scope of application without additional carbon dioxide consumption and capital investment is limited. The ratio of magnesium to chloride in SWRO product water was estimated in the course of the study and was found to be approximately half of the ratio in Standard Seawater, suggesting that under operational conditions (giving 1500 mg/L from first pass reverse osmosis) rejection of magnesium was significantly greater than rejection of sodium.