A cost effective method for improving the quality of inland desalinated brackish water destined for agricultural irrigation

Abstract

Desalinated brackish water is becoming an important water source for agricultural irrigation, a fact which raises the incentive to include a minimal Mg 2+ concentration in the product water, which would minimize fertilization requirements. Both the mineral content and chemical stability of desalinated water are attained through the post treatment stage at the desalination plant. A modified post treatment process is introduced, in which Mg 2+ ions are separated from the brackish water fed to the desalination plant via a selective cation exchange resin, and subsequently released to the product water through further exchange with Ca 2+ ions previously generated in a common H 2SO 4-based calcite dissolution process. Calcite dissolution provides the water with carbonate alkalinity, Ca 2+ and SO 4 2 −, while the ion exchange step adds Mg 2+. Laboratory and theoretical investigations of the process show that it results in a high water quality, attained in a cost effective and environmentally friendly manner. Three case studies are presented in which the additional cost of supplying Mg 2+ concentration of either 12 or 18 mg L − 1 using the process is estimated at less than $0.006 m − 3 product water. The results indicate that the process is feasible for brackish waters characterized by Mg 2+ to Ca 2+ concentration ratio higher than 1:1.