Establishment of the Underlying Rationale and Description of a Cheap Nanofiltration-Based Method for Supplementing Desalinated Water with Magnesium Ions

Abstract

The importance of supplying drinking water with a balanced mineral composition, including a minimal concentration of Mg(II) ions, has been recently acknowledged by many publications, as well as in official WHO guidelines. The issue is relevant to naturally occurring soft waters and lately to the rapidly increasing volume of supplied desalinated water. This paper presents an enhancement of a recently developed nanofiltration-based method for the selective separation of soluble Mg(II) species from seawater. The generated rich-Mg(II) brine is demonstrated to be suitable for supplementing soft waters with magnesium ions. The brine, generated using a commercial membrane (DS-5 DL, Osmonics) at various operational conditions is characterized by high Mg(II) concentrations (~8.5 g/L) and low Cl:Mg and Na:Mg molar concentration ratios (1.6 and 0.6, respectively, at 28-bar operation). A food-grade antiscalant is dosed to the feed seawater to prevent scaling; however, since the Mg(II) concentration in the brine is high, for attaining 10 mg Mg/L of desalinated water, the dilution ratio with the desalinated water is ~1:850, resulting in maximal additional concentrations of 0.024 antiscalant, 34.9 Cl(−I), 12.9 Na(I), 0.05 Sr(II) and 0.003 B (all concentrations in mg/L). The overall cost of 1 kg of Mg(II) separated by the presented process amounts to between $0.05 and $0.07, i.e., much cheaper than the estimated costs of alternative processes for Mg(II) addition to desalinated water.

Keywords

• nanofiltration
• post-treatment
• desalination
• magnesium ion
• drinking water
• cost assessment
• water quality