Implementation, design and cost assessment of a membrane-based process for selectively enriching desalinated water with divalent seawater ions

The paper describes results from operating a new 3-step membrane-based process targeted at separating Mg²⁺ from seawater in an inexpensive way, with the purpose of using it to enrich desalinated water with magnesium, with as little as possible Cl⁻ and Na⁺ addition. To this end, seawater undergoes a series of processes aimed at increasing the Mg²⁺ concentration from ~1350 to ~4000 mg/L through nanofiltration while the monovalent ion concentrations are reduced by a nanofiltration-diananofiltration sequence, in which the diluent is RO produced water from a desalination plant. A dense ultrafiltration (UF) step precedes the nanofiltration-diananofiltration (NF-DiaNF) cycles. In this step sulfate in seawater is rejected better than divalent cations hence the retentate of this step has a ratio of total hardness to sulfate (([Ca²⁺] + [Mg²⁺])/[SO₄²⁻] → 1) which enables attaining an almost complete washout of monovalent ions in the DiaNF step. The paper is concluded with presentation of general design of the process steps and a cost assessment, which shows the process to be both flexible in the quality of the rich Mg solution generated, and cost competitive, relative to other alternatives.