DiaNanofiltration-based method for inexpensive and selective separation of Mg²⁺ and Ca²⁺ ions from seawater, for improving the quality of soft and desalinated waters

A new technique is introduced for extracting Mg²⁺ (and Ca²⁺) ions from seawater to produce a solution characterized by high concentrations of required ions (i.e. Mg²⁺, Ca²⁺ and SO₄^2-) along with minimal concentrations of unwanted species (i.e. Cl^-, B, Br^- and Na⁺), to be dosed to soft waters. The technique comprises of nanofiltration of seawater (or 1st-pass seawater reverse osmosis brine) followed by a DiaNanofiltration step, applied on the nanofiltration brine. DiaNanofiltration is a procedure involving dosage of low-TDS water to the nanofiltration feed solution, to improve the "wash out" of unwanted components. The purpose of the preceding nanofiltration step is to produce low-volume brine retaining most of the Mg²⁺, Ca²⁺ and SO₄^2- from the seawater, thereby minimizing the cost of the DiaNanofiltration diluting solution. The paper describes results of experimental and theoretical examination of the process under various operational conditions. The results show that the Cl^- to Mg²⁺ concentration ratio in the produced solution varies between 1.52 and 3.27 (w:w), depending on the operational alternative. A cost assessment (operational and capital expenses) is provided, showing the cost of all examined scenarios to be lower than $0.01 for supplying soft (desalinated) water with 10 mg/l of Mg²⁺.