Multistage nanofiltration system for supplementing Shoaiba Ph.4 product water with magnesium extracted from seawater

Abstract

A multi-stage nanofiltration (NF) system was developed with an inter-stage dilution feature for efficient magnesium (Mg) harvest from seawater or brine. The CaMg/NaCl and Mg/Cl ratios at each stage were used to design and evaluate its performance, while keeping the CaSO4 scale deposition risk within the controllable range using anti-scalant. The pilot experiment was conducted using commercial-size 8-inch NF membranes, demonstrating its performance of divalent ions concentration while lowering the concentration of monovalent ions. This new NF system (NF-Mg) was then firstly introduced on a commercial scale in the Shoaiba Ph.4 SWRO desalination plant in Saudi Arabia, to supplement ≥15 mg/L Mg in the Shoiaba Ph.4 product water of 400,000 m3/d. During the NF-Mg commissioning period, the CaMg/NaCl, Mg/Cl, and stage-wise ion rejections by the NF membrane were evaluated, and operating conditions were identified to meet the desired Mg supplementation level. Finally, the Shoaiba NF-Mg system could demonstrate successful performance through its long-term operational data from Jun. to Dec. 2022. A techno-economic study shows the cost-effective supplement of Mg (13.2–23.4 (average 18.3) mg/L) in drinking water at the cost of 0.009–0.019 USD/m3.