Multi-objective optimization and process intensification of multistage nanofiltration with structural synergy for brine separation

Potable water production through reverse osmosis is a key application of membrane technology, with increasing focus on the efficient processing of reverse osmosis brine due to the health benefits and market potential of magnesium. In this study, we performed the first in-depth analysis and multi-objective optimization of a multistage nanofiltration system for brine separation in collaboration with the Saudi Water Authority, the largest desalination company in the world. Pareto fronts between purity, recovery, and power demand were evaluated via constraint-handled multistart optimization, revealing synergistic enhancement of separation performance through interstage dilution and permeate recycling. Structural intensification enables up to 233% increase in separation factor, or 9% energy demand reduction, projecting significant sustainability and cost benefits. Alternatively, optimal intensification can yield a 7% increase in divalent cation production, equivalent to a global increase of up to 124,200 tons per year. The proposed modeling framework offers a robust approach for future endeavors in ion separation optimization.