Desalinated brackish water with improved mineral composition using monovalent-selective nanofiltration followed by reverse osmosis

Typical product water from desalination plants is depleted of salts and important minerals that are essential for its final consumption as well as its conveyance in the distribution system. In this study, we propose a hybrid nanofiltration (NF)-reverse osmosis (RO) filtration scheme that can produce desalinated brackish water with improved mineral composition. Here, the monovalent-divalent ion selectivity of NF membranes is used in a preliminary step to remove and “store” divalent minerals in the NF retentate stream, which are later dosed back to the mineral-free RO permeate to replenish the product water. We combine experimental data from pilot-scale NF with RO simulation to demonstrate the feasibility and economic viability of the proposed treatment scheme. In particular, we show the potential to operate the hybrid filtration scheme at high recovery ratios (>85%) and minimum saturation indices of potential scaling solids. A cost assessment further supports the applicability of the proposed treatment scheme, estimating the total capital and operating cost at $0.24/m 3 product, which is on par with existing brackish-water desalination costs. • A hybrid nanofiltration-reverse osmosis scheme for desalination is explored. • A monovalent-selective nanofiltration removes and stores minerals in a side stream. • The minerals removed by nanofiltration are dosed to the reverse osmosis permeate. • Operation at high recovery ratios and minimum scaling potential is demonstrated. • Capital and operational costs are comparable to those of existing plants.