Conventional ultrafiltration (UF) pretreatment in seawater reverse osmosis (SWRO) removes particulate and colloidal matter but provides little control over dissolved scaling precursors. This study evaluated nanofiltration (NF) as a selective pretreatment for modifying the ionic composition entering SWRO under high-salinity Arabian Gulf conditions. One UF membrane (UP150) and four commercial NF membranes (NP010, NP030, XN45, and NF45) were evaluated in single-run batch screening experiments with real Arabian Gulf seawater, and their permeates were subsequently used as RO feeds. Hydraulic resistance, ion and dissolved organic carbon (DOC) partitioning, carbonate saturation state, brine-stream differentiation, and effective ion transport were assessed. NF45 provided the strongest softening, with batch-averaged apparent rejections of 99.3% SO₄2⁻, 96.2% Mg2⁺, 90.2% Ca2⁺, 65.0% HCO₃⁻, and 94.9% DOC, whereas XN45 combined 97.8% SO₄2⁻ rejection with higher average apparent permeability. XN45 and NF45 recovered 86.3% and 85.1% of sulfate entering the UF/NF stage in the NF retentate; of sulfate entering the subsequent RO stage, 91.5% and 84.7% was recovered in the corresponding RO retentates. Using NF45 permeate as RO feed increased RO apparent permeability at 5% recovery from 0.22 to 0.48 L/m2/h/bar and was accompanied by lower hydraulically reversible resistance. PHREEQC/Pitzer calculations indicated lower carbonate supersaturation in NF permeates but greater supersaturation in NF retentates. Under the tested batch conditions, NF redistributed rather than eliminated solute and thermodynamic scaling burdens.
