For pressure-driven membranes such as nanofiltration (NF), a procedure based oil the modified film-theory model (MFTM) is presented to predict actual ion rejection (R-a) and ion concentrations at the membrane Surface (C-m) in the presence of significant concentration polarization effects. Reported NF data for the desulfation of Paradox Valley brine (PVB) is employed to evaluate the MFTM. The predicted values of R-a and C-m are essential to determine: (1) actual osmotic pressures differences across the membrane; and (2) saturation degrees of sparingly Soluble scale species that precipitate on the membrane surface and inside the pores of the membrane. The operating cost for desulfating the PVB by NF is directly proportional to the cost of. (1) power consumption for applying higher operating pressures to exceed osmotic pressures across the membrane and achieve acceptable levels of throughput; (2) chemicals to inhibit and/or dissolve scale precipitates; and (3) membrane cleanup and replacement.