Secondary treated wastewater from a steel company in Korea contains very low organics but high conductivity (2,700-3,000 mu s/cm), which is attributed to the high concentration of many dissolved monovalent and divalent ions. Since they cannot be removed by conventional treatment methods, a commercial polyamide reverse osmosis (RO) membrane was used in this work for steel wastewater reuse. Concentration polarization in RO has been evaluated on the basis of the mass transfer coefficient determined. An indirect measurement technique for mass transfer coefficient was employed by the nonlinear parameter estimation method. The experimental data obtained in a disk-shaped flat RO cell were put into a combined film theory-solution-diffusion model. A 2.5" spiral-wound pilot-scale membrane system with a processing capacity of 1.47 m(3)/d, comprised of back-washable microfiltration (BMF) and RO membranes, was applied as the secondary wastewater treatment. It was found that the RO membrane system was capable of successfully eliminating the monovalent and divalent ions dissolved in steel wastewater when a suitable and effective pretreatment system was used. Combining the BMF and RO membrane system reduced flux decline and enhanced the removal efficiency over 95 %.