High Permeance or High Selectivity? Optimization of System-Scale Nanofiltration Performance Constrained by the Upper Bound

Membrane separation performance is constrained by a permeability-selectivity trade-off, also commonly known as the upper bound. To date, the upper bound lines for gas separation membranes and reverse osmosis membranes have been well-documented. However, the upper bound is far less established for nanofiltration (NF) membranes. In addition, existing literature appears to be inconclusive on whether high permeance or high selectivity is more preferred to achieve optimized NF separation performance. In this study, we first analyze the permeance, water/NaCl selectivity, water/Na2SO4 selectivity, and NaCl/Na2SO4 selectivity for polyamide NF membranes reported in the literature, which results in a set of comprehensive upper bound lines defining the state-of-the-art NF separation properties at the material level. We further establish a comprehensive framework that examines the system-scale performance including product water quality, specific energy consumption (SEC), and system stability in relation to the upper bound. Our analysis reveals the risk of high SEC for more selective membranes yet poorer product water quality for more permeable membranes, and an optimized NF system generally requires a combination of moderate permeance and water/salt selectivity to ensure desirable combinations of SEC, water quality, and system stability. Our work provides critical guidelines for the future development of NF membranes and optimization of NF systems.