The membrane interception molecular weight (MWCO) is defined as the MW of 90% of the organic tracer trapped by the membrane, which is determined by drawing the relationship between the organic tracer interception rate and the tracer MW. Figure 1A shows an overview of membrane MWCO from nanofiltration/reverse osmosis (NF/RO) to microfiltration (MF) and the corresponding pore sizes.
MWCO for UF membranes typically ranges from 1-500 kDa, while MWCO for NF membranes is usually below 1 kDa (0.15-0.3 kDa). With a MWCO of 0.5-2 kDa, "loose" NF membranes have unique properties, including high retention of charged organics such as dyes and organics, low salt retention (univalent and multivalent), and lower operating pressures (<6 bar) than NF. MWCO is a dimensional exclusion parameter that averages the pore size and does not take into account molecular interactions (e.g., adsorption, charge interactions, solute - solvent interactions) that are associated with separation between ultrafiltration and nanofiltration processes.