In this work, we used the SEDE (Steric, Electric and Dielectric Exclusion) model to investigate the rejection rate of asymmetric electrolytes by nanofiltration membranes. The SEDE model predicts that the rejection rate of asymmetric electrolytes with divalent counter-ions is a non-monotonous function of the volume charge density. Because the Donnan exclusion screens the interaction between ions and their images, it was shown that there is a range of membrane volume charge densities for which the increase in the Donnan exclusion is overcompensated by the strong decrease in the interaction between the ions and their images. As a result, the rejection rate decreases even if the membrane volume charge density increases. At very high membrane charge densities, the image charge interaction vanishes and the rejection rate tends to the value predicted by the Donnan theory. The separation performances of two NF polyamide membranes were investigated. It was shown that they cannot be described without taking into account the dielectric exclusion.