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Polyelectrolyte multilayer based membranes are highly promising systems to create stable and versatile nanofiltration membranes. One very popular and well‐studied polyelectrolyte pair, is the polycation poly(diallyldimethylammonium chloride) (PDADMAC) and polyanion poly(sodium 4‐styrenesulfonate) (PSS), due to its excellent separation properties and high chemical and physical stability. Membrane charge can be easily controlled by simply terminating the multilayer by either PDADMAC or PSS. Unfortunately, a phenomenon that occurs during multilayer coating, is overcompensation by PDADMAC. In this study, it is shown that overcompensation of PDADMAC results in a positive surface charge even when the multilayer is PSS‐terminated. In addition, it is shown that this leads to poorer membrane separation properties with sulfate retention decreasing from 94 to 39%. At the same time, it is demonstrated that a so‐called annealing cycle with a high salt concentration leads to recovery of the negative surface charge, increasing the sulfate retention from 39 to 95%. Even for multilayers at which no irreversible positive surface charge is measured, separation properties improved substantially (increasing sulfate retention from 94 to 97%, at a higher membrane permeability) after salt‐annealing. It is concluded that post‐treatment by salt‐annealing results in an improved membrane performance and allows an additional degree of control over the membrane separation properties.

The use of forward osmosis (FO) for water purification purposes has gained extensive attention in recent years. In this review, we first discuss the advantages, challenges and various applications of FO, as well as the challenges in selecting the proper draw solution for FO, after which we focus on transport limitations in FO processes. Despite recent advances in membrane development for FO, there is still room for improvement of its selective layer and support. For many applications spiral wound membrane will not suffice. Furthermore, a defect-free selective layer is a prerequisite for FO membranes to ensure low solute passage, while a support with low internal concentration polarization is necessary for a high water flux. Due to challenges affiliated to interfacial polymerization (IP) on non-planar geometries, we discuss alternative approaches to IP to form the selective layer. We also explain that, when provided with a defect-free selective layer with good rejection, the membrane support has a dominant influence on the performance of an FO membrane, which can be estimated by the structural parameter (S). We emphasize the necessity of finding a new method to determine S, but also that predominantly the thickness of the support is the major parameter that needs to be optimized.