Removal of sodium and micropollutants - Performance and stability improvement of layer-by-layer (LbL) membranes

Since a long time, reverse osmosis (RO) membranes have been used for seawater desalination to produce safe drinking water. Although RO membranes retain almost all solutes, they fail in chlorine resistance meaning that membrane modules need to regularly be replaced. These operational costs are combined with high energy costs due to high operating pressures. In order to limit such costs, nanofiltration (NF) membranes using lower pressure are used as shown in Figure 1. NF membrane fabrication is often based on the interfacial polymerization. This requires the use of chemicals, such as hexane, which poses problems according to the Registration, Evaluation, Authorisation and CHemicals (REACH) regulations.

Figure 1: Membrane crossflow filtration principle for salts and micropollutants removal.

A new class of NF membranes assembled via the LbL (layer-by-layer) technique making use of water based coating solutions, are of significant interest. [1]. Individual layers, alternatively positive and negative, are assembled by the dip-coating method on the hollow fiber membrane substrate as depicted in Figure 2. Membrane properties are tailored by defining both the coating conditions and the number of bilayers.

Figure 2: Layer-by-layer deposition process

[1]        L. Y. Ng, A. W. Mohammad, and C. Y. Ng, “A review on nanofiltration membrane fabrication and modification using polyelectrolytes: Effective ways to develop membrane selective barriers and rejection capability,” Adv. Colloid Interface Sci., vol. 197–198, pp. 85–107, 2013.