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Nanofiltration membranes (NF) have applications in several areas. One of the main applications has been in water treatment for drinking water production as well as wastewater treatment. NF can either be used to treat all kinds of water including ground, surface, and wastewater or used as a pretreatment for desalination. The introduction of NF as a pretreatment is considered a breakthrough for the desalination process. NF membranes have been shown to be able to remove turbidity, microorganisms and hardness, as well as a fraction of the dissolved salts. This results in a significantly lower operating pressure and thus provides a much more energy-efficient process. Similar to other membrane processes, a major problem in NF membrane applications is fouling. Several studies have investigated the mechanisms of fouling in NF membranes and suggested methods to minimize and control the fouling of NF membranes. For NF membrane characterizations and process prediction, modeling of NF processes and the use of atomic force microscopy (AFM) are very important. The ability to predict the performance of NF processes will lead to a lower number of experiments, saving of time and money, and help to understand the separation mechanisms during NF. A comprehensive review of NF in water treatments is presented including a review of the applications of NF in treating water as well as in the pretreatment process for desalination; the mechanism as well as minimization of NF membrane fouling problems; and theories for modelling and transport of salt, charged and noncharged organic compounds in NF membranes. The review will also address the application of AFM in studying the morphology of membrane surfaces as part of the NF membrane characterization.

A comprehensive review is given of (1) the use of NF membranes in water and wastewater treatment, (2) NF membrane fouling, (3) the mechanisms of separation, (4) NF modelling, and (5) use of AFM. It was shown that NF membranes can be used to treat water by removing contaminants and organics such as natural organic matter, disinfection by-product, pesticides, dissolved uranium, arsenite, chromate, and other metals. In addition, NF was used as a pretreat- ment in the desalination process to prevent SWRO membrane fouling, prevent scaling (both in SWRO and MSF) and lower the required pressure to operate SWRO plants. Fouling problems will lead to higher operation costs: higher energy demand, increase of cleaning and reduced life time of the membrane elements. The separation mechanism of NF has been identified mostly due to charge and steric effects. A large majority of modeling works on NF has been by models based on the extended Nernst-Planck equation. A good model will lead towards better prediction and optimization of NF membrane processes. Finally, state-of-the-art AFM has been mentioned in this review, which can significantly characterize the surface of NF membranes. Surface morphology, pore size distribution and particle adhesion of NF membrane were able to be quantified by AFM.