Chapter 4 - Nanofiltration membrane technologies

Abstract

With the increasing world population, the shortage of fresh water resources has become a serious issue. Thus the demand of new water treatment technologies increases. The development of nanofiltration (NF) membrane technology led to water treatment at lower operating pressures than that of reverse osmosis (RO). Due to its versatility as separation tool and with the properties between those of UF and RO, NF membrane is showing continuous interest since past two decades. Typically, NF membranes have pore size of the order of 1nm, sodium chloride rejection between 20% and 80% and molecular weight cut-offs for dissolved organic solute of 200–1000Da. Although some NF membranes are cellulose acetate based, some are also found as composite, especially, polyamide ones are used in water treatment. NF typically have high rejections of multivalent inorganic salts and small organic molecules at low applied pressure. Thus NF separation process is highly competitive in terms of selectivity and cost benefit. For efficient NF in water purification application, it should have a thin and porous selective layer with a narrow pore size distribution to ensure high selectivity. Salt rejection phenomena via NF membranes is due to effect of Gibbs–Donnan and dielectric effects. Therefore in this technology, salt rejection and water permeability depend on various factors, such as the pore distribution, size and interactions of the ions, and hydration at the nanopores and the membrane surface. NF has some advantages over RO, such as (1) high water fluxes, (2) low operating pressures, and (3) high rejection of divalent ions. NF plays an important role in different industrial applications, such as water treatment and resource recovery. Therefore many efforts have been dedicated to work on fouling reduction. This review summarizes the recent developments in this field and epitomizes the direction for potential future approaches to improve the separation efficiency of NF.