Introduction

Nanofiltration (NF) is the fourth class of pressure-driven membranes born after microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO). It was first developed in the late 1970s as a variant of RO membrane with reduced separation efficiency for smaller and less charged ions such as sodium and chloride. As the term NF was not known in the 1970s, such membrane was initially categorized as either loose/open RO, intermediate RO/UF, or tight UF membrane. The term NF appears to have been first used commercially by the FilmTec Corporation (now The Dow Chemical Company) in the mid-1980s to describe a new line of membrane products having properties between UF and RO membrane. Owing to the uniqueness and meaningfulness of the word NF, other membrane scientists have begun using it. The widespread use of this word today is testament to the need for just such a descriptor in the membrane lexicon (Schafer et  al., 2005).