Fouling in nanofiltration
According to Koros et al.  fouling is “the process resulting in loss of performance of a membrane due to deposition of suspended or dissolved substances on its external surfaces, at its pore openings, or within its pores”. Fouling is also decribed as flux decline which is irreversible and can only be removed by, for example, chemical cleaning . This is different to flux decline due to solution chemistry effects or concentration polarisation which is described in more detail later in this chapter. Those flux declines can be reversed with clean water and are hence not considered as fouling.
Schaefer, A., Andritsos, N., Karabelas, A., Hoek, E., Schneider, R. & Nystrom, M. (2004). Fouling in Nanofiltration. In A. Schafer, T. Waite & A. Fane (Eds.), Nanofiltration - Principles and Applications (pp. 169-239). UK: Elsevier Ltd.