Nanofiltration flux, fouling and retention in filtering dilute model waters

Fouling of membranes decreases the applicability of the nanofiltration (NF) process, and thus a deeper understanding of membrane fouling is needed. Fouling and retention of different NF membranes by model feed waters was investigated in a laboratory-scale filtration unit. The model waters were composed so as to imitate the characteristics of chemically pre-treated surface water. No differences were seen in membrane flux declines when filtering feed waters containing the studied organic compounds of different characteristics. However, organic matter containing feed waters resulted in remarkably lower flux than the metal ions containing feed waters. An additional decrease in flux was seen when both organic matter and metal ions, especially silica, were present in the feed water. An increased feed water organics concentration increased the retention of organic matter, but the addition of metal ions to organics containing feed water caused a decrease in the retention of both organic matter and conductivity. The different behaviour is most probably caused by the differences in the fouling layers and foulant characteristics as well as by the eleetroneutrality effect. Generally, it is difficult to change feed water composition to non-fouling, but the operating pH can be used to some extent in optimising membrane performance.