Nanofiltration is applied to remove multivalent ions (like heavy metals) and is often used for decolorisation, desalting, reducing the amount of organic matter... With addition of antiscalants, NF can also be used as water softener.
As a filtering technique nanofiltration is situated in between ultrafiltration and reverse osmosis. Un ultrafiltration will not separate ions, whereas a NF will remove the larger ions. An RO, on the other hand, will remove nearly all the ions.
A NF has the same setup as an RO: a large stream flowing by the membrane while only the filtrate stream goes through the membrane. Nanofiltration also works according to this crossflow principle.
It is important for the feed water of a nanofiltration not to contain any floating matter, because this can easily block the membrane. Therefore, ultrafiltration is often used as pre-treatment.
Working pressure for NF is usually around 5 à 6 bar.
It is also important that the incoming water is softened or an antiscalant is added, otherwise the membrane will rapidly clog.
PCA can help you with this technology, both in design and construction of the installation.