Nanofiltration is one of the newest pressure membrane processes. Nanofiltration membranes have pores of about 1-3 nm and usually carry functional groups with a negative charge, which helps prevent membrane fouling and also affects the selectivity of the membrane for the removal of individual salts. Nanofiltration separates low molecular weight organics, with the size of the separated substances being reported in units of molecular weight rather than size. Nanofiltration separates substances down to approximately 102-104 Da, corresponding to molecules such as sugars, dyes, pesticides and herbicides. The pressure requirements are higher than for ultrafiltration, due to the fact that osmotic pressures start to become apparent, up to 10 bars. The largest applications, due to their ability to selectively trap ions with a higher charge, are water softening, removal of organic matter and water colouring, followed by pesticide removal in drinking water production, treatment of organic synthesis wastewater, desalination of dyes after diazotization and landfill water treatment. The modules used are almost exclusively spiral wound, rarely plate or hollow fibre, but almost always made of polymeric material and containing a functional group. The operation of nanofiltration is continuous, and it must be taken into account that, like reverse osmosis, it produces, in addition to a permeate stream (purified water), a concentrate stream containing the separated compounds in concentrated form. It is also necessary to include suitable pretreatment for the separation of colloidal compounds, preferably ultrafiltration.
Benefits
- Removal of substances: organic substances, pesticides, bacteria, viruses, nitrates, sulphates, polyvalent ions, colour, odour
- Water softening
- Excellent quality of treated water regardless of raw water quality
- Lower operating pressures compared to RO (typically 4 - 10 bar)
Resources
| OPAMEM_Nano.pdf |