Nanofiltration is a membrane separation process whose cut-off lies between that of reverse osmosis and ultrafiltration. The commonly used operating pressure is in the range of 0.3 – 4.0MPa. Cut-offs are specified as being between 180 and 2000 Dalton.

The selectivity of a nanofiltration membrane is determined by two different parameters. Firstly, retention depends on the compound size i.e. from molecular weight. As far as nanofiltration membranes are concerned, retention and permeability are also a function of electric charge and the valence of the salts and compounds in the solution. Diluted solutions of monovalent ions can mostly pass through a nanofiltration membrane unhindered while most of the multi-valent ions (such as sulfate and carbonate) are retained. As for anions, retention by a nanofiltration membrane increases in the order: nitrate < chloride < hydroxide < sulfate < carbonate < phosphate. Cation retention by a nanofiltration membrane increases in the order: proton < sodium < potassium < calcium < magnesium < copper < iron.

Similar to reverse osmosis membranes, nanofiltration membranes are composite membranes consisting of a carrier structure as well as a thin selective layer.

Membranes with good chemical resistance are now available for the treatment of acids, bases and oxidative media, thus making the process suitable for a number of separation operations:

  • Softening of process, product or drinking water
  • Separation of organic impurities (pesticides, insecticides) in drinking water treatment
  • Filtration of acids and bases
  • Desalination of organic materials (pharmaceutical industry) in the diafiltration mode

Not only membrane materials but also several module configurations are available for the different applications.

Spiral would configurations are preferred for water treatment operations. Little space is required to treat large volume flows because of the plants’ compact structure and high packing density. One condition for the use of spiral wound elements is that the feed solution should be of high quality with a low solids concentration or should undergo elaborate pre-filtration. Spiral wound modules are also mostly used in diafiltration processes in the pharmaceutical industry.

Tubular modules are used for flows with higher viscosity or high solids concentration. Because of their free flow cross-section, treatment can take place either without any pre-filtration or with a greatly simplified technique.