Nanofiltration

The nano filtration technique is mainly used for the removal of two valued ions and the larger mono valued ions such as heavy metals. This technique can be seen as a coarse RO (reversed osmosis) membrane.
Because nano filtration uses less fine membranes, the feed pressure of the NF system is generally lower compared to RO systems. Also the fouling rate is lower compared to Ro systems

Applications for NF systems are:

• softening
• specific removal of heavy metals from proces streams for reuse of water
• Reduction of salt contents of slightly brackish water

Typical membrane performance for NF membranes  are 50% NaCl removed and 90% (or more) for CaSO₄

There are two types of membranes:

• Spiral membranes, cheapest but more sensitive for polution
• Tubular/ straw membranes, the most used membranes seen the costs and effect, shall not easily be poluted

The surfaces from the filter determine the capacity from the filter. Spiral membranes have l the biggest surface area in general and are therefore the most cheapest in use. The surface area from Tubular/ straw membranes is less in general. 

The pre purifying of the feedingwater has a influence on the performance of the installation. The need of pre purifying depends on the feedingwater quality.

Installing pre cleaning has the following advantages:

• long-life
• long production of the installation is possible
• simple management

besides pre cleaning, a chemical doses can be taken place to prevent scaling, precipitation on the surface from the membrane.

A membrane system consists of several basic compounds:

• Feed pump
• membrane elements compiled in pressure vessels
• Pipes
• cleaning system