Introduction to nanofiltration
Nanofiltration (NF) is a process that uses membranes with nanometer-scale perpendicular cylindrical pores. Membrane materials include thin films of polyethylene terephthalate, polyamide and aluminium. The membranes are most commonly configured as spiral wound flat sheets or tubular forms.
NF membranes can benefit from pre-treatment, and chemical dosing, but this is unnecessary with many waters.
Applications of nanofiltration
- Softening – the removal of calcium and magnesium (hardness) – predominantly done to protect downstream thermal processes from scaling
- Sulphate removal – a major application is treatment of water for oil production water injection
- Removal of other polyvalent ions (divalent & larger)
- Heavy metal removal
- Organic carbon removal, particularly disinfection byproduct precursors
Nanofiltration for desalination
Salt rejection rates for NF membranes are much lower than for RO membranes. As such NF only tends to be used for the desalination of brackish waters, where relatively low salt removal is required and the higher permeability allows much lower pressure, hence lower operating costs.
NF is however extensively used as a pretreatment step for RO as they can reduce scaling and otherwise protect the RO membranes.