Membranes can be classified into 4 categories according to their filtration properties:
- Microfiltration, range 0.1 - 1 micron (1 micron = 1/1000 mm);
- Ultrafiltration, range 0.001 – 0.1 micron;
- Nanofiltration, range 0.0001 – 0.001 micron (0.001 micron = 1 nanometer);
- Reverse osmosis, range < 0.0001 micron.
As the pore size gets smaller, more pressure will be needed to start the filtration process. Surfaceproperties also play an important role. For a hydrophobic (water rejecting) surface, more pressure will be needed than with a hydrophilic (water attracting) surface.
Microfiltration and Ultrafiltration are typical particle filters, whereas Nanofiltration and Reverse osmosischange the chemical or ionic composition of the water, for instance by removing dissolved minerals. See the filtration spectrum below.