The cross-flow filtration membrane technology is used to physically separate from an aqueous solution even extremely small soluble substances (ions, molecules), whose size is measurable in nm (nanometers).
The degree of filtration depends on many factors:
- the size of the pores of the membrane and the material of which it is made;
- the chemical characteristics of the fluid to be treated;
- operating parameters (temperature, pressure, pH, etc.);
- the fluid dynamic regime (increasing or decreasing the circulation velocity of the fluid that flows tangentially to the surface it is possible to alter the performance of the membrane in terms of rejection).
Nanofiltration membranes are similar to those of reverse osmosis, but leave monovalent ions, indissociated acids and low molecular weight organic substances pass into permeate, while retaining divalent ions and more complex organic substances (in the order of 200-500 D as molecular weight).