Nanofiltration may be compared with reverse osmosis as the two processes have a similar concept, being both pressure-driven methods of separation that employ principles of cross-flow filtration across a semi-permeable membrane. The main difference is that nanofiltration is generally aimed at removing just the divalent ions and the larger ions. Monovalent ions, the examples being chloride and sodium, have no problems passing through the nano membrane used in the process and are therefore retained in the output fluid. For this reason, nanofiltration is often used at the de-salting stage in the process stream of industrial filtration and in water treatment technologies.
Processes that include nanofiltration can reduce water hardness by up to 97% and are often referred to as membrane softening for precisely this reason. These various methods operate from75-150 psig, which means that they are an economical alternative when softening is needed, for example in commercial water filters.
It is possible for ultrafiltration (UF) membranes to have extremely high fluxes. However, in most practical situations the flux ranges between 50-200 GFD. This is when an operating pressure of 50 psig is presumed. In contrast, the membranes used in reverse osmosis only reach 10-30 GFD at pressures of 200-400 psi. UF membranes have pore sizes that stand between those of the membranes used in microfiltration and nanofiltration,. The membranes used are predominantly made of thin polymer films. Common materials include polyethylene terephthalate or metals, including aluminium.
There are two parameters that determine the selectivity of the membrane used in nanofiltration. Retention is entirely dependent on compound size,which is determined by the molecular weight. Where nanofiltration membranes are concerned, permeability and retention are functions of the electric charge and salt and compound valence in the solution. More dilute solutions which include monovalent ions may largely pass through a nanofiltration membrane without difficulty. However, most multi-valent ions (including sulphate and carbonate) do not pass through and are retained by the membrane.
When it comes to anions, the nanofiltration membrane’s retention increases according to a specific order of molecules: nitrate and chloride are followed by hydroxide, sulphate and carbonate and finally phosphate. Cation retention will be increased according to similar principles: proton and sodium, then potassium, followed by calcium, magnesium and copper, with iron in the final place.