The Nanofiltration (NF) is a process of separation on membrane with nominal pore diameter of about 1 nanometer (10 Angstroms), operating between UF and RO.
This membrane is not a complete barrier for the dissolved salts. In fact it retains organic molecules with molecular weight greater than 200-400 g/mol, dissolved salts with rejection from 20 to 98%, salts with monovalent anions (for example sodium chloride or calcium chloride) with rejection from 20 to 80%, and salts with divalent anions (ex. magnesium sulphate) with rejection higher than 90-98%.
The filtering process takes place thanks to the application of a difference of pressure that allows the membrane to act as a sieve: substances smaller than the diameter of the pores of the membrane pass with the solvent (permeate) while the larger ones are retained (concentrate).
Since the Nanofiltration membranes are less fine than RO ones, the supply pressure and also the rate of fouling have generally a lower value. Really, the market offers different types of Nanofiltration membranes, which differ in the material type and conformation. If we choose a membrane with low permeability, the difference in osmotic pressure between the two compartments can become high as for a RO system, while with a membrane with high permeability, the osmotic pressure is drastically reduced. Anyway, the efficiency of the system depends strongly also on the chemical-physical characteristics of the water to be treated and the working conditions (pH, temperature).
The applications of this technology include:
- The removal from surface waters of color and total organic carbon (TOC);
- The removal of hardness (polyvalent cations) from well water;
- The overall reduction of total dissolved solids (TDS) and separation of organic compounds from inorganic ones in food applications;
- The re-use of solutions with high salinity in wastewater treatment.
The last application allows to treat the rejections of an RO plant, obtaining decolorized brines, characterized by salinity between 10 and 30 g/l; therefore, they are suitable for re-use in the dyeing, both for the preparation of new dyeing baths, both in the case of solutions of NaCl for the regeneration of the softening resins.
Thus, this type of process would offer the dual advantage of reusing the electrolytes placed in the dyeing baths and to drastically reduce the amount of dissolved salts in wastewater.
The system works automatically, modulating the pressure and the flow rate in inlet, using inverters, analogic instruments and a PLC system.
Through the use of instruments such as pressure transmitters, flow meters, Ph-meters and conductivity, it is possible to control strictly the process and optimize the system, improving energetic efficiency.