to as dielectric exclusion, a reference to the dielectric constants (energy) associated with a particles precense in solution versus within a membrane substrate. Solution pH strongly impacts surface charge, providing a method to understand and better control rejection. The transport and exclusion mechanisms are heavily influenced by membrane pore size, solvent viscosity, membrane thickness, solute diffusivity, solution temperature, solution pH, and membrane dielectric constant. The pore size distribution is also important. Modeling rejection accurately for NF is very challenging. It can be done with applications of the Nernst–Planck equation, although a heavy reliance on fitting parameters to experimental data is usually requried. In general, charged solutes are much more effectively rejected in NF than uncharged solutes, and multivalent solutes like (valence of 2) experience very high rejection. Typical figures for industrial applications Keeping in mind that NF is usually part of a composite system for purification, a single unit is chosen based on the design specifications for the NF unit. For drinking water purification many commercial membranes exist, coming from chemical families having diverse structures, chemical tolerances and salt rejections. NF units in drinking water purification range from extremely low salt rejection (<5% in 1001A membranes) to almost complete rejection (99% in 8040-TS80-TSA membranes.) Flow rates range from 25–60 m3/day for each unit, so commercial filtration requires multiple NF units in parallel to process large quantities of feed water. The pressures required in these units are generally between 4.5-7.5 bar. For seawater desalination using a NF-RO system a typical process is shown below. Because NF permeate is rarely clean enough to be used as the final product for drinking water and other water purification, is it commonly used as a pre treatment step for reverse osmosis (RO) as is shown above. Post-treatment As with other membrane based separations such as ultrafiltration, microfiltration and reverse osmosis, post-treatment of eitherpermeate or retentate flow streams (depending on the application) – is a necessary stage in industrial NF separation prior to commercial distribution of the product. The choice and order of unit operations employed in post-treatment is dependent on water quality regulations and the design of the NF system. Typical NF water purification post-treatment stages include aeration and disinfection & stabilisation. Aeration A Polyvinyl chloride (PVC) or fibre-reinforced plastic (FRP) degasifier is used to remove dissolved gases such as carbon dioxide and hydrogen sulfide from the permeate stream. This is achieved by blowing air