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Nanofiltration (NF), in water treatment, “bridges” a gap between UF and RO membranes. It is sometimes referred to as a loose or lower rejecting RO membrane. It could also be called a tight UF, as NF membrane pore size ranges from .01 to a little less than .001. Generally, an NF membrane system lets more salt pass than an RO membrane. In addition, an NF membrane element will produce the same quantity of product at 50 to 70% the applied pressure as an RO water treatment system.
There are applications where a 75% rate of salt rejection is preferable to 95 to 99%, especially when it is achieved using only half of the energy. Unlike with RO membrane elements, the NF offerings of the major membrane manufacturers are quite different in performance from company to company. With RO membrane elements, any model with less than 99.5% sodium chloride rejection, especially with seawater membranes, is considered inferior. In the case of NF, there is a “place” for any membrane with a NaCl rejection rate of 40%, or more. Nanofiltration has also been called a “softening” membrane as while its salt rejection may be 80% or less, hardness rejection will often remain well over 90%.
Like RO, and unlike UF, a Nanofiltration Systems will recover a percentage of the feed water. Most all NF membranes are of a spiral wound configuration.