Nanofiltration is a form of filtration that uses
membranes to preferentially separate different fluids or ions.
Nanofiltration is not as fine a filtration process as reverse osmosis, but
it also does not require the same energy to perform the separation.
Nanofiltration also uses a membrane that is partially permeable to perform
the separation, but the membrane's pores are typically much larger than the
membrane pores that are used in reverse osmosis.
Nanofiltration is most commonly used to separate a
solution that has a mixture of some desirable components and some that are
not desirable. An example of this is the concentration of corn syrup. The
nanofiltration membrane will allow the water to pass through the membrane
while holding the sugar back, concentrating the solution. As the
concentration of the fluid being rejected increases, the driving force
required to continue concentrating the fluid increases.
Nanofiltration is capable of concentrating sugars,
divalent salts, bacteria, proteins, particles, dyes, and other constituents
that have a molecular weight greater than 1000 daltons.
Nanofiltration, like reverse osmosis, is affected by the charge of the
particles being rejected. Thus, particles with larger charges are more
likely to be rejected than others. Nanofiltration is not effective on small
molecular weight organics, such as methanol.
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