such NF membranes. The loose RO membrane chosen required
less pressure because it allowed monovalent ions like sodium to
pass through, yet still removed color molecules (and probably
unrealized at the time, tri-halo-methane precursor molecules).
This "membrane softening" application was born before the
term nanofiltration was known.
In 1983, the first documented
process NF membrane (as opposed
to water treatment) I have found was
commercialized for the purpose of
desalting a small food-grade dye —
an advantageous purification step in
a critical manufacturing process. This
membrane is documented well because,
coincidentally, the patented use it was
developed for became the basis of a
major patent law interpretation case,
which made it to the U.S. Supreme
Court after 12 years in the lower courts
(decided in 1997).
A New Membrane Class Is Born
In 1984, Dr. Peter Eriksson, in a
marketing contribution to differentiate
such new membranes, coined the term
nanofiltration, which he based on
the estimated size of the pores in a
membrane with these types of removal
characteristics. Thus, a fourth class of
pressure-driven membranes was born.
This term and the fractionation
process came into widespread use long
before the "nano" materials storm that
has recently swept the technology world.
The only connection the NF membrane
name has to nanomaterials is through
the smallest size of uncharged solutes
they tend to separate — a weak link.
The ability to separate some small
solutes from others is a very important
membrane characteristic. Keep in mind
that there are two types of solutes
separated by differing mechanisms: ions
based chiefly on their valence state
in water (charge) and sieving based
on molecular weight if uncharged.
This is a simplified definition that
is generally not universally accepted.
It seems that those industry groups
that mainly purify water think only
of the ionic separation performance
of NF as important and often ignore
the uncharged solute aspect in their
definitions and common usage.
Yet some of the most creative and technical applications for NF
involve removing one size uncharged molecule from a larger or
smaller solute, to achieve an otherwise difficult separation. Such a
process step is called "fractionation" and is employed in technical
applications for food and beverage ingredients, fine chemicals,
oil and gas (i.e., fracking), pharmaceutical, and biomedical
wateronline.com
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Water Innovations
MEMBRANES
9