Water & Wastewater Asia January/February 2023

SINGAPORE FOCUS
NANOPURE:
Novel nanofiltration
membrane for water
and wastewater
treatment
Fig. 1: NanoPure
hollow fibre NF
membrane and
module
Nanofiltration (NF) membrane
process is a pressure-driven filtration
process that employs membrane with
a pore size in the range of 0.5-5nm
which can reject solute via steric,
dielectric and/or Donnan exclusion.
In general, the NF membrane
process would benefit from a higher
water flux at a lower operating
pressure as compared to the
reverse osmosis (RO) membrane
process. Its applications in water and
wastewater treatment encompass
water softening, micropollutant
decontamination, colour removal,
heavy metal removal, etc.
In addition, NF membrane process
also exhibits great potential in
replacing the ultrafiltration (UF)
membrane process for RO pretreatment
due to its better organic and
salt rejection for effective mitigation
of fouling and scaling issues of the
subsequent RO membrane process.
To date, most of the commercial NF
membranes are available in spiralwound
modules that are consisted of
flat-sheet NF membranes.
The major suppliers of spiral-wound
NF modules in the market are DuPont
(FilmTec), Hydranautics, Suez (GE-
Osmonics), and Toray, to name a few.
These NF membranes are usually
made up of polyamide and produced
through interfacial polymerisation
process which forms a thin skin layer
with pore size down to 200 Dalton
molecular weight cut-off (MWCO) on
top of a substrate membrane.
Since the chemical composition and
module configuration of these NF
membranes are fairly similar to those
of the RO membranes, they share
numerous similarities in terms of the
membrane characteristics, application
conditions, as well as the technical
limitations. It is well-known to the
industry that the omnipresence of
chlorine in water is detrimental towards
the polyamide spiral-wound NF or
RO membranes since the polyamide
bond can be easily degraded by free
chlorine, which leads to performance
loss over the long-term operation.
Moreover, polyamide membranes
barely withstand the high and low pH
conditions because of the hydrolysis of
polyamide bond.
NANOPURE HOLLOW FIBRE
NF MEMBRANE
Herein, Mempure offers an alternative
option with the novel hollow fibre
NF membrane, i.e., NanoPure, for
water and wastewater treatment
applications. In contrast to the
spiral-wound membrane module, the
configuration of NanoPure membrane
module (Fig. 1) is much simpler and
identical to the traditional hollow fibre
UF membrane module which exhibits
several advantages e.g., low operating
pressure, high packing density, etc.
The operating pressure for NanoPure
membrane module is designed at
three to four bar that consumes much
less energy as compared to that of
the spiral-wound configuration that
usually requires more than seven
bar. Notably, NanoPure is backwashable
that reduces the membrane
fouling tendency owing to its robust
membrane structure. The fabrication
of NanoPure membrane utilises
charged-polymeric solutions that does
not comprise any polyamide-related
material and thus able to handle tough
feed water conditions.
For instance, the membrane
performance is able to sustain
under the chlorine exposure up to
250,000ppm/hr contact time, which
is significantly higher than that of the
polyamide NF membrane, typically at
1,000ppm/hr.
Hence, the membrane is able to
operate with 1-5ppm free chlorine
in the feed water and the free
WATER & WASTEWATER ASIA | JANUARY/FEBRUARY 2023 11