INTRODUCTION >
Reverse Osmosis
By applying pressure to a fluid
on one side of a semi-permeable
membrane, it is possible to reverse
the natural flow of pure water from
an area of high salt concentration
to one of low concentration. This
process is called Reverse Osmosis.
Osmosis
In the process of Osmosis, water
naturally moves from an area of low
salt concentration through a semipermeable membrane to an area
of high salt concentration.
OSMOSIS
Water flows through a
membrane from the side
of low salt concentration
to the side of high salt
concentration.
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EQUILIBRIUM
Osmotic Pressure is
the pressure required
to stop the water flow
and reach equilibrium.
APPLIED PRESSURE
MEMBRANE
OSMOTIC
PRESSURE
The movement of a pure water to
equalise salt concentrations on each
side of the membrane generates a
pressure called ‘Osmotic Pressure’.
REVERSE OSMOSIS
By applying an
external pressure
greater than Osmotic
Pressure, the flow
of water is reversed.
Water now flows
from high to low salt
concentration.
Reverse
Osmosis
Reverse Osmosis (RO) is a filtration
method that removes many types
of large molecules and ions from
solutions by applying pressure to
the solution when it is on one side
of a selective, or semi-permeable,
membrane. The result is that salts
are retained on the pressurised side
of the membrane, and the pure water
is allowed to pass to the other side.
To be ‘selective’, the membrane
should not allow large molecules or
ions through the pores (holes), but
should allow smaller components of
the solution (i.e. the water itself) to
pass freely.
RO is most commonly known for its
use in drinking water purification
from seawater and brackish or waste
water, removing salts and other
unwanted substances from the water.
It also provides the highest levels of
all available filtration methods, and is
often used in combination with other
methods in order to optimise the
overall performance and operating
parameters of the system, based on
the feed water quality.
INTRODUCTION > Reverse Osmosis
Sustainable
Flux
In fluid dynamics, ‘flux’ is the rate of
volume flow across a unit area. Typical
units of measurement are: gallons per
square foot per day (gfd), or litres per
square metre per hour (lmh).
All RO systems look to maximise flux
(i.e. the volume of feed water passing
through a unit area of the membrane)
while minimising scaling and fouling
of the membrane. Scaling and fouling
lead to greater energy consumption,
poorer quality water being produced,
and higher maintenance costs.
Every membrane filtration system
has a critical flux, determined by the
nature of the feed water and the set of
system operating parameters. ‘Critical
flux’ is defined as the point where the
combination of the nature of the feed
water and the set of system operating
parameters results in a loss of flux due
to fouling, scaling or particle deposits
on the surface of the membrane.
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‘Sustainable flux’ is therefore the
flux that can be maintained over
an extended period for the given
feed water and system operating
parameters, with minimal scaling and
fouling of the membrane surface.
Membrane manufacturers supply
membrane elements with a set of
‘wet test’ parameters obtained via
tests on fouling and scaling free
water. In order to ensure stable
operation – which in most cases is
well below the wet test parameters –
the recommended system operating
parameters that will create a
sustainable flux are chosen to fall
below the critical flux.
For instance, a low pressure RO
element (8” or 16” diameter) may
show 30 gfd (~51 lmh) flux in the
manufacturer’s wet test for a given
membrane type. In the case of
8” diameter elements, however,
the critical flux may be just 12 gfd
(~20 lmh) under a normal set of
operating parameters for secondary
waste water – well below the wet
test flux.
If the membrane were to be
incorporated in a conventionallydesigned 16” diameter element, the
results would be identical, or even a
little lower. The operating parameters
would therefore typically be chosen at
10 gfd (~17 lmh) with the purpose of
operating within a safety margin.
Simply put, increasing the sustainable
flux rate of a RO system reduces the
membrane surface area required and,
therefore, the number of membrane
elements and pressure vessels needed.
This reduction in membrane elements
translates to a smaller plant size, and
lower capital and operating costs as
membrane replacement and general
maintenance costs are reduced.
A more efficient plant means
lower water-cleaning costs.
INTRODUCTION > Reverse Osmosis
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THE FILTRATION SPECTRUM
A comparison of the rejection capabilities of reverse osmosis with other membrane technologies.
NuWater RO
Treatment
Plants
NuWater RO treatment plants
utilise large diameter 16” pressure
vessels and membrane elements,
incorporating our proprietary
Integrated Flow Distributor (IFD)
and Electromagnetic Field (EMF)
device, as well as an innovative
membrane element configuration. This
combination of technologies radically
improves hydraulic conditions within
the pressure vessels and membrane
elements, resulting in a substantial
increase in critical flux, and therefore
sustainable flux.
NuWater 16" plants are
simply more efficient than
conventional 8" plants.
In the case of secondary waste water,
the critical flux in a NuWater RO
module is around 28 gfd (~48 lmh)
and the sustainable flux can therefore
be comfortably chosen at 24 gfd
(41 lmh). This is significantly better
than conventional 8” systems where
the corresponding sustainable flux
is in the region of 10 gfd (17 lmh).
Similar improvements in hydraulic
conditions are achieved for NuWater’s
03
seawater desalination plants. The
wet test data for a seawater element
typically indicates flux in the region
of 20 gfd (34 lmh). Most conventional
SWRO plants using conventional
8” elements operate at a sustainable
flux of 6 to 10 gfd (~10 to 17 lmh).
NuWater SWRO large diameter
16” plants consistently operate at
a significantly higher sustainable flux
of 13 to 16 gfd (~22 to 27 lmh).
The improved hydraulic conditions in
NuWater plant RO modules is achieved
by far greater control over fouling,
scaling and particle deposits on the
elements, with the IFD greatly reducing
– or completely eliminating – microbial
fouling in the front end of the plant.
This type of fouling is generally the
limiting parameter for sustainable
flux in conventional plants using 8”
diameter elements. The IFD ensures
an even distribution of the feed flow
and creates higher cross-flow velocity,
allowing flux to be increased without
the normally-associated fouling of the
membrane surface.
Our EMF device further reduces
fouling potential by causing microbes
(particles) to aggregate, thereby
preventing them from depositing
on the membrane surface. The EMF
device also changes the morphology
of potential scaling substances that
can form in the last few elements
of the vessel where solubility of the
substances may be exceeded. In a
NuWater plant, for instance, calcium
sulfate will precipitate as a ‘fluffy’,
instead of a crystalline substance,
which can form a boundary (or cake)
layer on the membrane. The fluffy
substance is easily swept away by the
concentrate flow without depositing
on the membrane surface, avoiding
harmful build-up of scale.
NuWater’s innovative 16” plants also
use a maximum of four elements per
pressure vessel, ensuring optimum
concentrate flow and facilitating a
simple ‘clean-in-place’ (CIP) process
that runs without interrupting
production. Membrane and plant
performance levels are therefore
maintained. Conversely, it is generally
accepted that conventional 8” RO
plants require a thorough off-line
cleaning procedure when the plant has
lost 10% to 15% flux due to fouling,
which results in a reduction in plant
capacity.
NuWater plants therefore
achieve something new and
unique; a high sustainable
flux over long periods of
time without performing
tedious, expensive and
time-consuming cleaning
procedures.
INTRODUCTION > Reverse Osmosis
NuWater
RO Benefits
The 16” RO technology incorporated
in NuWater’s water treatment plants
has confounded academics and
competitors alike, as it defies long-held
views on achievable sustainable flux.
NuWater’s plants consist of coupled
trailer-mounted modules, making them
scalable, more cost-effective and smaller
than conventional 8” RO plants.
These higher sustainable flux rates,
and the use of large diameter 16”
pressure vessels and membrane
elements, mean that NuWater plants
have footprints significantly smaller
than conventional 8” plants. This
allows NuWater large capacity plants
to be modular and mobile, providing
customers with the most flexible
options to address their water-cleaning
requirements.
Smaller and more efficient
plants allow NuWater to
offer competitively priced
fully managed watercleaning services with the
highest levels of customer
service that are difficult for
competitors to match.
Challenge us to clean your water.
Tap Tomorrow
04
INTRODUCTION > Reverse Osmosis
Tap Tomorrow
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WEBSITE
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Almost 70% of Earth’s fresh water is frozen in
ice sheets, glaciers, snow and permafrost, with
Antarctica holding about 90% of this water. The
only rivers in Antarctica are meltwater streams.