Desalination with Reverse Osmosis
Desalination Installation Design
Scaling and Antiscalant
Softening Water and Filtering Water
Before Buying a Water Treatment Solution
What does a typical R.O. take out of water?
Desalination is the removal of salts
from sea water or brackish water to produce drinking water.
Normal sea water can contain up to a
few percents of salt, while drinking water contains only more than
10 ppm of salt.
The most commonly used technique to
desalinate is the use of reversed osmosis. This technique is able to
remove the salts in one step.
Due to the high osmotic pressure of sea
water the working pressure can be very high (up to 100 bar) for
reversed osmosis desalination systems.
Alternative desalination techniques are
distillation, electrodialysis, ion exchange and carbonate removal.
Desalination with Reverse Osmosis
Osmosis is a natural phenomenon,
without which life would be made impossible. Osmotic processes
enable plants to absorb nutrients from the ground. Our kidneys
purify the blood in our bodies by means of osmosis.
Although osmosis has been discovered
and studied as early as 1850, it has taken us until 1960 to be able
to apply the process for water desalination.
Membrane filtration has been approached
as a futuristic, expensive and complex process for a very long time.
However, in the past fifteen years this process has developed to a
mature and reliable technique, which is very usable for (drinking)
When we are dealing with a system,
consisting of two fluids that are separated by a semi permeable
membrane (permeable, just not for salts) and we add salt on one side
of the system, pure water will start flowing through the membrane.
This flow will continue until the pressure is equal on both sides of
the membrane. Afterwards the water level will be higher where the
salt was added. The difference in water level, caused by the
addition of a specific amount of salt, is called osmotic pressure.
The osmotic pressure of seawater is around 26 bar.
We can explain the term “reverse
osmosis” as follows:
To desalinate water, we must create a
flow through a membrane, causing the water to leave the salty side
of the membrane, flowing into the unsalted side. To achieve this,
pressure must be created upon the water column on the salt side of
the membrane; firstly, to remove the natural osmotic pressure and
secondly, to create extra pressure on the water column, in order to
push the water through the membrane. For the desalination of
seawater, the pressure must be about 50-60 bars.
There are several different techniques
that can be applied for water desalination. Examples are reverse
osmosis, electro dialysis, distillation and ion exchange (see image).
Reverse osmosis is the most economic
process for the desalination of brackish water and seawater. When we
compare this process to the traditional thermic process of
distillation, the capital investments and the energy use are much
Reverse Osmosis modules
There are four different types of
reverse osmosis modules, which are used for reverse osmosis
processes, mainly desalination processes. These are the tubular,
plate & frame, spiral, and hollow fiber modules.
Tubular, plate & frame >> hollow fiber,
Spiral > hollow fiber > plate & frame >
Required system space:
Tubular >> plate & frame > spiral >
Susceptibility to fouling:
Hollow fiber >> spiral > plate & frame
Tubular > plate & frame > hollow fiber
The system costs for the treatment of
mineral water are the same for spiral modules and hollow fiber
modules. Pre-treatment costs for the purification of surface water
are higher when hollow fiber membranes are used. These membranes
require a more specific pre-treatment, because they are more
susceptible to fouling.
The use of tubular modules and plate &
frame modules may be more expensive than that of hollow fiber
modules or spiral modules. The costs of the use of tubular modules
and plate & frame modules are approximately equal.
The required system space for the
modules is very high when hollow fiber modules or spiral modules are
used. Tubular modules take up much less space.
It has been said that for reverse
osmosis systems for the desalination of seawater, spiral membranes
are used most often.
A desalination installation consists of
pre-treatment system, a
desalination unit and a subsequential treatment. A high-pressure
pump increases the feed water pressure to the required workload. The
workload is considerably higher than the osmotic pressure, in order
to bring about a satisfactory flow through the membrane. A higher
workload will result in a higher amount of treated water. A
A high-pressure switch regulates the pressure within the membranes.
The installation also contains flowmeters and pressure meters for
process regulation. Furthermore, there are a number of security
measures such as a low-pressure switch and a conductivitymeter with
a security barrier.
Membrane element with pressure vessel
The desalination unit in a desalination installation consists of a
number of Reverse Osmosis membranes in several pressure vessels,
which are placed in a certain order. The feed water that is pumped
into the module will be separated into water with a low salt content
and water with a high salt content.
The water with a low salt content is called ‘permeate’ and the water
with a high salt content is called ‘concentrate’. The concentrate
flow is regulated by a pressure vessel, which is placed within the
concentrate. The pressure vessel regulates the percentage of feed
water that leaves the module through the concentrate flow.
spiral module that consists of
multiple membranes situated in a pressure vessel, the feed water
flows within the pressure vessel, through the spiral channels of the
* graphic images come from FILMTEC membranes
The spiral membranes in the pressure vessel that are interconnected
will make sure that the feed water becomes more concentrated. When
the concentrate passes through the last membrane, it will reach a
pressure lid, which releases pressure.
The permeate of each membrane element is collected in a tube that is
place centrally in each spiral membrane. Outside the pressure vessel
a drain catches the permeate off the spiral membranes.
A specific benefit of
spiral membrane units is the fact
that they can be arranged serially within any pressure vessel. This
means that the feed water flows through a membrane with one tubular
system multiple times.
Hollow fiber units do not have this benefit, which means that they
need separate drains for the feed water, the permeate and the
concentrate. Hollow fiber installations that are built on a grand
scale will have very high system costs, mainly for the built-up of
the drainage system.
Desalination Installation Design
The desalination installation is a
complete system, with feed water input and separate discharge pipes
for concentrate and permeate. The data of the input and output pipes
should always be compared with
water analysis, feed water pressure
The designer of a Reverse Osmosis system will aim at the lowest
possible membrane pressure and installation costs and maximum
recovery and salt retention.
The recovery of a desalination installation for brackish water is
around 85%. It depends on the solubility of suspended solids that
are present in the feed water. During seawater desalination a
recovery of forty to fifty percent is desirable. The recovery of
seawater desalination depends on the osmotic pressure of the feed
water and the applied membrane types during the desalination
A membrane filtration system is usually
designed to attend a continuous process. The choice for a continuous
process can be made, because the process conditions, such as feed
water flow and permeate flow, are continuous.
Schematic representation of a continuous process
The same goes for Reverse Osmosis
systems. These are also designed to attend a continuous process with
a continual permeate flow and a system recovery that is steady.
Variations in water temperature and fouling degree of the feed water
are compensated by adjustments of the feed pressure.
Systems that consist of more than one
stage are called multi-stage systems. These systems can reach higher
system recoveries, without exceeding the single element recovery
limits. To gain a recovery of up to 70%, two stages must be
implemented in the feed water treatment system.
Schematic representation of a two-stage system
For higher recoveries, three stages
must be used. These values are based on the assumption that standard
pressure vessels with six elements are used. For shorter vessels the
number of stages must be doubled.
When the system recovery is higher, more membrane elements have to
be connected in series. A typical two-stage system uses a stage
ratio of 2:1 for the desalination of seawater with high dissolved
Plug-flow and concentrate
The plug-flow concept is the standard
Reverse Osmosis system design for water desalination applications.
The feed water is passed through a plug-flow system only once. A
fraction of the feed water passes a membrane to produce permeate.
The rest of the feed water is not derived of salts and will become
more and more concentrated.
When the number of membrane elements in a plug-flow system is too
low to achieve a high enough system recovery, concentrate
recirculation may be implicated. During recirculation part of the
concentrate is directed back to the feed water side of the module.
The recycled concentrate mixes with the feed water and will be
treated once more.
Schematic representation of an
installation with concentrate recirculation options
The number of elements in each
Reverse Osmosis systems are usually
designed for a specific permeate flow. To achieve this flow, a
number of membrane elements is required. The number of membrane
elements that is placed within the installation depends upon the
For the desalination of seawater the limiting factor is the maximum
feed pressure; this may not exceeds 69 bars.
Based on the designed flux, the production per unit membrane can be
Production per element = flux * element surface
Number of elements = permeate flow / production per element
Number of pressure vessels = number of elements / number of elements
By means of the permeate flow and the required recovery, the feed
water flow is calculated:
Feed water flow = permeate flow / recovery
Feed water pressure
A certain feed pressure is required,
depending on the system design. The flux, the energy loss in the
system and the osmotic pressure determine the feed pressure that the
system requires. The required feed pressure will increase when the
membrane elements are becoming contaminated through the years. A
feed pump that enables a higher flow than the flow that is
theoretically required will than be implicated to keep the feed
pressure continual. A feed pump that increases the feed pressure by
25% will be satisfactory in practice.
When the system is started up, the initial situation is recorded.
All relevant parameters should be registered and noted in a log.
Based on this data the performance of the installation can be
examined and regulated after the system has been put into action.
During monitoring of the system,
measurements will take place of the flow, pressure and conductivity
of the water. To check the hydraulic affectivity of the system the
feed pressure per stage and the permeate flow need to be measured.
The feed pressure depends upon the temperature of the feed water.
When feed water temperatures are low, more pressure is required to
achieve the same recovery that would be reached when feed water
temperatures are high. When water temperatures fluctuate, one needs
to normalize the permeate flow, to enable comparison with the
When the installations function correctly the conductivity of the
permeate is low, because of the removal of univalent and bivalent
ions. When a leak is situated in the membrane element, the
conductivity will increase. That is why conductivity measurements
These measurements take place in the permeate collection drain.
Measurements can be performed for each individual stack or for all
A sufficient monitoring of the system will enable the user to know
when the system needs cleaning.
Within desalination installations,
there is a course environment when it comes to corrosion of separate
parts of the system. Because of this, the material needs to possess
a certain resistance to corrosion. This goes for external parts,
which are exposed to a salty atmosphere (spillage, leaks), as well
as for internal parts. Corrosion of external system parts can
usually be prevented by providing them with a surface layer
(painting, galvanizing) and by periodic maintenance of the system
and closing of leaks.
Despite the fact that materials are protected against potential
corrosion, they also need to be able to be resistant to pressure,
vibrations and changing temperatures.
To prevent corrosion and chemical reactions in the part of the
system where pressures are low (<10 bar), such as in membrane
elements and pressure vessels, non-metals such as
PVC and fibreglass are often
applied. For high-pressure parts (10-70 bar), such as pumps, drains
and lids, one needs to use metals to provide them with the same kind
PVC and some metals cannot
sufficiently resist corrosion. When they start corroding they can
contaminate membranes. When corrosion protection is in order, we
need to keep this in mind.
The main material that is used for high-pressure parts is stainless
steel. The benefit of stainless steel is that it is resistant to
corrosion and erosion corrosion. Stainless steel is rarely stricken
by galvanic corrosion.
Pipes and parts of the installation should follow up demands of the
company where the installation is located. Pipes and components of
the installation are usually built using the following materials:
Candle filters and pressure vessel:
polypropylene filter in
PVC or stainless steel
Pumps: stainless steel
High-pressure pipes: stainless steel
Cleansing system: PVC or other chemical-resistant synthetic material
In a desalination system the
concentrate is released under high pressure, that is why it is
important to win back energy from the concentrate flow. This can be
done by application of a pressure exchanger. The concentrate flow
from the membranes is directed through the pressure exchanger, where
it directly transfers energy to part of the incoming feed water with
The feed water flow now has the same volume as the concentrate flow.
It will be directed to a small booster pump that corrects the
hydraulic losses of the flow.
The upgraded feed water flow will join the feed water flow from the
In an installation that uses a pressure exchanger, the high-pressure
pump will give off 41% of the energy, the booster pump will give off
2% of the energy and the pressure exchanger will give off the
remaining 57%. The pressure exchanger does not use any external
energy, so the total energy savings will be 57%.
By practising pressure directly to incoming seawater in a membrane
system, a reduction of 60% in size of the high-pressure pump can be
achieved. This does not only save energy; it also saves purchase
costs of high-pressure pumps.
* Graphic images come from FILMTEC
For the preservation of the affectivity
and life span of a Reverse Osmosis (RO) installation, a sufficient
pre-treatment is required. A proper selection of pre-treatment
methods for feed water will improve affectivity and extend the life
span of the system by preventing or minimizing biofouling, scaling
and membrane plugging.
To perform an uninterrupted and
reliable pre-treatment of the feed water a special approach is used.
A pre-treatment that is not geared to the installation may cause a
system overload. When this occurs the system parts need cleaning
much more often to restore productivity and salt retention. Cleaning
costs, system performance and standstill time are very significant
in that situation.
The kind of pre-treatment system that
is used greatly depends on feed water quality. Consequentially,
sufficient feed water pre-treatment is dependent on:
The source of the feed water
The composition of the feed water
The function of the feed water
When the source of the feed water that
needs treatment is specified, a complete and exact water analysis is
performed. This action is an important step for the design of a
pre-treatment system and the entire Reverse Osmosis system, because
this often determines the type and size of the pre-treatment.
Feed water analysis
Most water types that are treated in a
Reverse Osmosis system are either:
Brackish water with a low salt
content and a total dissolved solids content of up to 5,000 ppm,
Brackish water with a high salt
content and a total dissolved solids content of between 5,000
and 15,000 ppm, or
Seawater with a total dissolved
solids content of around 35,000 ppm
Seawater with a total dissolved solids
content of around 35,000 ppm is regarded standard seawater, because
of its global abundance. Globally, the differences in total
dissolved solids content of seawater can be very sufficient. The
Baltic Sea for example, has a total dissolved solids content of
about 7,000 mg/L, whereas the total dissolved solids content of the
Red Sea is 45,000 mg/L.
Standard values for the concentrations
of dissolved solids in seawater have been determined. During
seawater analysis, one should always keep in mind that land
influences are of great significance for the composition of
seawater. A sample that is taken on open sea can have a very
different composition than a sample that is taken near the shore.
The limiting factor for seawater
treatment with a Reverse Osmosis system is the osmotic pressure,
caused by a high total dissolved solids content. The limiting factor
for the treatment of brackish water with a Reverse Osmosis system is
mainly its chemical nature. This means precipitation and scaling
(caused by calcium carbonate or sulphates). The chemical composition
of brackish waters varies greatly and is very location specific.
To create a satisfactory process
design, a very accurate water analysis should be carried out. On a
water analysis form the concentrations of inorganic salts and other
data of the feed water are collected. The water analysis should be
fully balanced. When this is not the case sodium or chlorine ions
should be added, in order to neutralize the water balance.
Scaling prevention and control
Scaling is the accumulation of
(partially) insoluble salts in a membrane. When a Reverse Osmosis
installation has a recovery of 50%, the concentration of salts in
the concentrate flow is double the concentration of salts in the
feed water flow. When recovery increases, the chances of scaling
increase, as well. Because of this it is of great importance that
the saturation limits of the (partially) insoluble salts are not
Partially insoluble salts that cause
problems in a Reverse Osmosis system are mainly calcium carbonate
(CaCO3), calcium sulphate (CaSO4) and silica, but also calcium
fluoride (CaF2), barium sulphate (BaSO4) and strontium sulphate
Calcium carbonate scaling can be
prevented by the addition of acids. Adding anti-scalents can prevent
the precipitation of barium and strontium salts, silicates and iron.
One must always keep in mind that anti-scalents can add to
biofouling (microrganisms fouling). This can be prevented by
tightening the demands of applied chemicals and by using chemicals
with a different composition.
The use of anti-scalents cannot always
be avoided. Decreasing recovery by 25% will reduce the chances of
over-saturation of precipitating salts (scaling).
The membrane elements will need
replacement within one or two years, because that causes the use of
anti-scalents to be limited. This method is applied for relatively
small installations, like installations that produce drinking water
from seawater for domestic use.
Another cleaning method is opening the
concentrate lid to a forward flush under low pressure.
Prevention of fouling by colloids
In Reverse Osmosis elements colloidal
pollution can seriously diminish the performance by decreasing
productivity. An early sign of this pollutant is usually an
increasing pressure gradient. The sources of this pollution in feed
water can vary greatly. They are usually bacteria, clay, and iron
Chemical products that are used during
pre-treatment may also cause fouling of membranes. The best
available technique for the determination of the fouling potential
of feed water by colloids is the MFI (Modified Fouling Index)
measurement. This is an important type of measurement that takes
place prior to the design of a pre-treatment system. This
measurement must be done regularly when the Reverse Osmosis system
is put to use.
In order to maintain a MFI of (less
than) five, there are a number of pre-treatment methods to decrease
Sand filtration and candle
Ultra filtration and micro
Coagulation and flocculation
All surface water contains
microrganisms, algae, fungi, viruses and higher organisms.
Microrganisms are colloidal pollutants, because they are never
larger than 1 to 3 nm. Because of this, bacteria can be removed with
techniques that remove colloids.
The consequences of biological growth
within a membrane system negatively influence the system, just like
other contaminants. The symptoms of fouling can vary from a higher
pressure at the supplier side of the membrane, to telescoping
(shoving apart of the membrane) and membrane damage. Sometimes bio
film can develop in the permeate; causing it to become polluted
The number of microrganisms in the
surface water, in the feed water and in the concentrate can provide
us with valuable information on the degree of contamination of the
water. The types and amounts of nutrients present in the feed water
are factors that determine bio film growth. Despite the fact that
there are several investigators that determine the growth of bio
films, it has not been fully comprehended yet.
The best way to discover biofouling
during its development phase is to check a testing surface in the
feed water flow. The ‘Robin Sampler’ is a simple device, which
exposes small testing surfaces to water. These surface check-ups can
be performed regularly and can be tested for the growth and
attachment of bio film.
A careful periodic inspection of candle
filters and pipelines can also be useful. The presence of mucous or
strange smells can be an indication of the occurrence of bio film.
Membrane technology has become a
dignified separation technology over the past decennia. The main
force of membrane technology is the fact that it works without the
addition of chemicals, with a relatively low energy use and easy and
well-arranged process conductions.
Membrane technology is a generic term
for a number of different, very characteristic separation processes.
These processes are of the same kind, because in each of them a
membrane is used. Membranes are used more and more often for the
creation of process water from groundwater, surface water or
wastewater. Membranes are now competitive for conventional
techniques. The membrane separation process is based on the presence
of semi permeable membranes.
The principle is quite simple: the
membrane acts as a very specific filter that will let water flow
through, while it catches suspended solids and other substances.
There are various methods to enable
substances to penetrate a membrane. Examples of these methods are
the applications of high pressure, the maintenance of a
concentration gradient on both sides of the membrane and the
introduction of an electric potential.
Membranes occupy through a selective
separation wall. Certain substances can pass through the membrane,
while other substances are caught.
Membrane filtration can be used as an
alternative for flocculation, sediment purification techniques,
adsorption (sand filters and active carbon filters, ion exchangers),
extraction and distillation.
There are two factors that determine
the affectivity of a membrane filtration process; selectivity and
productivity. Selectivity is expressed as a parameter called
retention or separation factor (expressed by the unit l/m2·h).
Productivity is expressed as a parameter called flux (expressed by
the unit l/m2·h). Selectivity and productivity are
Membrane filtration can be divided up
between micro and ultra filtration on the one hand and nano
filtration and Reverse Osmosis (RO or hyper filtration) on the other
When membrane filtration is used for
the removal of larger particles, micro filtration and ultra
filtration are applied. Because of the open character of the
membranes the productivity is high while the pressure differences
When salts need to be removed from
water, nano filtration and Reverse Osmosis are applied. Nano
filtration and RO membranes do not work according to the principle
of pores; separation takes place by diffusion through the membrane.
The pressure that is required to perform nano filtration and Reverse
Osmosis is much higher than the pressure required for micro and
ultra filtration, while productivity is much lower.
Membrane filtration has a number of
benefits over the existing water purification techniques:
It is a process that can take place
while temperatures are low. This is mainly important because it
enables the treatment of heat-sensitive matter. That is why
these applications are widely used for food production.
It is a process with low energy
cost. Most of the energy that is required is used to pump
liquids through the membrane. The total amount of energy that is
used is minor, compared to alternative techniques, such as
The process can easily be expanded.
Process management of membrane
Membrane filtration systems can be
managed in either dead-end flow or cross-flow. The purpose of the
optimization of the membrane techniques is the achievement of the
highest possible production for a long period of time, with
acceptable pollution levels.
The choice for a certain kind of
membrane system is determined by a great number of aspects, such as
costs, risks of plugging of the membranes, packing density and
cleaning opportunities. Membranes are never applied as one flat
plate, because this large surface often results in high investing
costs. That is why systems are built densely to enable a large
membrane surface to be put in the smallest possible volume.
Membranes are implemented in several types of modules. There are two
main types, called the tubular membrane system and the plate & frame
membrane system. Tubular membrane systems are divided up in tubular,
capillary and hollow fiber membranes. Plate & frame membranes are
divided up in spiral membranes and pillow-shaped membranes.
During membrane filtration processes
membrane fouling is inevitable, even with a sufficient
pre-treatment. The types and amounts of fouling are dependent on
many different factors, such as feed water quality, membrane type,
membrane materials and process design and control.
Particles, biofouling and scaling are
the three main types of fouling on a membrane. These contaminants
cause that a higher workload is required, to be able to guarantee a
continuous capacity of the membranes. At a certain point the
pressure will rise so much that it is no longer economically and
Scaling and Antiscalant
Scaling means the deposition of
particles on a membrane, causing it to plug. Without some means of
scale inhibition, reverse osmosis (RO) membranes and flow passages
within membrane elements will scale due to precipitation of
sparingly soluble gas, such as calcium carbonate, calcium sulfate,
barium sulfate and strontium sulfate. Most natural waters contain
relatively high concentrations of calcium, sulfate and bicarbonate
In membrane desalination operations at
high recovery ratios, the solubility limits of gypsum and calcite
exceed saturation levels leading to crystallization on membrane
surfaces. The surface blockage of the scale results in permeate flux
decline, reducing the efficiency of the process and increasing of
The effects of scale on the permeation
rate of RO systems are illustrated in the following figure.
Following an induction period, plant flow decrease rapidly. The
length of this period varies with the type of scale and the degree
of super saturation of the sparingly soluble salt.
As it is evident from the graph, the
induction period for calcium carbonate is much shorter than that for
sulfate scales, such as calcium sulfate. It is economically
preferable to prevent scaling formation, even if there are effective
cleaners for scale. Scale often plugs RO element feed passages,
making cleaning difficult and very time consuming. There is also the
risk that scaling will damage membrane surface.
There are three methods of scale
control commonly employed:
ion exchange softening
Acidification: acid addiction destroys
carbonate ions, removing one of the reactants necessary for calcium
carbonate precipitation. This is very effective in preventing the
precipitation of calcium carbonate, but ineffective in preventing
other types of scale. Additional disadvantages include the
corrosively of the acid, the cost of tanks and monitoring equipment
and the fact that acid lowers the pH of the RO permeate.
Ion exchange softening: this method
utilizes the sodium which is exchanged for magnesium and calcium
ions that are concentrated in the RO feed water, following the
Ca2+ + 2NaZ => 2Na+ + CaZ2
Mg2+ + 2NaZ => 2Na+ + MgZ2
represents the sodium exchange resin(
When all the sodium ions have been
replaces by calcium and magnesium, the resin must be regenerated
with a brine solution. Ion exchange softening eliminates the need
for continuous feed of either acid or antiscalant.
they are surface active materials that interfere with
precipitation reactions in three primary ways:
Threshold inhibition: it is the
ability of an antisclant to keep supersaturated solutions of
springly soluble salts.
Crystal modification: it is the
property of an antiscalants to distort crystal shapes, resulting
in soft non adherent scale. As a crystal begin to form at the
submicroscopic level, negative groups located on the antiscalant
molecule attack the positive charges on scale nuclei
interrupting the electronic balance necessary to propagate the
crystal growth. When treated with crystal modifiers, scale
crystals appear distorted, generally more oval in shape, and
Dispersion: dispersancy is the
ability of some antiscalants to adsorb on crystals or colloidal
particles and impart a high anionic charge, which tends to keep
the crystals separated. The high anionic charge also separates
particles from fixed anionic charges present on the membrane
During the past two decades new
generations of antiscalants have emerged commercially, in which the
active ingredients are mostly proprietary mixtures of various
molecular weight polycarboxylates and polyacrylates.
Calculation procedures exist for
predicting the likelihood of scale formation. Use of these
predictors depends upon an up-to-date water analysis and a knowledge
of system design parameters. The ions contained in the feed water
concentrate though the RO system, the point of maximum scale
potential is the concentrate stream. Antiscalant type and dosage is
therefore based upon the mineral analysis at this point.
It is important to find the
optimization of antiscalant treatment with respect to type and
dosage, identifying the proper antiscalant to use and the
dosage-induction type relationship for the extended level of super
Lenntech can help you in the
selection of the best antiscalant for your particular application.
Acid addition is not very cost
effective because of the cost of acid, tanks and monitoring
equipment. Unless removed by degasification, excess of carbon
dioxide contained in the permeate of acid-fed systems increases the
cost of ion exchange regeneration.
Antiscalants are relatively cheap
products and have no additional costs.
When compared to either acid or
antiscalant addition, the main disadvantage to softening is cost
factoring in equipment costs. Through a present worth analysis there
is no level of hardness in which softening competes economically
with antiscalants addition.
The following table gives a cost
comparison between softening and antiscalant treatment options for
different levels of hardness, on a basis of a RO system designed to
produce 75 gpm (17 m3/h) of permeate at 75% recovery.
There are a number of cleaning
techniques for the removal of membrane fouling. These techniques are
forward flushing, backward
flushing, air flushing and chemical cleaning, and any
combination of the methods.
several different membrane cleaning methods, such as forward flush,
backward flush and air flush.
forward flush is
applied, membranes are flushed with feed water or permeate
forward. The feed water or permeate flows through the system
more rapidly than during the production phase. Because of the
more rapid flow and the resulting turbulence, particles that are
absorbed to the membrane are released and discharged. The
particles that are absorbed to membrane pores are not released.
These particles can only be removed through backward flushing.
Backward flush is a
reversed filtration process. Permeate is flushed through the
feed water side of the system under pressure, applying twice the
flux that is used during filtration. When the flux has not
restored itself sufficiently after back flushing, a chemical
cleaning process can be applied.
chemical cleaning process,
membranes are soaked with a solution of chlorine bleach,
hydrochloric acid or hydrogen peroxide. First the solution soaks
into the membranes for a number of minutes and after that a
forward flush or backward flush is applied, causing the
contaminants to be rinsed out.
cleaning method is the so-called
air flush or air/ water flush.
This is a forward flush during which air is injected in the
supplier pipe. Because air is used (while the water speed
remains the same), a much more turbulent cleaning system is
flush is applied in a membrane, the barrier that is responsible for
dead-end management is opened. At the same time the membrane is
temporarily performing cross-flow filtration, without the production
The purpose of a forward flush is the removal of a constructed layer
of contaminants on the membrane through the creation of turbulence.
A high hydraulic pressure gradient is in order during forward flush.
backward flush is applied the pores of a membrane are flushed inside
out. The pressure on the permeate side of the membrane is higher
than the pressure within the membranes, causing the pores to be
cleaned. A backward flush is executed under a pressure that is a
bout 2.5 times greater than the production pressure.
Permeate is always used for a backward flush, because the permeate
chamber must always be free of contagion. A consequence of backward
flush is a decrease in recovery of the process. Because of this, a
backward flush must take up the smallest possible amount of time.
However, the flush must be maintained long enough to fully flush the
volume of a module at least once.
Air flush or air/ water flush
the membrane surface needs to be removed as effectively as possible
during backward flush. The so-called air flush, a concept developed
by Nuon in cooperation with DHV and X-flow, has proved to be very
useful to perform this process. Using air flush means flushing the
inside of membranes with an air/ water mixture.
During an air flush air is added to the forward flush, causing air
bubbles to form, which cause a higher turbulence. Because of this
turbulence the fouling is removed from the membrane surface.
The benefit of the air flush over the forward flush is that it uses
a smaller pumping capacity during the cleaning process.
above-mentioned cleaning methods are not effective enough to reduce
the flux to an acceptable level, it is necessary to clean the
During chemical cleaning chemicals, such as hydrogen chloride (HCl)
and nitric acid (HNO3), or disinfection agents, such as
hydrogen peroxide (H2O2) are added to the
permeate during backward flush. As soon as the entire module is
filled up with permeate, the chemicals need to soak in. After the
cleaning chemicals have fully soaked in, the module is flushed and,
finally, put back into production.
Cleaning methods are often combined. For example, one can use a
backward flush for the removal of pore fouling, followed by a
forward flush or air flush.
The cleaning method or strategy that is used is dependent on many
factors. In practise, the most suitable methods is determined by
trial and error (practise tests).
A biofilm is a layer of microorganisms
contained in a matrix (slime layer), which forms on surfaces in
contact with water. Incorporation of pathogens in biofilms can
protect the pathogens from concentrations of biocides that would
otherwise kill or inhibit those organisms freely suspended in water.
Biofilms provide a safe haven for
organisms like Listeria, E. coli and legionella where they can
reproduce to levels where contamination of products passing through
that water becomes inevitable.
It has been proven beyond doubt that
removes biofilm from water systems and prevents it from forming when
dosed at a continuous low level. Hypochlorite on the other hand has
been proven to have little effect on biofilms.
Softening Water and Filtering Water
Filtering water involves separating
mineral particles, like manganese, iron, hydrogen sulfide or other
organic matter, from pure H2O. By passing water through a "filter
bed," or "media bed," these granular particles are trapped - and
clean, purified water passes through the bed.
Softening water involves something
called "ion exchange" to remove dissolved minerals - like calcium,
magnesium, iron and manganese - that can't be trapped in a filter
bed. Softeners use fresh resin beads with sodium attached to the
resin. As water enters the tank, dissolved calcium and magnesium are
attracted to the resin. The resin passes up the sodium in exchange
for the dissolved chemicals and the water is then rid of these
Free of contaminants and minerals,
water treated with softeners or filtration systems will make an
impression on you and your household, and at the work place.
Whether you use a filter or a softener
depends on whether the contaminates in your water are particles or