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DESALINATION
Desalination 143 (2002) 207-2 18
www.elsevier.com/locate/desal
Distillation vs. membrane filtration: overview of process
evolutions in seawater desalination
Bart Van der Bruggen
*, Carlo Vandecasteele
Depart ment of Chemi cal Engineeri ng, Uni versit y of Leuven, W I e Croy laan 46, B - 3001 Heverl ee, Bel gium
Tel. +32 16) 32 23 40; Fax +32 16) 32 29 91: emai l : bar t .vanderbr uggen@cit .kul euven.ac.be
Received 19 November 2001; accepted 15 January 2002
Abstract
The worldwide need for fresh water requires more and more plants for the treatment of non-conventional water
sources. During the last decades, seawater has become an important source of fresh water in many arid regions. The
traditional desalination processes [reverse osmosis (RO), multi stage flash (MSF), multi effect distillation (MED),
electrodialysis (ED)] have evoluated to reliable and established processes; current research focuses on process
improvements in view of a lower cost and a more environmentally friendly operation. This paper provides an
overview of recent process improvements in seawater desalination using RO, MSF, MED and ED. Important topics
that are discussed include the use of alternative energy sources (wind energy, solar energy, nuclear energy) for RO or
distillation processes, and the impact of the different desalination process on the environment; the implementation
of hybrid processes in seawater desalination; pretreatment of desalination plants by pressure driven membrane
processes (microfiltration, ultrafiltration and nanofiltration) compared to chemical pretreatment; new materials to
prevent corrosion in distillation processes; and the prevention of fouling in reverse osmosis units. These improvements
contribute to the cost effectiveness of the desalination process, and ensure a sustainable production of drinking
water on long terms in regions with limited reserves of fresh water.
Keywords:
Seawater; Reverse osmosis; MSF; MED; Electrodialysis; Pretreatment; Environmental impact; Hybrid
processes; Fouling
1 Introduction
fresh water is a fundamental need for most aspects
The supply of fresh water is a key element for
of life. Fresh water is needed in agriculture, as
all societies. Together with the supply of energy,
drinking water, or as process water in various
industries. Groundwater and/or surface water is
*Corresponding author.
not always sufficiently available, and the scarcity
001 l-9164/02/ - See front matter 0 2002 Elsevier
Science
B.V. All rights reserved
PII: SO0 1 I-9 164(02)00259-X
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is expected to increase in the future. Therefore,
alternative sources of water such as wastewater,
brackish water and seawater will gain importance
compared to the more traditional water sources.
Wastewater reuse after purification helps to
overcome water shortages, but it also decreases
the volume of wastewater to be discharged, which
is of high importance in view of new legislations
for wastewater discharge. Wastewater reuse is a
relatively new concept, but already used in many
industries [l-3], and even for drinking water
purposes [4]. The technique to be used depends
largely on the specific application, and in many
cases more research is needed to conclude on the
right technique to be applied, and on the process
parameters.
Seawater desalination, on the other hand, has
become a reliable method for water supply all
over the world. It has already been practised
succesfully for many decades and the technical
and economical feasibility is obvious. However,
the common processes for seawater desalination
[multi-effect distillation (MED), multi-stage flash
(MSF), reverse osmosis (RO), and electrodialysis/
electrodialysis reversal (ED/EDR) for treatment
of brackish water] have evoluated from expensive
techniques requiring large quantities of energy
to a sustainable method for drinking water supply
[5,6]. The cost decreased to 0.50-0.80 /m3
desalinated water and even to 0.20-0.35 /m3 for
treatment of brackish water. These figures may
further decrease by new improvements in process
technology (especially the application of alter-
native energy sources). Automation and control
techniques are useful in the design and the operation
of expensive plants and should avoid cost increases
by keeping the process paramaters within the
specifications [7]. The desalinated water has
always been of excellent quality, practically
regardless of the influent quality. Analyses of the
permeate show that potable water can be
produced even without remineralisation [8].
However, problems may occur when e.g. the silt
density index (SDI) of the influent is too high,
which may cause membrane fouling in RO;
corrosion is another recurrent problem, mainly
in MSF.
This paper reviews the important advances in
seawater desalination in view of lowering the total
cost, and of decreasing the impact on the environ-
ment. These advances should allow producing
drinking water at an affordable cost and a minimal
impact on the environment, so that large-scale
water production is feasible and that regional
economic development is not hindered by water
scarcity.
2.
Traditional desalination methods
2 1 Multi-e@ect distillation MED)
The MED process is the oldest technique for
seawater desalination, and the first reports of
MED date back to the middle of the 19th century
[9]. MED [5] is based on heat transport from con-
densing steam to seawater or brine in a series of
stages or effects (Fig. 1). In the first effect, primary
steam is condensed for the evaporation of
preheated seawater. The secondary steam that is
generated in this way is brought to a second effect,
operated at slightly lower temperature and pressure;
the primary steam condensate is recycled to the
steam generator. High heat transfer rates can be
achieved in the MED process due to the thin film
boiling and condensing conditions [6]. The design
can be horizontal (HTE) or vertical (VIE). In
the horizontal design the feedwater is sprayed
over the outside of the tubes, while condensation
occurs inside the tubes. Spray nozzles or per-
forated trays are used to distribute the feedwater
evenly over the heat transfer tubes. The vertical
design uses steam condensation outside the tubes,
with feedwater flowing down as a film on the
inner side of the tubes.
Problems that may occur with MED are
related to corrosion and scaling of oversaturated
compounds such as CaSO,. These problems can
be very important because of the intense contact
between both steam and brine with the heat
exchangers. The performance ratio of water
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seawater
steam
1 evaporator
steam 95C
I-----1
2d evaporator
90C
I-----I
cooling water
Fig. I. Principle of MED (multi-effect distillation).
reshwater
production to steam consumption is generally
very high in MED, dependent on the number of
effects and approximately equal to the number
of effects minus one. The number of effects is
limited by a maximal temperature of about 120C
in the first effect (because of the risk of scaling)
and a minimal temperature in the last effect that
allows heating of the incoming seawater.
Additionally, a minimal temperature difference
of 5C is needed in each effect. Therefore, the
number of effects is usually between 8 and 16.
2.2. Multi-stage flash MSF)
MSF came into practice in the early 1960s and
became the most common process for seawater
desalination for the next few decades, due to its
reliability and simplicity [lo]. The principle of
operation in MSF is based upon a series of flash
chambers where steam is generated from saline
feedwater at a progressively reduced pressure
(Fig. 2). The steam is condensed by heat exchange
with a series of closed pipes where the seawater
Steam heater
Brine recirculation
Fig. 2. Principle of MSF (multi-stage flash).
SC
out
:awater in
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to be desalted is preheated. Collector trays are
used to gather the condensate, which is obtained
as the desired product. The exhausted brine is
partly recirculated to obtain a higher water
recovery, and partly rejected to the sea.
The main advantage of the MSF process is
the ease and reliability of the process. Heat exchange
with the saline water does not occur through heat
transfer surfaces, so that there is no risk of reduced
heat transfer by scaling. Precipitation of inorganics
may happen within the chambers, and can be
reduced by applying acid or antiscalants. The top
brine temperature is limited to about 110C by
the risk of scaling. Biocides may be added as well
to reduce growth of bacteria; these products will
not end up in the product water because of the
concept of the process. MSF is also insensitive
to the initial feed concentrations and to the
presence of suspended particles. The product
water contains about 50 ppm of total dissolved
salts.
Corrosion is easier to control with MSF com-
pared to MED, because the design is less complex.
The most important disadvantage of MSF is
the lower performance ratio, limited at about 11.
This results in a much higher energy consumption,
which makes MSF a more expensive technique
than MED and only economically competitive
when energy costs are very low [6]. However,
Seawater intake
MSF is still an important process for seawater
desalination, although there is a clear tendency
towards MED and RO.
2.3. Reverse osmosis
Brackish water desalination was the first
succesful application of reverse osmosis [ 1 I], and
the first large-scale plants appeared already in the
late 1960s. In the next decade, new RO membranes
with considerably higher permeability appeared,
which made RO suitable for seawater desali-
nation. In the 1980s RO became competitive with
the classical distillation techniques.
Reverse osmosis is a membrane separation
process in which the seawater permeates through
a membrane by applying a pressure larger than
the osmotic pressure of the seawater (Fig. 3). The
membrane is permeable for water, but not for the
dissolved salts. In this way, a separation between
a pure water fraction (the permeate) and a con-
centrated fraction (the retentate or concentrate)
is obtained. Pressures needed for the separation
were as high as 120 bar in the early days of RO,
but are nowadays usually in the range of 50 bar
for seawater, 20 bar for brackish water. Most RO
membranes are polymeric thin-film composite
membranes, consisting of a very thin separating
layer and a number of supporting layers with
Cartridge filter
A
Module
Storage tank
Fig. 3. Principle of desalination by reverse osmosis (RO).
u
@
Product water
High pressure
pump
Brine
- NaHS03
(discharge)
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much lower resistance against mass transport
[ 121. The membranes are usually configured in
spiral-wound modules, where the seawater flows
between two flat membrane sheets wrapped around
a central tube. An alternative are the hollow fiber
membranes, where membrane tubes of approxi-
mately 0.5 mm are used.
The advantage of reverse osmosis is the low
cost of the product water, which can be around
0.50-0.70 US /m3, compared to 1.0-I .4 US /m3
for MSF and MED, depending on the energy cost
[6,13]. Energy consumption in RO is low compared
to distillation processes, although pumping costs
are still considerable. The permeate quality is very
good, with total dissolved solids concentrations
between 100 and 500 ppm. Pollutions of small
organic molecules or e.g. carbon dioxide may occur,
to be avoided by aerating.
The disadvantage of RO is the sensitivity of
RO membranes to fouling by e.g. suspended solids,
and to damage by oxidized compounds such as
chlorine or chlorine oxides. Pretreatment is usually
needed to ensure a stable performance of the
module; optimization of the pretreatment is one
of the most critical aspects of RO. Scaling of e.g.
CaCO,, CaSO, and BaSO, is another possible
problem, depending on the recovery ratio of
permeate production and feed. At the usual
recovery of 50%, scaling can be effectively pre-
vented by adding antiscalants to the water;
increasing the recovery has a negative impact on
membrane scaling.
2.4.
Other techniques
Among the other techniques for seawater or
brackish water desalination, electrodialysis (ED)
or electrodialysis reversal (EDR) is still the most
promising technique, although the expected
breakthrough has never been realised. ED/EDR
is based upon transport of the dissolved salts through
a stack of cationic and anionic membranes by
applying an electric potential, so that a diluted stream
is obtained (Fig. 4). The cost for desalination
largely depends on the concentration of salts to
Calho
(-1
node
+)
Fig. 4. Operation principle of ED/EDR.
be removed. The process becomes ineconomical
for large salt fractions, but is competitive for brackish
water desalination. For water with low salt con-
centrations, ED/EDR is considered to be the most
advantageous technique.
Vapor compression (VC) is a technique that
is used for small-scale plants. The technique is
comparable to MED, but it is based on compression
of the vapor generated by evaporating water
instead of condensation [5], so that the latent heat
of the vapor can be efficiently reused in the
evaporation process. Vapor compression can be
seen as a variation of MED, but technically
somewhat more complex, so that application is
limited to smaller plants. However, a better
process control might result in a shift towards
MED and VC.
The use of a simple
solar desalination,
consisting
of a transparant cover allowing sun radiation,
where seawater evaporates under the cover and
is collected on the sides after condensation on
the glass, has been frequently considered, but is
economically not feasible since only 3 1 of
permeate are obtained per m*. A recent study,
however, claims that solar distillation of seawater
can be economical on a large scale in a cost
effective way, by optimizing materials and system
design [ 141. Other experiments involved freezing
of the salts from the seawater or extraction with
organic solvents, but these techniques have never
passed the experimental stage.
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3 Hybrid desalination processes
The possibility to combine different desali-
nation processes in view of a synergetic effect
has already been suggested over a decade ago
[
15,161. The benefits of RO in particular could
be used in combination with distillation plants
(usually MSF, possibly also MED or VC). This
should allow a greater flexibility in dual-purpose
plants for the cogeneration of water and elec-
tricity, because the RO facilities can cover the
water demand when the electricity needs are low
[ 171. The RO operates at maximal permeability,
because of the positive influence of preheating
the seawater (optimization of energy reuse - a
flux increase of 2.5% per degree Celsius tempera-
ture increase is to be expected). In practice, the
water flux has an upper limit because of fouling
considerations. The desired flux at elevated temp-
eratures is obtained by decreasing the trans-
membrane pressure, so that energy consumption
is lower at the same production level.
The RO facility can be operated in a single
step; the permeate can then be used for blending
the distillation product, so that the required
freshwater quality is obtained without the need
for using local groundwater. The next step in this
evolution is the replacement of the RO by low
pressure RO units or even nanofiltration units.
The resulting permeate quality will be lower, but
the final blended product would still meet the
quality requirements for freshwater. Pilot plant
results indicate that significant improvements in
RO product water flow rate and overall energy
savings can be obtained without decreasing the
product quality [17]. These cost savings also
allow using the dual-purpose plant in areas where
energy cost is relatively high [18]. The hybrid
system MSF/RO is now considered to be a valuable
and economic alternative for desalination in dual-
purpose plants [19], whereas the use of MSF in
single-purpose plants is decreasing. Real-scale
hybrid plants are still not common for desalina-
tion, but experiments show the feasilibility of the
process. The development of hybrid MSF/RO
processes is considered one of the most important
advances in seawater desalination during the last
years [201.
4.
Alternative energy sources for desalination
Water desalination is a process that requires
large quantities of energy. This implies that
desalination can be very economical when the
energy cost is low, as is the case of a number of
Middle East countries. However, large arid areas
exist where no traditional energy sources are
available; the cost for fresh water in these areas
is too high to ensure the water supply for popu-
lation and the development of a local economy
(including agriculture). Furthermore, traditional
energy resources on earth are limited, and energy
costs may change significantly. In this view, the
research about the use of alternative energy
sources is an important future-oriented project.
Two different approaches can be used for the
implementation of seawater desalination with
lower energy costs: optimization and minimi-
zation of the energy consumption, or the use of
alternative energy sources. Minimization of
energy consumption can be done by using dual-
purpose plants and hybrid processes, as discussed
above, or by slight changes in the design of
traditional processes. Dual-purpose plants are
usually based on power plants, but other examples
can be found, such as the coupling of MED
seawater desalination to the (highly exothermic)
production of sulphuric acid [21]. Examples of
changes in the design of traditional processes are
the use of combustion gas turbines instead of a
steam turbine, condenser and cooling tower in
the initial stage of a MED plant [22], and energy
reuse in RO [23], particularly by the use of a
pressure exchange system (PES) for RO [24], in
which the energy content of the high-pressure
brine is transferred to the feed by a hydraulic
mechanism, as an alternative to the mechanical
energy recovery system based on turbines.
The use of alternative (renewable) energy
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sources for desalination purposes has been
extensively studied, but the market share for such
techniques are to date still marginal. However, a
number of interesting possibilities has been
suggested, for which the technical and econo-
mical feasibility seems promising. The use of
solar energy for seawater distillation was the first
option that was explored, as an improved com-
bination of solar distillation and MED [25,26].
Solar energy can be used for preheating the
seawater, or for steam generation. Different
systems can be used, among which the salt
gradient solar ponds [27] and the parabolic trough
[28-301 are the most common. The cost effect-
iveness of salt gradient solar ponds and dual-
purpose electric power stations for MED and a
hybrid MED/RO system [3 l] depends largely on
the site of the plant; partial solar systems with
conventional energy back-up are the most cost-
competitive (for continuous operation). To this
date, solar energy can still not compete favorably
with fossil energy at current crude oil market
prices, except for (sunny) remote areas where
solar energy can be an attractive alternative [32].
The combination of RO with photovoltaic
cells involves the conversion of thermal energy
to mechanical energy, which seems to be a more
complex than the respective distillation processes.
However, due to the smaller energy consuption
in RO, the use of solar energy has proven to be
very cost effective for sunny areas by introducing
a secondary steam cycle powered by solar energy
[33]. A small photovoltaic/reverse osmosis plant
with a capacity of around 1 m3/d was installed on
the island of Gran Canaria and is currently
succesfully operated [34]. The coupling with
photovoltaic systems would also be feasible with
electrodialysis [35].
Wind-powered desalination is another option
that seems to be attractive, especially for use on
(windy) islands, where many options exist for
exploitation of wind power. Gran Canaria (Spain)
is a typical example of such a location; two wind-
powered RO systems are operated on different
islands of the archipelago [36]. Wind power can
significantly reduce the unit cost of produced
water in RO, provided that the regional wind
mean velocities are higher than 5 m/s [37].
The possible use of nuclear energy for sea-
water desalination has been explored by the
International Atomic Energy Agency (IAEA)
[38]. This should be seen as a dual-purpose plant
for the production of electricity and fresh water,
where part of the energy is used for the desalina-
tion process. This coupled process has no technical
impediments, and the desalination of seawater
using nuclear energy seems to be a cost competitive
and feasible option for potable water production.
Another possibility to decrease the consumption
of conventional energy sources is the use of
ambient energy [39]. The basis of this system is
an innovative endothermic energy harvesting
collector, which consists of a liquid-filled roof
or wall cladding that is in thermal contact with
the atmosphere. Heat energy originating from the
atmosphere is redistributed by a heat pump and
can be used for e.g. desalination processes. Experi-
ments with flash evaporation at low temperature
show that desalination using ambient energy is
feasible, although this technique seems to be
especially suitable for small-scale projects.
5. Pretreatment of seawater
Feed pretreatment is one of the major factors
determining the success or failure of a desalina-
tion installation. This is particularly imperative
for RO [40], but for distillation processes it is
also highly important. Traditional pretreatment
is based on mechanical treatment (media filters,
cartridge filters) supported by an extensive
chemical treatment, including chlorination,
flocculant dosing (FeCl,), chlorine scavenger
dosing (NaHSO,), and acid (H,SO,) dosing for
scaling prevention. Specific additives have to be
used for prevention of corrosion, and for the
preservation of the membranes in case of an RO
system. This results in a complicated system of
reagent addition at various points in the process,
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in which problems with e.g. biofouling after
addition of NaHSO, [40], or fouling by organic
compounds. Seasonal variations in seawater quality
further cause difficulties in process control [41].
Moreover, frequent chemical cleaning is needed
to prevent efficiency loss in the process. As a
result, the pretreatment may account for a
significant part of the total costs [42].
Conventional pretreatment can be minimized
if a beachwell intake is used [43]. However, this
is not always technically possible and it is very
susceptible to breakthrough. Pressure driven
membrane processes (microfiltration, ultra-
filtration, nanofiltration) are the new trend in
designing pretreatment systems. Microfiltration
(MF) is an obvious technique for the removal of
suspended solids and for lowering the silt density
index (SDI). Energy consumption in MF is
relatively low, so that the total costs for the MF
pretreatment are comparable to beachwell intake
[43], whereas the cost for a corresponding
conventional pretreatment is more than double.
MF generally provides an RO feedwater of good
quality, with (slightly) lower COD/BOD, and a
lower SD1 in comparison to the untreated
seawater, although there is a large influence of
the feedwater quality. Good quality seawater may
be used for large SWRO plants with a minimal
pretreatment and at relatively low cost.
Further improvement of the RO feedwater can
be obtained by replacing MF by ultrafiltration
(UF). In UF, not only suspended solids and e.g.
large bacteria are retained, but also (dissolved)
macromolecules, colloids and smaller bacteria.
Somewhat larger pressures have to be applied, in
the range of l-5 bar, so that the cost is higher
than for MF, but competitive with conventional
pretreatment and even allowing a cost reduction
of about 10% by an increase in recovery rate and
permeate flux [44]. Values of 0.07 to 0.09 c /m3 for
UF pretreatment have been reported [45], On the
other hand, the UF permeate (the RO feed) is
significantly improved. Turbidity and suspended
solids are completely removed, SD1 values are
always well below 2, and the COD/BOD is
decreased by the removal of (large) dissolved
organics. If beachwell is fed to the UF, the
permeate will have the highest quality due to the
preceding sand filtration [46].
The use of MF and UF, however, optimizes
only the pretreatment in view of lower capital
and operating costs, or the applicability of the
RO treatment on a wider variety of sources [47].
The introduction of nanofiltration (NF) as a
pretreatment, on the other hand, will lead to a
breakthrough in the application of RO or MSF
because it has implications on the desalination
process itself, and not only on the quality of the
feed water. Turbidity, microorganisms and
hardness are removed in the NF unit, as well as a
fraction of the dissolved salts. Multivalent salts
are effectively removed, and monovalent salts are
reduced by 1O-50%, depending on the NF mem-
brane type. This results in a significantly lower
osmotic pressure, so that the RO unit can operate
at lower pressure (and thus requiring much less
energy) and at a higher recovery [48]. The process
is more environmentally friendly, because less
additives (antiscalants, acid) are needed. A second
RO stage can be omitted since the permeate in
the first RO stage has a TDS of around 200 mg/l.
These effects will allow producing fresh water at
a 30% lower cost compared to conventional RO
1491.
In the case of NF as a pretreatment to MSF,
the improved feed quality should result in the
possibility of an enhanced top brine temperature
(TBT). A TBT of 120C is feasible, and a TBT
as high as 160C may even be possible [50].
6. Environmental impact of desalination
processes
The environmental impact of desalination
processes is often neglected, although desalina-
tion may have a significant influence on the
environment. Two important emissions should be
considered: the discharge of the brine, and
atmospheric emissions [51]. In brackish water
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desalination, the discharge of the brine can be
avoided by using the concentrate for e.g. blending
with raw seawater to be desalinated by an RO
facility [52]. Emissions to the atmosphere result
from generating power for the pumps used in RO,
or from the generation of steam and auxiliary
power in seawater distillation. The concept of
desalination requires an input of thermal or
mechanical energy in order to achieve the separa-
tion; this leads to emissions related to energy
production. The use of nuclear power plants may
solve the problem of atmospheric emissions,
especially the emissions of carbon dioxide [53],
but at the same time it would cause other environ-
mental problems (nuclear waste), which may not
be beneficial on long terms. Other atmospheric
discharges are found in the deaeration and
degassing of feed and product water (with SO,
and NOX as the most important contaminants). A
comparison between MSF and RO [54] showed
that the emissions in RO are smaller, mainly
because of the lower energy consumption in RO.
Thus, the shift towards a larger application area
for RO has benefits for the environment as well.
The discharge of the brine shows a more
complicated picture. Three aspects are important:
(1) the temperature of the brine to be discharged;
(2) the salinity of the brine; and (3) the additional
chemicals discharged with the brine. Evidently,
the thermal impact of the MSF brine is much more
important than for RO. MSF results in a tempera-
ture increase in the order of 10C whereas the
RO concentrate remains at the same temperature.
The temperature rise may have a negative influence
on the oxygen level of the receiving water; the
same effect is found for a salinity increase. MSF
has an inlet seawater flow of 8-10 times the fresh
water production, whereas this ratio is around 3
for RO. Thus, the impact of RO on the salinity is
much larger. On the other hand, one may argue
that the salts that are discharged into the seawater,
originally were taken out of the water, so that no
additional compounds are added. The impact of
the brine discharge should thus be seen as a local
impact on the receiving water.
Additional chemicals, on the contrary, are a
real contamination of the receiving water. They
can be divided into three major categories: (1)
biocides, which can be used in all desalination
techniques; (2) scale control, in RO as well as in
distillation; and (3) anti-foams, used in distillation
plants. New trends are in the development of
environmentally-friendly products the same
Examples are use of additives
based maleic anhydride, a reduced
for eutrophication, biodegradable anti-foams
on ethoxylated chain aliphatic
compounds with toxicity [51].
biocides are needed and difficult to
by products a lower
7.
Erosion and corrosion in desalination
systems
Desalination systems invariably face a highly
corrosive medium and are therefore extremely
sensitive to erosion and corrosion. Apart from the
seawater, the materials also have to operate in
extreme conditions during chemical cleaning
(removal of scale). Corrosion problems are one
of the major reasons why MSF replaced MED in
new desalination plants in the 1960s. During the
last decade, however, new materials have been
developed with significantly better resistance
against corrosion. Most materials are based on
stainless steel [54], although for critical parts such
as heat exchanger tubes often other metals such
as titanium are used [55]. The latter material shows
a very low corrosion rate even under extreme
conditions of operation. Other stainless steels have
a variable resistance against corrosion, mainly
depending on the dominant alloy used [56]. A
ranking of different stainless steels can be made,
so that the optima1 material can be chosen, taking
economic and technical considerations into
account. The use of these new materials has led
to a revival of MED, and to a longer lifetime of
desalination plants. A lifetime of 40 years is
nowadays realistic, if the operating conditions and
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B. Van der Bruggen, C. Vandecasteele /Desalination 143 2002) 207-218
the materials used are carefully selected [57]. This
will also affect the final cost of the desalinated
water in future plants; the higher cost of more
expensive corrosion-resistant materials is expected
to be regained by the longer lifetime of the plants.
However, more research about materials and their
effect on corrosion and erosion in desalination
plants is still needed [58,59], and the optimal
operation of desalination plants [60], for the
further improvement of construction materials
that may lead to an extended lifetime.
8 New membranes for seawater RO
Polymer and membrane research during the
last decade resulted in significant improvements
in membrane materials. Two trends can be
distinguished: the development of low pressure
reverse osmosis membranes, operable at rela-
tively low pressures, and the development of
membranes operable at high pressure, with
improved water recovery [61]. Low pressure
reverse osmosis is similar to nanofiltration, so
that the general idea of a hybrid NF/RO system
is supported. The low pressure reverse osmosis
or nanofiltration unit can be used in the first stage,
whereas the second high pressure stage results in
a high quality permeate.
Another improvement of membrane materials
is the development of fouling resistant RO mem-
branes [62]. Fouling should be considered in
relation to the pretreatment system; the pretreat-
ment should involve a total system approach for
continuous and reliable operation [63]. For surface
water, this requires a thorough control of the water
quality because of seasonal factors. Problems in
the pretreatment will usually lead to membrane
fouling by precipitation of sparingly soluble salts,
by organic matter, or by the growth of a biofilm
at the membrane surface. New membrane types
may partially solve this problem because they
have an inherent resistance against fouling.
Technical-economic research showed that fouling
resistant membranes such as the FilmTec
SW30HR-320 membrane may allow savings of
25% in energy consumption and up to 4% for
cleaning costs [64]. Additional technical improve-
ments resulted in savings of 20% for installation
costs. Other membrane manufacturers also aim
for improved membrane types, which should
reduce the cost of desalination [65,66]. Further-
more, surface modifications of existing membranes,
resulting in a more hydrophilic polymer, may also
lead to fouling resistant membranes [67].
9. Conclusions
During the last decade, seawater desalination
has evoluated to a reliable, cost-effective source
of fresh water. MSF is still the standard technique
for large scale applications, but MED and
especially RO have an increasing market share.
ED/EDR and ion exchange are still limited to
brackish water applications. Major improvements
in process design, energy sources, pretreatment
possibilities, and materials used, resulted in an
environmentally-friendly process that may be the
most important source of fresh water during the
next century in many areas of the world. The new
challenge is to make the desalination processes
technically and economically feasible without
large investment and operation costs, in view of
the economical development of areas with less
water and energy resources.
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