memebrane distillation fundamentals and principles

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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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B. Van der Bruggen, C. Vandecasteele /Desalination 143 2002) 207-218

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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B. Van der Bruggen, C. Vandecasteel e /D esali nat i on 143 2002) 207-218

209

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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211

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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B. Van der Bruggen, C. Vandecasteele /D esal i nat i on 143 2002) 207-218

213

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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214 B. Van der Bruggen, C. Vandecasteele /Desalination 143 2002) 207-218

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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216


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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