distillation vs. membrane filtration: overview of process evolutions in seawater desalination

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

Department of Chemical Engineering, University of Leuven, WI de Croylaan 46, B - 3001 Heverlee, Belgium Tel. +32 (16) 32 23 40; Fax +32 (16) 32 29 91: email : [email protected]

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

208 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 alternative 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 environment. 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 condensing 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

B. Van der Bruggen, C. Vandecasteele /Desalination 143 (2002) 207-218 209

seawater

steam

1” evaporator steam 95°C

I-----1

2”d evaporator 90°C

I-----I

cooling water

Fig. I. Principle of MED (multi-effect distillation).

l freshwater

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 120°C 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 5°C 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

i i Brine recirculation

Fig. 2. Principle of MSF (multi-stage flash).

- SC

out

:awater in


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 110°C 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 compared 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 desalination. 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)


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 approximately 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 concentrations, 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.


3. Hybrid desalination processes

The possibility to combine different desalination 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 electricity, 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 temperature 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 desalination, 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 minimization 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


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 economical feasibility seems promising. The use of solar energy for seawater distillation was the first option that was explored, as an improved combination 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 effectiveness 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 seawater 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 desalination 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 desalination 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,


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, ultrafiltration, 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 1 O-50%, depending on the NF membrane 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 120°C is feasible, and a TBT as high as 160°C may even be possible [50].

6. Environmental impact of desalination processes

The environmental impact of desalination processes is often neglected, although desalination 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


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 separation; 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 environmental 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 temperature increase in the order of 10°C 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


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 relatively 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 membranes [62]. Fouling should be considered in relation to the pretreatment system; the pretreatment 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 improvements 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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distillation vs. membrane filtration: overview of process evolutions in seawater desalination

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