Techno-economic assessment and environmental impacts of desalination technologies

Desalination 266 (2011) 263–273

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Techno-economic assessment and environmental impacts of desalination technologies Toufic Mezher ⁎, Hassan Fath, Zeina Abbas, Arslan Khaled Masdar Institute of Science & Technology, PO Box 54224, Abu Dhabi, UAE

a r t i c l e

i n f o

Article history: Received 23 May 2010 Received in revised form 25 August 2010 Accepted 26 August 2010 Available online 12 October 2010 Keywords: Desalination technologies Environmental impact Cost Energy requirement UAE

a b s t r a c t This paper presents a comprehensive review and assessment of desalination technologies such as thermal which includes multi-stage flash (MSF) and multiple effect distillation (MED), membrane reverse osmosis (RO), and hybrid (MSF/MED-RO). The assessment includes energy requirements, water production cost, technology growth trends, environmental impact and potential for the technology improvements. Comparison and technology matrix of commercial technologies are highlighted. The global desalination policies for the major desalination user countries, Kingdom of Saudi Arabia (KSA), United States of America (USA), Spain, China and Kuwait are given. More detailed analysis of desalination, cogeneration, and water situation in the United Arab Emirates (UAE) with some related recommendations for future policy and plans are also presented. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The role and the importance of water for sustenance of humanity require no introduction. Similarly the significance of water desalination, in providing the domestic water requirements in the Middle East region in general and UAE in particular, is well established. With the increasing water demand and the scarcity of renewable natural water resources, the dependence on desalination will continue to grow and consequently its energy consumption and impact on environment will increase without bounds. Therefore any technology policy encompassing the issues of energy and water must include a comprehensive coverage of desalination. The objectives of this paper are two-fold: 1. Development of a working understanding of different desalination technologies with a comparison in terms of various influencing parameters affecting the technology selection process. Desalination technologies comparison matrix will be a guide to modeling and formulation of future technology policy.

Abbreviations: ADWEC, Abu Dhabi Water and Electricity Company; BW, brackish water; CAPEX, capital costs; ED, electro-dialysis; EDR, electro-dialysis reverse; GCC, Gulf Cooperation Council; GHG, green house gases; IWPP, independent water and power producers; KSA, Kingdom of Saudi Arabia; MED, multiple effect distillation; MF, microfiltration; MSF, multi-stage flash; NF, nanofiltration; OPEX, operational costs; RO, reverse osmosis; R&D, research and development; SW, seawater; TBT, top brine temperature; TDS, total dissolved salts; TVC, thermal vapor compression; UAE, United Arab Emirates; UF, ultrafiltration; UN ESCWA, United Nations Economic and Social Commission for Western Asia; USA, United States of America; VC, vapor compression. ⁎ Corresponding author. Toufic Mezher, Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, UAE. 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.08.035

2. Reviewing the global desalination policies of major user countries and analyzing the status of desalination technologies in UAE in order to develop a knowhow of the desalination policies. Energy requirements, water production cost, technology growth trend, and environmental impact are the set of parameters used as the base of comparison between the major desalination technologies. The energy requirement of any given process has a very important effect on the overall process economics that is more prone to suffer from variations in the cost of fossil fuels. Moreover more energy intensive processes generally mean more carbon dioxide emissions. The technology matrix given in the following section compares the technologies by their specific energy requirements expressed in terms of electrical equivalent in kWh/m3. This is done in order to compare thermal processes that require heat input (along with electricity) with membrane processes that require electrical energy input only. On the other hand, the desalinated water production cost depends on a number of factors affecting both capital (CAPEX) and operational (OPEX) costs. Some technologies have high CAPEX (land, engineering, unit purchase, transportation, installation, etc. till commissioning) whereas others are more in OPEX (labor, maintenance and spare parts replacement, energy, and chemicals). The unit cost used is $/m3 of water produced. The differences in water cost estimation, in literature, can be attributed to factors such as differences in (i) fuel cost, (ii) material and construction cost, (iii) feed water properties (salinity and turbidity) and (iv) methods of cost calculation. The technology growth trends or future market potential indicate which technologies are developing and which have the potential to substitute others.

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Finally, the environmental impact assesses how desalination can cause considerable damage to the environment in a number of ways including (i) the uncontrolled discharge of concentrated brine that can contaminate water aquifers and damage aquatic ecosystems. The brine discharge may also contain pretreatment chemicals, corrosion materials, nuclear contaminants (if linked to nuclear plants) etc. (ii) Desalination plants get the thermal and electrical energy from an attached power plant. This energy used in the process amounts to a certain carbon dioxide emission, which results in environmental pollution. Generally the lesser the energy requirement by desalination technology the lesser this indirect environmental impact is going to be. And (iii) a desalination plant may cause noise pollution, gaseous emissions and chemical spills. As far as the harmfulness of discharged concentrate is concerned, total dissolved salts (TDS), temperature and specific weight (density) of the discharge are of critical importance as they result in damage to the aquatic environment. TDS discharge is directly proportional to the recovery ratio of the plant. The increased temperature can also harm the aquatic life. The increased density results in the sinking of the discharge causing harm to certain parts of the ecosystem [1]. The life cycle emissions of MSF, MED and RO are analyzed by Raluy et al. [2]. The authors concentrate on the emissions due to energy used and do not consider brine discharge or noise as environmental hazards. A sustainability analysis was carried out by Afgana et al. [3] and compares four desalination scenarios; single purpose MSF, dual purpose MSF (cogeneration), RO with local energy consumption and RO with photovoltaic (PV) electric energy production. Fuel consumption, environmental impact and cost of water are the indicators that form the basis of the comparison. Reverse osmosis using grid energy comes out as the most sustainable option due to its balanced mix of environment, energy and economy. 2. Desalination technologies The commercial desalination technologies can be divided into two main categories: thermal distillation (MSF and MED) and membrane separation (RO) [4]. Also, there are hybrids plants which integrate thermal and membrane technologies [5]. In addition, there are other commercial technologies of less application due to their small units' size such as vapor compression (VC) or their application of low salinity such as electro dialysis (ED). Moreover, there are different emerging technologies which are still under research and development (R&D), including forward osmosis (FO), membrane distillation (MD), capacitance deionization (CDI), and gas hydrates (GH), freezing, humidification dehumidification (HDH) and solar stills. Other supporting technologies include ultra/nano/ionic filtration (UF/NF/IF respectively) [6]. Fig. 1 shows the global desalination capacity by process, highlighting the high capacities shares of RO and MSF. As for the feed water quality used, seawater is the most used, followed by both brackish and river waters (Fig. 2) [7].

ED 3%

Other 11%

MED 8% RO 53%

MSF 25% Fig. 1. Global desalination capacity by process [7].

River water 8%

Waste water 6%

Brackish water 19% Seawater 67%

Fig. 2. Worldwide feed water quality used in desalination [7].

Table 1 summarizes the technology matrix and compares the major desalination processes including MSF, MED, and RO. The data is taken from the various sources as indicated and a more detailed analysis as presented below. 2.1. Thermal technologies MSF and MED are the major commercial thermal desalination technologies. Historically, thermal technologies have dominated the desalination market, particularly in the Middle East, where the low energy costs and large scale cogeneration plants have guaranteed the ascendance of thermal processes. These processes mimic the natural water cycle of evaporation and condensation and produce output water with very low salt concentration. Thermal desalination plants suffer, however, from formation of scales (as calcium carbonates/ sulfates and magnesium hydroxide) which limits the top brine temperature (TBT) [16]. 2.1.1. Multi-stage flash (MSF) MSF produces pure water by boiling and then condensing saline water. In MSF, the saline feedwater first passes through a series of tubes. This essentially preheats the water before entering the brine heater. It is then heated in the brine heater using any given form of thermal energy. The heated water is than introduced into a vessel (called stage) where the ambient pressure is lower than the brine heater. This low pressure results in sudden boiling (flashing) of the saline water. The vapors, formed during the boiling, condense on the tubes carrying input saline water and the distillate is collected. Only a small percentage of the heated water is converted into steam depending upon the pressure maintained at each stage. The remaining water is then introduced to the next stage with an even lower pressure, and the process continues until the saline water (now brine) is cooled down and discharged. MSF plants may contain between 4 and 40 stages, but usually they comprise of 18 to 25 stages [16]. The process explanation is taken from Buros [17] and Cooley et al. [16]. MSF has proven to be the most reliable thermal desalination technology as it has dominated the thermal desalination market during the 1980s and 1990s. However with the improvements in rival technologies as RO, the installation of MSF plants is on a downward trend and with present 25% worldwide capacity share (Fig. 1). Fig. 3 shows the worldwide contracted capacity for MSF-based desalination plants. The most recent 2-year period between years 2008 and 2009 shows a decrease in the number of contracted MSF plants. Also, the overall trend is reducing due to the emerging of RO and MED (except in the Gulf region). However, MSF still dominated the thermal technologies, especially in the Gulf region, in spite of the emerging growth of MED. There are 45 MSF plants, 32 MED plants, and 41 RO plants in the Gulf [19].

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Table 1 Desalination technologies matrix of major desalination processes.

Energy requirement (kWh/m3)

MSF

MED

RO

Electrical (SA or CG)1: 3.5–5.0 kWh/m3 SA: Thermal: 69.44–83.33 kWh/m3 CG: Thermal: 44.44–47.22 kWh/m3 [7]

SA: Electrical: 1.5–0.5 kWh/m3 CG: Electrical: 1.5–2.5 kWh/m3 SA: Thermal: 41.67–61.11 kWh/m3 CG: Thermal: 27.78 kWh/m3 [7] Around 1 $/m3; 0.827 $/m3 for Jubail II plant; the cost reduces with cogeneration use of thermal VC (TVC) and unit capacity [10,12] High Is a mature technology Brine discharge and temperature rise are similar to MSF [15]

Seawater (SW): 4–8 kWh/m3

Cost of water ($/m3)

0.9–1.5 $/m3; the cost reduces with cogeneration and unit capacity [9]

Technology growth trend

Moderate Is a mature technology Discharge is 10–15 °C hotter than ambient, TDS increase of 15–20% [15]

Environmental impact 1

Brackish water (BW): 2–3 kWh/m3 [8] 0.99 $/m3 for seawater RO; 0.53 $/m3 for Ashkelon 0.2–0.7 $/m3 for brackish water [11,13,14] High, with membrane technology growth RO will become more and more economical Brine discharge at ambient temperature TDS increase of 50–80% [15]

SA stands for Stand alone; CG stands for co-generation.

MSF plants require both electrical and thermal energy with relatively high specific energy consumption as compared to other competing technologies as MED and RO, Table 1, and R&D is very much needed to decrease these figures. The MSF specific water cost estimates made by different authors vary a lot; the variation can be attributed to inconsistent methodologies in cost calculation and market variation in fuel and material costs [16]. A recent study by Mabrouk et al. [20] shows that for subsidized fuel cost of 20 $/Barrel, the MSF water cost is 1.0 $/m3; however, for IWPP project with international fuel cost of 100 $/Barrel, the water cost may reach 4 $/m3 (Fig. 4). Since MSF has relatively high specific energy consumption, more greenhouse gas (GHG) emissions are expected. Combined cycle coupled with MSF desalination has a high thermal efficiency and, therefore, a low environmental impact [21]. Table 2 shows the carbon dioxide emissions of different MSF configurations using natural gas as a fuel as well as their cost impact. The situation becomes worse if coal or oil is used as fuel. On the other hand, the brine discharge from a usual MSF plant is 7–15 °C hotter than the feed water temperature. The brine discharge produced by MSF can be as 15–20% more saline than the feed water [15]. Although many authors indicated that MSF reached its maturity and has no margin for future improvement, MSF performance and economy can be more superior if the TBT and number of stages are increased. The use of high TBT anti scalant, and NF for feed water pre treatment can increase TBT and improve the systems performance. In addition, the partial recycle of the low pressure vapor can also add to the improvements of MSF performance and reduction of driving thermal and electrical energies. Therefore, these could reduce specific CAPEX, OPEX and water production costs. 2.1.2. Multiple-effect distillation (MED) MED takes place in a series of vessels or effects and uses the principle of evaporation and condensation at reduced ambient pressures. The

Contracted Capacities by year

plant designs for MED vary in process details. Usually the feed water after being preheated in the final condenser is fed in equal proportions to the various vessels. The water is sprayed on the evaporator surface (tubes) after being heated to boiling point. The evaporator surfaces in the first effect are heated by steam from the steam turbines of a power plant or boiler where the steam inside condenses as the water sprayed on the evaporators vaporizes. The vapors (steam) from the first effect are used to heat the surfaces of the succeeding effects. The vapors produced in the last effect are condensed in the final condenser – cooled by incoming saline feedwater – to yield pure water. Not all of the water is converted to steam; in each effect, the remaining water forms the brine solution. In some designs, this water is fed to the input of the next effect. MED plants employ a separate vacuum system for maintaining different ambient pressures in different vessels. In upcoming applications of MED plants a vapor thermal compression cycle is added to the plant to reduce the number of effects and the surface area required. Similar to MSF, The process explanation is taken from Buros [17] and Cooley et al. [16]. MED has been used, for a long time, in many process industries; traditional uses included the sugarcane and salt production industry. However in desalination, MED failed, in the past, to compete with MSF due to the scaling problem and the larger CAPEX and OPEX [16]. New designs with operation at lower TBT (b66 °C), use of cheaper material and the use of TVC solved this problem [17]. Similar to MSF, MED plants require both electrical and thermal energy. MED electric energy requirement is lower than MSF and as a consequence, MED started to gain ground and compete MSF (Table 1). Typical cost of recent very large plant of Ras Azzor (KSA) shows that MED and MSF specific water production costs are very close and near 1.0 $/m3 [12]. However, MED costs more in CAPEX and less in OPEX than MSF. The TDS of the output brine stream and outlet water temperature is similar to that of MSF. In terms of CO2 emissions MED is ranked lower than MSF as the specific electrical energy consumption is less than that of MSF but higher than RO (Table 2). Fig. 3 shows the global contracted capacity of MED plants. It indicates that between years 2000 and 2008, there was a steady trend, but from year 2009,

water unit cost

4000000 3000000 2000000 1000000 0 2000

2002

2004

2006

2008

Year MSF

MED

RO

water unit cost, $/m3

Capacity (m3/d)

5000000

4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

water unit cost

0

20

40

60

80

100

Oil price, $/Barrel Fig. 3. Contracted capacity of desalination plants worldwide for different technologies [18].

Fig. 4. Water cost at different values of oil price [20].

120

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Table 2 CO2 emissions and their abatement cost for different Desalination Processes [2,7,22]. Configuration (Fuel)

Non-cogeneration, natural gas (NG) Cogeneration, steam cycle, NG Cogeneration, combined cycle, NG Cogeneration, hybrid RO, steam cycle, NG Cogeneration, hybrid RO, combined cycle, NG

MSF

MED

RO

kg/CO2/m3

CO2 abatement ($/m3)

kg/CO2/m3

CO2 abatement ($/m3)

20.4–25.0 13.9–15.6 9.41 9.45 5.56

0.41–0.50 0.28–0.31 – – –

11.8–17.6 8.2–8.9 7.01 7.33 4.38

0.24–0.35 0.16–0.18 – – –

there was an increase in the contracted capacity. This shows that MED is gaining market share. In the future, the trend towards MED may be reinforced by its greater compatibility with solar thermal desalination. Some experts believe that MED is recognized as a major candidate for capturing the MSF dominated distillation market [23]. MED, at present, uses low TBT of b66 °C. However, with the use of NF as pretreatment, TBT could be as much as 130 °C [24]. However, the cost of introducing NF on the overall CAPEX and OPEX is not well established as the experience of UAE (Sharjah) plant was stopped due to some operational problem. 2.2. Membrane technologies Membrane processes work by prohibiting or permitting the passage of certain salts ions. Reverse osmosis (RO) is the major commercial membrane process. RO is used for both brackish and sea water. 2.2.1. Reverse osmosis (RO) In RO process, the saline water is pumped into a closed vessel where it is pressurized against a membrane; pure water permeates through the membrane whereas the brine left behind is discharged. The brine discharge may have a concentration ranging from 20% to 70% depending upon the salt content of the feedwater [17]. An RO system is made up of a pretreatment process, high-pressure pump, membrane assembly and post treatment process. Most modern reverse osmosis plants have an energy recovery system. The brine discharge is usually at very high pressure whereas the fresh water is at low pressure. The pressure energy in the brine is fed back to the feedwater using pressure exchangers. The usage of energy recovery systems and improved membranes has resulted in an overall reduction in the cost of RO-based desalination. RO is a pressure-driven process which uses membranes for saltswater separation. The saline water is pumped at high pressure into a closed (high pressure) vessel against a membrane; pure water permeates through the membrane whereas the brine left behind is discharged as reject. An RO system requires a pretreatment process, high-pressure pump, membrane assembly, and post treatment process. Pressure exchangers can be used to partially recover the energy from outgoing brine. RO is nowadays the world leading technology of a 53% world share in desalinated water production (see Figs. 1 and 3). The energy required for RO unit, Table 1, varies based on different parameters, but mainly water salinity. Specific energy consumption is the lowest as compared to other commercial technologies. The operating costs of RO have reduced over the years due to development of low-cost efficient membranes and usage of pressure recovery devices. Seawater RO cost has gone down to about 0.53 $/m3 in large plant of Ashkelon at the Mediterranean Sea [13]. However, in the case of high water salinity, high turbidity, high feed water temperature, high marine life presence (as in some Gulf sites), the cost will be high (near to MSF and MED) as the RO unit will need expensive pre treatment system. As the specific energy consumption is less, the green house gas emissions of RO will be less than emissions by an MSF or MED. Table 2 shows the exact CO2 emission figures for different configurations. The

kg/CO2/m3 Steam cycle Internal combustion engine Combined cycle

2.79 2.13 1.75

waste discharge from RO could have high salinity due to the higher recovery rate of about 30–50% and TDS increases from 50% to 80% above feed water [15]. Moreover, the chemicals added for the pretreatment add to the toxicity of the brine discharge. In RO, the brine is discharged at ambient temperature; i.e. no thermal pollution as in thermal technologies. On the other hand, RO employs large pumps for generating high pressures which results in noise pollution [25]. Fig. 3 shows the increasing trend between years 2006 and 2009 for RO-based processes. RO has a dominating presence in the brackish water desalination and increase towards the use of RO for seawater desalination. The trend of growth in RO will continue and may be strengthened by the growth in water demand and the development of low pressure membranes. Integration of NF for pretreatment, preheating of feed water, improves pressure recovery efficiency, and development of larger size and low pressure membranes improves the RO recovery and performance and specific energy consumption [15]. 2.3. Hybrid desalination Hybrid desalination systems combine thermal and membrane processes to add economical and technical features of the integrated system. Some of the advantages of this hybridization include flexibility in operation, less specific energy consumption, low intake/out fall construction cost, high plant availability and better power and water matching [5]. Normally MSF-RO and MED-RO combination are used in commercial applications. The presence of RO compensates for the inflexibility of MSF/MED to follow variable demand and allows for blending of RO and MSF/MED product which reduce (or eliminates) the post treatment costs. Two large hybrid plants are installed in the UAE and will be presented in Section 4 below. 2.4. Other commercial technologies (VC and ED) 2.4.1. Vapor compression distillation (VC) VC is a thermal process in which the external heating energy comes from compression of part of the produced vapor. The vapors can be compressed using either TVC or mechanical vapor compression (MVC). Vapor compression methods are used, in general, inconjunction with other technologies in particular with MED. However, small scale plants employing VC with one or two effects. According to Table 1, the energy required for VC is about 7.5–13 kWh/m3, i.e. less than MSF. The cost of water produced using VC is in general larger than MSF and MED as the VC unit capacity is smaller. If connected to MED, the unit cost will be similar to MED. The environmental impact of VC is relatively low regarding the concentrate effect, because its concentration is less than 10 mg/L TDS and has around 50% recovery. However, heating sources and maintenance procedures produce CO2 emissions which increase global warming [26]. No data could be found about technology growth trend; however VC world share is very small. The hybridization and partial use of waste heat for feed water preheating could improve the VC unit performance and reduces the specific fuel consumption.

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2.4.2. Electro-dialysis (ED and EDR) Electro-dialysis (ED) or electro-dialysis reverse (EDR) is an electricity-driven process; it uses electrical energy to move salts ions selectively, through a membrane leaving fresh water behind. ED was commercially introduced in the mid 1950s [16]. ED units are normally used to desalinate low salinity (brackish) water. In electrodialysis reversal (EDR) the direction of flow of ions is reversed (with the reverse of electricity polarity), after certain time interval, to reduce scaling and membrane fouling. The energy required for seawater ED is 17 kWh/m3; brackish water ED requires 3–7 kWh/m3 [8]. ED is primarily applied on brackish water with low TDS hence the cost is low (about 0.6 $/m3). ED has a high recovery of 85–94% and a concentrate of 140–600 mg/L TDS. This means that the concentrate has a great impact on the environment, and also there is a possibility of leakage in membrane stacks [26]. According to Table 2, ED has been on a steady low trend since year 2000. Limited surface of electrodes should be increased to increase the possible attraction of more salts ions or/and reduce the specific electrical energy requirements leading to the possible use of ED for sea water. A new technology is developed using very small (nanotubes), known as CDI. However, still under R&D phase and suits only brackish feed water [27]. The specific energy required is claimed to be 0.6 kWh/m3 [28]. 2.5. Supporting technologies 2.5.1. Microfiltration, ultrafiltration and nanofiltration These processes are not desalination techniques; however, they are used in the pretreatment processes of desalination plants [23]. Microfiltration (MF) is used to reduce turbidity and remove suspended solids and bacteria. MF operates via a sieving mechanism under a low pressure. Ultrafiltration (UF) is used for removal of contaminants that affect color; for example, high-weight dissolved organic compounds, bacteria, and some viruses. UF also operate via a sieving mechanism. Nanofiltration (NF) is used for water softening, organics, sulfate and virus removal. Removal is done by combining sieving and solution diffusion. NF is a major process for desalination in USA [16]. 2.5.2. Ion exchange Ion-exchange process uses resins to remove undesirable ions in water. At much diluted concentrations, ion exchange is used for the final polishing of waters that have had most of their salt content removed by other desalination technologies [16]. 2.6. High salinity water 2.6.1. Properties of high salinity water High salinity water (or concentrate) and clean water are the products of desalination processes. Brine is a concentrate stream which has a high TDS concentration of more than 36,000 mg/L. Concentrates are one of the aspects that affect the environment and the marine life due to their disposal. The intensity of the hazard of concentrates depends mainly on their temperature, TDS and density. The relations are as follows: the higher the temperature of the concentrate, the less its impact; the higher the density of the concentrate, the more its impact; the higher the TDS of the concentrate, the more its impact. If the concentrate's density is high, it will sink to the bottom of the sea and will harm the marine life, as opposed to a low density where it will float and cause less damage. The temperature and density are related similarly. Recovery rate, which is defined as the ratio of the product water to the feed water, also affects the TDS of the concentrate. The higher the recovery rate, the higher the TDS of the concentrate [1]. Of course the concentrate does not solely include brine; on the other hand, it includes chemicals used from pretreatment and posttreatment, which are added to the pipe of the concentrate before its

267

Table 3 Chemicals used in pretreatment and post-treatment of desalination plants [1,29]. Pretreatment

Post-treatment

Chemical

Purpose

Chemical

Purpose

NaOCl

Prevention of biological growth Flocculation and removal of suspended matter from water

Enzymes

Biocides Chelators

Breaking down of bacteria Resuspension of particulate matter and dissolving of organic material and silica Killing bacteria Removing scale

Acids

Dissolving inorganics

FeCl3/AlCl3

H2SO4/HCl NaHSO3 Scale inhibitors

Adjustment of pH Neutralization of chlorine in feedwater Prevention of scale formation

Detergents, surfactants and caustics

disposal and add to the negative impact of the concentrate. These chemicals are shown in Table 3 with their purposes [1]. Concentrates have more severe effects in membrane processes (RO and ED) than thermal processes (MSF, MED and VC). One of the reasons is because the concentrate of the thermal processes is mixed with a stream of cold water to dilute its salt concentration before discharging it, while this is not the case for membrane processes. There are properties other than the temperature of the concentrate that differ from process to process, Table 4. 2.6.2. Dealing with high salinity water The usual way of dealing with high salinity water is disposing of it. There are six ways of disposal which include: surface (both surface water and submerged), sewer system blending (front and end of wastewater treatment plant), land application, deep well injection, evaporation ponds and zero liquid discharge [1]. The selection of disposal method depends on eight factors, which are: volume of concentrate, quality of concentrate constituents, geographical location of discharge point of concentrate, availability of receiving site, permissibility of the option, public acceptance, capital and operating costs, and ability of facility to be expanded [30]. A brief description of each method as well as their environmental concerns and alleviation methods are explained below, Table 5. Due to all of the factors of selection, different countries use some of these disposal methods and do not use others. Table 6 below gives a list of some countries and the disposal method(s) that they adopt. 2.6.3. Concentration of hazardous materials As mentioned above, the concentrate is comprised of salts and chemicals used from pretreatment and post-treatment. Their concentration in the concentrate depends on the type of desalination process used. Some of these chemicals are toxic while others are very low in toxicity. The concentrations of the toxic chemicals in the concentrate are shown below in Table 7 [38]. The low-toxicity chemicals include: metals (iron, nickel, chromium and molybdenum), antiscalants (policarbonic acids, phosphonates, polyphosphates and sulfuric acid), coagulants (ferric-III-chloride and polyacrylamide), anti-foaming agents (polyethylene and polypropylene glycol) and cleaning chemicals (alkaline solutions, acidic solutions, detergents, oxidants, disinfectants and inhibitors, for example, dodecylsulfate, dodecylbenzene sulfonate, sodium perborate, sodium

Table 4 Properties of concentrate in different desalination processes [1]. Properties

Recovery rate Temperature Final concentration

Process RO (BW)

RO (SW)

MSF/MED (SW)

60–85% Ambient 2.5–6.7

30–50% Ambient 1.25–2.0

15–50% 10–15 °F w N ambient b 1.15

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Table 5 Disposal methods, their environmental concerns and alleviation methods [1]. Disposal Option

Method

Environmental Concerns

Alleviation Method

Surface

Surface water: disposal of concentrate at the surface of freshwater, streams, oceans, etc. Submerged: disposal of concentrate underwater via pipes stretched far into the ocean Disposal at the beginning or at the end of the waste treatment process Disposal of concentrate using spray irrigation, infiltration trenches, and percolation ponds Injecting concentrate into non-drinking water aquifers Ponds constructed for concentrate to be poured in so as to vaporize the water and accumulate the salts in the base of the pond Technique to solidify liquid concentrate and put the product in a landfill

Contamination of receiving water

From raw water

Sewer system blending Land application

Deep well injection Evaporation ponds

Zero liquid discharge

From pretreatment

Contamination of ultimate receiving water Contamination of underlying groundwater and soil Contamination of overlying drinking water aquifers (well leakage) Contamination of underlying higher quality aquifers (pond leakage)

Contamination of underlying higher quality aquifers (landfill leakage)

hypochlorite, hydrogen peroxide, formaldehyde, glutaraldehyde, isothiazole and benzotriazole derivates). 2.6.4. Potential applications of concentrate The concentrate has been found to have a good purpose in membrane processes for the removal of GHGs (SO2 and CO2). A study was performed by Luis et al. [39] to discuss the different methods of removal of SO2 and CO2 from air because of their environmental risks. It was shown that selective absorption is the best method of removal of a compound from a mixture of gases. The absorption agent used in this study is the combination of supported liquid membranes and ionic liquids (SILMs). The advantages of SILMs are high stability, nonvolatility, high affinity towards SO2, transport properties, easy synthesis and low cost (due to ionic liquid addition). The experiments performed also show that SILM is a barrier for air, permeability of air is one order magnitude lower than CO2, permeability of air is lower than that of a mixture of air and 10%vol SO2 and the best combination of SILM for the highest permeability of SO2 and CO2 is [BIM][ace] (1-butylimidazolium acetate). Another study performed by Lee et al. [40] studied the use of different gas and liquid flows, namely NaOH, Na2CO3, Na2SO3 and NaHCO3, which are used for SO2 removal in hollow fiber (HF) membrane contactors. The results showed that o As concentration of NaOH increases, SO2 is removed more efficiently.

Table 6 Some countries and their disposal method usage [1,30–37]. Country

Disposal method

UAE Oman KSA USA

Surface water Land application and Evaporation ponds Evaporation ponds Surface water, sewer system blending, land application, and evaporation ponds and deep well injection (in decreasing order of % use) Evaporation ponds Surface water Land application Evaporation ponds Land application and evaporation ponds Land application and evaporation ponds Surface water Surface water Sewer system blending and Land application

Australia Spain China Kuwait Jordan Qatar Japan Algeria UK

Blending; diffusers; mixing zones; aerate; degasify

Use non-toxic additives; increase pH before discharge From concentrate Diffusers; mixing zones; blending Reduce recovery rate; membrane type selection Reduce recovery rate; blending membrane type selection

Relocating disposal position/alternate means of disposal Double lining with leachate collection system

Double lining with leachate collection system

o As gas flow rate increases, SO2 removal efficiency decreases. o As pore size increases, SO2 removal efficiency increases. 3. Global desalination policies The total capacity of desalination plants around the globe is 59.9 million m³/d. There was an increase of 6.6 million m³/day since year 2008, and is the largest amount of desalination capacity brought online in a single year. Fig. 5 below shows the annual new contracted and commissioned capacity of plants from year 1980 up to the first 6 months of year 2009 [41]. The largest plant commissioned was the 880,000 m³/d Shoaiba 3 project in Saudi Arabia, which was one of 700 new plants worldwide. There are now 14,451 desalination plants online, with a further 244 known to be under contracting or under construction. Also, the installed capacity of seawater desalination plants has expanded by an enormous amount of 29.6% to 35.9 million m³/d [42]. Table 8 shows the installed capacities and percentages of the top 10 major desalinating countries [7]. The problem with this source is that it includes online plants, presumed online plants and plants under construction before year 2008. However it does give a fair idea of where the world stands in terms of desalination capacity. The desalination policies of some of the major countries are described below. 3.1. KSA KSA is the largest producer of desalinated water in the world with 17% of world water production. With a growing population, KSA needs to add another 6 million m3/day for the next 20 years [43]. Considering the present process costs, the building of this desalination capacity amounts to around 200 billion dollars [44]. MSF and RO are the two major processes, with MSF having around 64.2% of the total

Table 7 Concentrations of toxic chemicals in the concentrate [38]. Chemical

Thermal process

RO process

Chlorine/free and combined chlorine residuals (FRC) Trihalomethanes (THMs), e.g. bromoform and haloacetic acids Heavy metals (copper)

200–500 μg/L 10–25% of dosing concentration First source: up to 9.5 μg/L Second source: up to 83 μg/L

20–50 μg/L

In the range that can affect natural copper concentrations

–

15–100 μg/L

8 7 6

Pure 7% Brine

Commissioned Contracted

1% Waste 9%

5 4

269

Unknown 1%

Seawater 7%

3 2 1 0 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Capacity (million m3 /d)

T. Mezher et al. / Desalination 266 (2011) 263–273

Year

River 25%

Fig. 5. New desalination capacity 1980–2009 [41].

installed desalination capacity in KSA. KSA has the largest desalination plant in the world which is the facility at Al-Jubail, which produces around 2 million m3/day of desalinated water [7]. The country is also trying to manage its water consumption to stop the depletion of its aquifers. In future the Kingdom can look towards other sources such as nuclear or solar energy to sustain its growing water and energy demand. An ambitious initiative was recently announced to convert KSA desalination plants to solar driven ones in 10 years [45]. 3.2. USA USA is the third largest desalinating country in the world. Desalination plants have been built in all of the states. Water is desalinated for municipal as well as industrial use. Brackish water and river water are the major sources both of which have low salinity levels as shown in Fig. 6; therefore the energy consumption and cost is low. Moreover, RO is the major desalination method in USA as shown in Fig. 7. NF is the next biggest technique. Thermo desalination exists in a very diminutive quantity, unlike in the Gulf Countries. USA recognizes that by year 2020, desalination and water recycling will be a major contributor to the stability of an adequate water supply. Therefore US Bureau of reclamation and Sandia have outlined a roadmap to counter the water needs of the nation and desalination is a major component of the solution. USA plans, therefore, to invest heavily in the R&D of water reuse and desalting technologies [45]. 3.3. Spain Spaniards built the first desalination plant in Europe 40 years ago in Canary Islands [46]. Spain uses desalinated water to nurture a thriving agricultural sector and an ever green tourism industry. From year 2000 to 2004 the desalination production was doubled and the government expected it to double again in the following five years [46]. The country suffered a drought in year 2005. Therefore the desalination in Spain is state supported and the government focuses on making the desalinated water as cheap as common household

Fig. 6. USA desalination capacity by feed water source [16].

water. Seventy percent of desalinated water is produced by RO and some 23% is produced by MSF-MED. 3.4. China China had the twelfth highest desalination capacity in the world in year 2005, and increased its capacity drastically by around 4.6 times by year 2008. Because of the lack of potable water, it started to install more desalination plants. It then became the fifth highest desalination country with 4% of the global desalinated water production. MSF was first installed industrially in year 1989 and supplied 6000 m3/ day to Tianjin city, and another MSF device was installed in year 1997 and supplied 1200 m3/day and more MSF installations after that [47]. Also, a MED was installed in year 2004 in Qingdao city with a specific power consumption of 1.6 kWh/m3 [47]. RO was first installed on a large-scale in year 1997 with a yield of 500 m3/day of fresh water from sea water, and seven others till year 2001, with the latest having a yield of 18,000 m3/day of fresh water from brackish water. In year 2007, China signed a $119 million contract to build the largest sea water desalination plant in China (in Beijing) to provide 100,000 m3/day of fresh water [48], which had started to operate in August 2009 [49]. 3.5. Kuwait With 4% of desalination capacity, Kuwait is a major desalination country. It was the first country in the GCC region to invest in desalination, starting in year 1953 with an MED plant with a capacity of 9,200 m3/day. Its desalination capacity increased almost exponentially between years 1950s and 2010 (Fig. 8). MSF was introduced in the 1970s in a great amount, making the capacity of desalination reach around 2.4 million m3/day in year 2008. This increase was around 107% [7]. Because of the success of MSF technology, all the plants installed after year 1960 were MSF [50]. All major desalination plants in Kuwait are dual purpose cogeneration

VC 3%

Table 8 Highest 10 desalinating countries [7]. Country

Capacity (m3/day)

Share of global production (%)

KSA UAE USA Spain China Kuwait Qatar Algeria Australia Japan

10,598,000 8,743,000 8,344,000 5,428,000 2,553,000 2,390,000 2,049,000 1,826,000 1,508,000 1,153,000

17 14 14 9 4 4 3 3 2 2

Brackish 50%

Other 2%

NF 15%

MED 1%

ED 9% MSF 1% RO 69% Fig. 7. USA desalination capacity by process [16].

270

T. Mezher et al. / Desalination 266 (2010) 263–273

3,000,000

Table 10 Domestic water use for 2005, with projections to 2025 for GCC countries [52].

Capacity (m3/d)

2,500,000 2,000,000

Country

Domestic water use as a percentage of total water use (%)

Total domestic use in 2005 (million m3)

Projected domestic use in 2025 (million m3)

Bahrain Kuwait Oman Qatar KSA UAE

43.6 89.8 15.1 45.9 10.4 32.3

139.1 405.5 205.0 80.0 2100.0 943.0

204.4 642.3 302.8 123.6 3178.3 1500.2

1,500,000 1,000,000 500,000 1950

1960

1970

1980

1990

2000

2010

Year consumption. KSA and Oman have relatively similar GDP per capita, but KSA has much higher domestic water consumption [7].

Fig. 8. Historical growth of desalination capacity [7].

plants, using MSF desalting system [51]. Several old units from the 60s and 70s have been refurbished to continue operation for another 10 years. The evaluation of the trends in Kuwait reveals that MSF will continue its monopoly in the coming 20 to 30 years. Advances in membrane science, maturing of technology and increase in energy consumption may force the authorities to switch towards RO. Moreover environmental issues and advances in renewable energy may result in integration of green energy in the desalination industry [50]. Also, Kuwait is producing more desalinated water annually than is available from its national renewable water resources, and it also reuses 20–40 m3/c/yr of wastewater. Moreover, Kuwait uses between 50% and 70% of its water for agriculture, with most of the remaining water being used in domestic consumption. The latest plant planned in Kuwait, with year 2010 being the operation year, is Shuwaikh plant with a capacity of 136,000 m3/day using RO technology [7]. 4. Desalination in the UAE 4.1. Water consumption and population growth rate in GCC countries According to ESCWA [52], the population growth rate has slowed down since 1995–2000 and it is expected to be lower in 2020–2025 as shown in Table 9 in most of the GCC countries, namely: Bahrain, Kuwait, Oman and KSA. Although the growth rate is projected to increase at a lower rate from year 2005 to year 2025, the domestic water use is projected to increase, almost doubling for some of the countries, Table 10. UAE is shown to have the highest per capita domestic water use and Oman the lowest. Surely, the per capita factor is dependent upon the population size. Also, Kuwait is shown to have the highest percentage of domestic water use of total water use whereas KSA has the lowest percentage. Furthermore, KSA has the highest total domestic use whereas Qatar has the lowest [52]. When related to the GDP per capita, the GCC countries are described as follows (Fig. 9). Qatar is seen to have the highest GDP per capita and relatively high domestic water consumption. Bahrain has a relatively low GDP per capita but the highest domestic water consumption among all the GCC countries. The UAE and Kuwait are relatively similar in terms of GDP per capita but Kuwait has higher domestic water

4.2. Desalination UAE has a 14% share in the global desalinated water capacity and it is ranked second in the world just after KSA, Table 8. Fig. 10 shows the distribution of desalination technologies used in the UAE. Like the rest of the Mideast, MSF is the major process in UAE with 63% share of the desalinated water is produced. In UAE, all the major plants use MSF with cogeneration of water and power and the fuel for power generation and desalination is predominantly natural gas. The reasons for the heavily reliant on MSF are the relative inexpensiveness of energy, good reliability of MSF, large capacity requirement, the high TDS of Gulf seawater and cogeneration of MSF with power generation. As for MED, it plays a relatively minor role in UAE and accounting for 6% of the whole installed capacity. For example, Jebel Dhana plant in Ruwais is an MED-based plant with a capacity of 18,180 m3/day. Also, Al Mirfa plant exploits the MED technology and has a production capacity of 9,080 m3/day [18]. MED was also used in the renewable energy desalination project in Umm Al Nar (UAN). The share of VC in the desalination capacity of UAE was almost 9.2% [8]. This value may have changed over the years, and it is

Fig. 9. Domestic water consumption for GCC vs GDP per capita [7].

Other 19%

Table 9 Trends in population growth rate in GCC countries [52]. Country

Bahrain Kuwait Oman Qatar KSA UAE

Growth rate (%) 1995– 2000

2000– 2005

2005– 2010

2010– 2015

2015– 2020

2020– 2025

2.80 5.48 2.30 2.85 2.80 5.76

1.56 3.73 1.00 5.86 2.69 6.51

1.79 2.44 1.97 2.11 2.24 2.85

1.56 2.04 1.95 1.76 2.05 2.13

1.35 1.77 1.81 1.49 1.84 1.85

1.18 1.55 1.58 1.16 1.62 1.64

RO 12%

MED 6%

MSF 63%

Fig. 10. Distribution of desalination technologies used in the UAE [53].

T. Mezher et al. / Desalination 266 (2011) 263–273

Abu Dhabi IWPP v Non-IWPP Desalination Capacity 700 IWPP Non-IWPP

600

Gross MGD

500 400 300

271

Table 12 Peak water forecasts in Abu Dhabi [56]. Global Demand

Forecast (MIGD)

Actual(MIGD)

Forecast-Actual

Percent

2005 2006 2007 2008 2009 2010

533 572 590 613 649 748

527 560 572 614 664 Later

6 12 18 1 15

1% 2% 3% 0% 2%

200 100 0 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Year Fig. 11. Comparison between IWPP and Non-IWPP in Abu Dhabi [56].

not available in the United Nations Economic and Social Commission for Western Asia(UN ESCWA) report of year 2009. However what can be interpreted from Fig. 8 is that VC share is less than 19%, as it is part of the “other's” share. The share of RO in the UAE desalination capacity is as low as 12% [8]. This is an 8.4% increase than it was in year 2001 [8]. This shows that due to the recent improvements in the RO technology, it is gaining importance. In UAE, there are plants that are RO based, and others that have RO units installed along with other desalination technologies. For example, Fujairah I is an online RO plant with a production capacity of 170,000 m3/day. Also, Hamriyah IV in Sharjah is a planned stand-alone seawater RO plant with a capacity of 455,000 m3/day [18]. As for plants with RO installed with other technologies, a very important hybrid plant in UAE is Fujairah I, which is the largest hybrid plant in the world. It is an MSF-RO plant with water production of 100 MIGD or 454,600 m3/day, where 62.5% of the plant is MSF-based using five MSF units and 37.5% is seawater reverse osmosis-based [54]. Another large hybrid plant is Fujairah II Independent Water and Power Producers (IWPP), which is a mixture of MED 100 MIGD and SWRO 30 MIGD [55]. ED is used mainly for brackish water, which implies that it is not used as desalination in UAE is based on seawater [23]. ED has the smallest share in the desalination in the UAE, at about 0.12% [8]. Desalination in some of UAE emirates will be highlighted below. 4.2.1. Desalination in Abu Dhabi Abu Dhabi is the major desalination user in UAE. Like the rest of UAE and Middle East, cogeneration power plants with MSF form the backbone of desalination in Abu Dhabi. Abu Dhabi Water and Electricity Company (ADWEC) is responsible for water supply in Abu Dhabi. The desalinated water comes from a combination of IWPPs and non-IWPP plants. Fig. 11 shows an increasing trend towards IWPP desalination capacity being added to the water desalination infrastructure [56]. A sample list of desalination plants in Abu Dhabi is given in Table 11.

4.2.2. Desalination in Dubai and Sharjah Dubai has a total desalination plant capacity of 330 MGPD and a peak demand of 271 MGPD in year 2009 [62]. On the other hand, Sharjah has a total desalination installed capacity of 83.60 MGPD, a total available capacity of 72.47 MGPD and a total daily average production of 65 MGPD [61]. Table 11 gives a brief list of the major desalination plants in Dubai and Sharjah. 4.3. Co-generation The energy sector of UAE is dominated by cogeneration-based power desalination plants; use the low pressure/temperature steam from power plant as the heating source to desalination plant. Cogeneration plants normally work with MSF, MED, or hybrid MSF/MED-RO although using the preheated condenser cooling water as feed water of RO is also a possibility. Cogeneration decreases the specific energy requirements of a desalination process resulting in reduced GHG emissions [2]. On the economical side, cogeneration results in a decreased unit cost of water, making it a method of choice in the UAE where the infrastructure development requires both power and water. On the other hand, cogeneration ties the power and water production together, although in a practical scenario the water and power demand may not be perfectly matched. In the cases of increased water demand the desalination units require direct low pressure steam from auxiliary boilers to be fed to them which increases the specific energy requirement and the unit cost of water produced. One way to address the problems of cogeneration is the use hybrid MSF/MED-RO-Power plants allowing for greater flexibility of operation, demand matching and specific energy reduction. Most of the cogeneration plants in the UAE run on gas which is more efficient and environment friendly as compared to oil and coal. A switch to more efficient plant configurations such as of combined cycle plants can further improve the energy and environmental situation. 4.4. Water situation and scenario The continuously growing and evolving population of UAE demands water for its sustenance. Table 12 shows the peak water forecasts from year 2005 till the present, where it is clear that the actual production was very close to the forecasted value. Fig. 12 shows the forecasted water capacity versus demand for Abu Dhabi [57] from year 2007 up to 2030. The graph indicates that with

Table 11 Major Plants in the UAE. Abu Dhabi

Dubai

Sharjah

Project

Technology

Capacity

Project

Technology

Capacity

Project

Technology

Capacity

Shuweihat S1

MSF

100 MGD [57]

Jebel Ali L1

MSF

317,800 m3/day [18]

Layyah plant

63.5 MGPD [61]

Taweelah B extension Taweelah A1

98 MGD [58] 84 MGD [57]

Jebel Ali G Jebel Ali L2

MSF MSF

272,520 m3/day [18] 250,000 m3/day [18]

Saja'a plant Hamriyah plant

Taweelah B UAN west B

MSF Hybrid (MED + MSF) MSF MSF

Hybrid (MSF + MED + RO) [60] RO RO

75 MGD [58] 62.8 MGD [59]

Jebel Ali Jebel Ali M

MSF MSF

121,134 m3/day [18] 477,330 m3/day [18]

Kalba plant Khor Fakkan (KFK) plant

RO RO

5.50 MGPD [61] 2.50 MGPD [61]

UAN west Taweelah A2

MSF MSF

53.2 MGD [59] 50 MGD [57]

Jebel Ali K2 Jebel Ali K1

MSF MSF

182,000 m3/day [18] 125,000 m3/day [18]

5.50 MGPD [61] 1.15 MGPD [61]

272

T. Mezher et al. / Desalination 266 (2010) 263–273

ADWEC Winter 2007 / 2008 Water Capacity v Demand Forecast 1400

Mostly Likely Demand Forecast

1300 1200 1100

1,015 939 904

Peak MIGD

900

817

800

934

962

986

864

622

640

593

610

587 560

1,035

1,145

1,054

1,163

1,074

1,179

1,094

1,202

1,196

1,215

1,156

1,185

1,113

1,144

1,170

1,130

Required Capacity - Abu Dhabi Emirate ONLY

708

Demand - Abu Dhabi Emirate ONLY Northern Emirates' Contracted Demands 817

631

1,011

1,124

1,305

Available Capacity

779

400 300

900 831

743

700

500

1,077

979

1000

600

1,047

1,098

1,290

1,244

1,276

1,229

1,261

1,219

685

685

20

20

868

893

893

893

893

893

893

893

893

893

893

30

30

30

30

30

30

30

30

30

30

30

854

854

854

854

854

854

854

854

854

30

30

30

30

30

30

30

30

30

200 100

12

30

0 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 Fig. 12. Water demand forecast [54].

the increase in the demand there will be an increasing gap between the capacity and demand. ADWEC plans to fill this gap using conventional desalination technologies. This expansion in the water capacity will come with an increase in the CO2 emissions and at the same time it will result in consumption of valuable energy resources such as natural gas. The following approaches can help in reducing this unsustainable dependence on fossil fuels: • Waste water treatment and reuse can – economically and environmentally – be a better solution as compared to desalination at least for agricultural use and landscaping water. Therefore water recycling should be encouraged on a governmental scale. • UAE efforts should be directed towards reduction in water consumption. This can be achieved by the two pronged approach of using water efficient technologies and conducting public awareness schemes. At the same time water loss due to improper infrastructure and piping network should also checked. • Using efficient combined cycle plants in power generation and desalination plants can alleviate the environmental problems. • Renewable energy (RE)-based desalination can be a solution in remote areas. However, this does not mean that the country should not focus on large scale RE desalination plants. Converting the ever present solar energy to water is a promising solution in the future. Any policy for provision of water in UAE should not focus on a ‘silver bullet’ solution. Water management should involve conservation, recycling and desalination. According to state of the environment, agricultural sector improvements and tariff increases are among the measures to improve the current water situation. 5. Conclusions Conventional desalination technologies, distillation and membrane based, rely on RO and MSF, followed by MED as it is steadily gaining popularity. Distillation technologies are often coupled with power plants to increase overall efficiency and reduce cost and their usage is very common in the Middle East. RO has progressed a lot in the recent era due to improvements in membranes. Hybrid MSF/MEDRO cogeneration plants are also gaining ground for large water and power capacity due to their flexibility and efficiency. The selection of a particular desalination technology involves many factors, such as economics, environmental impact, energy

requirement, and dependence on feed water quality. A proper mix of technologies at a national level can greatly help in solving the water and power to water ratio problems. An analysis of the pros and cons of technologies can help in achieving this optimum mix. UAE is one of the major desalinating countries in the world, namely the second. The major desalination configuration in UAE is MSF with cogeneration of water and power. The authorities have planned to tackle the water challenge by building large centralized plants. Though a workable solution it will pose a great strain on the environmental footprint and energy economics of the country. The technology policy of an arid and rapidly developing country like UAE can never neglect the importance of stable water and power supply. With dwindling water aquifers and scrimpy rainfall the country has to rely on desalination. It is approximated that the fuel requirement of desalination will be doubled by year 2030. Therefore water demand and desalination pose important challenges to the country's drive towards energy and environmental sustainability. Finally, it must be realized that desalination is one part of the solution. Only a synergistic approach involving desalination, water management, consumption reduction and water reuse can efficiently confront the water challenge. Acknowledgements The support of MASDAR Institute of Science and Technology (MIST) in Abu Dhabi, UAE, is gratefully acknowledged. References [1] T. Younos, Environmental issues of desalination, Journal of Contemporary Water Research & Education 132 (2005) 11–18. [2] R.G. Raluy, L. Serra, J. Uche, A. Valero, Life-cycle assessment of desalination technologies integrated with energy production systems, Desalination 167 (2004) 445–458 753. [3] N.H. Afgana, M. Darwish, M.G. Carvalho, Sustainability assessment of desalination plants for water production, Desalination 124 (1–3) (1999) 19–31. [4] AMTA (America's Authority in Membrane Treatment), Improving America's waters through membrane treatment and desalting, 2007 February Retrieved from, http://www. membranes-amta.org/amta_media/pdfs/8_WaterDesalinationProcesses.pdf. [5] O.A. Hamed, Overview of hybrid desalination systems—Current status and future prospects, Desalination 186 (1–3) (2005) 207–214. [6] J.W. R&D TF(Joint Water Reuse, Desalination Task Force), Water Innovation Symposium, 2005 October 17-21 Retrieved from, http://www.coloradowaterquality.com/ro/docs/water_innovation_symposium.pdf.

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