Price For Sw-660k-18880 2500 Cubic Per Day Swro With Energy Recovery

5.4 % Australia Pacific 461,266

brackish water waste water

Figure 5 Global desalination capacities in cubic meters per day. For example, the installed capacity in Southern Europe is 4,405,024 m3/day. This figure includes all source water types. Seawater desalination accounts for most of the production in Southern Europe, brackish water for about one-fourth of the production and wastewater desalination plays a relatively minor role (pie diagram). The figures next to the pie diagram give the contribution to the global production, that is, the seawater desalination capacity in Southern Europe represents 10.6% of the global seawater desalination capacity. The brackish water capacity - though it is less than half the seawater desalination capacity in Southern Europe - represents 12.8% of the global brackish water desalination capacity. Primary data from Ref. [5] (see plate 2 in color plate section at the end of this book).

4.1 The Gulf region

In terms of sea areas, the largest number of seawater desalination plants can be found in the Gulf with a total desalination capacity of approximately 12.1 Mm3/day - or a little less than half (44%) of the worldwide daily production (Fig. 6). The main producer in the Gulf (and worldwide) is Saudi Arabia with 25% of the worldwide seawater desalination capacity, of which 11% are located on the Gulf shore and 12% on the Red Sea coast (2% unaccounted for), followed by the United Arab Emirates (23%) and Kuwait (6%).

Thermal desalination processes dominate in the Gulf region (about 94% of all production), as water and electricity are often generated by large cogeneration plants that use low value steam and electricity from power plants as a heat source for desalination. Most of the water (81%) in the Gulf is produced by the MSF distillation process. Minor processes are MED distillation and RO, which account for 13% and 6% of the production, respectively (primary data from Ref.[5]).

4.2 The Red Sea

In the Red Sea region, desalination plants have a combined production capacity of3.6Mm3/day (13% of the worldwide capacity, Fig. 7). Similar to the Gulf, most of the water is produced by large cogeneration plants, mainly on the Saudi Arabian coast in the locations of Yanbu, Rabigh, Jeddah, Assir, and Shoaiba, where the world's largest desalination complex with a capacity of 1.6Mm3/day is located. Saudi Arabia accounts for more than 92% of the desalinated water production from the Red Sea, with 2.6Mm3/day (78%) produced by thermal plants. Egypt, the second largest producer of desalinated water in the region, accounts for only 7% of the production from the Red Sea, with 90% (0.2Mm3/day) coming from smaller RO plants on the Sinai Peninsula and in the tourist resorts along the Red Sea coast.

4.3 The Mediterranean Sea

In the Mediterranean, the total water production from seawater is about 4.0Mm /day (14% of the worldwide capacity, Fig. 8). Spain, with about 8% of the worldwide desalination capacity, is the largest producer of desalinated water in the region with an installed capacity of 2.2 Mm /day. About 65% (1.4 Mm /day) of the Spanish capacities are located on the aThe figure of 44% includes only those plants located on the shores of the Gulf. In contrast to the figure of 61%,which is given for the GCC states above, the figure of 44% does not include plants in Oman and on the Red Sea coast of Saudi Arabia, but it does include plants in Iran.

Desalination capacity in the Arabian Gulf in m3/day

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Figure 6 Cumulative MSF, MED, and RO capacities in the Arabian Gulf in cubic meters per day by site location (dots) and by country (triangles). The map shows all sites with an installed capacity >1000 m3/day and displays sites with a capacity >100,000 m3/day by name and capacity. The map was first published in Ref. [17,28] and updated using raw data from Ref. [5] (see plate 3 in color plate section at the end of this book).

Figure 7 Cumulative MSF, MED, and RO capacities in the Red Sea in cubic meters per day by site location (dots) and by country (triangles). The map shows all sites with an installed capacity >1000m3/day and displays sites with a capacity > 100,000 m3/day by name and capacity. The map was first published in Ref. [17,28] and updated using raw data from Ref. [5] (see plate 4 in color plate section at the end of this book).

Figure 7 Cumulative MSF, MED, and RO capacities in the Red Sea in cubic meters per day by site location (dots) and by country (triangles). The map shows all sites with an installed capacity >1000m3/day and displays sites with a capacity > 100,000 m3/day by name and capacity. The map was first published in Ref. [17,28] and updated using raw data from Ref. [5] (see plate 4 in color plate section at the end of this book).

Mediterranean coast and the Balearic Islands, and 25% on the Canary Islands. The Spanish A.G.U.A. program will further augment water supply on the Mediterranean coast by increasing the desalination capacity to over 2.7 Mm /day until 2010. While thermal processes are dominating in the Gulf and Red Sea, 70% of the Mediterranean and 99% of the Spanish

2The program ''Actuaciones para la Gestion y la Utilizacion del Agua'' was introduced by the Spanish government in 2004 following the decision not to divert the Ebro river to Southern Spain. The package of measures includes desalination but also water saving and efficiency of use and water reuse.

Desalination capacity in the Mediterranean Sea in mVday

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Figure 8 Cumulative MSF, MED and RO capacities in the Mediterranean Sea in cubic meters per day by site location (dots) and by country (triangles). The map shows all sites with an installed capacity >1000m3/day and displays sites with a capacity > 50,000 m3/day by name and capacity. The map was first published in Ref. [17,28] and updated using raw data from Ref. [5] (see plate 5 in color plate section at the end of this production on the Mediterranean coast is produced by the process of SWRO.

Larger numbers of distillation plants are only found along the coasts of Libya and Algeria in North Africa, and also in Italy. However, new plants in these countries are also often SWRO plants. A tremendous expansion of capacities is currently taking place in Algeria, North Africa's fastest growing desalination market, where the first large SWRO plant (200,000 m3/day) was opened in February 2008 [11]. It is the first in a series of other projects with capacities between 50,000 and 500,000 m3/day, which will increase the country's desalination capacity to 4 Mm3/day by 2020 [2].

On the Mediterranean coast of Israel, two large SWRO are currently in operation, the Ashkelon plant with a capacity of 330,000 m /day - the world's largest SWRO project to date - and the Palmachin plant (83,000m /day). Desalination presently accounts for approximately 8% of Israel's water supply. According to original plans, this would have been increased to more than 30% (1.8Mm3/day) by 2020 [12]. In 2008, however, the Israeli government approved a new, even more ambitious emergency program to address the country's growing water shortage, which will raise the target for desalinated water production to 1.6 Mm3/day by 2013 and to 2.1 Mm3/day by 2020, which may also reach 2.7 Mm3/day depending on water demand and other alternatives [13]. Several large SWRO desalination plants with capacities up to 274,000 m3/day are currently being planned along Israel's Mediterranean coast [14]. Furthermore, it is planned to sharply increase the use of the country's brackish water resources, from presently around 16,500 m3/day to somewhere between 220,000 and 274,000 m3/day [15]. Other measures include more water efficient practices, fixed water quotas, greater enforcement of water restrictions, and upgrading wastewater treatment capacities in order to increase recycling of wastewater from 75% at present to 95% in 5 years [13].

4.4 Other regions

While seawater desalination is already a well-established technology in the above-mentioned sea regions, the era of large-scale desalination projects is about to start in other parts of the world, such as California, Australia, or China, just to name a few.

In California, a potential for 15-20 new desalination projects is expected until 2030 with a combined production of 1.7Mm3/day (Fig. 9). The two most advanced and largest projects are the 200,000 m3/day facilities in Carlsbad and Huntington Beach, which will presumably start operation in 2009 [5].

Figure 9 Seawater desalination projects in California (green: in operation or construction, blue: in planning). Adapted from Ref. [25].

In Australia (Fig. 10), the first large SWRO plant with a capacity of 144,000 m3/day became operational in Perth in 2006. Another project currently under construction is the Sydney plant with an initial capacity of 250,000 m3/day, which can, if necessary, be expanded to 500,000 m3/day. Further projects include the Melbourne, Brisbane, and South East Queensland plants, with projected capacities up to 400,000 m3/day each, and projects in Adelaide, the Upper Spencer Gulf, and a second plant near Perth, with capacities between 120,000 and 140,000 m3/day each.

A third impressive example is China. The country is expected to dramatically expand its desalination capacity and might establish itself as another important market in the near future. In order to alleviate expected severe water shortages, China's desalination capacity may be increased 100fold by 2020 - i.e. from presently around 366,000 m3/d to 36Mm3/d. Besides desalination of seawater, wastewater treatment is a serious option under consideration [2].

5. ENVIRONMENTAL CONCERNS OF SEAWATER DESALINATION

The desalination industry has undergone many gradual changes since its beginnings in the early 1960s. Today, the trend is towards large,

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Figure 10 Seawater desalination projects in Australia (green: in operation or construction, blue: in planning). Based on Refs. [14,26].

industrial-sized facilities with production capacities in the range of 100,000 m3/day or more. The implementation of large desalination facilities is no longer limited to a few water-scarce but oil-rich countries of the Middle East. Desalinated water has become a commodity that amends and diversifies conventional water supplies in many parts of the world. Due to the growing desalination activity in many sea regions and the growing number of large facilities, concerns over potentially negative impacts of the technology on the environment are being raised. The main environmental concerns of desalination activity revolve around the emissions of greenhouse gases and air pollutants, the concentrate and chemical discharges into the sea, the use of large quantities of seawater for cooling purposes and as feedwater, causing the impingement and entrainment of marine organisms, and construction-related impacts on the coastal and near-shore habitats. A brief overview of the main concerns is given in the following sections. More details can be found in recent literature surveys (e.g., [16-18]).

5.1 Intakes

Seawater desalination plants can receive feedwater from different sources, but open seawater intakes are the most common intake option. The use of open intakes may result in losses of aquatic organisms when these collide with intake screens (impingement) or are drawn into the plant with the source water (entrainment). The construction of the intake structure causes an initial disturbance of the seabed, which may result in the resuspension of sediments, nutrients, or pollutants that may affect water quality. After installation, the structures can affect water exchange and sediment transport, act as artificial reefs, or may interfere with shipping routes or other maritime uses. Alternatives are beachwell intakes and infiltration galleries, which are placed below the seabed.

5.2 Discharges

All seawater desalination processes produce large quantities of a saline waste stream (the concentrate), which may be increased in temperature (thermal plants), contain residues of pretreatment and cleaning chemicals, their reaction (by-)products, and heavy metals due to corrosion (Table 2).

Chemical pretreatment and cleaning is a necessity in most desalination plants, which typically includes the treatment against biofouling, scaling, foaming, and corrosion in thermal plants, and against biofouling, suspended solids, and scale deposits in membrane plants. The chemical residues and byproducts are typically washed into the sea along with the concentrate. The concentrate of distillation plants is increased in temperature and salinity and typically contains residual chlorine and chlorination by-products, antiscalant, and antifoaming agents and certain heavy metals such as copper or nickel. The concentrate of SWRO plants is increased in salinity and typically also contains antiscalants, but residual chlorine is removed by dechlorination with sodium bisulfite to protect the RO membranes from oxidation. The concentrate of SWRO plants does not contain antifoam agents or significant levels of metals from corrosion, but it is often used to dilute other intermittent waste streams such as high-turbidity backwash waters from media filters that contain natural solids and coagulants several times per day or chemical cleaning solutions several times per years. To conclude, the discharge is a mix of these different pollutants, which may have potentially synergistic effects on marine life, such as for example the synergistic effect of chlorine residues and increased temperature which is well documented [19]. The discharge volume depends on the process recovery rate and the size of the facility. Also, the composition and concentration of residual pollutants from the pretreatment process is process- and plant-specific.

Table 2 Effluent properties of RO, MSF, and MED distillation plants, assuming conventional process design [16,27]

Physical parameters

Reverse osmosis (RO)a

Multistage flash (MSF)b

Multieffect distillation (MED)b

Salinity (S) (depending on ambient salinity and recovery rate)

Cooling water: ambient salinity (e.g., 40 g/L) Brine: 60-70 g/L Combined: 45—50 g/L

Cooling water: ambient salinity

Temperature (T)

If subsurface intakes used: may be below ambient T due to a lower T of the source water If open intakes used: close to ambient

If water is received from cooling water discharges of power plants: may be above ambient

Brine: 3—5 °C above ambient Cooling water: 8—12 °C above ambient

Cooling water: 8—12 °C above ambient, up to 20 °C possible

Combined: ~ 10-20 °C above ambient

Plume density (p)

Lower than ambient (negatively buoyant plume)

Plume can be positively, neutrally, or negatively buoyant depending on the process design and mixing with cooling water before discharge, typically positively buoyant

Dissolved oxygen (DO)

If subsurface intakes used: may be below ambient DO due to a lower DO of the source water

If open intakes used and if oxygen scavengers for dechlorination are not overdosed: close to ambient

Brine: below ambient because of deaeration and use of oxygen scavengers

Cooling water: close to ambient (minor effects on DO because of changes in temperature)

Combined: mixing of brine with cooling water increases the DO content of the combined effluent close to ambient; turbulent mixing allows oxygen take-up from air

Table 2 (Continued)

Chemical parameters

Reverse osmosis (RO)a

Multistage flash (MSF)b Multieffect distillation (MED)b

Biofouling control additives and by-products

Oxidants

• Typically dosage of 1—2 ppm to the feed water in all plants operating on open seawater

• Mainly chlorine

• Chlorine dioxide used in some plants

• Oxidants typically removed to prevent membrane damage, using sodium bisulfite (two to four times higher dosage than oxidizing agent dose)

• Discharge level is about 10—25% of dosage due to chlorine demand of the seawater

• Both the brine and the cooling water contain residual chlorine

• Chlorine typically not removed by a dechlorination step inside the plant

Halogenated organic

• Use of chlorine dioxide reduces the risk of by-product formation

by-products such as trihalomethanes

(THMs)

• May form during chlorination, but levels are assumed to be low due to dechlorination

• Chlorination of seawater results in varying composition and concentrations of halogenated (chlorinated and brominated) organic by products, mainly THMs such as bromoform

Removal of turbidity (suspended solids)

Coagulants

• Often iron (III) salts

• If filter backwash is discharged to surface waters: may cause turbidity and sedimentation in

• Treatment not applied • Treatment not applied

Coagulant aids (e.g., Polyacrylamide)

• e.g. Polyacrylamide

the discharge site and iron salts may cause effluent coloration (red brines)

Scale control additives (used

in

all desalination processes, can be a blend of several different antiscalants in combination with acid treatment)

Polymeric antiscalants

Mainly used in RO • Antiscalant only present in the brine, but not in the cooling water

(e.g., polymaleic acids) and phosphonates • Dosage: 1—2ppm

• •

Dosage/discharge concentration below toxic levels to invertebrate and fish species; some products are classified as being harmful to algae, presumably due to a nutrient inhibition effect Slow degradation (some products classified as 'inherently' biodegradable) with presumably increased residence times in surface waters

Phosphates • Dosage: 2ppm

Still used at a limited scale • Not stable at high temperature (blends of polymeric antiscalants and phosphonates preferred)

May cause eutrophication near outlets, as easily hydrolyzed to orthophosphate, a major nutrient for primary producers

Acid (H2S04) • Dosage: 30— 100 ppm

• • •

Lowers the pH from around 8.3 (natural pH of seawater) to pH 6—7

Effective against calcium carbonate scales but not against sulfate scale, therefore more effective in seawater RO and MED processes where calcium carbonate is the main scale forming species The acidity is quickly consumed by the natural alkalinity of seawater, so that the pH quickly returns to normal

Foam control additives

Antifoaming agents (e.g., polyglycol)

Treatment not applied • Typically low dosage (0.1 ppm) below harmful levels

• Used in all distillation processes, but primarily in MSF

• Antifoam only present in the brine, but not in the cooling water

Corrosion

Heavy metals

• •

Metallic equipment made • Metallic equipment made • Metallic equipment made from corrosion-resistant from carbon steel, stainless from carbon and stainless stainless steel steel, copper nickel alloys steel, aluminum and Concentrate may contain low aluminum brass, titanium, or levels of iron, chromium, copper nickel alloys

Table 2 (Continued)

Chemical parameters

Reverse osmosis (R0)a

Multistage flash (MSF)b Multieffect distillation (MED)b

nickel, molybdenum if low-quality steel is used

• Concentrate may contain iron • Lower corrosion rates than in and copper, copper levels can MSF be an environmental concern • No data on brine contamination available

Corrosion prevention

• Not necessary besides choice of materials

• As the feed water is deaerated, the brine is also deaerated before mixing with cooling water, which is not deaerated

• In MSF, the feed water (but not the cooling water) may also be treated with oxygen scavengers (e.g., sodium bisulfite), which may also remove residual chlorine

Cleaning solutions (only present if cleaning solutions are discharged to

suface waters)

Cleaning chemicals (used intermittently)

Alkaline (pH 11-12) or acidic (pH 2—3) solutions with additives, e.g.:

- Detergents (e.g., dodecylsulfate)

- Oxidants (e.g., sodium perborate)

- Biocides (e.g., formaldehyde)

Acidic (low pH) washing solution which may containing corrosion inhibitors such as benzotriazole derivates

No use of cooling water in the process, but RO plants may receive their intake water from cooling water discharges.

Assuming that the two waste streams from the desalination process are combined, that is, the brine is diluted with major amounts of cooling water from the desalination process; further dilution with cooling water from power plants may occur but is not considered here.

No use of cooling water in the process, but RO plants may receive their intake water from cooling water discharges.

Assuming that the two waste streams from the desalination process are combined, that is, the brine is diluted with major amounts of cooling water from the desalination process; further dilution with cooling water from power plants may occur but is not considered here.

Negative effects on the marine environment can occur especially when wastewater discharges and pollutant loads coincide with sensitive ecosystems. The impacts of a desalination plant on the marine environment depend on both the physical and chemical properties of the reject streams and the hydrographical and biological features of the receiving environment. The concentrate of SWRO plants is negatively buoyant due to higher than ambient salinity values, with the potential of plume sinking and seafloor spreading. The concentrate of distillation plants can be negatively, positively, or neutrally buoyant, depending on the salinity and temperature values and the amount of cooling water co-discharge, which results from the desalination process itself and co-located power plants. It is most likely positively buoyant due to large cooling water flows with a higher than ambient temperature. The concentrate of SWRO and distillation plants therefore affects different realms in the marine environment. Seafloor spreading may negatively affect benthic ecosystems such as seagrass meadows or macroalgae stands and associated benthic species such as sea urchins or shrimps, whereas neutrally or positively buoyant plumes spread in the water column and could affect nektonic species such as fish, turtles, or mammals. As these are mobile species, they can be assumed to avoid the discharge site, which could result in a loss of habitat, such as foraging, resting or reproduction areas, for the affected species. Enclosed and shallow sites with abundant marine life can generally be assumed to be more sensitive to desalination plant discharges than exposed, high-energy, open-sea locations [20], which are more capable to dilute and disperse the discharges. Environmental baseline studies thus provide important information for project planning and site selection, while monitoring during construction and operation is useful for compliance and effect monitoring. Although the number of publications discussing the potential for negative environmental impacts of effluents from desalination facilities has been steadily increasing over the last years, a surprising paucity of useful experimental data, either from laboratory tests or from field monitoring still exists. Therefore, a considerable amount of uncertainty still exists about the environmental impacts of desalination [2].

5.3 Energy demand

Desalination of seawater consumes significant amounts of energy (Table 1), either directly in the form of steam (distillation processes) or indirectly through electricity use from the electricity grid. Energy supply is consequently an important factor in the planning of new facilities. The main environmental concern associated with energy demand, both directly and indirectly, is the emission of air pollutants. Air quality may be affected by emissions of greenhouse gases (mainly CO2), acid rain gases (NOx, SOx), fine particulate matter (PM), and other air pollutants that are produced when fossil fuels are used for electricity/steam generation. The production of greenhouse gases is relevant in the context of national and international efforts to limit these emissions to minimize the impacts of climate change. Significant local impacts may further occur if emissions conflict with applicable air quality standards or management plans, contribute substantially to other existing or projected air emissions (cumulative impacts) in the vicinity and expose the resident population to increased pollutant concentrations [18]. Concerns may also arise due to more indirect impacts, such as the cooling water requirements of power plants or the increasing risk for accidents associated with the transport of fuels. When existing power plant capacities are increased or new plants constructed in order to provide additional electricity for desalination, these indirect impacts will likely be intensified.

As the treatment and distribution of water from conventional sources and by conventional processes also requires energy, it is necessary to consider both the total energy increase caused by desalination processes and the relative increase compared to other water supply options.

Reference values are often used to put the energy demand of desalination into perspective, which may influence how we perceive and evaluate the significance of energy demand and associated environmental impacts, for instance by comparing it to energy demand on a local, regional, or national level or to other energy consumers. Some examples [21]:

• On the Canary Islands, desalination accounts for 14% of all energy demands [22].

• The SWRO plant of Carboneras (capacity of 120,000 m3/day) on the Mediterranean coast of Spain consumes about one-third of the province's electrical energy [23].

• The Spanish Agua program shall increase desalination capacity on the Mediterranean coast of Spain from 1.1Mm3/day (2005) to over 2.7 Mm3/day (2010). This will require additional 11 GWh/day of electricity assuming an energy demand of 4 kWh/m3 of desalinated water as foreseen in the Spanish National Hydrological Plan [9] and will cause a 1.4% increase over 2005 national electricity generation levels (805 GWh/d or 294 TWh in 2005 [24]). It would result in additional CO2 emissions of 5475 tons/day, which represents a 0.6 % increase in national CO2 emissions compared to pre-2005 levels of 326 million tons CO2 in 2004.

• For California, it is estimated that the currently proposed desalination plants with a total capacity of 1.7Mm3/day would increase the water-related energy use by 5% over 2001 levels assuming an average energy use of 3.4 kWh/m3 [1]. The total water-related energy use was 48,012 GWh in 2001, representing 19% of the total energy use in California [25]. In another source [26], an average energy use of 2.9 kWh/m3 is assumed to produce the 1.7 Mm3/day by desalination in 2030, which is realistic as further energy savings are to be expected in the future. Desalination would thus increase the water related-energy use by 1800 GWh/year or about 4% over 2001 levels.

• The Sydney desalination plant with an initial capacity of 250,000 m3/day is expected to result in a 1.2% increase of New South Wales' electricity demand if upgraded to a capacity of 500,000 m3/day [27]. The Perth SWRO plant in Western Australia is responsible for about 0.67% of the energy demand in the region (at peak power consumption of 3574 MW in summer), compared to 30% as used for air-conditioning in Perth [28].

• In Kuwait, co-generation plants produce 443 Mm3 of desalinated water (90% of the national water supply) and 42,257 GWh of electricity per year, using 462 million GJ of energy, which is 54% of the national fuel use. About 10% of the national fuel use and the national emissions are thus attributed to the production of desalinated water and 43% to electricity generation. As the plants use mainly heavy oil (78%) and crude oil (20%), air pollution from cogeneration plants is significant and amounts to 7 million tons of CO2, 0.13 tons of SO2, and 0.02 tons of NOX per year for water production, and 30 million tons CO2, 0.54 tons SO2 and 0.06 NOX per year for electricity production. 62% of the total fuel energy (290 M-GJ) are rejected to the atmosphere (46 M-GJ) and to the sea (243 M-GJ) as cooling water. 60% ofthe cooling water discharges are attributed to the power plants and 40% to the MSF plants [29].

To conclude, desalination can be a significant energy consumer in some parts of the world, which depend heavily on desalinated water. As seen in the aforementioned examples, desalination accounts for 14% of the energy demand on the Canary Islands or for 10% of the national fuel use in Kuwait. On the mainland of Spain, however, desalination accounts for only about 1.4% of national electricity generation, and this value would even be lower if the energy use of desalination would be compared to the total Spanish energy demand taking emissions for example from transportation or heating into account. The value of 1.4% is similar in magnitude to the reference values given for Sydney (0.6% of the regional electricity demand for a single 250,000 m3/day facility) and Perth (0.67% of the regional peak energy demand for a single 140,000m3/day facility). Taking these latter values into consideration, energy use seems to be a minor energy consumer on a regional or national level in industrialized regions. However, environmental impact assessments may still find energy use to be a significant factor, which may entail some form of impact mitigation. For example, the projects in Sydney and Perth compensate the electricity demand by renewable energy projects.

5.4 Impact mitigation measures

A widely recognized and accepted approach for investigating, evaluating, and mitigating impacts of development projects on the environment is the environmental impact assessment (EIA). To date, only a handful of EIA studies have been carried out for desalination plants and made publicly available. In some cases, the investigations are carried out under immense time constraints. For instance, only 4 months were set aside for an EIA study for a 200,000 m3/day SWRO plant in Algeria [21]. This shows that environmental concerns can be of secondary importance when a ready supply of freshwater is urgently needed. The opposite is also true: comprehensive environmental studies are currently being carried out for the large SWRO projects in Australia, and environmental concerns are the major hurdle in the permitting process of new projects in California, where the planning and permitting process of the first large plant took more than 10 years.

A central element of all EIA studies is the comparison of alternatives, such as alternative project sites or technologies in order to identify the option with the least environmental footprint. Especially the selection of a suitable project site for a new desalination project can be a very effective way of minimizing and preventing impacts on the environment. Furthermore, several technical options can be implemented to mitigate the environmental effects of the waste discharges. For example, advanced diffuser systems can achieve a maximum dilution with a minimum salinity increase of one unit above background levels in the sea. Negative impacts from chemicals can be minimized by treatment before discharge, by substitution of hazardous substances, and by implementing alternative nonchemical treatment options. For instance, backwash waters from pretreatment filters can be dewatered and deposited on land, or membrane cleaning solutions can be treated on-site in special treatment facilities or discharged to a sanitary sewer system [16].

The use of alternative pretreatment methods may be considered where feasible. Prefiltration with ultrafiltration (UF) or microfiltration (MF) membranes may reduce the need for chemical pretreatment. The UF/MF membranes usually require chemically enhanced backwash and periodic cleaning. The process is therefore not entirely "chemical-free," but an advantage of intermittent cleaning over continuous pretreatment is that wastewaters are produced in smaller volumes and can be treated effectively.

A nonchemical treatment option is irradiation of the intake water with UV-light at 200-300 nm wavelengths for disinfection. A major advantage of UV-light is that storage, handling, and disposal of toxic chemicals are avoided; however, UV-irradiation has not been found to be an effective pretreatment for large desalination plants to date.

Air pollutant emissions can be minimized by increasing the energy efficiency of the desalination process. For instance, use of energy recovery devices allow for a reduction of the specific energy demand in seawater RO plants to 2-3kWh/m3, which may be decreased further in the future. Furthermore, air emissions can be controlled at the source - the power plant - as emissions depend on the fuel source (e.g., gas, coal), the technology and efficiency of the power plant, as well as on any exhaust purification equipment installed (e.g., scrubbers capturing sulfur emissions). When electricity is taken from the electricity grid, the composition of the energy mix must furthermore be taken into account when estimating the indirect air emissions of a single desalination project.

Finally, the potential for renewable energy use (solar, wind, geothermal, biomass) may be investigated to minimize impacts on air quality and climate. This may be in the form of desalination systems directly driven by renewable energy, or as an indirect compensation measures such as the installation and use of renewable energy in other localities or for other activities. For instance, the large SWRO projects in Perth and Sydney, Australia, compensate for their energy demand through wind farm projects.

6. SUMMARY AND CONCLUSION

In a nutshell, 63% of the worldwide (44.1 Mm3/day) desalination capacity is produced from seawater sources. Of this water, 61% is produced by thermal processes. The MSF distillation process is almost exclusively used for the desalination of seawater in the Gulf countries. The RO process is the second most important process for treating seawater on a global scale, but it is the first choice in many industrialized and developing countries that are now starting to consider seawater desalination. Eighty-three percent of the treated seawater is for municipal use. Sixty-six percent of the seawater desalination capacity is attributed to industrial scale facilities, with production capacities in single MSF distillation plants up to 1.6Mm3/day, while proposed capacities for single SWRO plants approach 500,000 m3/ day. Seventy-nine percent of the global seawater desalination capacity is located in the Middle East, North Africa, and Southern Europe, with 71% being located in the Gulf, the Red Sea, and the Mediterranean Sea. The enclosed nature of these sea areas makes them especially susceptible to any form of pollution, and desalination plants have been classified as a main contributor to land-based pollution in the Gulf and Red Sea [22,23].

Only 19% of the global desalination capacity is presently produced from brackish water sources and 5% from wastewater sources, with 84% of the brackish water and 79% of the wastewater being treated by RO. This share increases to 98% and 85%, respectively, if one includes the other membrane-based processes, that is, NF and ED, as well.

Although brackish water and wastewater treatments offer a great future potential, desalination of seawater will remain the dominant process for some time. This is mainly because Saudi Arabia and the United Arab Emirates will continue to be the largest desalination markets in the foreseeable future, where seawater desalination plays a prominent role. MSF distillation will therefore continue to be the main desalination process, but will presumably lose further market shares to MED and RO. While thermal cogeneration facilities predominate in the oil-rich countries of the Middle East, which produce both electricity and water, RO is usually the preferred process where cheap fossil energy or waste heat is not available, due to its lower energy demand. Consequently, most countries outside the Middle East choose RO for seawater desalination.

As the need for desalination accelerates in many parts of the world, the problem spreads from water scarcity to energy use and airborne emissions [9,24], and from overused polluted freshwater bodies to the marine environment. Due to the environmental concerns associated with the desalination of seawater, this option should therefore only be considered after other alternatives have been tapped to the full potential, such as water saving and water reuse. Examples such as Spain or Israel (cf.Section 4.3), however, show that desalination developments are often only one aspect of a whole package of water management measures, and not necessarily the first and only choice to satisfy the ever-growing demand for water and to reduce the burdens of drought. To negate the need for desalination in countries such as Israel or Spain would also mean that societies in the North would have to make concessions, as much of what we eat and wear is grown in sunny but water-scarce regions.

The question is not if desalination will provide the ultimate solution to the world's water problems. In the end, decisions about desalination developments revolve around complex evaluations of local circumstances and needs, economics, financing, environmental and social impacts, and available alternatives [1]. The question is rather which mitigation measures are necessary to reduce the environmental burden of desalination to acceptable levels. Many useful ideas have been put forward in recent literature to minimize the environmental footprint ofdesalination. The best project design, however, can only be identified in project- and site-specific studies. A catalogue of best available techniques (BAT) and best environmental practices (BEP) may be useful in guiding practitioners, consultants, and decision makers in their choices when undertaking new desalination projects. Furthermore, there is need for ongoing research and demonstration projects to gain experience, knowledge, and trust in new environmentally friendly technologies, as well as political incentives through policies or financial support to implement state-of-the-art technologies. Some of these measures will increase the price of desalinated water production; however, technological advances will most likely result in a lower energy consumption and production cost of desalinated water in the future. Sustainable desalination is not a utopia, but requires a commitment to providing water at a reasonable price, which includes not only the construction and operating costs, but also the costs to mitigate environmental impacts, including the costs for environmental studies, advanced technology, or compensation measures.

In the end, some advantages of wastewater desalination over seawater desalination should be highlighted. Water reuse is practiced in many parts of the world, but the use of desalination technologies in water reuse has been limited so far. The world's largest desalination facility treating waste water with an output capacity of 310,000 m3/day is located in Sulaibiya, Kuwait. It uses ultrafiltration followed by reverse osmosis to treat secondary effluent waste water. The main advantage of treating waste water is that it is cheaper and the energy demand is lower than for seawater RO. An expansion of waste water desalination is therefore expected in the future. Second, most of the waste is already where it is most needed, that is, near urban areas. Even if the decision is made not to use the purified wastewater for direct potable use (though from a technical point ofview, the product can comply with WHO standards), it can be used for industrial use or landscaping activities in urban areas. And third, wastewater and some of its contaminants, including nutrients, metals, or micropollutants such as pharmaceutical and personal care products, are still a burden for many rivers, estuaries, and coastal seas. Purifying and reusing wastewater does not only produce a new source of water supply, but can eliminate a waste product if the waste stream from the desalination process, which is about 15% of the original waste water volume, is treated instead of discharged. Zero liquid discharge (ZLD) technologies could be used for this purpose. While some media vilify reclaimed wastewater by negative headlines, public education programs using terms such as "new" or "purified" water can help to establish a positive attitude.

REFERENCES

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