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Essay Desalination Water

1. Introduction

Ever since desalination was originally invented in antiquity, different technologies have been developed. Back in the 4th century BC, Aristotle, the Hellenic philosopher, described a desalination technique by which non-potable water evaporated and finally condensed into potable liquid. Likewise, Alexander of Aphrodisias in the 200 AD described a technique used by sailors, as follows: seawater was boiled to produce steam, and that steam was then absorbed by sponges, thereby resulting in potable water [1]. Since then, the technology of seawater desalination for the production of potable water evolved rapidly and has become quite popular [2].

The most reliable desalination processes that can currently be exploited at the commercial scale can be divided in two main categories:

(a)

thermal (or distillation) processes like multi-stage flash distillation (MSF), multi-effect distillation (MED), thermal vapor compression (TVC), and mechanical vapor compression (MVC) processes; and

(b)

membrane processes: reverse osmosis (RO) and electrodialysis (ED) processes. ED is mostly used for brackish water installations, while RO can be used for both, brackish and seawater [3].

Over the last few years, a large number of desalination plants began to operate globally. Moreover, the production cost of desalinated water has been considerably decreased and is expected to decrease even further [4,5]. This is mostly due to the recent improvements in membrane technology, but also due to the increase of the energy conversion coefficiency for desalination processes [6].

In this paper, a short review of water desalination is provided before cost data are examined and processed. This paper focuses on water desalination processes and projects in Greece.

2. Desalination is Growing around the World

Desalination is growing so fast globally that it is more than certain that it will play a significant role in water supply in the years to come. Desalination is growing particularly in parts of the world where water availability is low. Annual desalination capacity seems to increase rapidly as years go by.

A sharp increase in the number of desalination projects to supply water is indicated. This rose from 326 m3/d in 1945 to over 5,000,000 m3/d in 1980 and to more than 35,000,000 m3/d in 2004 [7]. In 2008, the total daily capacity was 52,333,950 m3/d, from some 14,000 plants in operation globally [8]. In 2011, the total capacity was about 67,000,000 m3/d, while in 2012 it was estimated at about 79,000,000 m3/d from some 16,000 plants worldwide [9].

The Gulf Region (Middle East) has the biggest number of desalination plants in the world, followed by the Mediterranean, the Americas, and Asia [10]. The percentages of desalination plants for each geographical area are shown in Figure 1.

The global capacity of desalination plants, including renewable desalination, is expected to grow at an annual rate of more than 9% between 2010 and 2016. The market is set to grow in both developed and emerging countries such as the United States, China, Saudi Arabia (SA) and the United Arab Emirates (UAE), as shown in Figure 2. A very significant potential also exists in rural and remote areas, as well as in islands (Figure 2, rest of world (ROW)), where grid electricity or fossil fuels to generate energy may not be available at affordable costs. About 54% of the global growth is expected to occur in the Middle East and North Africa (MENA) region [11], where the 21 million m3/d of desalinated water in 2007 is expected to reach 110 million m3/d by 2030, of which 70% is in SA, the UAE, Kuwait, Algeria and Libya [11].

Figure 1. World desalination plants per geographical area (%). Adapted from [10].

Figure 1. World desalination plants per geographical area (%). Adapted from [10].

Figure 2. Global installed desalination capacity, 2010–2016. Adapted from [11].

Figure 2. Global installed desalination capacity, 2010–2016. Adapted from [11].

The majority of the largest desalination plants (in operation or under construction) use seawater and are located in the Middle East. The biggest desalination plant is the Ras Al-Khair in the city of Ras Al-Khair (also called Ras Al-Zour or Ras Azzour) SA, which uses both membrane and thermal technology with a capacity over 1,000,000 m3/d, in operation since 2013. The Ras Al-Khair plant supplies Maaden factories with 25,000 m3 of desalinated water and 1350 MW of electricity. It also supplies with water the capital city of Riyadh and several central cities with a total need of 900,000 m3/d [12,13]. Another example is the 880,000 m3/d MSF Shuaiba 3 desalination plant that is located along the east coast of SA and supplies with potable water the cities of Jeddah, Makkah, and Taif. SA also hosts the Ras Al-Zour unit, producing 800,000 m3/d of water [14]. Table 1 shows some of the biggest desalination plants in the world.

Table 1. The biggest desalination plants around the world. SA: Saudi Arabia; and UAE: United Arab Emirates. Adapted from [12,13,15].

LocationCapacity (m3/d)FeedwaterOperation year
Ras Al-Khair, SA 1,025,000N/A2013
Shuaiba, SA880,000Seawater2007
Ras Al-Khair, SA800,000Seawater2007
Al Jubail, SA 730,000 Seawater2007
Jebel Ali, United Arabic Emirates600,000Seawater2011
Al-Zour North, Kuwait567,000Seawater2007

As far as the membrane technologies are concerned, especially the RO desalination technology (one of the most renowned), there are big plants around the world with great potential in energy saving and reasonable production cost (Table 2) [16]. The largest membrane desalination plant in the world is the Victoria Desalination Plant in Melbourne, Australia with a capacity 444,000 m3/d, in operation since 2012. However, larger units will soon operate, like the Magtaa plant in Algeria and the Soreq plant in Israel, with capacities of 500,000 m3/d and 510,000 m3/d, respectively [13].

Table 2. Major reverse osmosis (RO) desalination plants in the world. Adapted from [13,16].

LocationCity, CountryCapacity (m3/d)
Soreq desalination plantRishon Letzion, Israel510,000
Magtaa desalination plant Oran, Algeria500,000
Victoria desalination plantMelbourne, Australia444,000
Point Lisas desalination plantPoint Lisas, Trinidad109,019
Tampa Bay desalination plantTampa, FL, USA94,635

3. Production Cost of Desalinated Water

The overall cost of desalination can be divided into investment cost and maintenance-operation cost. Investment cost involves land, edifices and equipment, as well as transportation cost, insurance, construction, legal fees and unforeseen costs. Maintenance and operation cost is divided in energy cost, maintenance, repairs, personnel/staff, spare parts, and reconstruction when required. Energy cost is the higher contributor to operation cost and thus to the overall cost. In many cases, energy cost can reach almost the 60% of the operation and maintenance cost. A comparison of the total cost of the RO and MSF technologies is given in Table 3 [17].

Table 3. Cost percentage in conventional RO and multi-stage flash distillation (MSF) of the same capacity in Lybia. Adapted from [17].

Desalination technologyInvestment cost (%)Energy cost (%)Maintenance & repair cost (%)Membrane replacement cost (%)Personnel cost (%)Chemicals cost (%)
RO (membrane)3126141397
MSF (thermal)42418072

Cost evaluation for desalination is a difficult process as data are influenced by different factors such as energy cost, materials and labor. Those factors differ significantly from place to place. Moreover, cost is influenced by elements like desalination technology, total dissolved solids (TDS) concentration of the raw water used to feed the plant, and other economic parameters that related to local conditions [18]. In conclusion, desalination cost is significantly decreasing when brackish water is used instead of seawater and when the capacity of the plant is increased (Table 4).

Table 4. Desalinated water production cost from seawater and brackish water.

Capacity (m3/d)Cost (€/m3)
Sea waterBrackish water
3,8000.970.50
7,6000.700.27
19,0000.540.21
38,0000.500.17
57,0000.490.15

Membrane desalination technologies such as RO are known for their lower energy demands compared to thermal technologies which can be further reduced using energy recovery systems. Such limited energy demands have direct effect on the cost of the produced desalinated water, which in most cases is lower than the cost of water produced by thermal technologies. The RO cost per feeding water and capacity is shown in Table 5, compared to the cost related to the most common thermal desalination technologies.

Table 5. RO desalination production cost compared to thermal desalination technologies cost, per feeding water and production capacity. MED: multi-effect distillation; and VC: vapor compression. Adapted from [19].

FeedwaterPlant capacity (m3/d)Cost (€/m3)
Brackish water RO<204.50–10.32
20–1,2000.62–1.06
40,000–46,0000.21–0.43
Seawater RO<1001.20–15.00
250–1,0001.00–3.14
1,000–4,8000.56–1.38
15,000–60,0000.38–1.30
100,000–320,0000.36–0.53
MSF<1002.00–8.00
12,000–55,0000.76–1.20
>91,0000.42–0.81
MED23,000–528,0000.42–1.40
VC1,000–1,2001.61–2.13

Hybrid desalination systems, which are used to combine desalination technologies, are suitable for big installations in order to accomplish scale economies that reduce the production cost. In such plants, membrane technologies can be combined with thermal technologies and vice versa. To give an example, in a plant where the brine flow of RO is the feed flow of membrane distillation, the cost is 0.94 €/m3. Had the system used only RO, the respective cost would be 0.94 €/m3, whereas the same system operating under membrane distillation would have a production cost of 0.99 €/m3. In other words, when technologies are combined the double quantity of water is produced at the same cost or less compared with the alternatives. To give another example, a MSF used in a desalination plant of 528,000 m3/d, produces water at a 0.32 €/m3, whereas when it is combined with RO, its cost is reduced by 15% [18

Desalination, also called desalting, removal of dissolved salts from seawater and in some cases from the brackish (slightly salty) waters of inland seas, highly mineralized groundwaters (e.g., geothermal brines), and municipal wastewaters. This process renders such otherwise unusable waters fit for human consumption, irrigation, industrial applications, and various other purposes. Existing desalination technology requires a substantial amount of energy, and so the process is expensive. For this reason it is generally used only where sources of fresh water are not economically available.

The desalting of seawater is an ancient notion. Aristotle described an evaporation method used by Greek sailors of the 4th century bce. An Arab writer of the 8th century ce produced a treatise on distillation. In the 19th century the development of steam navigation created a demand for noncorroding water for boilers, and the first patent for a desalination process was granted in England in 1869. The same year, the first water-distillation plant was built by the British government at Aden, to supply ships stopping at the Red Sea port. The first large still to provide water for commercial purposes was built in 1930 in Aruba, near Venezuela. By 2005 more than 10,500 desalination plants producing a total of more than 55 billion litres (in excess of 14.6 billion gallons) of potable water per day were in operation throughout the world.

Desalination processes

Desalination methods can utilize either thermal processes (involving heat transfer and a phase change) or membrane processes (using thin sheets of synthetic semipermeable materials to separate water from dissolved salt). Multistage flash distillation is a thermal process for desalting relatively large quantities of seawater. Based on the fact that the boiling temperature of water is lowered as air pressure drops, this process is carried out in a series of closed tanks (stages) set at progressively lower pressures. When preheated seawater enters the first stage, some of it rapidly boils (flashes), forming vapour that is condensed into fresh water on heat-exchange tubes. Fresh water is collected in trays as the remaining seawater flows into the next stage, where it also flashes, and the process is continued. One of the largest of these systems, located in Al-Jubayl, Saudi Arabia, can produce more than 750 million litres (200 million gallons) of desalted water per day.

In small communities where salt water and intense sunlight are both abundant, a simple thermal process called solar humidification can be used. The heat of the Sun partially vaporizes salt water under a transparent cover. On the underside of the cover, the vapour condenses and flows into a collecting trough. The principal difficulty in this process is that large land areas are required, and energy is needed for pumping the water. Another thermal process makes use of the fact that, when salt water is frozen, the ice crystals contain no salt. In practice, however, objectionable amounts of salt water remain trapped between the crystals, and the amount of fresh water needed to wash the salt water away is comparable to the amount of fresh water produced by melting the crystals.

Membrane processes for desalting include reverse osmosis and electrodialysis. Of the two, reverse osmosis is the more widely used, particularly for desalting brackish waters from inland seas. The salt content of brackish inland water, though undesirable, is considerably below that of seawater. Electrodialysis uses electrical potential to drive the positive and negative ions of dissolved salts through separate semipermeable synthetic membrane filters. This process leaves fresh water between the filters. In reverse osmosis salt water is forced against the membranes under high pressure; fresh water passes through while the concentrated mineral salts remain behind. To conserve space, the membranes are packaged in multiple layers in a collection of long tubes. One of the largest reverse-osmosis desalination plants now in operation is located in Ashqelon, Israel, and can produce some 300 million litres (80 million gallons) of desalted water per day.

Global production

In many areas of the world, particularly in densely populated arid regions, desalted water is the main source of municipal water supplies. Desalination is used in more than 100 countries, and about three-quarters of all desalted water is produced in the Middle East and North Africa. By 2010 the largest producers of desalinated water were Saudi Arabia, accounting for about 17 percent of total global output, and the United Arab Emirates, with 13.4 percent. The United States is third, accounting for roughly 13 percent of the total output (mostly in Florida, Texas, and California). The majority of all desalination plants are reverse-osmosis systems, with multistage flash distillation being the second-ranking process.

In general, a population usually can afford to pay about 7–10 times as much for water for domestic purposes as it does for agricultural water. Large-scale desalination facilities promise to lower the cost of desalted water at the desalination sites to a level that most industries and some agricultural enterprises can afford. In the future it can be expected that the ocean will become an increasingly important source of fresh water. If production and transportation costs can be lowered sufficiently, it may be possible to produce fresh water to irrigate large areas that border the oceans in many parts of the world.