Water demand and supply have become an international issue due to several factors: global warming (droughts are more often in arid areas), low annual rainfall, a rise in population rates during last decades, high living standards, and the expansion of industrial and agricultural activities. Fresh water from rivers and groundwater sources are becoming limited and vast reserves of fresh water are located in deep places where economical and geological issues are the main obstacles. Therefore, it has turned into a competition to get this vital liquid and to find more feasible and economical sources that can ameliorate the great demand that the world is living nowadays and avoid water restrictions and service interruptions to domestic water supply.

The oceans represent the earth’s major water reservoir. About 97% of the earth’s water is seawater while another 2% is locked in icecaps and glaciers. Available fresh water accounts for less than 0.5% of the earth’s total water supply . Even though seawater is not suitable for human use and agricultural purposes, desalination

Desalination

Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...

 has emerged as a potential solution to remove salt from seawater and become a quasi-unlimited source of fresh water as the conventional water supply is turning very expensive due to over-exploitation of aquifers and the high costs related to wastewater treatment. Desalination has the great advantage that it does not depend on climate conditions such as rainfall catchment. However, seawater temperature, ocean currents and salinity are important that should be considered during the desalination process.

Desalination is an excellent alternative for getting fresh water, especially in dries places such as Middle East

Middle East

The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...

 and North Africa

North Africa

North Africa or Northern Africa is the northernmost region of the African continent, linked by the Sahara to Sub-Saharan Africa. Geopolitically, the United Nations definition of Northern Africa includes eight countries or territories; Algeria, Egypt, Libya, Morocco, South Sudan, Sudan, Tunisia, and...

, in regions where are surrounded by seawater through their boundaries such as Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 and some of the islands in the Caribbean Sea

Caribbean Sea

The Caribbean Sea is a sea of the Atlantic Ocean located in the tropics of the Western hemisphere. It is bounded by Mexico and Central America to the west and southwest, to the north by the Greater Antilles, and to the east by the Lesser Antilles....

. Desalination of seawater has been in practice for nearly 50 years and is considered as a main source of fresh water and many countries highly depend on this main source. It is estimated that over 75 million people on the world obtain water by desalinating water..

Current water supply in Australia

Until some decades ago, Australia was meeting the water demands of its population. Dams and water catchments were the main source of fresh water. However, during the last decade it has been observed a significant drop in water stored in water reservoirs in almost every state due to the lack of rainfall . The most affected cities have been the capitals, where there is high water demand. For instance, in 2007 Sydney

Sydney

Sydney is the most populous city in Australia and the state capital of New South Wales. Sydney is located on Australia's south-east coast of the Tasman Sea. As of June 2010, the greater metropolitan area had an approximate population of 4.6 million people...

, which is the capital city of the state of New South Wales

New South Wales

New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

, experienced a dramatic drop of its main dam (Warragamba) in the level of stored water to approximately 33% of its total storage capacity .

Australia is the driest inhabitable continent on earth and despite this, its installed desalination capacity is still around 1% of the total world’s desalination capacity. The Department of Agriculture, Fisheries and Forestry (Australia)

Department of Agriculture, Fisheries and Forestry (Australia)

The Department of Agriculture, Fisheries and Forestry is an Australian government department. The Australian Department of Agriculture, Fisheries and Forestry’s role is to develop and implement policies and programs that ensure Australia's agricultural, fisheries, food and forestry industries...

 (AFFA), has considered two main desalination technologies processes in Australia :

• Major processes, which include membrane (reverse osmosis
  Reverse osmosis
  Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
  
   (RO) and electrodialysis
  Electrodialysis
  Electrodialysis is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell...
  
   (ED)), distillation (multi-stage flash distillation (MSF), multiple-effect distillation (MED) and vapor-compression desalination
  Vapor-compression desalination
  Vapor compression desalination refers to a distillation process where the evaporation of sea or saline water is obtained by the application of heat delivered by compressed vapor. Since compression of the vapor increases both the pressure and temperature of the vapor, it is possible to use the...
  
   (VC)).
• Alternative processes, which include renewable energy powered conventional desalination, solar humidification
  Solar Humidification
  The solar humidification-dehumidification method is a thermal water desalination method. It is based on evaporation of sea water or brackish water and consecutive condensation of the generated humid air, mostly at ambient pressure...
  
  , freezing and membrane distillation.

Australia’s first desalination plant was constructed back in 1903 to treat saline groundwater in the gold fields of Western Australia (WA) at Kalgoorlie . A lot of desalination plants were built in Australia between 1960 and 1980 where there was a world membrane technology revolution. Most of them were RO plants, followed by VC and MSF plants. However, by 2002 just two main RO desalination plants were still working, the first in Kangaroo Island and the second in Rottnest Island . Seawater reverse osmosis (SWRO) is the only type of desalination technology currently used for future large-scale desalination plants in Australia , the most important SWRO desalination plants are located in Perth and Sydney.

Desalination is still considered as an expensive alternative to obtain fresh water compared to with the existing metropolitan water. The situation could change by 2020 as a result of available mainstream technologies or by 2010 if alternative technologies currently under investigation, become available at industrial scale .

Despite the fact that there are many drawbacks of desalination in Australia, it is considered as a realistic solution that can ameliorate the shortage of fresh water in this country, especially in Western Australia where main shortages have been observed.

Description of main desalination technologies

Desalination refers to a suite of technologies designed to produce freshwater from saline sources such as seawater or brackish groundwater. There are a considerable number of different types of desalination technologies including: distillation, ion exchange, membrane processes, freezing, geothermal, solar humidification, and seawater greenhouse. Of these, however, distillation and membrane processes are most typically utilized.

Distillation, involves boiling salt water, evaporating off fresh water and then condensing the vapor while membrane processes use a relatively permeable membrane to trap and remove salts and minerals. Below are brief descriptions of the main desalination processes :

Distillation Technologies

Multi-stage flash distillation (MSF)

MSF is a thermal distillation process that involves evaporation and condensation of water. The evaporation and condensation steps are coupled to each other in several stages so that the latent heat of evaporation is recovered for reuse by preheating incoming water.

• Advantages. MSF plants, especially large ones, produce a lot of waste heat and can therefore often be paired with cogeneration .
• Disadvantages. It requires high operating costs when waste heat is not available for distillation and also it has high rates of corrosion .

Vapor-compression desalination

Vapor-compression desalination

Vapor compression desalination refers to a distillation process where the evaporation of sea or saline water is obtained by the application of heat delivered by compressed vapor. Since compression of the vapor increases both the pressure and temperature of the vapor, it is possible to use the...

 (VC)

It is a process which uses its own steam after it has been compressed as its primary heat source. In this process, it is necessary to provide a mechanical energy source, specifically a mechanical compressor

• Advantages. The operating costs are low compared to multi-stage or multi-effect flash distillation systems and also the equipment is smaller than the multi-stage flash or multi-effect flash distillation systems.
• Disadvantages. The maintenance of compressors and heat exchangers is greater than those of other systems. The energy consumption is high and capital costs are high.

Multiple-effect distillation (MED/ME)

The process consists of multiple stages or "effects". In each stage the feed water is heated by steam in tubes. Some of the water evaporates, and this steam flows into the tubes of the next stage, heating and evaporating more water. Each stage essentially reuses the energy from the previous stage.

• Advantages. The process involves low operating costs when waste heat is used for the distillation process and the quality of the feed water is not as important as for a reverse osmosis system . The multi-effect flash system can utilize hot water instead of steam for the distillation process.
• Disadvantages. It involves high operating costs when waste heat is not available for the distillation process and the multi-effect flash system operates at high temperatures that increase corrosion and scale formation.

Membrane Technologies

Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure

Osmotic pressure

Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....

.

Reverse Osmosis

Reverse osmosis

Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...

 (RO)

This process recovers water from a saline solution that is pressurized to a point greater than the osmotic pressure of the solution. In essence, membrane filters hold back the salt ions from the pressurized solution, allowing only the water to pass.

• Advantages. It has long membrane lifetime and high efficiency (up to 94% water recovery, usually around 80%).
• Disadvantages. The process requires more pretreatment of the seawater and more maintenance than MSF plants. RO membranes are sensitive to pH, oxidizers, a wide range of organics, algae, bacteria, particulates and other pollutants . Hence pretreatment of the feed water is an important consideration and can a significant impact on the cost of RO.

Forward osmosis

Forward osmosis

Forward osmosis is an osmotic process that, like reverse osmosis, uses a semi-permeable membrane to effect separation of water from dissolved solutes...

 (FO)

Instead of employing hydraulic pressure as the driving force for separation in the RO process, FO uses the osmotic pressure gradient across the membrane to induce a net flow of water through the membrane into the draw solution, thus efficiently separating the freshwater from its solutes.

• Advantages. The main advantages of using FO are that it operates at low or no hydraulic pressures, it has high rejection of a wide range of contaminants, and it may have a lower membrane fouling propensity than pressure-driven membrane processes.
• Disadvantages. Forward Osmosis cannot produce pure water but only concentrated solutions and hence can be used mainly for pretreatment.

Membrane distillation (MD)

MD involves the transport of water vapor from a saline solution through the pores of a hydrophobic membrane.

• Advantages. MD has a number of potential advantages over conventional desalination including low temperature and pressure operation, reduced membrane strength requirements, compact size, and 100% rejection of non-volatiles.
• Disadvantages. Large air flows are required to achieve significant water yields, and the costs associated with transporting this air are very high.

Nanofiltration

Nanofiltration

Nanofiltration is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening and removal of disinfection by-product precursors such as natural organic matter and synthetic organic...

 (NF)

Nanofiltration membranes are made of semi-permeable material that allows almost nothing larger than pure water molecules to pass through. Because they are larger than water molecules, most salt molecules and other materials are left behind.

• Advantages. NF membranes have the ability to remove turbidity, hardness, fluoride and nitrate as well as a significant fraction of dissolved salts . Desalination can be performed with a significantly lower operating pressure and becomes a much more energy-efficient process.
• Disadvantages. It requires high capital cost, unknown lifetime of membrane, no large-scale plant built yet.

Electrodialysis reversal (EDR)

Electrodialysis (ED) utilizes a direct current source and a number of flow channels separated by alternating anion and cation selective membranes to achieve the separation of water and dissolved salts. In the EDR process, the membrane polarity is reversed several times an hour.

• Advantages. The advantage of EDR is the ability to take EDR membrane stacks apart and clean them by hand . The process is more economical and easier to control. Since there is not a flux of water through the membranes, ED can treat water with a higher level of suspended solids than RO.
• Disadvantages. ED units are subject to fouling, and thus some pretreatment of the feed water is usually necessary . ED is generally limited to brackish waters containing only a few thousand ppm of dissolved solids.

Main Factors affecting desalination cost

There are some factors that should be considered and are related to the cost and selection of a desalination technology. Some of these factors are :

• Performance ratio. It is the ratio of freshwater to the amount of energy consumed, either steam or heat.
• Plant life. Amortization period which affects the capital costs and also the selection of the performance ratio.
• Installation costs. Costs related to the installation of equipment in the desalination plant.
• Maintenance costs. They vary depending on how often maintenance is implemented to the plant and load operation in which the plant is working.
• Interest rates. Financial aspect that can affect the investment of the plant itself.
• Site costs. They are linked to the place where the desalination plant will be installed. Besides, it is also considered the distance that the freshwater is going to travel once it is produced.
• Seawater intake and outflow. It refers that depending on the distance between the feed water source and plant, the capital costs can increase or not.
• Feed water quality. The quality of seawater intake can affect the desalination process.
• Freshwater quality required. The required quality could vary depending on the final usage and local standards and this could affect the selected desalination process.
• Energy sources. Desalted water costs are susceptible to the variation of energy prices.
• Pretreatment. Seawater intake could be pretreated in order to optimize the desalination process. If it is required, it will raise the cost of desalted water.
• Plant load factor. It is the total production for the year measured as a percentage of the rated capacity. It gives a general overview of plant utilization.
• Availability of skilled labor. Employees could need training in order to operate the desalination plant so some costs can be related to this.
• Storage and distribution of final freshwater. Costs linked to the delivery of final product (freshwater) to storage or customers.
• Plant capacity. In general, the cost of desalted water usually decreases when plant capacity increases. This is due to the fact that operational, labor and maintenance costs can be allocated over a larger capacity.

Costs of the major processes

There can be several configurations of technologies using different energy sources. Therefore is difficult to make non-case specific comparisons and it is presented a comparison between three of the most applicable technologies.
The financial characteristics of a desalting plant are usually expressed in two ways: the capital costs and total annual operating water costs per unit of installed or process capacity and are usually measured in units such as dollars per kL per day . The table below shows a comparison among the three major desalination processes.

Summary of application of desalination technologies

Parameter	Seawater RO	Brackish RO	MED	EDR
Capital Cost Capital cost Capital costs are costs incurred on the purchase of land, buildings, construction and equipment to be used in the production of goods or the rendering of services, in other words, the total cost needed to bring a project to a commercially operable status. However, capital costs are not limited to...  [A$/kL/day of product water]	1,600-2,500	600-1,800	2,500-3,900	570-3,250
Operating cost Operating cost Operating costs can be described as the expenses which are related to the operation of a business, or to the operation of a device, component, piece of equipment or facility.-Business operating costs:...  [A$/kL/day of product water]	1.89-2.20	0.65-1.50	With Waste Heat 0.55-0.95 Without Waste Heat: 1.8-2.80	1.00-2.80

Studies have shown that the least expensive forms of RO desalination are the most cost competitive with traditional mains water supplies, but only in those instances where users are already paying a yearly average of approximately $0.60/kL for their mains water. The least expensive RO plants are those that are desalinating low salinity feedwaters at reasonably large quantities (50 kL/day or enough for all daily uses of 250 people)
As long as desalination is developed and used, its cost will decrease and will become a source of fresh water where water supply is limited.

Sustainability of desalination: renewable energies

The context of energy sustainability and the related greenhouse offsets for desalination plants is of growing global concern. Considering industrialized countries with higher per-capita emissions and ever growing energy demands, renewable energies must contribute to the electricity provision of desalination processes to decrease emissions . There are a number of ways in which renewable energy can be harnessed for desalination. Direct employment of renewable energy is possible, such as in a solar still which uses sunlight to evaporate water, or through converting to mechanical energy to drive water pumps . Although indirect use of renewable energy through the provision of electrical energy via photovoltaic’s, solar thermal, wind turbines, water turbines, wave or tidal power is much more common.

As seawater reverse osmosis (SWRO) is the only technology employed or proposed for future Australian large-scale plants the provision of electricity through renewable energy is the currently the only option for offsetting the energy demands of desalination plants.

Desalination plants in Australia by state and location

State	Location	Capacity (ML/d)	Project date
WA	Kwinana Beach, Western Australia Kwinana Beach, Western Australia Kwinana Beach is an industrial suburb of Perth, Western Australia, located within the Town of Kwinana.Kwinana is firstly a local indigenous Australian word meaning either "young woman" or "pretty maiden". The ship SS Kwinana was wrecked on Cockburn Sound in 1922 and blown onto Kwinana Beach. The...	144	Operating
	Bunbury, Western Australia Bunbury, Western Australia The port city of Bunbury is the third largest city in Western Australia after the State Capital Perth and Mandurah. It is situated south of Perth's central business district...  (Binningup)	137	To be completed 2011
	Esperance, Western Australia Esperance, Western Australia Esperance is a large town in the Goldfields-Esperance region of Western Australia, located on the Southern Ocean coastline approximately east-southeast of the state capital, Perth. The shire of Esperance is home to 9,536 people as of the 2006 census, its major industries are tourism, agriculture,...	45	Proposed 2011
	Karratha	175	Planning
	Undecided	150	Planning
SA	Port Stanvac Port Stanvac Desalination Plant The Port Stanvac Desalination plant is a SWRO desalination plant currently under construction at Lonsdale, South Australia which will provide the city of Adelaide with a significant amount of drinking water...  (Adelaide Adelaide Adelaide is the capital city of South Australia and the fifth-largest city in Australia. Adelaide has an estimated population of more than 1.2 million... )	150	Startup in Dec 2010
	Whyalla(possible)	180	Planning
Vic	Wonthaggi	400	To be completed 2011
NSW	Kurnell	250	Operating
Qld	Tugun	125	Operating
	Sunshine Coast, Queensland Sunshine Coast, Queensland The Sunshine Coast is an urban area in South East Queensland, north of the state capital of Brisbane on the Pacific Ocean coastline. Although it does not have a central business district, by population it ranks as the 10th largest metropolis in Australia and the third largest in...	unknown	Suggested after 2028
	Bribie Island	Unknown	In dispute
	North Stradbroke Island North Stradbroke Island North Stradbroke Island is an Australian island in the state of Queensland, 30 km southeast of the capital Brisbane. Before 1896 the island was part of the Stradbroke Island. In that year a storm separated it from South Stradbroke Island, forming the Jumpinpin Channel. It is known...	Unknown	In dispute

In Australia many desalination plants are utilizing wind farms to produce enough energy to operate nearby desalination plants. For example, the Kurnell Desalination Plant

Kurnell Desalination Plant

The Sydney Desalination Plant is a drinking water supply project operated by Veolia Water Australia Pty Ltd on behalf of Sydney Water, Sydney's State-owned water supply corporation. It is located in the Kurnell industrial estate area, in Sydney's southern suburbs. The desalination plant itself was...

, with a capacity of producing 250 million liters (ML) of drinking water per day, supplies 15% of Sydney’s water needs via RO technology and is powered using “100 percent renewable energy” from the 140 MW Capital Wind Farm .

The availability of renewable resources as well as their fluctuation in electricity production from region to region requires a customized design for each desalination facility. In order to maintain steady-state operations many facilities utilize renewably produced energy while connected to a smart grid, importing or exporting energy to the plant as required. The Perth Seawater Desalination Plant utilizes this strategy where 48 wind turbines produce 80MW on the Emu Downs Wind Farm to provide an overall 24MW to the desalination plant . Electrical energy from the renewable energy can also be stored in storage batteries and utilized when needed. As seen in the PV-powered RO system in Gillen Bore, Australia; producing 1,200 L/d . Or if the plant is not required full-time, it can operate using the power as it becomes available. In 2005 a PV-powered hybrid UF/RO filtration system providing 764 liters per day tolerated well power variation from changing weather conditions .

However, there are limitations in the ability of renewable sources to provide for desalination facilities. Desalination is a continuous process while renewable energies provide inconsistent power. For any new desalination installation to claim it will be powered by renewable energy, additional renewable energy should be generated.

The Perth Experience

About 1.5 million people occupy the Perth region in the south western corner of Western Australia. The city increased its water supply capacity through implementation of the Perth Seawater Desalination Plant (PSDP) in late 2006 to produce up to 45 gigalitres of potable water per year. The energy consumption of PSDP is carbon neutral, meaning that it is 100% offset with renewable energy. In addition, its brine discharge has been shown to have no adverse impact on the environment. The plant buys its power from electricity generated by the Emu Downs Wind Farm, located 200 kilometers north of Perth. The 83 megawatt wind farm consists of 48 wind turbines and contributes over 272 giga-watt-hours (GWhr) per year into the grid, fully offsetting the Perth SWRO Plant’s estimated electrical requirement of 180 GWhr per year . The plant has attracted a lot of interest among the world’s water industry and media and has won numerous national and international awards including the International Desalination Association’s International Desalination Plant of the Year in 2007.
Another seawater desalination plant on the coast about 160 kilometres south of Perth is in construction. It is designed to have an initial annual output of 50 gigalitres, slightly higher than that of the Perth plant with the potential to double this to about 100 gigalitres. The Water Corporation intends purchasing the energy requirements and associated environmental credits for the southern plant from renewable energy generators using a combination of traditional and currently commercially unproven renewable technologies.

Conclusions

As the present rate of annual precipitation is diminishing and population density in major cities increases, desalination technology applications in Australia will likely increase. Despite the high cost of fresh water produced by desalination, the scarcity of natural water sources make desalination a necessity .

Membrane technology has undergone a rapid development and has become the leading water purification method. . Water treatment processes in the future may employ integrated membrane processes that treat fresh, brackish, and saltwater. In addition, risk assessments relative to security and supply vulnerability may also influence the selection of membrane processes.