How Desalination Works
Photo via Reiner Schubert via Flickr CC
As our global water crisis persists, we're hearing more and more about the use of desalination as a viable source of drinking water. Desalination used to be written off as an energy-intensive way to separate salt from water, used on ships as the only option for supplying fresh water, or for communities in the Middle East where both seawater and fossil fuels are plentiful. However, with new technologies reducing both the energy requirements and environmental impacts of desalination plants, and with ever greater demand for diminished supplies of fresh water, desalination is moving into the spotlight. But what exactly is desalination? When is it a viable option? How popular will it be? We're exploring everything about this often controversial topic.
What Is Desalination?
Simply put, desalination is removing salt and other minerals to create fresh, drinkable water. Desalination is used where fresh water supplies are short but seawater is plentiful, to supply a community with potable water for households, manufacturing or agriculture.
While it is typically an expensive and energy-intensive solution that often supplements wasteful water use practices, it is sometimes the only solution for keeping a community going. This is where the controversies start up -- is it worth the damage to the environment to run a desalination plant if other options for water collection and conservation are available? But as fewer options are at people's disposal as groundwater supplies diminish and rivers and lakes are tapped to the limit, desalination is becoming a more attractive option.
The most popular methods used in desalination are multi-stage flash distillation, which uses heat to evaporate water, leaving the salt behind, and which accounted for 84% of desalination in 2004; and reverse osmosis desalination, which is less energy intensive but still requires a great deal of energy to pump water through filtration membranes.
Despite improvements, the energy requirements of desalination are still high, and other technologies that use less energy are being explored, including forward osmosis and low temperature thermal desalination.
Forward osmosis is a mix of membrane and thermal purification, using a solution of removable solutes that draws water towards it. Yale has created a mix of ammonia and carbon dioxide gasses dissolved in water that pulls water from saline feeds. The salts created from ammonia and carbon dioxide are heated into gasses again, leaving purified water behind. It is far more energy efficient than other processes, and Yale has used it successfully in a pilot plant in conjunction with Oasys.
Low temperature thermal desalination boils water at relatively low temperatures, with cooling water pumped through coils to cool the evaporated water vapor, gathering it back into purified water in liquid form. This type of technology allows for a process low-energy enough to utilize solar power or other renewable energy -- or possibly even using waste heat from places like power plants -- to decrease the environmental impact of desalination.
Important technological advances to improve desalination don't stop at the water filtration system. They also include how to power the plants. Ideas such as utilizing the waste heat from facilities like power plants to run adjacent desalination plants have been put to the test, but in an effort to decrease our reliance on fossil fuels, making renewable energy work for desalination is of great interest. Australia has successfully built desalination plants that run on renewable energy, but less conventional ideas are -- quite literally -- being floated. Check out this idea by Alatico:
A Smarter Water Filtration System?
As with any technology, the costs have to be measured by looking at the variables -- when is it cheaper and more efficient to transport water from an existing freshwater source than using a desalination plant. With desalination, factors such as a community's distance from seawater, distance from other freshwater sources, the sustainability of those freshwater sources, the fuel choice for running the desalination plant, the cost of infrastructure and so on have to all be weighed to discover if desalination is a smart choice for providing fresh water to a community.
Other factors that have to be weighed are the environmental costs of the process. Drawing water directly from the sea results in mortality of fish eggs and larvae and plankton, and depositing brine into the sea increases the salinity of the seawater and can damage the marine flora and fauna living near the output. The ways in which water is drawn and deposited must be considered. Some plants draw from farther inland, pulling brackish water at the water table, rather than directly from the sea, and depositing the brine through a system of pipes that dispurse it, or mixing it with outgoing water from places like wastewater treatment plants or power plants that use water cooling systems.
Here is an interview with City Engineer Richard Simonitch, of Sand City, California, which recently built a new desalination plant. For this community, desalination seemed to be the only possible solution for accessing fresh water. Some of the factors for their decision, as well as ways they kept the facility as environmentally friendly as possible, are discussed:
Ultimately, desalination is a possible option for some communities with ideal proximity to both ocean water and energy sources. However, it doesn't mean inexpensive fresh water for most areas, including those with the most significant water shortages. For many areas, water filtration systems like rainwater harvesting and purification and water recycling systems are the more viable solutions to water shortages.
Where We Can Expect to See Desalination PlantsDesalination is gaining ground among water-strapped cities. A recent survey showed that spending on desalination is set to double over the next decade, with Saudi Arabia, the United Arab Emirates, the United States, China, and Israel as the top five markets. Even London has flipped the on switch to a desalination plant on the Thames to provide water to its citizens as supplies grow short. Australia also has desalination plants in several areas, and has been making progress on reducing their environmental footprint, including running them on renewable energy.
In fact, renewable energy will be an important part of making desalination work. Areas that are close to coastlines and have plenty of access to sunshine or even wind power will have the best shot at using desalination as a water source.
In the United States, coastal areas with crunched water supplies will be the first to use desalination, and California is leading the way. From a massive plant planned for Carlsbad in the southern part of the state, all the way up to Monterey County in the bay area, which is looking at switching to desalination as a primary water source, California cities are considering how to provide water for citizens living in the drought-impacted state.
Rainwater Harvesting and Water Recycling: Alternatives to DesalinationDesalination is a possible solution for only those communities with the right mix of proximity to seawater and energy. For areas that are far from water sources, or don't have the financial ability to build and run a desalination plant, other options have to be considered.
Some communities are relying more on recycled water to make the most of a limited resource. Graywater recycling is ideal for some communities to treat and reuse wastewater for purposes like irrigation. In the United Arab Emirates, where desalination is already heavily used, alternatives like water recycling are being more seriously considered:
Stormwater harvesting and rainwater capture and storage are also possibilities -- though these too come with their own special brand of controversy, as we've seen sparked up in places like Colorado. Areas like the western United States that are struggling for ways to alleviate water woes are also going to have to struggle with the pros and cons of even seemingly simple solutions like storing rainwater.
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More on Desalination
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