Desalination Research

By Isadora Rivera-Janer

What Is Desalination? How is it Done?

Desalination is one the world’s oldest forms of water treatment. It involves, as the name suggests, the conversion of saline water to freshwater, which the drinkable form for most living organisms. Saline water refers to water that contains significant concentrations of salt, like the water you would find in the ocean. Salinity is measured by ppm or ‘parts per million.’ The current classification of different water types goes:

  • Freshwater - Less than 1,000 ppm

  • Slightly saline water - From 1,000 ppm to 3,000 ppm

  • Moderately saline water - From 3,000 ppm to 10,000 ppm

  • Highly saline water - From 10,000 ppm to 35,000 ppm

As freshwater sources have become scarcer over the decades, desalination has become more popular for many countries to adopt. Though it can seem rather simple, removing salt from water usually done by boiling water and then capturing the condensation, it can be rather expensive and difficult to do it on the larger scale that it would take to adequately serve human populations. There are many processes that are being used nowadays in order to create desalination plants on a greater scale.

The most used desalination system is reverse osmosis. Reverse osmosis consists of applying pressure to a salt water solution and putting it through a semipermeable membrane. This allows the passage of the water (solvent) but not the salts (solute). The passage of water goes through the membranes on a gradient, going from where there is more salt to where there is less.The salty solution is minimized in favor for freshwater.

Another method, is distillation. The involves heating water to a point where it evaporates and then condensing it into freshwater.

There are less common methods of desalination. Freezing involves putting sea water in a freezer at low pressure, which results in the formation of ice crystals over brine which can then be separated. Flash evaporation is when water in the form of fine drops is put into a chamber at below saturation pressure. Some of these water drops immediately convert into vapor, which later condenses to form desalinated water. The remaining water enters another chamber at lower pressure than the former and repeats the process. Hydrate formation consists of adding hydrocarbons to the saline solution, forming complex crystalline hydrates which are later separated to obtain desalinated water. Electrodialysis is an electric current is passed through an ion solution. The positive ions migrate toward the negative electrode while the negative ions head for the positive electrode (anode). Semi-permeable membranes are placed between both electrodes so that only Na+ or Cl- can pass through, and the water contained in the centre of the electrolytic cell is progressively desalinated obtaining freshwater.

What are its Effects?

Desalination has become much more prevalent as water-scarce areas are having population booms, such as China, India, and Western USA. Also, as the effects of climate change accelerate, so does the necessity for desalination, as droughts become more frequent. The costs of desalination have come down as technology has advanced. This has led to more than 300 million people across the world getting water from desalination plants, according to the International Desalination Association. Currently, only 1% of the world’s population depends on desalination for water, but by 2025 it is predicted that it will be 14%. Fun fact, Kuwait depends on desalination for 100% of it’s water uses, with its use of desalination plants dating back to 1951.

However, desalination can lead to serious problems for the environment. Leading desalination experts see that in order for the process to grown, problems such as large energy usage, some of which includes fossil fuels, will need to be solved. Using fossil fuel could produce a feedback loop, with desalination-related fossil fuel usage causing more greenhouse gases to be in the atmosphere, which warms the planet, which increases the necessity for desalination. Desalination can also cause damage to marine ecosystems from the extra-salty waste water, as if the disposal is not done properly, the salty water can deplete oxygen in the ocean and hurt sea life. There is also issues with the intake of water by the plants, as small organisms in the ocean can be sucked in as well, killing things like fish larvae, eggs, and plankton.

What Direction is Desalination Taking?

Some solutions to this involve trying to reuse brine as a resource. In a Stanford study, the researchers designed and tested a device that splits the components of brine through a method called electrochemical water-salt splitting. When it is done, sodium and chlorine combine with other elements to form new chemicals including sodium hydroxide, hydrogen and hydrochloric acid. These three products are ingredients in many common implements at home, such as deodorant, soaps, or a component of leather processing respectively. The benefit of this, despite none of these products being available for commercial use, is that they can help visualize how to cut brine disposal costs, which can account for up to a third of total desalination expenses, and avoid damaging environmental impacts.

Desalination by solar evaporation array (SEA) panels is one of the latest developments in solar technology. SEA panels are self-contained water purification devices that can be set up in minutes and are cheaper than traditional desalination methods for small-scale applications. In 2020, researchers at MIT and Shanghai Jiao Tong University showcased a method for passive solar-powered desalination. This system could provide more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area and guarantee water security in water-scarce areas. It involves a multilayer solar still device that relies on a set of evaporating and condensing components. It uses flat panels to absorb heat and then transfer that heat to a layer of water so that it begins to evaporate. The vapor then condenses on the next panel. That water gets collected, while the heat from the vapor condensation gets passed to the next layer.The multilayer evaporator makes sure that released heat flows to the next evaporating layer which recycles the solar heat and boosts overall efficiency of the device.

Other forms of renewable energy, like wind, are also possible solutions. In Australia, a wind-powered desalination plant has been providing the town of Perth with nearly 40 million gallons of drinking water every day for twelve years. This plant and others are connected via a grid to local wind farms, lowering overall energy costs while reducing carbon emissions.

Scientists have also started experimenting with algae as a desalination agent. According to the U.S. Environmental Protection Agency, certain species of salt-tolerant algae (called halophytic algae) can absorb significant amounts of salt from ocean and brackish water, and then the spent algae can be used as a base for biofuels. Complete desalination of ocean and brackish water cannot be achieved by algae absorption alone but they have found that a combination of algae treatment and reverse osmosis results in considerable energy savings.

Of course, all of this is rather small-scale, but the steady reduction of production costs, coupled with increasing costs of water treatment driven by more stringent regulatory requirements, are expected to accelerate the current trend of increased reliance on the ocean as a water source. This will further establish ocean water desalination as a reliable, drought-proof alternative for many coastal communities worldwide.

Technology advances are expected to reduce the cost of desalinated water by 20% in the next five years, and by up to 60% in the next 20 years.

Sources

  • https://www.usgs.gov/special-topic/water-science-school/science/desalination?qt-science_center_objects=0#qt-science_center_objects

  • https://www.acciona.com/water-treatment/desalination/

  • https://e360.yale.edu/features/as-water-scarcity-increases-desalination-plants-are-on-the-rise

  • https://news.stanford.edu/2020/06/01/desalination-solution/

  • https://www.eesi.org/articles/view/whats-the-deal-with-desalination

  • https://iwa-network.org/desalination-past-present-future/

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