Desalination is the process of removing minerals from saline (salty) water, and producing fresh water from it. Scroll down or click the button to learn more!

Page Last Updated: March 12, 2020 at 12:17 p.m.

Desalination:

What You Didn't Know

Written by Charlie Huang. Coded in HTML 5.

Desalination is the process of removing minerals, especially salt, from saline or salty water. This is achieved in various ways, but the main two methods are using thermal distillation and reverse osmosis. In this article, you will discover all about this amazing process of generating freshwater!


Why Should I Care About Desalination?

There is a miniscule amount of freshwater in the world, compared to the total amount of water on earth. Only 2.5% of all the water on earth is freshwater, and of that, 0.3% is in liquid form on the surface of the earth! No, not 0.3% of all water on earth, but 0.3% of the 2.5% of water on earth. That means only 0.0075% of all the water is freshwater in the drinkable form at the surface of the earth! And, a lot of that 0.0075% is polluted because of us humans, so even less is clean water!

If our poor human practices continue and we continue to pollute the water, our growing population will bring even more people to pollute the water, yet at the same time bring a greater need for clean water. This cycle repeats, and eventually, there won’t be enough clean freshwater to supply our growing population, especially for countries experiencing droughts, where freshwater is extremely scarce. Eventually, we may rely heavily on desalination to get the freshwater we need from the saline ocean water.

Many countries already heavily rely on this process. Saudi Arabia gets 70% of their water through desalination, and islands in the caribbean almost completely rely on this process. Other countries include United Arab Emirates, Spain, Italy, India, China, Japan, and Australia. In the United States, there are 11 desalination plants in California alone, with more to come in the future.

It is important to improve desalination processes, and make the technology more efficient, fast, accessible, and reliable, as this is a very important freshwater supply option that many heavily rely on.

Methods of Desalination

There are various methods to achieve desalination, but the main two are thermal distillation and reverse osmosis. For thermal distillation, the main two types are vacuum distillation, and the traditional separation by evaporation. Here you will learn about Thermal Distillation, Vacuum Distillation, and Reverse Osmosis, and their pros and cons.

Thermal Distillation

The 'traditional' form of thermal distillation is the oldest form of desalination, and has been around for thousands of years. It uses separation by evaporation, using heat to boil and evaporate saline water (water with salt in it). The steam produced is collected and cooled back down, turning into freshwater through condensation. The salt is left un-evaporated and separated from the rest of the water.


Pros:

  • The amount setup work and equipment required for this method of desalination is less than the other methods, as this process is much more simple.

Cons:

  • The standard method of thermal distillation uses significant amounts of energy, making it extremely costly with not much economical sense.
  • This process is extremely slow as the water must slowly evaporate.

Vacuum Distillation

Vacuum distillation, a more modern form of thermal distillation, utilizes heat to evaporate water. However, unlike the 'traditional' thermal distillation, which boils water in normal atmospheric pressure, vacuum distillation utilizes a low-pressure or vacuum chamber. The boiling temperature of water falls as the ambient atmospheric temperature falls, so pumping water into a vacuum chamber would lower its boiling temperature, thus reducing the amount of energy required. For example, normal atmospheric pressure is around 1000 mbara, and if we tenth that, the boiling point of water at 100 mbara is only 45.6 degrees C. Once the water evaporates, the vapor is collected, condensed, cooled, and freshwater is produced!


Pros:

  • Much more energy efficient than the 'traditional' thermal distillation method.
  • A reliable and simple desalination method.
  • Often, instead of using more energy to heat the water to its boiling point, people use waste heat from power plants. This greatly reduces the energy required to run this desalination process, and also puts the wasted heat to use.

Cons:

  • Building a large low-pressure vacuum chamber for water is extremely expensive to build and maintain.
  • As vacuum distillation is still a form of thermal distillation, it can only produce freshwater as fast as the water evaporates, which can be quite slow.

Reverse Osmosis Desalination

Desalination by reverse osmosis, abbreviated as RO, is a process where pressure is used to drive water through a filter. Osmosis is the process of a solvent moving from an area of lower concentration of solutes to an area of higher concentration of solutes. So, by the name "reverse osmosis", this is where a solvent, in this case, water, moves >out of an area of high concentration of solvents, in this case, salt, and becomes fresh water. Saline water is pushed through semipermeable membranes, which have pores big enough to let water molecules pass, but small enough to capture and block larger molecules, such as dissolved salt and minerals. The water continues through the filtration device, coming out as freshwater, while the salt and other minerals are blocked by the filter.


Pros:

  • This method of desalination uses significantly less energy than the other two.

Cons:

  • Reverse osmosis involves pumping water through a filter, so the water can only flow as fast as it can fit through the semipermeable membranes. Especially with water with higher concentrations of salt, this can be quite slow.
  • Produces quite a bit of waste water.
  • The reverse osmosis desalination process requires another filtration system if handling polluted water, as the semipermeable membrane can get clogged easily if there are lots of debris in the water.

Issues/Challenges with all three of these methods of desalination:

  • As water evaporates, only the water molecules turn into vapor. This means that not only is the salt being extracted from the water, but so are all the minerals. While many people prefer water without minerals, they are still beneficial for your health.
  • As with any form of desalination, once you remove the water, you have large amounts of extremely concentrated salt water leftover. Disposal of this solution can be an issue. If you simply dump it back into the ocean, the high salt concentration will change the salt and oxygen levels in the water around the dump site. This sudden increase in salt can stress and kill marine animals and plants in the water. This extremely salty water must be disposed of accordingly.

Despite all the innovative refinements that has occurred to desalination technology, state-of-the-art best desalination facilities still require 7 to 30 kWh (kilo-watt-hour) of energy per 1000 gallons (3785 litres) of desalinated water. When scaled up to supply water to billions of people, that is a tremendous amount of energy required. In the future, there is much room for more innovation in desalination technologies.

As the world's population continues to grow, and as we continue to pollute our water, our demand for freshwater increases dramatically. Existing water supplies become increasingly insufficient, and desalination of sea water continues to become an increasingly important source of useable water.




References:

  1. Wikipedia contributors. "Water distribution on Earth." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 5 Mar. 2020. Accessed 11 Mar. 2020. https://en.wikipedia.org/wiki/Water_distribution_on_Earth
  2. Parise, Tom. “Water Desalination.” Water Desalination, Stanford University, 16 Dec. 2012. Accessed 11 Mar. 2020. http://large.stanford.edu/courses/2011/ph240/parise2/
  3. Wikipedia contributors. "Reverse osmosis." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 6 Feb. 2020. Accessed 11 Mar. 2020. https://en.wikipedia.org/wiki/Reverse_osmosis
  4. Wikipedia contributors. "Desalination." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 10 Mar. 2020. Accessed 11 Mar. 2020. https://en.wikipedia.org/wiki/Desalination#Solar_distillation
  5. Seawater, Desalination. “Desalination Worldwide.” Huntington Beach Desalination Project, Poseidon Water, 2018. Accessed 11 Mar. 2020. https://www.hbfreshwater.com/desalination-worldwide.html


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