TL;DR Science: Water Desalination

Written by Thomas P. | Wednesday, July 7

As climate change continues to progress, the Western United States comes closer each year to running out of water. Each year, the West comes closer to the worst drought recorded. Each year, the Colorado River and Lake Mead (two bodies of water many Western cities rely upon for their municipal water) become shallower. As drought conditions become more and more common, it thus becomes necessary to push for new efforts to research water desalination, in order to find new reliable sources of freshwater and avoid the drastic consequences of running out of water.

The Basics of Water Desalination

Basically, water desalination is filtering out all of the brine (which consists of salt) in sea water or that in brackish water (water that contains more brine than freshwater, but less than sea water). Sea water as well as brackish water cannot be drunk by humans, only freshwater can; the process of water desalination allows us to drink this formerly undrinkable water by turning it into freshwater.  The process sounds simple enough, but water desalination is far more than merely sieving (ie. filtering) out the brine. The fact that it takes two gallons of saltwater to make one gallon of fresh water (the other gallon of water is brine and cannot be drunk) makes the process extremely expensive. In addition, the methods by which water is desalinated makes it exceedingly energy intensive. Currently, in Texas (which is quite arid) water desalination costs between $1.25 per 1000 gallons and $5.80 per 1000 gallons depending on whether sea water (which has more brine) or brackish water (which has less brine) is used.

Reverse Osmosis

Water desalination is primarily completed through a mechanism called reverse osmosis. The process of reverse osmosis rids the salty or brackish water of the salt that makes it undrinkable by forcing the water through a semipermeable* membrane (ie. a filter), which only water molecules can move through. The salt/brine left from the formerly brackish/sea water is then removed from the mechanism as waste. 

Pros and Cons of Water Desalination

Pros 

  1. Water desalination will allow us to move through into the future with confidence, if adopted, as we will not have to worry about having shortages of water or the effects of droughts (caused primarily by climate change in recent decades). 

Cons 

  1. The mechanism by which water can be desalinated (reverse osmosis) is extremely energy intensive, and expensive as previously mentioned. 
  2. Water desalination generates waste and pollutants; the largest waste generated by process being brine - which must be disposed of. 
  3. The amount of energy necessary to desalinate water creates a demand for fossil fuels, which creates a carbon footprint. A process created in response to climate change that progresses climate change would create a feedback loop. The solution to this problem is to power water desalination plants using sustainable energy sources (e.g. solar or wind). 
  4. Water desalination intake systems can sometimes trap fish or their eggs, as well as other marine organisms. The solution here is to take the water from the seafloor. 

An Alternative to Water Desalination

The most notable among alternatives to water desalination is water reclamation. Water reclamation primarily involves the use of water that has not been treated (in order to be potable) for purposes other than drinking. For example, reclaimed water could be used for agriculture and industrial purposes where river water may not be necessary. Water reclamation thus conserves the city’s drinkable water supply by ensuring that it is only used for drinking purposes. 

However, resorting to this solution in some cases may not be enough. If a city has a decreasing supply of water and it resorts to water reclamation as the permanent solution, it will only delay the inevitable event often called “Day Zero”, when a city will have no water left. Hence, water desalination in most places is being considered among the premier solutions for saving water. 

TL;DR Water desalination is the process by which seawater or brackish water (which cannot be drunk), which has salt (aka brine), is turned into freshwater. 

* Semi permeable means only certain types of molecules can pass through. 

References 

About the Author

Thomas is a student at Eastside High School.