I recently read an article about how some cities and communities in California are considering a new technology to deal with the four-year drought they’ve been suffering through, called desalination. Desalination is the process of removing salt and other minerals from water; specifically, in this case, sea water. With desalination back in the news, I wanted to dig a little deeper to help everyone understand what it is and how it works.
What Is Desalination?
As I said, it’s a process to remove salt and minerals dissolved in water through a variety of different techniques. Desalination is a naturally occurring process through evaporation: water falls to the earth in the form of rain, picks up salts and minerals from the ground and ends up in the ocean. When the sun evaporates the water, the salt and minerals are left behind. Rain water is desalinated water.
You can observe this process simply at home – take a large clear bowl, fill it with water, and add salt. In the center of the bowl, place an empty cup, and cover the bowl with plastic wrap. Use a safety pin to poke a small hole in the plastic above the cup, and place the bowl in the sun. The water will evaporate and collect on the plastic cover and at the point of the hole, it will drip into the cup. Eventually, you’ll have salt deposits on the bottom of the bowl, and fresh water in the cup.
There are other desalination processes, two of which are the most popular and prevalent: the reverse osmosis method and the multistage flash method.
The Reverse Osmosis Method
Reverse osmosis is the process of removing salt from water using membranes to allow water to pass through while capturing larger molecules, like the salt in the water. This process was originally developed in 1955 and is often used in military contexts, specifically in submarines. Sea water is passed through a membrane, providing desalinated water for those aboard the submarine to drink.
Compared to the multistage flash method, the reverse osmosis method of desalination is relatively low energy, but still requires a tremendous amount of power to purify the water.
The Multistage Flash Method
In a process similar to the at-home experiment discussed above, the multistage flash method is essentially a process of rapid distillation, where the water is repeatedly heated and steam is collected and cooled to make pure water. The problem with the multistage flash method is that heating the water takes a lot of energy – about four times the amount of energy as the reverse osmosis method – to purify water, and only about 15% of the water captured is purified.
Objections to Desalination
There are a number of concerns about the desalination process – the environmental effects of capturing the water and disposing of the salty brine left behind and the amount of energy required to desalinate water. Capturing water from the ocean can result in sea life being captured as well, or hurt or damaged by the process. The brine byproduct left behind after desalination can damage the sea life and affect the balance of salt and minerals in the water when returned to the ocean.
For some countries and communities, desalination is the only viable option for clean, healthy water. There are challenges that face this new technology, but new technologies and developments in existing solutions help make desalination a hopeful technology for the future.