Guest Post: How much energy is in your water bottle? by Ryan Kingsbury

We don’t often think about the energy it takes to satisfy our thirst, but where we get our drinking water has huge consequences for how much energy is needed. In many parts of the world, fresh water sources like lakes or aquifers are becoming scarce, forcing residents to settle for supplies that aren’t as clean. And the dirtier the water, the more energy it takes to purify.

Salt is especially hard to remove. In desert or coastal regions with limited sources of freshwater, residents must use a process called desalination to turn salty groundwater or seawater into drinking water. We’ve already covered how mixing saltwater with freshwater releases a lot of energy,  so, to do the reverse– to remove salt from water– consumes a lot of energy.

Exactly how much energy is required depends on the method of desalination. Distillation (which involves boiling)  is a simple way to desalinate water, but it’s also one of the most energy-intensive. By using a technology called reverse osmosis, we can desalinate water using about 1/10th as much energy as distillation. So compared to boiling the water, using reverse osmosis is much more efficient. But reverse osmosis still requires about 100x more energy than treating fresh surface water or groundwater. In fact, you could charge your smartphone with the energy it takes to desalinate just 1 gallon of seawater!

Despite its energy demands, desalination is widely used around the world. There are more than 18,000 desalination plants in 150 countries, including about 250 in the U.S. About half of these plants use reverse osmosis, and about a third use distillation.

In the U.S., most plants are located in Florida, Texas, and California (shown on this cool map), but there are about a dozen here in North Carolina. If you’ve ever visited the Outer Banks, your drinking water probably came from a reverse osmosis desalination plant.

There are so many reverse osmosis plants in the world, that together they produce 4x as much water in one year as refineries do oil! But virtually all of these plants were built as a “last resort,” in areas where there simply isn’t enough freshwater to meet the needs of consumers, industry, and agriculture. When it’s available, treating freshwater is always preferable to desalination.

The more we have to rely on seawater and other salty water resources, the more energy it will take to slake our thirst. So next time you take a drink of water, remember that you’re not just drinking ounces, you’re drinking watts.

Want to learn more about reverse osmosis desalination? Check out this animated video from the Seven Seas Water Corporation.

Ryan Kingsbury, P.E., is a PhD student at the University of North Carolina at Chapel Hill where he is a member of the Coronell Research Group.  Orlando Coronell, PhD, and his research team study membrane-based processes for water purification and energy production and storage, with applications in municipal, industrial, and household systems. Ryan studies salinity gradient energy which you can read more about here.

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