[Note that this article is a transcript of the video embedded above.]
This is the Carlsbad Desalination Plant outside of San Diego, California. It produces roughly ten percent of the area’s fresh water, around 50 million gallons or 23,000 cubic meters per day. Unlike most treatment plants that clean up water from rivers or lakes, the Carlsbad plant pulls its water directly from the ocean. Desalination, or the removal of salt from seawater, is one of those technologies that has always seemed right on the horizon. It might surprise you to learn that there are more than 18,000 desalination plants operating across the globe. But, those plants provide less than a percent of global water needs even though they consume a quarter of all the energy used by the water industry.
I live like 100 miles away from the nearest sea, so it’s easier for me to mix up my own batch of seawater right here in the studio. There are two main ways we use to desalinate water, and I’ve got some garage demonstrations to show you exactly how they work. Will the dubious chemistry set or the cheapest pressure washer I could find work better? Let’s track the energy use and other complications for both these demos so we can compare at the end of the video. Dumping that salt into a bucket of water may seem like no big deal, but reversing the process is a lot more complicated than you might think. I’m Grady, and this is Practical Engineering. In today’s episode, we’re talking about desalination.
Earth is a watery place. Zoom out and the stuff is practically everywhere. It doesn’t seem fair that the word “drought” is even in our lexicon. And yet, the scarcity of water is one of the most widespread and serious challenges faced by people around the world. The oceans are a nearly unlimited resource of water with this seemingly trivial caveat, which is that the water is just a little bit salty. It’s totally understandable to wonder why that little bit of salt is such an enormous obstacle.
How much salt is in seawater anyway? You’ve heard of “percent,” but have you ever heard of “per mille”? Just add another circle below the slash and now, instead of parts per hundred, this symbol means parts per thousand, which is the perfect unit to talk about salinity. The salinity of the ocean actually varies a little bit geographically and through the seasons, but in general, every liter of seawater usually has around 35 grams of dissolved salt. In other words, 35 parts per thousand or 35 permille. That means, for this bucket, I need about this much salt to match the salinity of seawater.
I didn’t get it dead on, but this is close enough for our demo. Looks like a lot of salt, but I could dissolve about 10 times that much in this water before the solution becomes saturated and won’t hold any more. So, compared to how salty it could be, seawater isn’t that far from freshwater. But, compared to how salty it should be (in order to be okay to drink and such), it has a ways to go. Normal saline solution used in medicine is 9 parts per thousand because it’s approximately isotonic to your blood. That means it won’t dehydrate or overhydrate your cells. But (unless it’s masked by a bunch of sugar) even that concentration of salt in water isn’t going to taste very good.
Most places don’t put legal limits on dissolved solids for drinking water, but the World Health Organization suggests anything more than 1 part per thousand is usually unacceptable to consumers. It doesn’t taste good. 500 parts per million (or half permille) is generally the upper limit for fresh water (and that includes all dissolved solids combined, not just salt). But that means seawater desalination has to remove (or in industry jargon, reject) more than 98 percent of the salt in the water. That’s the reason why there are really only two main technologies in desalination. But neither of them are particularly sophisticated, at least in their simplest form, so I’m going to try some do-it-yourself desalination to show you how this works.
The oldest and most straightforward way to separate salt and water is distillation, and this is my very basic setup to do just that. All you chemists and laboratory professionals are probably shaking your heads right now, but this is just to illustrate the basics. On the left, I have a flask of my homemade seawater sitting in sand, in a pot, on a hot plate. Salt doesn’t like to be a gas, at least not under the conditions we normally live in on earth. Water, on the other hand, can be convinced into its gaseous state with some heat from a conventional hotplate. And that’s what I’m doing here, just adding some heat to the system. And I’m tracking exactly how much heat using this Kill A Watt meter.
Once the water is converted to steam, it is effectively separated f
