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How do we make seawater drinkable? And can that technology save California?!

Hosted by: Michael Aranda
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Michael: Maybe you’ve heard that California has been having some water trouble lately.

And by lately, I mean for the past five years. California is experiencing a serious drought, and if it continues, it could cause permanent damage to its ecosystems, threaten its precious freshwater resources, and ruin the state’s many, many crops. But maybe you are thinking, how can a state that shares 1350 kilometers of coastline with an ocean be out of water? Well, ocean water is full of salt, that’s how.

But it doesn’t have to stay that way. Because there’s a process called desalination, which removes the salt from seawater to make it drinkable. It’s already used in some parts of the world on a large scale. So now, both scientists and citizens are wondering whether “desal,” as it’s sometimes called, can help California out of its drought.

So let’s take a look at how desalination currently works. There are five different methods used facilities around the world, to extract pure water from seawater. But over 80% of the world’s desalinated water is produced by just two of these methods: They’re known as reverse osmosis and multi-stage flash. Reverse osmosis uses pressure to remove salt, other minerals, and microbes from seawater. The water is pumped through a bunch of semi-permeable membranes, which contain tiny pores that let water molecules pass through, but filter out other particles if they’re too big or charged.

The water that gets forced through the membranes becomes usable freshwater. But the leftover water is so salty and full of other junk that it’s too expensive to try and extract any more freshwater, so it’s sent off to a separate treatment plant. This extremely salt-saturated water is known as concentrate or brine.

In multi-stage flash, seawater is brought to a boil multiple times as it travels through a series of chambers, creating some water vapor and leaving the dissolved salts in a liquid. Freshwater is collected as the water vapor rises to the tops of these chambers and condenses. And the leftover brine is either recycled in other parts of the system, or disposed somehow.

These technologies have been effectively used in thirsty countries around the world. For example, the Ras Al Khair Desalination Plant in Saudi Arabia uses both reverse osmosis and multi-stage flash processes. It’s capable of pumping out about 1 million cubic meters of desalinated water every day.

In Israel, meanwhile, Sorek Desalination Plant, uses reverse osmosis exclusively, and can supply around 620,000 cubic meters of freshwater per day to the country’s water system. So why don’t we use this stuff to solve California’s drought problems?

Well, even the biggest desal plants in the world are only supplemental – they don’t make enough water to support an entire population. The Carlsbad Desalination Plant in California, for instance, is helping produce freshwater through reverse osmosis. But, at best, it provides less than 10% of regional water demands.

Desalinating seawater can also be twice as expensive as treating other water sources, like rainwater or wastewater. On a large scale, most of that expense comes from the energy that’s needed to heat the water, or apply pressure to it.

According to some estimates, it takes about 25,000 US dollars’ worth of electricity per month to produce enough desal water for only 1,200 homes. Plus, there’s the problem of how to process all that leftover brine. You can’t just dump it back into the ocean, because the extreme levels of salt have been shown to damage fish, coral, and sea grass.

And finally, there are logistics to consider. Most of California’s water is used for agriculture, but the state’s growers would be largely unaffected by any efforts to desalinate seawater. Why? Because most California farms are inland, and not even close to the ocean, which is generally where you’d want to put a desalination plant.

So, to help quench California’s thirst, we’re gonna have to try new desal technologies. One promising technique scientists are developing in California is solar desalination, which harnesses the thermal energy of the Sun to power a desalination plant. An example of this would be a multiple-effect distillation plant, which is a lot like multi-stage flash... except the steam from the first stage is used to boil water in the second stage.

And the steam from the second stage is used to boil water in the third stage – and so on. Solar desal may be able to make clean water at a lower cost than conventional desal, and it could produce less brine. Some companies even estimate around 93% of the saltwater being turned into freshwater.

For now, though, the cost and logistical problems of desalinating seawater are just too big to solve problems like the California drought. But with more research and ingenuity, desal plants could provide at least some relief to drought-stricken communities everywhere.

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