Why is desalination so difficult?

[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 from the salt. All I have to do is condense the vaporized water back into its liquid form. This pump moves ice water through the condenser to encourage that process… if the tube doesn’t slip out of the beaker and spill ice water all over the table.

In my receiving flask on the right, I should have distilled water that is nearly salt free. Testing it out with the meter, the dissolved solids are practically nil, just a few parts per million. But it took nearly 2 hours to get only 200 milliliters of water, and right about a kilowatt-hour of electricity too.

Water usage in the US varies quite a bit, but a rough estimate is 300 gallons (or 1,100 liters) per day per household. To produce that much water using my distillation setup here, I would have to scale it up nearly 500 times this size, and it would consume nearly 6,000 kilowatt-hours in a day (assuming the same efficiency I got in the demo). At the average residential US electricity price, it’s roughly 800 dollars per day! That’s an expensive shower. Could this be made more efficient? I don’t think so.

No, obviously it can. My garage demo has very little going for it in terms of efficiency. It’s about as basic as distillation gets. There’s lost heat going everywhere. Modern distillation setups are much more efficient at separating liquids, especially because they can take advantage of waste heat. In fact they are often co-located with coal or gas-fired power plants for this exact reason. And there’s a lot of technology just in minimizing the energy consumption of distillation, including reuse of the heat released during condensation, using stages to evaporate liquids more efficiently, and using pumps to lower the pressure and encourage further evaporation through mechanical means. But the thermal efficiency isn’t the only challenge with distillation.

Take a look at the flask that held the seawater after all the water boiled away and you can see the salt deposits building up, even after distilling only a small amount of water. These scale deposits reduce the efficiency of boiling because heat doesn’t transfer through them very easily, which means they would have to be cleaned off regularly. One alternative is a flash evaporator that sends the liquid stream through an expansion valve to force it to evaporate at temperatures lower than boiling, which minimizes the buildup of scale. Flash evaporators are the workhorses of desalination plants that use distillation, and especially in the middle east, plants like this have been reliably producing fresh water for decades now, but they’re not the only way to get the job done.

The other primary type of desalination uses membranes. You may have heard of the phenomenon called osmosis, where a solution naturally diffuses through a barrier. But you can reverse the osmotic process, moving a solution from high concentration to low with pressure… usually a lot of pressure. Let me show you what I mean. Luckily there are commercially available seawater membranes that don’t cost an arm and a leg. That’s because these systems are frequently used in boats and ships to make freshwater while at sea. But why spend thousands of dollars on a working watermaker when you have the rudimentary plumbing skills of a civil engineer?

Here’s the membrane I’m using for this demo. It’s wrapped in a spiral so you get lots of surface area in a small package. It is kind of like a filter that lets water pass through while holding back the dissolved solids, but at a much tinier scale. It’s generally a lot more efficient than thermal distillation, so most modern desalination plants use reverse osmosis (or RO) for primary separation. But, as you’ll see, it still uses a lot of energy, way more than a typical raw water treatment plant.

It takes a lot of pressure to force seawater through a membrane, in my case about 600 psi or 40 times normal atmospheric pressure. Even small RO systems use high-pressure pumps designed for continuous use, because this is not a fast process. Instead of springing for a nice pump well-suited for the application, I’m using the cheapest power washer I could find at the local hardware store. The instructions didn’t say not to run saltwater through it.

The membrane sits inside this high pressure housing that keeps it from unraveling under the immense forces inside. That’s if you hook everything up correctly… I had to redo a few connections when the housing sprung a leak during early testing.

A booster pump delivers the seawater from the bucket to the pressure washer, then the pressure washer sends it into the housing. Unlike a typical filter, not all the feed water flows through the membrane. Instead, most of it flows past the membranes and comes out on the other side just a little bit more concentrated with salt. This is called the brine and we’ll talk more about it in a minute. The water that does make it through the membrane, called the permeate, comes out in the center of the housing. You can see on my flow meters that, if I close the valve on the brine discharge line, it increases the pressure in the housing, forcing more of the water through the membrane. The meter on the left is brine discharge, and the one on the right is the permeate line. As I close the valve, the brine flow goes down and the permeate flow goes up. Of course I could close the brine flow all the way down, but you still need some water to carry the salt away or it will just foul up the membrane.

Typically you need to run water through these membranes for several hours before they settle into their best performance. My little power washer wasn’t quite up to the task of running for that long, but even after roughly half an hour, I was getting water with one to two parts per thousand of dissolved solids through this crude setup. That’s not high quality drinking water, but it’s definitely drinkable!

I ran this experiment a few times at different pressures, but the results didn’t vary too much. For this run, the combined power for the booster pump and the pressure water was around 1200 watts, and it took about five minutes to produce a liter (or quarter of a gallon). Going back to our residential household, it would take four pressure washers running non-stop and consume more than 100 kilowatt-hours in a day. That’s a huge improvement over the distillation demo, even considering the water quality wasn’t quite as good, but it’s still 15 dollars a day or more than 5,000 dollars per year just to separate salt from water.

It won’t surprise you to learn that, just like my crude distillation demo, my reverse osmosis via pressure washer demo is also not nearly as efficient as it could be on a larger scale. Modern RO plants use huge racks of high quality membrane units and high efficiency pumps. They also recover the energy from the brine stream before it leaves the system back out to sea, saving the precious kilowatt-hours already consumed by the pumps. To separate a cubic meter or 264 gallons, of seawater from its salt, my power washer RO system would take about a hundred kilowatt-hours. The newest RO plants can do it with just one or two.

But, even though the separation step is energy intensive, it’s not the only energy requirement in a seawater desal plant, and it’s definitely not the only cost. I’m using tap water in my demonstration, but these plants don’t start with that. Raw seawater not only has salt, but also dirt, algae, organic matter, and other contaminants too. Those constituents can foul or damage evaporators or membranes, so all desal plants use a pretreatment process to remove them first. That takes energy and cost to keep up with the various chemical feeds and filters before the water even reaches the salt separation process. And, even with good pretreatment, the RO membranes or evaporators have to be taken out of service for cleaning regularly, and eventually they have to be replaced. Additionally, you usually can’t send RO permeate or distilled water directly to customers. It’s too clean! It normally goes through a post-treatment process to add minerals, since most people prefer the taste over just pure water. Plus it gets disinfectant so that it can’t be contaminated on its way through the distribution system. And don’t forget about that brine.

All that salt that didn’t come out of the product stream is now packed into a smaller volume of water, making it more concentrated than before. Modern desalination plants generally recover about half of the intake flow, which means their brine stream is about twice the concentration of normal seawater. It’s a waste product that is actually pretty tough to get rid of. You can’t just discharge that super-saline waste directly back into the sea because of the environmental impacts, particularly on the plants and animals near the sea floor (since the concentrated solution usually sinks). To avoid environmental impacts, most brine discharge lines either use diffusers to spread out the salty solution so it dilutes faster or they blend the brine with some other stream of water like power plant cooling lines or wastewater effluent so it’s diluted before being released. When that’s not possible, some plants have to inject the saltwater into the ground (an expensive endeavor that only adds to operational costs).

With all the complications of separating salt from seawater, it’s easy to let one’s mind drift toward alternatives like harnessing renewable sources of energy. Like, what if we could use solar power to not only distill seawater but also carry it inland toward major cities and release it onto the ground where it could easily be collected. But now we’ve just re-invented the water cycle, which is already how we humans get the vast majority of the water we use to drink, cook, and bathe. It’s not like dams, reservoirs, canals, pumping stations, and surface water intakes don’t have their own enormous costs and environmental impacts. But, if mother nature isn’t dropping enough water for your particular populated area, you can build and operate a pretty long pipeline for the immense costs and energy required to desalinate seawater.

And that’s the problem with desalination. It’s kind of like the nuclear power of water supply. It seems so simple on the surface, but when you add up all the practical costs and complexities, it gets really hard to justify over other alternatives. It’s also harder to compare costs between those alternatives because of desal’s unique problems. It’s just a newer technology, so it’s harder to predict hidden technical, legal, political, and environmental challenges. For example, because of the high energy demands, desalination can strongly couple water costs with electricity costs. During a drought, the cost of hydropower goes up because there’s less water available, increasing overall energy costs and thus making desalination less viable right when you need it most.

Of course, desalination is a viable solution in many situations, especially in places with large populations and severe water scarcity. All the biggest plants are in middle eastern countries like Saudi Arabia and the UAE. That’s because they really have no choice. But it can also be viable in areas with a lot of variability in climate like California, Texas, and Florida. In these cases, a desalination plant is just one element in a diverse portfolio of resources, all with different risk profiles. Yes, the desalinated water is more expensive than other options like rivers, reservoirs, and groundwater supplies. But it can be more reliable too, providing water during drought conditions when the other sources are limited or completely unavailable. And, a lot of these costs and complexities get simpler when you’re not pulling salt out of seawater. There are sources of water that have some salt (but not as much as the ocean) like estuaries and brackish groundwater. In places where such a supply is available, desalination can be a much more cost effective source of fresh water.

Another way to make desal projects more viable is to let the private sector take on the risks. Many of the largest desalination plants are partnerships with private water companies rather than being financed, built, and operated by the utility like what’s done for a typical treatment plant. Partnering with a private company allows a utility to offload the financing costs and operational risks in return for the stability of a simple water purchase agreement. You pay for it, build it, and operate it, and we’ll just buy the water from you. This type of arrangement also keeps government boards from having to weigh in on complicated technical issues and innovations where there’s just not as much precedence to lean on as there is with more established types of water infrastructure projects.

The private company running the Carlsbad plant in San Diego County I mentioned earlier is working on a major project scheduled to finish in 2024: a new standalone seawater intake required after the power plant next door shut down in 2018. Bonds issued for the project were upgraded to rating of triple-B by Fitch, meaning the facility has a relatively stable outlook with a lower chance of defaulting. That’s just one rating agency’s assessment of just one project on just one membrane plant, but it gives some confidence that the technology of desalination is making progress, and that it might become a bigger and bigger part of the world’s limited supply of fresh water in the future.

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