Why Is Desalination So Difficult?

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 chem

istry 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 everyw

here. 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,” b

ut 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 thous

and 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 ge

nerally 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 oldes

t 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 wate

r 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 disti

llation 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  h

eat 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 encou

rage 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 phen

omenon 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 wh

en 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 us

es 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 t

o 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 dis

charge, 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 residentia

l 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 b

e 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 vario

us 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 th

e 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 s

ources 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. One of the cool things I learned about  desalination while researching this video is that there is a theoretical mi

nimum amount of  energy required to separate salt from freshwater. Even the most efficient RO or distillation  process can’t do beter than this limit, and there’s a great paper in the Journal  of Chemical Education explaining why. But, I have to be honest, trying to read this  paper was like reading gibberish to me. And that happens a lot to me actually, where  I’m researching something that leads me to the edge or beyond my understanding. And I  guess I could just give up at that point, but I w

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