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 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. One of the cool things I learned about  desalination while researching this video   is that there is a theoretical minimum 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 would much rather break through that lack of  context and understanding. It’s important to me,   and it makes me better at this job. And,  particularly for math and science subjects,   I’ve just found that today’s sponsor Brilliant.org  is the best way to teach yourself something new. Of course Brilliant has a huge library of  courses. One I found particularly useful   in understanding desalination was this fun set  of puzzles about chemical reactions. That just   tickles the right part of my brain. But what I  find far more important is that each lesson is   interactive. Most people, including me,  learn better when they combine seeing,   reading, and doing. That’s why I start at  Brilliant when I want to master a new skill. If that sounds useful to you, you should  go try it yourself and see if you agree at   Brilliant.org/PracticalEngineering. It’s  totally free to try for an entire month,   and you can get through a lot of courses in  that time. But if you do find yourself coming   back like I did, the link below will get  you 20% off an annual premium subscription.   They even have an app so you can try this  on your phone. I really believe we should   never stop trying to learn new things,  and Brilliant makes that so easy to do,   and it supports the channel too. Thank you  for watching, and let me know what you think!