Is it just me, or did this article dance around the question?
I am not a physicist but let me give it a stab: except for a few specialized steps like UV or oxidizing heavy metals, most filtration is mechanical. A series of filters with smaller and smaller pores capture more and more of the mess in the water like bacteria and particulates while UV breaks down viruses, the oxidizer precipitates out metals, and so on.
None of those methods work with salt. Salts in general disassociate through ion-dipole interactions - the water dipoles essentially rip the ionic compound apart and surround each ion in what is called a hydration shell. They're bigger than bare water molecules but not much bigger - much too small to target with pore size. This shell also puts them in a thermodynamically stable state and it takes energy to "jostle" the water molecules away from the ions either through evaporation, distillation, or through another chemical reaction that precipitates out the ions.
As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of.
You've pretty much nailed it except for one minor nit:
> It takes less energy but produces a concentrated liquid waste stream that must be disposed of.
This implies that creating a concentrated waste stream is a problem unique to reverse osmosis. It isn't. No matter what you do you're going to end up with a bunch of salt that you have to get rid of somehow.
The Israelis ran into this problem with their mineral extraction in the Dead Sea, so they're bulldozing the dry salt waste to build a physical wall for border security (more than 10 meters high) that's apparently hard to climb:
I did a bit of Googling. It’s a desalination plant on the Dead Sea. That is the Jordan River. The stretch they are building the salt wall on is barren desert land.
Jericho is the nearest town, 18,000 people, 8” of rain a year, 2 miles from the border. They presumably get their water from that desalination plant. No towns that far south are getting their water from it. The river barely even makes it to the Dead Sea anymore. https://en.wikipedia.org/wiki/Jordan_River#Main_environmenta...
Adding a bit more salt into the famously-salty Dead Sea are the least of their problems in that part of the world. Where else would you put the salt, anyway? Putting it all into the Dead Sea would be even worse —- at least this reduces the amount that makes it back there. Putting it into any other watershed would cause vastly more harm.
It seems like a good enough solution to me. If you think about it, it wouldn’t be in Israel’s interests to inflame tensions with Jordan over this, if it were really an issue.
They obviously are changing the environment. You'd need a proper environmental impact statement to determine what the negative aspects of these changes are. I wonder if one was done?
By that definition, when my kid goes to the beach and digs a hole in the sand and fills it with water they are "changing the environment". Do we need a proper environmental impact statement to determine the negative aspects of these changes?
Come on. They are literally moving salt a few meters.
It sort of does. RO produces relative large amount of waste water that are harder to deal with (e.g. storage, disposal). By spending more energy, you can turn the waste water into salt solids, which can be easier to dispose of.
You have that problem only if you decide to remove all the salt. You could decide to remove say 20% of the water and make that only H2O; effectively getting H20 and a higher concentration of salt water as outcomes.
I say this having recently read about the desalination plant in Dubai.
No reason to keep the 1:1 ratio. Use the salt to replace our current salt mines/outtakes and then use the water as an addition to our current freshwater usage.
The waste stream doesn’t contain that much more salt than seawater. Extracting salt from mines is much cheaper than extracting it from slightly brackish wastewater from water treatment plants.
is this true? I can understand why it would be true, but "sea salt" is a real thing that's been traditionally made in seaside communities through to today, and I find it hard to believe they're cleaning it at anything finer than a gross scale.
thank you for the information, for sure, but i was responding to the claim that sea salt "needs cleaning" to point out that that seasalt isn't being cleaned, so it puts an upper bound on the value of need as it concerns the commercial market including govt regulation. My suspicion was that where would be more to worry about from single-celled life detritus, heavy metals, forever chemicals etc, but the marketplace doesn't seem to be worried about that.
I go about my daily life not worrying about microplastics (I know they are ubiquitous, and I'm not in favor of them, I just don't worry about them) Are microplastics known to cause particular diseases, or just suspected on the grounds that they "couldn't be good"? plastic is pretty inert which is why it remains around for so long, and while it is made from toxic things, it's generally considered safe. I'm just curious about actual microplastic effects rather than the sort of "it's estrogenizing our boys, antivax...er-plastic" suspicions.
> plastic is pretty inert which is why it remains around for so long, and while it is made from toxic things, it's generally considered safe.
This is very ignorant. plastics release a wide variety of organic compounds.
"Most plastic products, from sippy cups to food wraps, can release chemicals that act like the sex hormone estrogen, according to a study in Environmental Health Perspectives. The study found these chemicals even in products that didn't contain BPA, a compound in certain plastics that's been widely criticized because it mimics estrogen."
Which is why you see plastic change - look at old plastics around you, in shoes, food containers and fabrics - it becomes brittle, changes colour, etc.
for instance, the "can release" weasel words here - technically correct, but also misleading.
it is not the case that all changes you see in plastics are because they're releasing nasty (((chemicals))) into the environment. it is routine to engineer plastics to meet arbitrary emission/contamination standards. different kinds of plastics also require different (or no) such additives. mostly, people are thinking of PVC when they worry about this.
The brine waste from RO is still mostly water. In order to extract the salt you'd need to evaporate the water which still takes a lot more energy. You could use evaporative ponds to let the Sun do the work but that takes a lot of space. In either case you're spending a lot more money per pound than just digging the salt out of a mine.
Actually solar powered desalination makes a lot more sence - solar is much cheaper and you don't need 24/7 properties of nuclear. so you are overpaying for no reason.
We cam store huge quantities of water, sometimes a year's worth. So intermittency is not a prohlem.
The idea of floating nuclear reactors used for whatever was floating around for a while, but won't happen with government support. It is really just another approach to modular reactors, not a terrible one, but free market wont do it.
What would make more sence, is making all our large cargo ships nuclear powered, and reducing emissions that way
Is not just a question of preferences, there are risks also.
If the problem is that we need lots of energy to create pressure to separate salt from brine, well... I figure out that there is a lot of free "all that you can eat" pressure in the sea bottom.
The problem would be to calculate if moving all this weight up and down the sea (and in open sea) would be economical or not. Or of would unavoidably lead to people cutting corners and release part of the salt to the deep water ecosystems. Deep water masses are salty yet so a small amount of salt would impact less here than if released on surface, thats for sure.
In any case, physic laws about density and mass are our friend. Things either float or sink without extra energy added.
Having all of this in mind, I would go further and propose to regulate by law changing the material of the deep sea submarine ballasts.
It seems technically doable and would have some benefits
1) Lesser impact on deep sea ecosystems. No human trace left behind.
Disposable loads of iron or concrete will remain forever in the bottom. If we use salt or sand instead the impact on fragile deep sea ecosystems seems reduced. The organisms there are adapted naturally to deal with very salty water. The sand or salt ballast could be released gradually over a bigger surface reducing even more the impact over a particular spot or colony
2) Improved economics?
To dump valuable iron made with valuable energy into the sea seems a suboptimal solution. Substitute it with some common by products that are yet in the area and don't need to be transported from a mine far away could save some money probably. Containers of ballast would be fully recyclable also.
Ships could be adapted to literally making part the ballast on the open sea while in campaign, instead to need to carry all of it from a port.
3) Extra safety.
If your load weight gets stuck by a net you are trapped in the bottom forever, With a ballast of sand or salt you have the extra possibility of open a few escape valves and let the concentrated salt go away. You can also release part of the weight much more gradually. After a while the submarine would tend to float and ascend automatically even if the energy supply would have been entirely lost in an accident.
Having the machine on the surface (or closer to the surface) would save millions and would made a big difference on humanitary and economical aspects of the rescue operation. A damaged submarine can be still repaired. Building another would be much more expensive.
Dunno about the possible negative aspects, more volume required for example, but would deserve thinking about it a little more.
> I figure out that there is a lot of free "all that you can eat" pressure in the sea bottom.
> Things either float or sink without extra energy added.
That seems intuitively wrong... where is the energy coming from in this scenario? It's like saying we could use the pressure at the bottom of the ocean to spin a turbine and get free electricity.
I guess the obvious problem that sticks out in my mind is that once you've filled this submersible with desalinated water, how do you surface? A typical submarine does that by pumping water out of the ballast tank, but doesn't that require the exact same pressure that you just used to fill the cabin with desalinated water?
Here's an even more interesting physics brain teaser: Imagine you put a reverse osmosis membrane at the bottom of the ocean and connected it to a pipe filled with fresh water leading to the surface. The pressure at the bottom of the pipe would be less than the pressure at the bottom of the ocean, since fresh water weighs less than salt water.
In this setup, would an endless supply of fresh water flow through the membrane and bubble out the top of the pipe? I'm guessing not, but I'm having a hard time understanding why.
Wild guess: maybe only the partial pressure[1] controls whether water will flow through the membrane, and the partial pressure of freshwater in this scenario is identical on both sides?
Laws of thermodynamics aren't really useful here. They are perfect for explaining universe, and notoriously bad for predicting local events when you zoom and focus at a fine grain level and add time. Life beings are apparently breaking or "delaying" this laws all the f*ng time.
In any case nobody is trying to make a perpetual movement machine.
The goal is to create a cheaper way to extract salt from saltwater, and use the pressure gradient at the sea to put apart brine and water could be a solution waiting for an engineer (in my opinion). It is assumed that will not be free in terms of energy. It doesn't matter as long as is slightly better than the current solution. Would be much faster than waiting for the sun so it does not need to be cheaper than that. It just needs to be able to replace the last energy-expensive phase of our current solution by another process that is cheaper or faster.
A corpse of freshwater immersed in saltwater would experience a force up because its lower density. The weight of the submarine itself cancels this but if we keep adding freshwater at some point we would cross a density threshold. Probably a big volume. Maybe too much to be practical. Dunno.
Even more, ice floats so in a case of live or death if we could freeze with liquid nitrogen or so a big enough volume of cold water while avoiding the effects of the increase in volume, in theory the submarine could emerge automatically. We can't do it in the main submarine (would explode and the non frozen parts would implode immediately), but maybe in an independent storage area attached and able to absorb the extra volume?... dunno
A way to lower the temperature just when the oxygen is about to end would add also some precious extra time. A corpse is dead only when is warm and dead. In any case I'm just digressing wildly about an extreme and hypothetical emergency case. I could be totally wrong or not practical. I prefer not to test it.
> I figure out that there is a lot of free "all that you can eat" pressure in the sea bottom.
Have you heard of the second law of thermodynamics?
There's also lots of all-you-can-eat heat in any piece of matter, eg sea water or rock. But that doesn't mean you can get at it for anything useful, without a heat differential (or a pressure differential).
Hum, I wonder where we could find a heat differential and a pressure differential in this planet... if we except the ocean, and the land, and the atmosphere, and any place with an organism able to fart...
Is that true when we take into account environmental externalities? I am not an expert in this field; I know that many forms of mining are capital-B Bad for the environment but I don't know how salt mines impact the area around them.
well, mines are bad on the short term. the long term damage is not of the actual digging, but of the separation processes. which all can be done as clean as we wish it just costs more.
Just pump it back into the ocean. The little bit of concentrated brine from a desalination plant will be quickly diluted; the ocean contains a LOT of water.
The people who need that water tend to shed it after some time.
Discarding waste water into the oceans via rivers is a huge idiocy. You essentially rely on the environment to "magically" sort it all out. Naively so and fraught with huge inefficiencies.
Proper treatment of that waste in the sense of recovering usable matter streams is the logical way to go.
I know that generally speaking, disposing of waste in the ocean and expecting it to disperse enough to be harmless is foolish and wrong. But in the case of salt, It would seem to me that the ocean can handle that amount of salt. Course, I haven't done the math. But it would seem to me that the back in == salt taken out. We'd only be changing the net salinity by the amount of water subtracted. Without having done the math, my gut reaction is to think that's something the ocean can handle.
The problem, as I see it, is localized concentrations. While the ocean at large might be able to absorb it, the localized concentrations can be very problematic.
The problem with brine from desalination is that it kind of behaves like a heavier liquid, sinking to the bottom. That causes it to stay together, taking longer to mix with the regular ocean; and the coastal seafloor there is a lot of life that doesn't appreciate water with double the salinity of regular ocean.
To solve that you can just dilute it more, either mixing with some other waste water stream or by releasing it over a larger area rather than a single outlet.
> Modern desalination plants generally recover about half of the intake flow, which means their brine stream is about twice the concentration of normal seawater.
I imagine this heavily depends on the actual plant design though. Also because of the above mentioned issues you generally don't discharge it like that but blend it with other water.
There are potentially some good reasons to just spray the brine into the air.
Seawater sprayed into the air becomes tiny salt crystals, which in turn help clouds to form, and cause increased rainfall. The rain produced has negligible levels of salt.
In places with dry climates, this often can turn desert land into farmland across an area hundreds of kilometers wide.
Surely the salt falling on the farmland would have some kind of deleterious effects, right? I imagine you’re not trying to grow sea cucumbers in this scenario, and I’m not sure most land plants would be crazy about a ton of salt.
Just ask the Carthaginians…
Guessing that you know something I don’t here, though.
I guess what I’m forgetting is that the water has a chance to wash salt away back to the ocean. But how much salt does one drip of water absorb? What if the spec of salt that starts in the water droplet is as much as the droplet can wash away?
I suppose it must be slowly becoming clear that I have no idea what I’m talking about (save for a nickel worth of Roman history).
Would this be worth doing straight from seawater? I.e. place enormous solar/wind powered rigs in the ocean to spray massive volumes of seawater into the air? What would be the effect (beneficial and otherwise) of that?
You can though. The amount of precipitation (averaged over a long enough period of time) is inversely proportional to the amount of evaporation and other water entering the atmosphere (averaged over a long enough period of time). Note that the two things being compared are _rates_ not _masses_. If all you do is cause water to fall sooner then:
1. The humidity in the air drops, increasing evaporation rates because of the lower partial pressure of water vapor in the air.
2. The humidity on the surface increases (dusty areas becoming moist, plant leaves uncurling to expose more surface area for other processes but incidentally increasing evaporation rates, reservoirs having more surface area, ...), increasing the evaporation rate.
There are limits of course, and that back-of-the-napkin analysis ignores 2nd-order changes in temperature and all of the other hairier bits of climate modeling, but it illustrates that things are more complicated than they appear anwyway.
Edit: "inverse" here just meaning a multiplication by -1
Its always been my understanding that any treatment to remove stuff from water is going to produce waste which needs disposing, just look at the Brita water jug filters, they need disposing.
I've often wondered why dont we have more pure water pumped through the water mains in various countries, and I think after reading about Super K the Japanese Neutrino detector [1] and how the water in the tank was so pure it had dissolved a spanner/wrench that was left in the bottom, years ago, I might have the answer.
Firstly there is health implications for drinking pure water, and whilst it probably wont dissolve your guts [2], it will drastically and quickly alter your chemistry [3] which in moderate doses may be a good way to calm down, I havent tried personally yet, but there is another problem.
The ultra pure water would probably dissolve the older ceramic and metal pipes used to deliver water around the countryside, from the inside out.
In fact I would even go so far to guess that water mains pipes last longer if its delivering hard water compared to soft water, and probably explains the pub culture as the water is standardised in various alcohol brands.
Either way I prefer soft water, its more relaxing and could well help to reduce a certain amount of anxiety in the population along with stress levels, that could be useful for built up cities, but watch the GDP levels of the region go down if that happened and the profile of crimes change [5], not to mention health conditions!
What's the process, though? The "dissolving" is presumably rusting and then motion of water washing away the rust, but rust requires an oxygen source for the chemical reaction, and apparently Super Kamiokande has dissolved oxygen specifically removed using a vacuum degasifier to prevent interference and growth of bacteria.
I'm not feeling particularly convinced by this anecdote. It sounds a bit urban legendy. Still, I won't claim more than a high-school knowledge of Chemistry so I'm eager for someone to correct me and supply an explanation.
People who want to recover old coins such as encrusted roman coins will soak them in distilled water to dissolve the minerals in a pretty short time. The metal usually is not affected.
> the water in the tank was so pure it had dissolved a spanner/wrench that was left in the bottom
Once it dissolved a tiny bit of the metal it would no longer be so pure. So this sentence makes no sense. It takes just a minuscule amount of mineral to replicate regular well-water.
-> I've often wondered why dont we have more pure water pumped through the water mains in various countries,
It has to do with the economics of water treatment. If you supply water you're incentivised to supply it at the lowest acceptable level of treatment. That's because people just want plain old water, and they don't want to pay a lot for it.
Have you read any of the sources you linked to? They debunk most of what you just said. "Ultra pure" water is very slightly more "watery" than regular tap water. Any effect that tap/bottled water has on anything, "ultra pure" water can at most have about 1.01x that same effect.
> water mains pipes last longer if its delivering hard water compared to soft water
This is the one thing that may actually be true, but it's because the minerals in the hard water build up on the inside of the pipes over time, creating a protective layer. This is especially great for lead pipes, as it can prevent the lead from leeching into the water.
> As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of.
There are no pores, so to speak. Polymer materials form amorphous solids with transient voids which open and close randomly due to thermal motion. They're not "pores" because they aren't permanent over long time scales. Rather, the polymer+water is modeled as a single fluid phase, the same as if you were modeling ethanol+water. The fact that the polymer is a "solid" doesn't affect the fact that it's actually a tangle of vibrating molecules just like any other mixture.
Other materials do have well defined pores, like MOFs and zeolites. In this case, the water does sorb as a liquid in the pore space, but is gated by transport between the pores in a similar manner.
This is made apparent because water does enter into polymers (even those which desalination) freely, with or without the presence of salt. It is not the case that "the pores are too small for water to cross normally". I can take a polymer that will swell with 50% of its own weight in water, and which has no "free" liquid water (as evidenced by the inability of the water in the polymer to form ice), yet make it reject >90% salt at very high pressures (>3000 psi). If you just let salt water sit on one side without pressure, salt and water will make their way through non-selectively. So it can't be that the water is being physically sieved from the ions to enter into the membrane. Rather, the pressure creates a change in the activity of water (due to the mechanical forces acting on the polymer near the low pressure/support material interface). Since the water is more soluble and more mobile in the polymer, it transports at a more rapid rate than the salt, resulting in desalination.
I don't think the article dodges the question at all. Did you properly read it or just skim it? It goes over multiple ways of desalinating water, distillation and osmosis - as you also cover. The most relevant paragraph to the question and a bit of a conclusion seems to be:
>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.
I only have a BS in Physics but you're basically correct. But to make it even more simple and divorced from method:
1) There's a large difference in energy and entropy between seawater and drinkable "fresh" water. This represents a bare minimum expenditure, below which you can never go, lest you attempt to create a perpetual motion machine.
2) No matter how you do it: Well, now you have a bunch of previously dissolved solids covering everything. How do you get them off of your surfaces and out of your tubes and "away" from everything else?
Once you stare at the first factor, then look at the second factor, then go back and forth, you come to your senses and realize that the dream of a jeroboam of colorless, tasteless water next to a little pile of fine powder is just not going to happen, and that the more sensible thing is to release some extra briny water back to your source and hope it doesn't kill too many fish.
A sensible thing to do is to turn the waste brine water into a resource. Since it's already been pumped up, pour it out into an evaporation pond to increase humidity in an area that could benefit from it, and then scoop up the salt to extract valuable minerals.
I guess there aren't that many valuable minerals in seawater.
For example, Fritz Haber, a German Nobel Prize winner in chemistry, tried to extract gold from seawater after WWI to pay for the war reparations... long story short, the concentration of gold in seawater is too small.
Also, the phase transition for H2O from liquid to gas requires a lot of energy and space (evaporation surface). In other words, it takes ages to evaporate all the water. Also, the larger your pond is, the more expensive it is to scoop up the salt. And then just one rainy afternoon can set you back a lot.
Really good question! The answer is: it's complicated.
When you dissolve a "salt" (the whole class of them, rather than just NaCl), there is a lattice energy (you are tearing these crystals apart) and a hydration energy, which are a little give and take from an energy standpoint. Most salts dissolving are slightly exothermic. NaCl dissolving is very slightly endothermic.
Seawater? Well, remember, there's a lot of dissolved solids in there, not just salts. So you have a summation of dissolving a whole menagerie of different things into your water. Last I heard, and it's been many years since I went near anything like that, yes, there's both an entropy and an energy cost, although I would personally dread trying to do calorimeter measurements to verify it experimentally.
Using a fractional distillation column you can also separate filtered sea water into water vapor and brine, at equal pressure, with the brine appropriately hotter so the vapor pressure of water over the brine is the same as over the salt-free water droplets that form on the cold end.
You'd then have to compress the water vapor until it condenses barely hotter than the brine, and use both distilled water and residual brine at their approximately equal temperature (water at higher pressure than brine, though) in ofc separate counter-flow heat exchangers to pre-heat the filtered source (sea) water to the column's operating temperature (i.e., where the source water just starts boiling at the column's operating pressure (you want a decent vapor pressure to have a reasonable vapor density and thus feasible power density for capex reasons)).
Thermodynamically this should match a reverse-osmosis process with equal input/output parameters (I left out that you need pumps/turbines to "losslessly" adapt liquid between ambient pressure and internal operating pressure).
One benefit would be that you could directly heat the brine with solar thermal collectors, to get away without having to compress the vapor to condense it, essentially an open-cycle Type-1 absorption heat pump, with solar feed. (Lacking an evaporator, with the condensed output being the desired pure water, and the absorber being fed with source sea water while the return from the generator after the heat exchanger is just warm brine for discharging. If water and brine need to be sub-ambient, you'd evaporate part of the condensed water to chill both the condensate and the brine output streams. That'd be partially-open-cycle.)
Distillation and reverse osmosis theoretically use the same amount of energy.
Practically, reverse osmosis tech is far closer to that ideal efficiency level, especially if electricity is your starting energy source.
But it doesn't seem out of the realm of possibility that someone will figure out efficient distillation in the future. distillation has the big benefit that it can make use of low grade heat which is waste from lots of industrial processes.
> As it turns out, doing that takes a lot of energy
The change in entropy between a batch of saline water and a batch of fresh water and enough saline water that its concentration don't change is about the same as letting that same fresh water fall for 200m and converting the resulting energy into heat (at 300K).
What means that desalination will take a lot of energy whatever method you use. There are distillation procedures close to perfect efficiency that wouldn't take much more energy than reverse osmosis; and of course, electrical separation is that one method with lots of promise but that stops due to material related problems every time it's tried. It just so happen that we know how to scale reverse osmosis up cheaply and reliably; but this looks like a feature of our technology and not anything intrinsic.
If there's one complaint I have about PE's content, it's that he often give ambiguous answers to the central question. Sometimes it's strung throughout the 10 minute video, but I really wish he'd end all his content with a concise summary.
It’s important to note that the energy needed for RO to work is due to the high pressures needed to ram that little H2O molecule through that virgin nanohole. 700-900wh for a trickle of 14gal/h. At least that what I’m getting on my sailboat.
It takes less energy but produces a concentrated liquid waste stream that must be disposed of.
I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere?
It’s toxic only because of concentration. AKAIK, there aren’t any compounds in the brine that weren’t present in the seawater to begin with. The solution is dilution, but I’m sure it’s easier for me to type that than it is to achieve in a desalination plant.
But, why not really-long-pipe-with-small-holes-along-the-length? That seems to me like a simple mechanism to send the brine back into the ocean without causing a local disaster on the sea floor. Is there maintenance required that makes it more expensive than I realize?
No, the salt water you're about to discharge it into.
Have a pump that draws in 10L of ocean water for every 1L of brine you need to dispose of, mix 'em up, and discharge the 11L of only-slightly-saltier water back into the sea.
Not sure when it makes more sense to do that vs. having a leach-field type of brine discharge. They both ultimately do the same thing, but one requires more mechanicals, the other requires more piping and "passive" infrastructure.
Sewage treatment outflows maybe? Wouldn't be enough in a community where all the freshwater is coming from desalination, because not all of the freshwater goes into the sewage system, but it might be workable in communities where desalination is augmenting other sources of freshwater.
No, fresh water just enters the water cycle. It will eventually evaporate or end up in a river and back into the ocean where it will be reunited with the salt. The overall salt concentration of the ocean would not be changed unless we sequestered the fresh water permanently. Even then it would take a tremendous effort to make even the tiniest difference in salinity.
It will change the salinity on the short term though at the release location and the amount a large plant will be discharging is enough to alter the local salinity so long as the plant remains operational which will negatively affect sea life in that area.
Only if the discharge is not diluted sufficiently.
There is nothing about reverse osmosis that is fundamentally more toxic or harmful than the typical evaporation that takes place naturally in the ocean. And it’s pretty dishonest to claim otherwise. If there’s a problem with too high salinity of discharge, that’s an engineering problem that should be fixed with greater dilution.
If you're drawing water from a bay and not putting all that water back in, then the salinity of the bay must depend on the rate it mixes with the outside ocean, since you're removing water from the system.
I assume that in practice the amount of water taken by a desal plan is tiny and most bays have high tidal inflow and outflow, but it's obvious that more than just dilution should be considered.
I almost addressed this point in my original post... you need a diffusion system that runs the brine out into the ocean and mixes it with seawater to a certain concentration... as long as it isn't super concentrated or at a very different temperature it won't create a brine pool and the salt will diffuse harmlessly.
Of course but read the comment I was replying to, to my reading they're saying the regular water cycle would take care of the excess salinity which is drastically wrong in the human scale and a complete misread of the issue. The problem isn't that we'll over salinate the whole ocean but that locally the released brine will kill some, potentially large, area of underwater life before it diffuses back down to the rough background levels.
You dilute it with more seawater. The salinity of the ocean changes significantly just due to evaporation. The band of variability is roughly 30 to 40 grams of salts per liter for sea surface salinity. So if your discharge is within that band, it's not going to be a significant ecological problem (especially if the discharge is at the right location). It's no more a "mass ecological problem" than surface evaporation of seawater is.
Mass dilution is going to be fairly expensive because you have to move a significant multiple of the water you actually process. The brine waste water is roughly double the salinity of normal sea water at 70 ppt so to get back to the 33-37 ppt that's the real average band you're looking at moving a lot of sea water to dilute the output. If you run the numbers you're looking at 15:1 of volume to get 70ppt to 37ppt using 35ppt water all while ensuring you're at a place with enough natural mixing that your inputs and outputs don't start feeding back into each other.
Even normal seawater has toxic concentrations of salt:
“seawater's sodium concentration is above the kidney's maximum concentrating ability. Eventually the blood's sodium concentration rises to toxic levels, removing water from cells and interfering with nerve conduction, ultimately producing a fatal seizure and cardiac arrhythmia.”
> I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere?
Too much salt can kill stuff (e.g. people, plants), so I suppose that makes it "toxic." Maybe there's a tiny amount old industrial pollution from anywhere an everywhere that concentrated in there, too.
However, if you're desalinating seawater, what's the problem with just dumping the brine back in the sea? Unless you introduced new stuff into it during the desalination process, you wouldn't be making anything worse.
There are several practical problems dumping the brine back in the sea.
If you dump it on a living ecosystem you tend to kill it. Living ecosystems are, unfortunately, concentrated right where we are desalinating and is cheap to dump.
Compounding this problem is that water mixes much more slowly than your intuition suggests. It can stay a coherent mass of high-salt water way longer than you'd think, killing as it goes. This is one of the more surprising things I've learned in the past few years, honestly. Your kitchen-scale-based intuition of how long it takes for liquids of different characteristics to blend together turns out to be way off.
Trying to pipe it away to somewhere where it is less of a problem is expensive.
In the long term just dropping it back into the ocean is not a big deal, but that short term is surprisingly destructive. You'd think you could just drop it in and maybe a few hundred feet from the outlet it would be all dissipated and harmless, but unfortunately the physics don't work out that way.
> water mixes much more slowly than your intuition suggests
As a visceral example of this in the other direction, the freshwater plume of the Amazon River extends more than 60km into the ocean. [0]
I would love to see these plants placed in areas where there's a nearby dry below-sea-level basin, into which the brine may be discharged. The Salton Sea in CA is one example, there's another similar location in Egypt I'm aware of. The advantage of such locations is they are usually extremely hot and arid, which means there's generally not much of a local ecosystem or human population, and there is ample solar power availability.
The Salton Sea is over a hundred miles from the ocean, though, with at least one mountain range in the way. It might be a good dumping ground (or maybe not - there at least were fish in it, if there still are, this would almost certainly kill them). But it would definitely take a lot of energy to pump the water there.
It would take one mildly expensive tunnel. As a bonus, you might be able to put hydro turbines on it, if they can handle the salt.
The Salton Sea's salinity is already well over that of the ocean (44g per liter compared to 35), and is getting saltier. Considering that the very existence of the Salton Sea is already an ecological disaster, I don't really see dumping salt into it to be another one.
The Salton Sea is 70 miles from the Pacific Ocean. If this were built, it would be only the third longest water tunnel in the world, and second in the US.
The "clever" version I've seen in some papers is to use reverse electrodialysis to recover energy as you dilute the waste brine with seawater. AFAIK, this has not been incorporated into any existing installations.
The extreme volumes produced in the Arabian Gulf is subject to a lot of studies (some plants produce one million m2 per day). This does cause issues. An in-depth look at possible futures can be found here.
> If you dump it on a living ecosystem you tend to kill it.
This is false and somewhat dishonest. This is simply a choice of not diluting it enough. There is nothing about the discharge from reverse osmosis that is any more fundamentally toxic than the natural process of evaporation.
Proper dilution is essential, and treating the discharge as fundamentally toxic actually undermines the engineering to do this proper dilution because people will figure “oh well, I guess there’s nothing we can do as it’s going to tend to kill no matter what.”
People need to stop misleading about discharge toxicity.
It's not waving a word around. There's natural variations of salinity of oceanwater, roughly 30 to 40 grams of salt per liter. So diluting the discharged brine so it's within that natural evaporation-driven band would be perfectly safe as it's in the natural variation of salinity.
It won't kill anything. The surface of the ocean has natural fluctuations of salinity all the time due to evaporation, and it doesn't cause mass die offs. It's literally a necessary part of the ecosystem (and in fact discharging freshwater can cause problems to the ecosystem). But if your discharged water is within the natural 30 to 40 grams of salt per liter range, there's no ecological problem here.
Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are.
What's wrong with pumping 10% CO2 into your office constantly from a compressed gas plant extracting oxygen and argon next door?
> Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are.
How large would those brackish areas near the outlets be? It seems to be that would be a big problem in an enclosed bay, but much less so on a shore facing open ocean.
Could they run pipes out a few kilometers with small, regular holes (maybe some modification of oil pipeline technology) to spread the discharge out and mitigate the concentration problem?
Could they make the waste output less concentrated? Maybe by either running the desalination process less (would that also increase energy efficiency?) or by pre-mixing the waste with some un-desalinated intake water?
The pipe with many holes is one of the solutions used today it's just imperfect and requires a lot of pipe to make sure the waste output isn't too concentrated in a single area.
So the obvious question to me is: is there no other physical property that can be used to separate the hydration shell + ion molecule combinations from the just-water molecules?
Different magnetic charge?
Different mass?
Different chemical reactivity?
¯\_(ツ)_/¯
I know the answer must be 'no' since if there were a better answer, none of what I'm thinking of requires more than high-school-level chemistry to discover/exploit.
To me it doesn’t seem to make sense to first claim that you can’t filter for salt in water, and then talk about semi permeable membranes and osmosis. Those membranes are filters for salt!
But we seem to have colossal amounts of essentially free solar energy, and that energy already evaporates large amounts of sea water. We just don't capture it well.
Imagine building a pipe that stands above shallow tropical coastal waters. Make the bottom of it into an almost flat funnel to cover more water surface, using transparent plastic or even glass. Now all the evaporated water and hot air go into the pipe.
Build the pipe a kilometer tall. Humans have adequate technologies already, and the pipe does not need to be bearing much internal load, unlike Burj Khalifa or World Trade 1.
At 1km, the air is cool enough. The hot air will shoot upwards, cooling on its way up and releasing fresh water. Lightweight collector pipes will bring it down into a reservoir. The remaining dampness of the air will help it produce clouds, and thus shadow, over the land.
With a tall enough pipe, we could even generate electricity by putting a turbine inside.
Why are we not building it? It's expensive, and most (sub)tropical countries that lack water are poor. They are also politically unstable, and such an installation would be a high-value military and terrorist target.
Maybe Singapore or Dubai would some day dare and build it. (California, unlikely; it would never pass an environmental review.)
I was surprised how cheap it is. Desalinated water costs ~50 cents per 1000 liters [1]. That's about the same amount of water as a typical American household uses per day.
50 cents per day for a fully desalinated water supply is... incredibly cheap.
If you're interested in water policy and water management / engineering, I cannot recommend enough reading the book "Let There Be Water: Israel's Solution for a Water-Starved World".
From my (limited) understanding of agribusiness, particularly in California, it will be hard to be cost competitive with current water sources moving forward as they are heavily subsidized (read: near-zero costs).
The upfront cost from my understanding is in drilling a deep well. Those wells keep getting deeper and costing more. But, past that, it's just the cost of running the pumps to drain the underground aquifers. IIUC the cost of water is free plus the cost of harvesting it from the commons.
I might be wrong here, but all the billboards that say "is growing food wasting water?" along the interstates in California don't really matter over long time horizons. They're advocating for draining the water tables. You can't do that forever. Doesn't matter if it was a "waste" or not, it'll be gone soon and they'll have to pay to pull water from somewhere else or stop growing crops there (or the state will pay to give them water).
When "free" water runs out, other water sources will suddenly be cost effective. But it's hard to compete with free.
Pumping is at cost to them, but irrigating they pay a whole $20 per acre foot from our local water district. The wells are used when they want more than allotted.
Many of these canals are quite old and while they do require some maintenance, the upfront costs of the dams, reservoirs , and canals are largely paid off. Those maintenance costs and any upgrades are paid by the district customers.
There are aqueducts between the CV and coastal areas. The water goes the other direction.
If desalination becomes widespread, I imagine the water not shipped to the coast could remain in the Central Valley. I don’t know one way or another if this would make political or economic sense.
Water rights are so complicated that they’d probably sell the water to the Central Valley before it ships to them. I think it very unlikely that they’d give them up.
There's no way the water would be affordable for agriculture if it had to pay for pumping it uphill/inland. There's a reason the rivers flow in the other direction.
In California, where we have persistent water shortages, residential, commercial and industrial water use ( including all landscaping, golf courses, etc) all put together still only amount to 10-20% of overall water use, depending on rainfall
No, you are being misled by “environmental water,” which is a fancy way to say it’s water we don’t use because were not allowed to. This doesn’t make any sense, how can you include water you don’t use in your accounting for water use?
The reason this is there is to downplay the outsized use in agriculture, and also to shift some blame to folks that voted not to allow this water to be used in the first place.
But still, if you want to cut spending and are looking for where your money is spent, you don’t include money you choose not to earn in your list of spending.
Freshwater withdrawals per capita in the US (which includes agricultural exports and animal feed such as alfalfa sent to Saudi Arabia) are around 1550 cubic meters per year.
So that is around $775 per person per year assuming no net change in water use. In contrast, Germany uses around 410 m^3, France around 475 m^3, and Australia around 724 m^3, so the US is a significant outlier.
That equates to around 4,000 liters per person per day.
Freshwater withdrawals is a very broad category, it also includes water released to turn hydropower turbines. But it's also unfair to compare across countries without taking into account water sent between countries in the form of produce and products.
You were quibbling about how 1,000 liters per day does not adequately account for all usage. Freshwater withdrawals is on average going to be a overestimate and the US is one of the worst outliers with significant agricultural exports which are one of the largest contributors to differential water withdrawals. Despite this, it would only be $2.00 per day for a fully desalinated water supply even if we safely overestimate usage. Despite it being 4x higher than the person you replied to said, that is still incredibly cheap.
It accounts for the water used in agriculture, industry, power generation, etc. Those uses completely dominate domestic water usage including for aesthetic purposes such as lawns and golf courses (e.g. around 80% of water usage in California is for agriculture and power generation if I remember correctly with only around 5-10% for all domestic usage).
As you will note, the average German consumes 1,000 liters per day by the same metric which is 2.5 baths per day which is obviously unreasonable if we were only considering direct domestic water consumption.
To be fair, the agriculture is being grown to feed people, so it is fair to include the consumed produce in each person’s water footprint. This is further exacerbated by growing animal feed for animals that are consumed which is even more “water inefficient”. The US is a major food exporter, so it is over-represented in these sorts of numbers, but it is a fair approximation to within a factor of maybe 2-3x of the embodied water consumption of the average American.
Maybe my comment wasn't clear. That figure isn't great - we just divided the total amount of freshwater diverted to some kind of use by the population. That even includes letting water flow past hydropower turbines. It's obviously too big.
But not counting the water used to put food on your table is too small. So this establishes some bounds.
This is an interesting number. Recently, I saw complaints that green hydrogen is impractical because it would use too much water. But if all the per capita energy use in the US went to electrolysis, it would use about 1% of this water per capita (and, of course, green hydrogen would be only a fraction of total energy use, due to all the preferred direct uses of electrical energy.)
That just goes to show that people will complain about anything instead of applying their brain.
There are plenty of valid objections to hydrogen as a fuel, but water use is a total non issue. Not only does it use so little, but when you get energy back out of the hydrogen, the waste product is water again. It’s literally a renewable cycle.
why not? we're growing heavy water using crops in places with little water. logic is not always the deciding factor if involved at all in a lot of modern things
There are a few very big salt mines below the great lakes. We could totally get a desalinization plant in Michigan by pulling water from the lakes, salt from the Detroit salt mine, and combining them in the input stream to the plant!
I think it sounds quite sensible. How is it different from using all fresh water as an all you can dump free-for-all public sewage system before wondering where the drink is coming from? In India they poop in the water then take a bath in it and cook their food in it. In the west we do our pooping upstream because it looks so much cleaner that way. NO! I fail to see how adding salt before desalination is any less sensible. Lets just do it.
you're desalinating water to have an abundant source of fresh water. why in the world does taking fresh water and adding salt being mined from the ground together to make salt water to then desalinate make any sort of sense?
We already have abundant fresh water. We use it to transport sewage, fertilizers and other crap to the ocean. It would be silly to try to clean it afterwards - but here we are?
Adding salt is a great idea, it pushes our collective stupidity to a noteworthy level of nonsensicalness.
It makes me wonder what other hard to remove poisons we could add to challenge ourselves. Perhaps design a new disease?
Clearly, you're not having a rational conversation with comments like this. If there was an abundance of fresh water, the western half of the US would not have been in severe drought conditions for the past however many years. This is where I leave you as you are just making things up like and not even having a good faith conversation
To be fair to the author, difficult here could just mean “more complex than it seems” which he does a good job of illustrating, specifically around the additional concerns that go in beyond the actual processing of the water.
That's not too bad. the normal baseline consumer costs are actually more expensive than that. normal base use in irvine ca is 1.78 per 748 gallons which is almost 3000 liters. (2831.488 liters)
I assume that's cost to make and not total cost to consumer post treatment plant distribution and maintenance so it would be more expensive than that but still in the ballpark of reasonable.
The biggest plant in the US, near San Diego (Carlsbad), cost $1B to build and produces 200,000 m^3 per day. Israel's biggest desalination plant, Sorek, cost $400MM and produces over 600,000 m^3 per day, so 40% the cost and 300% the water. So I assume a large part of the cost of the Carlsbad plant is just all the usual reasons everything is insanely expensive to build in California.
The Carlsbad plant is in Carlsbad, CA. If you look at the neighborhood it's in, the houses are 1,200-2,000 square feet and sell for $2-$5 million. Everything is expensive.
No, 1000L/day is about right for a typical American household. [1] claims "The average American family uses more than 300 gallons of water per day at home." 300 gal is 1136 L.
I know American toilets are comically big but 250L a day on flushing them seems insane to me. A cursory google suggests the average UK household uses 350L a day in total!
That's like 20 flushes. It's a lot more than I'd expect for "typical". Maybe a family of 5 does 20 flushes. Or maybe it's 2 people with strict anti-if-it's-yellow policy.
Slightly off topic, however, the post references the Carlsbad desalination facility. If you find yourself in San Diego and like oysters, I would highly recommend you checkout the Carlsbad Aquafarm. Take the tour and pick up some oysters.
What's really interesting and relevant to the topic is that the oyster farm serves as a pre-filter to the desalination plant and there's an symbiotic relationship between the plant and the oyster farm.
Pick up some oysters, for eating? If these oysters serve as a pre-filter for the plant, would you not want to eat them as these oysters would contain all sorts of pollution?
> This was the first oyster farm to feature an inventive “depuration and purification” process, which involves immersing the oysters in triple-filtered seawater once they reach full size. This ensures that the oysters are a completely safe, top-quality delicious shellfish product.
> That’s a really convoluted way to say they rinse them off in clean water.
It's more than rinsing them off. Oysters are filter feeders. They need to spend enough time in clean water to pump out any contaminants. It's an FDA regulated process:
If they were pulling disgusting water into the desalination plant, it would probably damage their equipment. If you watch the video, you'll find that macroscopic contamination is the first problem they have to solve, and the oysters should be fine for that.
Relatively high energy cost since you’re undoing an endothermic reaction, you need to do a lot of it since we use water in large quantities… but most of all, the planet naturally does a lot of desalination for us already through various geological processes, so our “price point” for desalination is $0 per liter (infrastructure to capture rain, dam rivers, or tap groundwater isn’t literally free, but it’s pretty close - especially when it comes to the marginal cost for the next liter). It’s not difficult to desalinate per se, it’s difficult to desalinate extremely cheaply and at huge scale.
In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. If there is a real need, and a will to address it, we have everything we need to to it. https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...
OK, so we got a country that needs machines to survive (no desalination plant, no water to drink).
Maybe it's time to understand that it's not a long term strategy ? That may be they should move to another place where water is naturally more abundant ?
It's not even remotely economical without huge government subsidies. Completely untenable with current technology for poorer countries, or anyone that cares at all about carbon emissions.
>Why must drinking water be a for-profit enterprise?
It's about sustainability, not profit. Of course wealthy nations can (literally) burn enough money turning fossil fuels into water to make their population comfortable and happy. But most can't, and the externalized cost is unimaginable at a global scale.
Wind powered desalination seems like a perfect combo seeing as it's pretty much always windy on the coast. California's May gray and June gloom makes me thing would keep solar from my first option.
>Clean energy exists, it would require more subsidies but so what?
Easy to say when your government can afford the subsidies. But the vast majority of freshwater-insecure nations will never be able to do this without a 10x technological breakthrough.
> Well if they can't afford to even subsidize fresh water, their choices are move or die.
Would you tell that to the Israelis if they couldn't afford it?
The point is that yes, the original article is correct. Current desalination tech is woefully inadequate to replace fresh water surface reserves without putting a massive burden on the society using it.
> Would you tell that to the Israelis if they couldn't afford it?
Yes? What else would I tell them, to pray for a miracle? If they don't have enough water, can't get water profitably and cannot even afford to subsidize water, what else is there to do besides find somewhere new to live? What would you tell them?
> without putting a massive burden on the society using it.
Yeah, in some places it will be necessary to subsidize water, placing a burden on society. But considering we're talking about water, that's obviously a burden that needs to be borne. Acquisition of water comes before literally anything else a population might want to spend money on. And if there isn't enough money around to acquire sufficient quantities of water, there isn't enough money to live there at all.
Israel has a strong enough economy, they can afford to make desalination work for them. You objected that they have to subsidize the desalination, but I don't see any sense in that objection. If that's what they need to do, that's what they'll do.
Some parts of the US have too much fresh water, other parts have too little. Fresh water is a regional matter; you're talking about the American Southwest, particularly California, specifically. But this conversation is about desalination generally, and particularly Israel.
Why not use nuclear power plants to power desalination plants? I even wonder if some of the salt from the brine could be fed into certain types of nuclear reactors (Molten Salt Reactors and the like, possibly) making it an even more symbiotic relationship
Massive-scale desalination with nuclear has been proposed for decades by many nations. Here's a proposal (originally pushed by JFK) for using them to transform the Middle East into a luscious mecca, thereby solving any arable land scarcity issues (from 1967)
Is there a practical reason why it would be difficult to do this with solar power? Is this a process that does not adapt well to intermittent power sources?
I feel like I never get enough of the operational details to know. But intermittently running a capital intensive thing has bad economics. If the capital cost per m3 is $0.50 when the plant runs 24/7. It'll be $2.0/m3 if you cut it back to 6 hours a day.
However the details are important. You'd need to do a deep operations analysis to get an answer. That also would include energy storage as well.
One of desalination facilities I think is actually solar powered. It mix of evaporative desalination with power generation (giant tower that a bunch of mirrors focus light one). Not sure if it's in production now.
But in general in Israel solar has a couple of problems: very dusty (sand storms) and local electrical company which tends to create problems
This doesn’t make sense to me. You don’t need to store energy to desalinate water. You can store the final product. And water storage is a solved problem. At times where supply exceeds demand, use excess energy to desalinate more water. When energy demand is high, desalinate less.
I wasn't too clear about what I meant, but is storage even necessary here? For example, would there be an issue if the desalination process was left in an intermediate state for X hours / days while power is intermittent?
I wonder if there are more energy-expensive desalinization processes that are better to use with intermittent power sources, like solar.
desalination plants are built to produce specific amount of water in order to cope with demand. if you going to stop desalinating while there is no solar, you need more plants in order to desalinate more water during the day. also, in general, even during day, solar not always available.
Sure. So if for example your city has an average of 3400 hours of sun per year like Jerusalem does, and you know how much water you need to produce in the 8760 hours of the year, you can calculate how much to desalinate during sunny hours. Excesses can be used for excess water or sold back to the grid.
desalination plants don't serve one city. they serve country. also, in general, it's not economical to build plants that do not work 24/7 unless you are country that can spend x3 to overbuild and keep equipment idle
Israel btw supplies desalinated water to Jordan and PA.
I think we’re talking past one another. Solar is definitely economical in certain circumstances, and every country overbuilds to some extent. Whether building excess solar makes sense is a question of cost of a marginal unit of energy, since the water doesn’t care what powered its desalination. The energy isn’t wasted since it will eventually need to come from somewhere.
Another consideration is political sovereignty, since solar can’t easily be turned off by a foreign adversary.
Note that the large increase in the graph predates seawater desalinization. If you start at 2005 (first large seawater desalinization facility per wiki) and assume all the increase is due to desalinization (definitely untrue, but simplifying), the difference isn't so large.
A lot of public goods are that way. What you’ve stated is practically a tautology. “Government subsidy” just means the public is paying for it. Other things that fall into that category are universal education, the military, and police.
Fresh water isnt a "public good", technically or economically speaking. A "public good" is a commodity that is neither rival or excludable. This means that the quanity or quanity is not deminished by people using it, and that you cant prevent anyone from using it. A lighthouse or public radio are examples of public goods.
Fresh water, economically speaking, is a classic private good. It gets consumed as someone uses it, and can people can be easily restricted from acess if they dont pay.
on a serious note, it's a smallish research/etc facility. probably most of things are as deep underground as possible. it's not same thing as full blown nuclear power plant
Some years ago I toured a maple syrup operation that has the opposite goal: Concentrate the dissolved stuff in the water. Their first stage was reverse osmosis, but only to a point. Second stage is boil, but with aggressive heat recovery from the steam to preheat the incoming liquid. All this to keep the energy cost under control.
That's more or less the correct way to run anything energy intensive, scavenge as much of the waste heat as you reasonably can.
Theoretically, continuous distillation can be extremely efficient, as, you're removing as much heat as you're putting into the system. In reality, you get into diminishing returns fairly quickly, because insulation, pumps, heat exchangers, etc, are all far from free, especially at scale.
I'd guess that if anything they'd increase the pressure to raise the boiling point. That way things dissolve in it faster and the water doesn't evaporate away.
Part of the flavor of maple syrup is due to caramelization and the Maillard reaction of components of the sap. Just concentrating the sap would get you a syrupy substance sourced from a maple tree but it wouldn't be maple syrup.
I've never liked the flavour of maple syrup very much. At the same maple syrup place visit, we had some, on pancakes. WTF? This is plain table sugar! What kind of stunt are you pulling here? The guy supervising the boil said don't you know? The very best grade is like that, hardly maple-y at all! Turns out it really was about the sugar all along, with the initially undesirable maple flavour coming to be prized over time as a side effect.
Fun fact, early abolitionists marketed maple sugar as a replacement for cane sugar, since cane sugar was made by slaves while maple sugar was made by northern farmers.
So the old grading system rated the lighter colored plainer sugars and syrups higher than the darker more flavourful varieties. Since less maple flavour made for a better all-purpose sweetener and a more direct competitor to cane sugar.
Nowadays we usually use maple syrup for the flavour, so the grading system is non-judgemental that way. And the darker grades are more likely what you want.
The stronger colors and flavors occur later in the season. The warmer weather allows more bacteria growth in the sap, which metabolizes some of the sugar into other compounds.
Does this mean I can buy watery maple syrup that is 2x watery for less than half the normal cost like maybe a quarter of the cost? Or does the volume and weight for shipping and handling offset that savings or does no one want watery syrup so there is no market so they don't make it?
I think you're overestimating how much you could save. Rule of thumb is that it takes 40 units of sap to make one unit of syrup. (But this can vary widely depending on the sugar content of the sap.) So making syrup at half the concentration would require removing 38 units of water, instead of 39. That's not going to make much of a dent in the processing costs.
bro the more concentrated the syrup the harder it is to remove the next unit of water, that's why i was asking maybe provide a product that stops while it's still a little watery and not have to do the last whole stages
Not Canadian maple syrup, it's regulated. I am pretty sure the regulations started from the producers getting together, or at least has the full support of the producers to keep up quality/reputation.
Correct for things with that specific label, but other X syrup labels can be fair game:
> In the United States, table syrups can be sold under a name consisting of any word followed by the word syrup with the exception of maple, cane, and sorghum. Commonly used names are table syrup, pancake syrup, waffle syrup, and pancake and waffle syrup.[1]
MIT solar distiller in 2020 demonstrated a gallon and a half of fresh water in one hour using a square meter of close-quarter membrane distillation process.
So, 10,000 sq meter of this baby could pump 150,000 gallons of fresh water over a ten-hour solar shift.
Seems like the secret sauce is 1.2cm (or is that 80mm) separation between diffuser plates thus taking advantage of solar heating/condensation/collection in one area.
Of course, there remains an collection issue of brine discharge which could be removed gradually instead but in same but 3-peat manner (down to 2-3 permille, or 0.2-0.3% salinity level.)
At any rate, this MIT method has leapfrogged the passive solar method ahead of reverse osmosis (RO) method by quite a bit, in terms of energy required to extra fresh water. RO still holds the insurmountable lead in base (non-fluctuating) water output rate.
I'm not sure what the full story is, but in 2019 MIT was also looking at one of the other costs. They claim it's common to pretreat the water with sodium hydroxide, which is purchased. Here they claim they have a way to regenerate it from the effluent:
Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?
I wonder how that will affect the Amazon rainforest since it receives phosphorus and other nutrients blown of the Atlantic from the Sahara desert. If salt starts getting blown over too I wonder if that will ruin the soil fertility there.
Was going to say this, there are/have been many ideas to do just this - one was to build a huge canal and just then let the sea flood the area...
Also - there are lots of un-earthed treasures to be found under the saharan which was once a lush environ and have been covered with sand - so prior to flooding it, we need to lidar and excavate it.
What if we vaccuumed up all the sand and built a new island/continent with the material and just revealed everything underneath - then flooded it. (I believe UAE is in the market for more sand-built-land-masses)
One of the more feasible ones was a linear greenhouse with a canal in it.
I'm sure someone has done the math to figure out what the proper length is for such a canal, to avoid problems with scaling. And if you've ever been near an ocean for much time, you know the salt tends to get around. So I'm not sure how in a multi-year project you keep that to a minimum.
My guess is that no matter how much water you manage to evaporate, the scheme will be doomed by the original causes for why the area is a "hot dry desert", the wind and geography patterns.
So even if you evaporate a lot of water, it won't fall where you need it or where you can collect it.
Israel does this for a different reason: To collect minerals from the dead sea.
It doesn't increase rainfall.
Israel is actually desalinating water and rejuvenating a river, which eventually will reach the dead sea (although that's not why they are doing it, but it makes the point that evaporating water isn't doing much).
Maybe you mean on a far larger scale?
Well - the ocean itself is much larger, and water evaporates from its surface all the time. Humans aren't going to make an evaporation zone larger than the ocean.
Also: Water does not (mainly) evaporate because of heat, but rather because of wind. There's lots of heat in the desert, but not much wind. The ocean has a ton of wind.
Actually deserts can be quite windy, especially deserts near the ocean. Wind forms when there's a pressure differential, and the most common reason for a large pressure differential is when there are two adjacent areas with a significantly different temperature. So when you have a desert next to the ocean, the desert cools at night and then during the day, as the air in the desert heats up, it lowers the air pressure on land and pulls in air from the cooler air over the ocean. This phenomenon is why the SF Bay Area consistently has high wind and good sailing conditions during the summer, especially in the afternoons.
That said, none of this contradicts the overall point you were making.
I mean, that's actually not too much of a downside, since salt is a fairly valuable economic good - Indian salt farmers do exactly that to harvest the salt crystals.
We're coming to the inevitable conclusion that we can't keep doing this, as there's little existing ecosystems to destroy - I mean: we're looking at deserts and thinking: "why not?". Slightly more outlandish, we're looking at whole planets and thinking the same.
Some call that par for the course, others unsustainable. Still others don't make a distinction and see them as the same.
Deserts are far more alive and biodiverse than people think.
> The Sonoran Desert encompasses 120,000 square miles of southwestern Arizona, southeastern California, and in Mexico, northwestern Sonora and most of the Baja Peninsula. With nearly 3500 species of plants, 500 species of birds, and 1,000 species of bees, the Sonoran is the most biodiverse desert on earth.
Yes, but you picked the the most interesting desert. Doing this to one particular area that is already ruined (the salton sea) in a desert with far less wildlife is worth discussing.
You do have to consider though, how important are those ecosystems?
Because I see this often used as a reason we can't do something, but they never qualify it with reasons why one should care.
I also pose this as a completely honest question, as I don't really know if you wiped out every desert ecosystem with solar/desal/etc if it'd actually affect anything else.
>> I also pose this as a completely honest question, as I don't really know if you wiped out every desert ecosystem with solar/desal/etc if it'd actually affect anything else.
Presumably, you believe that there is something amongst all that "anything else" that has intrinsic importance. Perhaps for you it is humanity in general. Perhaps just yourself and your own family.
Whatever something you might value intrinsically is fundamentally arbitrary. Why not me and my family? Why not other great apes besides humans?
Arbitrarily, I value (desert) ecosystems as having value in their own right. Biodiversity is an intrinsic good, with no further justification required.
it's probably a good starting position to assume that since we all the share the same closed-ish system called planet earth, there's interconnections between different systems. Certainly the border areas between desert and not-desert aren't very crisply defined. Certainly (reference in other threads) nutrients can be blown by the winds from desert into non-desert areas far away. Certainly there exist some animals who go in and out of desert regions (birds, butterflies, ...). It's a really good idea to assume that things on this planet are connected to each other.
Sure, I mean, that's my assumption, the question is more of "how important is that connection" in a given instance. And, I suppose down that line of questioning, do we have the knowledge/systems/etc to overcome any losses?
Separately from "is this desert ecosystem useful", I would assert that the biodiversity of stuff that survives in arid environments is immensely valuable, representing umptillion creature-years of evolutionary R&D.
Even if our descendants committed to turning Earth into a uniformly-lush garden world, think of its value in terraforming other planets, or even just novel biochemistry and adaptations.
Using a desert isn't a great idea, but, using the sun to evaporate water works just fine. You're just replacing an expensive heat source with a 'free' heat source.
That's not unusual in desalination however, many facilities are combo plants, they're producing power and then using waste heat for desalination.
I think the key difficulty is condensing a very large quantity of water out of an even larger quantity of air, in the desert. The thermodynamic equilibrium of water vapor vs water-in-condensed-form isn’t going to work well for you here, even after nightfall. The very reason that it was possible to evaporate the water out (i.e. the air is very dry) cuts back the other way.
You know this is how the Phoenecians became the dominant culture, its also where the term "salary" comes from and "worth his weight in salt" -- as salt was the only known preservative of the massive amounts of Tuna the phoenecians were catching and shipping throughout the mediterrainian - and made them a super-power - they had control of the preservance of food over shipping distances...
Salt was used as money.
EDIT: Only a fn idiot without knowledge of history would downvote a comment... Jimminy Carter, what type of stupid are you trying to promote?
Probably (not me, I don't have downvote bits) because it turns out there's a lot of "game of telephone"/"folk etymology" about this legend which apparently started in the 1800s in english; it's not actually historical as you've expressed it.
The energy costs are a bit of a red herring depending on local conditions. In California we currently "curtail" i.e. discard a huge amount of renewable energy in the spring season. If we can seasonally apply that energy to desalination, and store the fresh water for later, it is essentially a huge time-shifting battery that stores excess spring energy for the summer.
There's lots to unpack here why this isn't workable at scale.
1) Renewable energy product still has a cost associated with it, even if it is at times, excess.
2) That excess capacity, and the times when there's more energy produced than consumed might not match with water demand.
3) There definitely isn't, and won't be, enough excess renewable capacity to distill even a fraction of the fresh water consumed.
4) This means that you still have to calculate a per kWh cost for the energy consumed to distill salt water to fresh water. The average kWh might not be the same as the market average kWh price, since if you make your distillation plants oversized so you can utilize any spare energy production, but there will still be a price.
5) This price will most likely mean that the per gallon cost of distilled water will be higher than RO, or water pumped through a pipeline.
Desalination is still an extreme measure taken when all other forms of fresh water are cost prohibitive.
With the amount of curtailed energy this year in California, using a state-of-the-art RO process, we could have desalinated about 880k acre-feet of water. This is roughly enough water for all domestic urban water use statewide for about half the year. It is already close to penciling out and our energy resources are still expanding.
> 1) Renewable energy product still has a cost associated with it, even if it is at times, excess.
This cost is already paid for in the infrastructure. You're not going to tear down extra solar panels when demand is low just to reinstall them an hour later.
> 2) That excess capacity, and the times when there's more energy produced than consumed might not match with water demand.
Water can be stored very easily in large quantities and over long periods of time. Replenishing an aquifer in the summer will still help you even when the dry season is winter.
> 3) There definitely isn't, and won't be, enough excess renewable capacity to distill even a fraction of the fresh water consumed.
You don't need to distill 100% of freshwater, you just need to make up the difference between what is naturally available and what is used. The difference is generally small, especially when combined with water conservation methods. California's water shortfall could be covered by using just 6% of it's current annualized electricity generating capacity for desalination.
> 4) This means that you still have to calculate a per kWh cost for the energy consumed to distill salt water to fresh water. The average kWh might not be the same as the market average kWh price, since if you make your distillation plants oversized so you can utilize any spare energy production, but there will still be a price.
You would presumably locate your desalination plant in an appropriate location and operate it at appropriate times such that your cost per kwh is substantially below normal market rate.
> 5) This price will most likely mean that the per gallon cost of distilled water will be higher than RO, or water pumped through a pipeline.
RO would be desalination. A pipeline is still taking water from somewhere else, the price depending heavily on where you're getting it from and the geography between you and the source. In many cases there isn't a suitable freshwater source to pull from. Certainly there are no fresh water sources so limitless and readily accessible as the world's oceans.
I assume this isn't done due to the large cost of building a desalination plant, which isn't paying for itself when it's not running. The money could be invested in something else that runs 24/7.
The water storage might be tough here. Especially in California, there will be a ton of evaporation (which will raise salt levels) and make it less efficient.
Exactly, desalination is very popular in arid countries. With renewable prices trending down, it's getting cheaper too. Basically, don't use consumer grid prices because those still include a fat profit margin for the energy suppliers and their sunk investment in legacy expensive generation using gas, coal, or nuclear. If you are within 40 degrees of the equator, which is where you'd find most arid places, solar is a very good option for generating lots of energy cheaply. Cents per kwh basically, possibly dipping below 1 cent per kwh in the not so distant future. And since you can store water in reservoirs, it's OK to not be desalinating 24x7.
A thousand liters takes about 3kwh. It's not really that expensive. If you run a very inefficient house in the US, that's actually what you'd need per day. You might consider some cost/water saving solutions if that worries you. But, either way, we're talking cents per day per household basically.
Not nothing. But cheap enough that it is a common solution to get water in places that have average incomes far below those common in places like the US where desalination is mostly science fiction.
Focusing on the financial side is okay, though I would mention that the energy cost is only half of the total cost, according to the literature. But desalination is not only about money. It has an impact on its environment, because it pumps fresh sea water and reject brine. These operations have a high cost for the local marine life.
A large desalination plant means large patches in the sea where life is not sustainable.
BTW, if an average US house really needs 1m^3 per day, that's appalling. These past years, my house has used less than 10m^3 per year and per person. 30× less. I'm afraid most US homes will keep wasting drinkable water and pressure society on building desalination plants, rather than halve (at least!) their water usage and protect the environment.
True in shallow waters, a literal drop in the ocean if you pump the salty water a bit further out where it is deeper. The Pacific coast in the US is pretty deep even close to the coast. The Atlantic is pretty deep as well. Of course pumps and pipes cost a bit extra so there is a tendency to cut corners there. But it's not a challenging problem technically.
The thing is, it doesn't have to be. The energy released by mixing salt into water is small, around 3.9kJ/mol.
Molar mass of salt is 58g/mol, and the average sea water salinity is around 3.6%
So a cubic meter of sea water will have 1000*0.036/0.058=620 moles of salt, and it'll require 2.4MJ of energy to remove the salt in a perfect desalinator.
In more common units, 2.4MJ is about 0.75 kWh. Around here electricity is ~10 cents per kWh, so the absolutely lowest price of one cubic meter of desalinated water would be around 8 cents.
> desalinating 35 g L–1 seawater at 50% water recovery has a theoretical minimum energy requirement of 1.1 kWh m–3 and a practical minimum of 1.6 kWh m–3.
SOTA is apparently ~3.7 kWh m-3. That's not a huge factor
> cubic meter of sea water will have 1000*0.036/0.058=620 moles of salt, and it'll require 2.4MJ of energy to remove the salt in a perfect desalinator. In more common units, 2.4MJ is about 0.75 kWh.
> A thousand liters takes about 3kwh.
So, if those numbers are right, desalination is currently at about 25% of theoretical energy efficiency. Is that correct?
Correct me if I am wrong as I am not a physicist. I see a point that is important to consider, that you have potentially overlooked. First, you assume that dissolution of salt is a completely reversible thermodynamic process, which is fine. But considering it a reversible process, in order to reverse the process we need to do a certain amount of work which you have calculated. In order to do work we need an engine. The most efficient possible engine is a Carnot engine. It is known that a Carnot engine can never be 100% efficient (unless we can achieve infinite or zero temperature). Given that you calculated the amount of work needed to reverse the process, you still need to bound the efficiency by the efficiency of a Carnot engine. Alternatively you need to factor in the efficiency of a Carnot engine to get the minimum required energy input.
You are correct. Although technically, dissolution is not a reversible process. That's why you need to input energy to reverse it.
Carnot cycle, technically, doesn't apply to all energy sources directly.
For example, solar panels have their "hot side" at around 6000K, so Carnot efficiency would be close to 100%. Real solar panels have other limiting factors, and I believe the absolute achievable theoretical maximum is around 80%.
On the other side of the spectrum, wind turbines have very lousy Carnot efficiency because they're exploiting a temperature difference of just a few degrees. However, the "Carnot tax" is not paid by us directly, so we don't really care about it.
According to the paper [1] he talks about in the video, the theoretical limit is a function of %salt removed and %waste-water.
For 90% salt removal with 50% waste-water, they say the limit is 1.09kWh per cubic meter (3.924 MJ)
NB: It is not 100% clear to me if the result is independent of the type of technology, but they do claim:
> We first derive the general expression of the thermodynamic minimum energy of separation determined by the Gibbs free energy, which is independent of the method of desalination
Most raw material processing involves some separation process which requires energy. First, there's gathering something that contains some of what you want. Then there's a phase that often involves breaking big stuff into little stuff and some mechanical separation of easily removed crud. Then there's some chemical step, such as smelting, leaching or distillation, which takes energy and feedstocks to pull the good stuff out of the bad stuff. Then there's getting rid of the bad stuff, which is the source of most industrial pollution. Now you finally have something that's mostly what you want, and go on from there. From desalinization to iron making to fertilizer to oil production, the front end looks like that.
All of those processes are energetically uphill, and all are routinely done on huge scales.
My point is, we shouldn't assume that fighting entropy isn't necessarily expensive. There's always a cost, and it may in fact be high, but we shouldn't assume it is.
Solar stills are one of those basic survival tools anyone living in an arid region near a salty body of water should understand how to rig. They’re dirt simple and as easy to build as a prison still with the added bonus of only requiring the sun as an external power source.
Similarly everyone should know how to rig a basic water purification system using gravel, sand, and charcoal in series.
Even just as applied science experiments to do with kids they’re worthwhile.
Edit: Water based solar power is generally an area I think that deserves more research. While photovoltaics have their advantages, water is cheap, clean, and reliable. Heating water with sun during the day and using it for household heating at night is the simple application that I’m most familiar with, but I wouldn’t be shocked if there’s some scale where an economically interesting Carnot cycle becomes possible.
I work in microfiltration (a pre-filter step for RO), and my view is:
1/ it’s as much an energy and water storage problem as it is a technical problem.
2/ commercially, because of 1/, RO is a municipal sale. It is a civil initiative, rather than a commercial one, which means it gets crowded out by other civil decisions.
The author have mentioned that water with more than 1-2 promille of salt is considered not drinkable. But I have a bottle of carbonated mineral water from the mountains and the label says it contains 6-9 g of salts per 1 liter. So I guess it depends on which salt it is.
Also, the author mentions that people in US use 1100 liters per day (which is too much in my opinion), but not all this water needs to be drinkable, one probably can not drink more than 3-4 liters per day, and the rest of the water can be salty.
Having a diversity of water supplies and using water fit for purpose reduces demand for drinking water. Toilet flushing, irrigation and even washing machines do not need high quality drinking water. I have 5,000L of rainwater storage that I use for toilet flushing and irrigation. Combined with a water efficient shower head (typically the largest domestic water use in my country) we use 100 L/d/person. In some areas of my city there is dual supply plumbing that delivers highly treated wastewater for these uses.
This one time on an episode of Survivorman, Kalahari iirc, I watched Les Stroud use a hole in the dirt and a clear plastic tarp covering it to act as a solar still. The condensation from his pee in said hole ran off to a collection container as pure H20 once evaporated.
I just remember thinking to myself "Bear Grylls drinking his own pee is such a philistine, here is an actual pro using science to remove all the water from that pee instead first." Genuine moment of awe personally.
RO does use a lot of energy to overcome the osmotic pressure and to create flux through the membrane. An interesting concept is using the reverse process, "forward" osmosis, to extract the energy where fresh water mixes with seawater, such as a river mouth. This is called pressure-retarded osmosis (PRO) and was tried at pilot scale by Norwegian power company Statkraft. Ultimately this trial was shelved due to being uncommercial [1], perhaps future membrane development will improve the viability of FO. And yes the membranes are quite different, RO membranes are relatively thick due to the transmembrane pressures required. FO requires a much thinner support for the active layer as there is no external pressure applied to push the water through (it is drawn through by the difference in salt concentration).
One thing that this article missed was that it was San Diego centric. In Israel, desalination is a much bigger part of the ecosystem. Over half of its domestic water comes from desalination. Quite a bit of the problem in California, as in almost every industrial application, is just that we make it hard to do anything with atoms.
Given the countless environmental challenges we are facing (and causing), we should more seriously and openly consider putting a stop to exponential population growth as an (at least short-term) solution.
It’s astonishing how some people preach blind faith in our ability to just find solutions for problems caused and exacerbated by never-ending population growth without identifying it as the root cause. Why is it a given that the earth can just withstand whatever we throw at it?
I think he fully gave the wrong answer. The main problem with desalination is capital cost. The Carlsbad plant cost one billion dollars to make. I bet very little of it is the cost of membranes, or the actual RO systems. It's simply a large plant, and building a large plant is expensive. The same problems that plague large nuclear power plants plague many other large construction projects, including large desal plants.
I like the analogy at the end regarding nuclear power vs piping in water from freshwater sources. The upfront costs are high, but over time it would be cheaper than desalination, but due to short term governments and borders, it's hard to justify the upfront costs. So instead we are somewhat stuck with more expensive short term solutions.
I’ve got an idea guys! Let’s build some artificial lakes and we can just let rain and river water pool in them! We can let natural precipitation do the desalination for us!
I was never able to find the publication again but I read an article one time about the work of 2 Australian professors (I forget which kind) who designed a giant circle shaped verical axle wind turbine with 2 tubes that suck sea water up along the sides and sprays it up in the air with the brine dropping near the turbine. An array of those they claimed could create dense clouds that produce a lot of rain at some (according to them) surprising ratio to cost.
Late to this thread but I have always asked why nuclear powerplants can't use seawater for cooling and the condense the steam to create desalinated water?
Ocean waves can be used to create electricity and it is also possible to create a dam that uses an artificial river sourced by the ocean and make hydroelectric plants to use that energy. Is it a cost issue in both cases that prevents using electricity generated that way to desalinate?
If you built a desalination system say… 500 feet under the ocean and have the pressure above pushing water through the filters, is it possible to lower the amount of required energy just a little? Then you’re more pumping water out of the system than pumping it through heavy filters
Wouldn't the energy of pumping the water all the way back up to the surface completely balance out the energy provided by the weight of that water to push it through the membrane?
For a blog about "practical engineering", it doesn't address the fact that in practice, saltwater isn't just "water and salt" but also all kinds of particules, pollutants and impurities that will clog any membrane after a while.
Difficult is not an absolute. It needs to be "in comparison to something".
Desalination is verrrrry easy if nature does it for us (sun on the salty sea -> clouds -> precipitation over land). Compared to this, doing it "with a machine" is hard.
Stupid question time, why not expose the sea water over a larger area and expose to sunshine? There's plenty of room in the UAE for this, somebody is even building a wall that could hold the water which could be dual purpose and act as cooling via evaporation...
I mean basically you're describing a solar pond (https://en.wikipedia.org/wiki/Solar_pond) or salt evaporation pond (https://en.wikipedia.org/wiki/Salt_evaporation_pond). There isn't an easy way to recover the evaporated water from that process; you would effectively have to build an artificial enclosed environment and it wouldn't work as fast as exposure to the elements. Basically you would need a still (as in for alcohol) the size of 20 football fields. But sun power wouldn't be enough to power it fast enough to be practical. Maybe if you moved it into outer space...
Search for OTEC (Ocean Thermal Energy Conversion). Not necessary to build wall, just a big enough pipe to collect cold water from the depth. Then a special contraption will yield pure water from evaporation. Can be done virtually everywhere there is a deep enough ocean.
This is how it is solved everywhere... but now you need to ship all this desalinetad water. And surprise! it's the second the most expensive part in the process.
You could use simple pricing to influence behavior. 1100 litres a day for each American is so damn much. When you hike and stay in the mountain huts you are charged 3$ for a 4 minute shower.
You could probably fix the drought situations by reducing consumption.
Should heat and cool it with standing soundwaves. Salty water takes the heat, cools and falls (brine-fall) rest is less salty.add membranes at intersection points..
The worry is a local increase of salinity levels, due to putting the brine back in the ocean. I think that is a solvable issue though. We can harvest some of the salt and then mix the resulting brine with reclaimed water. Then spread that out instead of dumping it in one area. We could also create inland salt deposits.
The biggest issue is cost for these plants and it isn't worth spending the political capital to build them yet.
Then all I need to do is desalinate drinking water.
“Distributed” home desalination for drinking water seems like the best approach in my mind, then people can pay as much or as little they need, but I have no real data to back this up.
I'm not even remotely knowledgeable about this, but I'd assume saltwater would wreck plumbing. The connection in and out would probably be degraded much quicker. Then the water treatment plants would have to deal with dirty salt water, which is probably more difficult.
On top of that all the brine that people produce in their homes would have to be disposed of, and I'm sure many people would just end up flushing it down the drain. So the water treatment plant would have to deal with highly concentrated, contaminated saltwater.
Interestingly, there is one house that I know of with both hot and cold freshwater plumbing as well as hot and cold salt water plumbing: the Breakers mansion, built by the Vanderbilts. I'm sure they spent fortunes maintaining that plumbing and think the tour guide said something to that effect, but everything was a show of wealth there. One room featured platinum wallpaper, because, why not?
Having worked on and plumbed boats, the bigger issue is actually growth and especially mineral deposits. Corrosion is less of an issue since most plumbing is actually plastic at this point. Although any water that goes into an appliance needs to be fresh water, so it would really ONLY be for showering.
Sewage in particular will create hard deposits in plumbing that needs to be dealt with every few years at a minimum.
Frankly, unless you are in a rather extreme environment, like a desert, or a boat where you have to carry or make all your own fresh water, saving a few gallons on showering and washing is pretty inefficient. You could have a far larger impact by changing habits, and ensuring low flow appliances.
Yes, you can shower in salt water, but you won't feel as clean in the end. You also use water for washing clothes and dishes, as well as washing your hands, watering plants, and various other tasks, for which salt water is unsuitable.
yeah but I usually feel quite refreshed after coming out of the waves, but keep in mind we don’t need clean feelings to survive, we do need fresh drinking water
You're likely better off treating and recirculating the same fresh water for bathing. You can also potentially save energy on heating the water with a proper setup.
I've been wondering if one solution is having dual plumbing in some houses, that includes a parallel system for non-drinkable (but otherwise clean) water.
Insightful. Have we been attacking the problem the wrong way? Gene editing is another field, in the future perhaps we'll see edits to the kidneys to save water.
> 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.
Yes, RO is energetically the best, but places with geothermal excess, like iceland might choose the heat based method as they have free heat = the still warm water discharged after power still is warm enough for distillation.
It's very easy to forget that we use orders of magnitude more water than comes out of our tap. A pound of beef is about 2,000 gallons, so that would be a nontrivial price increase.
Or maybe such water-intensive food production should require consumption of the planet's nearly unlimited seawater. Pass the greater expense to the cattle producers and/or consumers. Leave the planet's relatively limited freshwater sources for direct human consumption, or food production that's more sustainable, or other urgent and necessary activities. The beef lobby may not agree.
Israel desalination plant profitably offering a fixed price of 1.45 NIS per cubic meter. At current exchange rates that is around 0.40$ per cubic meter (1000L).
I don't think he gave any substantiation to the claim then either. (He dismissed delivery of clean water to the world as a problem not interesting enough for him to spend time on.)
Just look at this thread for price estimates. $2-5 is pretty reasonable, even assuming a cost breakdown with energy at 10% and capital/infrastructure at 90% of the final cost.
Some countries have floods in one part, and drought in the other.
The floods are so bad that large numbers of people die.
Here's a great challenge that should be worked on: how to capture the flood water and use it to mitigate the droughts. Could something like Elon Musk's Boring company concept fix this?
People are constantly talking about using pipes to solve the US desert southwest's water crisis, as if building a water pipeline for thousands of miles isn't some gigantic logistical and engineering nightmare -- as well as a bureaucratic nightmare, since most places with fresh water don't want people in the desert literally sucking them dry.
I am not a physicist but let me give it a stab: except for a few specialized steps like UV or oxidizing heavy metals, most filtration is mechanical. A series of filters with smaller and smaller pores capture more and more of the mess in the water like bacteria and particulates while UV breaks down viruses, the oxidizer precipitates out metals, and so on.
None of those methods work with salt. Salts in general disassociate through ion-dipole interactions - the water dipoles essentially rip the ionic compound apart and surround each ion in what is called a hydration shell. They're bigger than bare water molecules but not much bigger - much too small to target with pore size. This shell also puts them in a thermodynamically stable state and it takes energy to "jostle" the water molecules away from the ions either through evaporation, distillation, or through another chemical reaction that precipitates out the ions.
As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of.
Someone please correct any mistakes I've made