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.
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".
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
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.
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?
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...
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.
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.
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?
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.
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.
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.
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.
[+] [-] akiselev|2 years ago|reply
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
[+] [-] gamegoblin|2 years ago|reply
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".
[1] https://en.wikipedia.org/wiki/Desalination#Costs
[+] [-] bsder|2 years ago|reply
What it does not do is provide freshwater for argibusinesses. As California is also proving in the Central Valley. :(
[+] [-] Nifty3929|2 years ago|reply
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
[+] [-] iudqnolq|2 years ago|reply
Don't forget food, industrial, etc
[+] [-] Zetice|2 years ago|reply
He says it’s viable for many applications.
[+] [-] weaksauce|2 years ago|reply
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.
[+] [-] codedokode|2 years ago|reply
[+] [-] whiddershins|2 years ago|reply
[+] [-] yhgxfvx|2 years ago|reply
[deleted]
[+] [-] BeetleB|2 years ago|reply
I assume you meant per year?
[+] [-] dstainer|2 years ago|reply
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.
[+] [-] aeonsky|2 years ago|reply
[+] [-] beembeem|2 years ago|reply
[+] [-] fwlr|2 years ago|reply
[+] [-] nerbert|2 years ago|reply
[+] [-] MarkusWandel|2 years ago|reply
[+] [-] jtriangle|2 years ago|reply
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.
[+] [-] jeffbee|2 years ago|reply
[+] [-] egberts1|2 years ago|reply
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.
https://news.mit.edu/2020/passive-solar-powered-water-desali...
[+] [-] zokier|2 years ago|reply
[+] [-] jeffbee|2 years ago|reply
[+] [-] Maxion|2 years ago|reply
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.
[+] [-] pornel|2 years ago|reply
[+] [-] adgjlsfhk1|2 years ago|reply
[+] [-] Animats|2 years ago|reply
[+] [-] jillesvangurp|2 years ago|reply
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.
[+] [-] cyberax|2 years ago|reply
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.
[+] [-] dahfizz|2 years ago|reply
[+] [-] delecti|2 years ago|reply
[+] [-] jonny_eh|2 years ago|reply
[+] [-] vl|2 years ago|reply
[+] [-] unknown|2 years ago|reply
[deleted]
[+] [-] User23|2 years ago|reply
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.
[+] [-] unknown|2 years ago|reply
[deleted]
[+] [-] photonerd|2 years ago|reply
[+] [-] flybrand|2 years ago|reply
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.
[+] [-] codedokode|2 years ago|reply
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.
[+] [-] reyoz|2 years ago|reply
[+] [-] TheDudeMan|2 years ago|reply
[+] [-] im3w1l|2 years ago|reply
[+] [-] BenjiWiebe|2 years ago|reply
[+] [-] nullcontext|2 years ago|reply
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.
[+] [-] reyoz|2 years ago|reply
[1] https://www.powermag.com/statkraft-shelves-osmotic-power-pro...
[+] [-] ejz|2 years ago|reply
[+] [-] agnosticmantis|2 years ago|reply
[+] [-] credit_guy|2 years ago|reply
[+] [-] 1letterunixname|2 years ago|reply
1. Steam distillation + Product can be perfect DI type 1 water - Expensive: 300+ kJ/L
2. RO membrane + Cheaper - Slow - Wastes more water - Requires regular changing of membranes
The end.
[+] [-] leecarraher|2 years ago|reply
[+] [-] narag|2 years ago|reply