Submitted by MiloFrank76 t3_10paoln in explainlikeimfive
Every few years, you hear about some new revolution in the process, but nothing much after. Why is that? Is it very hard to scale up or...?
Also, is it going to become more viable as fresh water is becoming more scarce?
Thank you in advance.
Thank you all. So it was basically what I thought. It's not hard to do, but it is not really feasible due to many factors.
Are not some power plant near the sea for heating the water?
That explanation is good but I think it underestimates just how much energy it takes to evaporate water, especially at scale. Yeah, you can use a power plant but we're already struggling to generate the power that we need in ways that aren't destructive to the environment.
In any real world plant that operates via evaporation, most of the energy used to boil off the water is re captured via the heat exchangers used to condense the steam back into water. This heats the incoming water. (Most are multi stage running different parts at different temperatures/pressures.) Overall its a pretty efficient process, with reverse osmosis a bit more efficient.
However the amount of water that we use for not drinking purposes, irrigation, washing e.t.c is massive.
Most power plants do use water in their power generation, but it's largely a closed loop and freshwater is used. Fossil fuels/trash/radioactive decay is used to heat water which evaporates into steam and turns turbines, then cools and gets used in the cycle again.
If you decided to use saltwater in this process, the salt would be super super hard on the powerplant, piping, etc and dramatically reduce the lifespan and increase the maintenance costs of the power plant. You also wouldn't be able to generate water safe for use, as the water would likely be contaminated. You're better off with a dedicated power plant and a dedicated desalination plant.
Finally, you still need to figure out what to do with all that salt. If you dump it back in the ocean, you're going to kill off wildlife en masse and just generate more and more salt over time, as the salt concentration in the water you're pumping in increases. You can't dump it on land, it will completely destroy the ecosystem and you will need a TON of space. Unlike landfills, it won't decompose over time - just wash away (which means you're killing things elsewhere). You could potentially use old mines and such, but those aren't reliably structurally sound and you're still risking runoff or even worse, getting massive amounts of salt into the water table.
Regarding "what to do with the salt" wouldn't we be able to just use the salt from desalination plants and have that replace salt mining?
Obviously the concept is killed before even getting to this problem but I'm just wondering.
Most large scale desalination is reverse osmosis, which produces a very salty brine rather than solid deposits of anything. Further processing and refining of minerals from this would also be very resource intensive as well as complex. There is a lot of research going on in this area as well.
No. They are near large bodies of water for cooling. The ways which is turned to steam is rather pure water to start with.
Thank you. Do you think the need for fresh water increases in the future, that we'll figure out something?
The only thing I can figure out is to conserve water or eat less meat which uses more water per edible calorie compared to vegetables.
It isn't so much that it's hard to do, but it's hard to do cheaply enough to make it economically feasible. Most methods take a lot of energy, others are relatively slow.
In every case though, desalination on a large scale leaves you with all the salt and minerals that you remove from the water. It's difficult to find somewhere inexpensive to dump those without it screwing up the local ecosystem.
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Desalination isn't particularly hard at a small scale.
The issues with desalination - the area around the plant gets saltier than the rest of the ocean (you have to dump that excess salt somewhere,) which kills all the marine life in the vicinity of the plant. Also, humans need a ton of water, so the plants need to be very large to fulfill all the domestic demand. And since the plants are at sea level, you need large pumps to get all that desalinated water into pressurized pipes - which means you need a lot of power.
Exactly.
Ion-exchange resins are also used, but you need to change them regularly.
So, ultimately , it is not a technical problem, but an economic one.
It's not hard, in the sense that it's not difficult to do in principle. It's just energy-intensive and therefore expensive, and it requires facilities that are themselves expensive.
I wouldn't say it is hard more than it's energy intensive.
Even using the Sun's heat--a solar still-- which is free, takes quite a bit of energy: I guess it depends on scale as well.
To provide potable water for a medium sized city needs a lot of energy. Then, what to do with the salty sludge leftover.
Do you return this to the ocean? It will make the dump site toxic. Do you store all of it somewhere on land? Again, it'll make that area toxic to plants and you don't want rain bringing that salt to the water table, contaminating it.
>Then, what to do with the salty sludge leftover.
Clean it up and sell it as table salt or leave it dirty and sell it as road salt for the winter (of which there is also currently a shortage)
Sure, great ideas.
Besides sodium chloride, there are other salts as well:
Chloride (Cl-) 18.980 21.200 23.000 22.219 Sodium (Na+) 10.556 11.800 15.850 14.255 Sulfate (SO42-) 2.649 2.950 3.200 3.078 Magnesium (Mg2+) 1.262 1.403 1.765 742 Calcium (Ca2+) 400 423 500 225 Potassium (K+) 380 463 460 210 Bicarbonate(HCO3-) 140 - 142 146 Strontium (Sr2+) 13 - - - Bromide (Br-) 65 155 80 72 Borate (BO33-) 26 72 - - Fluoride (F-) 1 - - - Silicate (SiO32-) 1 - 1,5 - Iodide (I-)
Read more: https://www.lenntech.com/composition-seawater.htm#ixzz7rwm3A92W
Those are dissolved solids in mg/L. Did you notice how the top 2 are literally table salt? Potassium Chloride is NuSalt (a type of table salt), Magnesium Chloride is valued in industry, Flouride is valued in industry (especially dental products).
The only things I see in this list that are problematic are the Strontium and the Bromide, and I'm sure somebody has a use for them. This is really just an entrepreneur's opportunity waiting to happen
It takes a lot of pressure and therefore energy to pump salt water through salt-removal membranes.
Desalination is primarily done through high pressure filtration, no by phase change. It is more energy efficient, but still extremely demanding.
Also, the super salty brine needs to be disposed of, and can't just be dumped into one spot in the ocean or it creates a dead spot. You gotta pump it out into the open water in a current...
Basically, it's expensive. So you don't do it unless you absolutely must.
I operate in the climate space but am not an engineer or expert in water resources. So remember that as I give my understanding. It is very expensive. It uses a lot of energy. And it changes the salivation of ocean water around it which probably is not a good thing. The salt must go somewhere. And it can mess with groundwater. It is a solution of last resort.
The primary and least sexy way to begin the fix is conservation in a serious way. No more golf courses. No more home lawns bigger than x. Make wicked sure agriculture is using water in a sensible way. This is very hard but crucial - tackle the water rights monster and make it realistic and sensible. And a culture change. If your clothes aren’t dirty you wear them multiple days before washing. No washing hair everyday. Maybe a shower every few days with quick ‘army’ cleanups everyday. I know this sounds weird or far fetched but it is these type of mind shift that will help us manage in days ahead.
Thank you for this perspective.
California has strict rules about discharge and pollution. And there are desal plants. Their brine — which they don’t want you to call brine btw — gets pumped into a pipe that extends far out into the ocean, and has lots of holes in it so the salinity is dispersed over a wide area.
It is not hard, it is just very expensive because of how energy intensive it is. When it comes to anything, but especially water, cost is king, in regards to mass adiption.
If the water cost is double to irrigate crops, that is a big problem.
It's not hard or difficult. It's just that clean water is really really cheap under usual circumstances. So using electricity to make clean water usually ends up costing more than the water is worth.
Right now, in most parts of the world, there isn't an actual shortage of fresh water. The problem is managing and distributing the water that is available there, and managing the usage. So it's generally cheaper to buy & transport water, or reduce usage, than to build and operate a desalination plant. There are some exceptions, like some newly built cities in the Middle East.
This question was prompted by watching a show about Hawaii. So, yes, it is poor management, but that's where it came from.
Essentially, desal turns energy into water. You’re right that mass media has this tendency to write stories about breakthroughs, but they’re either tiny incremental improvements, or baloney.
In places that have almost no other water, like Israel, it makes sense. But if you have sources of water that cost half as much, it’s often cheaper to save a gallon of cheap water than it is to generate a gallon of desal water. So in places like Santa Barbara or northern San Diego County, it’s more like a last resort, the last water source they draws upon.
Desal is, at its heart, forcing water through a very fine filter, a filter so fine that it lets little more than water molecules though. The sort of pressure you need to force water through the filter is like pumping the water to a tank on a 1500’ tower. Which is doable, but at the scale of a city of 100,000 people it would be crazy expensive. At least compared to other cheaper sources of water.
There are consumable costs to an RO desal plant too, such as the filters themselves. They have to be replaced after a while and they’re not cheap. It’s great if those costs come down but they’re marginal compared just to the energy required to filter every single gallon of water. And that cost is really set by the global energy market.
Distillation and filtering are both energy expensive. So is electrolysis. If it weren’t we could produce hydrogen gas cheaply, run hydrogen fuel cells to generate electricity, and generate water as a waste product. There’s cheaper sources of hydrogen though, and a fuel cell bus emits just a tiny trickle of water from its exhaust. So if you try to use electrolysis to generate hydrogen to run a hydrogen fuel cell to power your electrolysis, it’s not going to give you free energy. It takes as much energy to break those bonds as is released when you form them.
Part 2: Santa Barbara famously built an expensive desal plant in the 90s which was promptly decommissioned because it started raining again. They’ve refurbished it and brought it back on line now.
If a city has multiple sources of water, they can turn to desal (or reverse osmosis of wastewater even) if they are running short. But the initial costs of building such a plant run into the hundreds of millions of dollars. Which raises the cost of water. And guess what? After a certain point necessary for cooking and hygiene, water consumers will cut back on water use if the cost rises. So a city could build a desal plant, then raise rates to pay for it, only to push their customers into conserving enough that they don’t have to run the desal plant. They could’ve just raised rates to manage demand, and not built the plant, saved $300 million, but in places like California it’s illegal to set rates like that.
A lot of California cities, 25%+ of water is used on lawns. That’s the culture after decades of underpriced water. Charge something approaching the real price, and people suddenly don’t want to be responsible for large swaths of ornamental sod any more. Places in the Middle East need desal. Most places in the US would have enough water if it was priced appropriately.
dkf295 t1_j6ja1cf wrote
Desalination itself is super easy. Take salt water, evaporate and recondense it to separate the salt and other minerals/etc from the water. You can do it yourself on a stovetop or a campfire.
The problem is that there's no way to scale it easy for the massive demands of human use (personal, industrial, and agriculture). It takes a decent amount of energy to heat up water, and you need to do something with all of the salt you've got left over - no matter where you dump it, you're going to cause environmental problems. Salt is also corrosive so there's longevity problems with equipment/piping/etc but those are relatively minor problems with partial solutions. You can't ignore physics, or just make giant piles of salt disappear.