Water is truly a near-infinite resource. If we can master desalination then humanity is in a great spot in regards to fresh water.
It also frames the challenge well. Desalinating a cubic mile gives you 120 million tons of leftovers. Another extremely difficult challenge.
The world's freshwater need is about 950 cubic miles a year. (https://www.wolframalpha.com/input?i=worldwide+water+use+in+...)
You can just put the leftovers back without worrying much about it.
Yes, just not all in one place/time. Separating seawater into pure water in one place and pure brine in another, you don’t want to let that brine out all at once in one spot, it’ll kill a lot of ocean life. Most desal plants that are attempting to do this right, will pump the brine into pipes that diffuse it over a wide area to avoid oversalinating. And it still kills a lot of ocean life.
Or we could have a premix station where we pump in sea water and mix it with brine at a certain ratio and then return that to the sea.
And with your second process, you still end up with a higher concentration of salt / brine around where it's returned to the sea, still killing animals.
I'm thinking of pumping the brine onto large evaporation lakes and harvest the salt or whatever - which also already happens to produce sea salt.
And if there's too much salt for the market, just stockpile it. Like underground salt mines.
The big problem with making salt from sea water is evaporating the water. It either takes a lot of energy, or a lot of time and area.
It takes a lot of time for the salt to just diffuse out over the ocean.
I think the best way to pay for X, is to use energy from an external source. External from the Earth. I'm obviously talking about the Sun.
We could use the removed material as a general filler. I'm sure we can get creative about it. I think the problem is lack of incentives and misalignment of goals amongst people.
I truly think if we can figure out how to how to use the most out of the Sun's rays - i.e. as most directly as possible - we will solve all our needs. The energy is truly free to the Earth.
There's a thermodynamic fundamental lower limit on the amount of energy needed to desalinate water, but it's absurdly small. Much smaller than the amount of energy we use in practice with current technologies.
(Just like there's a thermodynamic lower limit on how much energy a computer needs. But it's also extremely low. See https://en.wikipedia.org/wiki/Landauer%27s_principle )
Edit, found in another comment : For example, desalinating seawater with a typical seawater salt concentration of 35 g L–1 (corresponding osmotic pressure of 29.7 bar) and 50% water recovery (i.e., 50% of the feed stream becomes purified water and 50% becomes brine) requires at least 1.1 kWh per cubic meter of purified water. Regardless of the desalination technology, it is impossible to desalinate water using less energy than that determined by eq 12.
1.1 KWh per cubic meter is very much NOT a negligible amount, so the landauer analogy is incorrect.
I stand corrected in that case.
With all the chemical processing that would be needed to stabilize the salts, mechanical filtering and such, I think we're better off continuing to use bricks and ground sourced gravel and cement. At least the holes we dig can be repurposed into sanitary landfills.
Regardless, a polyp adds about 1mm to 1cm to the reef a year. You can get that right now just by throwing a shovel at the ground where I live.
Sodium is potentially useful towards two applications, off the top of my head. (1) Na2O is used in glassmaking, and it's possible that there are -- or that there can be discovered -- Na2O-enriched glasses that can be used in construction and as a filler substance, i.e. reduced to powder and added to cement. (2) Sodium-based zeolites can potentially be useful for carbon capture. Production of zeolites, however, also requires lots of alumina and silica.
I struggle to think of any large-scale application for all of that chlorine, though. Maybe vinyl chloride production? But the world doesn't need that much PVC...
Israel even turned into a water exported thanks to the technology.
See eg https://www.timesofisrael.com/how-israel-became-a-water-supe... and https://blogs.worldbank.org/water/israel-how-meeting-water-c...
Don't forget that we actually mine salt, a lot of which ends up in the sea. A million years from now we might regret that ;-)
https://www.energymonitor.ai/tech/can-desalination-save-a-dr...
A quick search popped up https://pubs.acs.org/doi/10.1021/acs.jchemed.0c01194
For example, desalinating seawater with a typical seawater salt concentration of 35 g L–1 (corresponding osmotic pressure of 29.7 bar) and 50% water recovery (i.e., 50% of the feed stream becomes purified water and 50% becomes brine)
requires at least 1.1 kWh per cubic meter of purified water.
Regardless of the desalination technology, it is impossible to desalinate water using less energy than that determined by eq 12.
There are so many examples in our collective technical history of overcoming these types of limitations not with brute force, but with finesse.
In this case, I expect it will be a combination of improved pumps, improved RO membrane technology, and finding synergies like making sea salt from the brine, collecting other useful minerals front the brine,etc. All of these things help to pay for the energy and development costs.
It's not a hard physics limit like the rule if squares or the speed of light. It's a complex engineering system that has may different dynamics and interactions between those dynamics, all opportunity for improvement.
Trying to pull gold out of the ocean to pay for pulling salt out of the ocean is --again not a chemist but-- probably thermodynamically worse.
Of course I'm not saying we need to break the laws of thermodynamics, in my house we obey the laws of thermodynamics.
In this case, the work needed is defined by the features of the RO membrane. It's conceivable that we could develop a RO membrane that requires less pressure or energy to operate. In fact they have been.
In that case, we would gain a more efficient process, while still obeying the laws of thermodynamics
It could easily be the case that RO is going to see only marginal improvements for the next decade or two (except perhaps some test membranes that are too expensive), and modern RO already uses energy recovery in the process.
Equation 12 is the energy you need to counteract the effect of the entropy of the ions in solution and separate the initial solution into one that does not have the ions. It’s pretty much a thermodynamic limit and does not depend on process or technology.
They explicitly assume that they have a perfect membrane when they introduce the equation. The floor will never be zero, it is a physical limit.
The energy required happents to go into separating the bonds between the salt ions and the water molecules. Those bonds are quite strong, so it takes some energy to break them.
The engineering problem is more one about capital investment. The price of water, vs the price of electricity are not what block desalination. It is the cost (and maintenance) of the machines that you need to earn back. That cost is what tends to make desalination un-economical. And that is an area where engineering has a lot of space to improve.
I'm confident that in our generation we'll see mass migrations due to water shortages in the west. California is already at risk, I gathered.
It's not the water that is in the wrong place. People are not supposed to live in places that can't sustain their numbers.