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.
Why is desalination so difficult?
401–410 of 644 posts
Re: Why is desalination so difficult?
#402I 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-Sta…
If we could make cheap small nuclear reactors I don’t think drinking water availability would even be a thing we talk about near coasts.
It seems as though many problems get solved once the cost of energy approaches 0.
Re: Why is desalination so difficult?
#403Earlier quoted context omitted.
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. https://ourworldindata.org/grapher/water-withdrawals-per-cap... 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…
> 1550 cubic meters per year 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.
Re: Why is desalination so difficult?
#404For 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.
This is discussed at ~12:10.
Re: Why is desalination so difficult?
#405For 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.
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Re: Why is desalination so difficult?
#406Earlier quoted context omitted.
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…
Double the salinity? Other comments suggest it’s only slightly more salty than sea water. Only a very small fraction of water is isolated from a very amount of sea water.
The article states
> 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.
Re: Why is desalination so difficult?
#407Earlier quoted context omitted.
Nuclear power in general would be the answer to a lot of energy problems. But the general public in most places doesn't like it.
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…
> 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?
Re: Why is desalination so difficult?
#408Earlier quoted context omitted.
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…
> 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'…
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.
Re: Why is desalination so difficult?
#409Earlier quoted context omitted.
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 goe…
> 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…
> the freshwater plume of the Amazon River extends more than 60km into the ocean.
It really doesn't, or perhaps only in the most pedantic sense.
That "ocean" is still the mouth of the Amazon itself, and most of it's less than 10 meters deep. It's really the estuary of the river itself. See: https://www.frontiersin.org/articles/10.3389/fmars.2017.0002...
Re: Why is desalination so difficult?
#410Earlier quoted context omitted.
> 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'…
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 memb…
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?