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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#71

Post by at_a_remove »

akiselev wrote: post #47

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. N…

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.

Re: Why is desalination so difficult?

#72

Post by LordDragonfang »

nerbert wrote: post #34
fwlr wrote: post #29

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…

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...

>85% of the country's drinkable water

"the country" in this context is Israel, since HN truncates the wiki link right before that.

Re: Why is desalination so difficult?

#73

Post by dghlsakjg »

beembeem wrote: post #59
bobbean wrote: post #55

Earlier quoted context omitted.

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 d…

Bingo. Corrosion. Everything would need to be "marine-grade" aka expensive to plumb :)

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.

Re: Why is desalination so difficult?

#75

Post by jeffbee »

Maxion wrote: post #63
jeffbee wrote: post #58

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 m…

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.

Re: Why is desalination so difficult?

#76

Post by LordDragonfang »

cratermoon wrote: post #26
zokier wrote: post #9

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?

A desert is a living ecosystem. Evaporating salt water would leave behind substantial salt solids. Think Great Salt Lake.

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.

Re: Why is desalination so difficult?

#77

Post by gamegoblin »

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".

[1] https://en.wikipedia.org/wiki/Desalination#Costs

Re: Why is desalination so difficult?

#78

Post by tibbon »

emptybits wrote: post #74

"Turning sea water into clean, drinkable water costs $2 to $5 for 1000 gallons. Less than half a penny per gallon is obviously absurdly cheap." - Elon Musk, 2023-05-07 https://twitter.com/elonmusk/status/1655262008898383872?ref_...

I'm looking at this and unclear what he's talking about. Essentially, citation needed Elon?

Re: Why is desalination so difficult?

#79

Post by tibbon »

luxuryballs wrote: post #35

Can I shower in salt water? 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'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.

Re: Why is desalination so difficult?

#80

Post by tln »

cyberax wrote: post #31
Animats wrote: post #3

It's not difficult. It's just energetically uphill. Undoing entropy costs energy. Second Law of Thermodynamics.

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…

The article / video mentions a paper discussing theoretical minimum.. I think it's this paper: https://pubs.acs.org/doi/10.1021/acs.jchemed.0c01194

> 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

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