
Scientists cook up new recipes for taking salt out of seawater - vinnyglennon
https://phys.org/news/2019-07-scientists-cook-recipes-salt-seawater.html
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fghorow
Having fancy chemistry as in the article is great, but there already exist
thermal desalination solutions that require heat (e.g. geothermal, industrial
waste heat) rather than work (e.g. electricity for R.O.). For example MED:
<[https://en.wikipedia.org/wiki/Multiple-
effect_distillation>](https://en.wikipedia.org/wiki/Multiple-
effect_distillation>) or MSF: <[https://en.wikipedia.org/wiki/Multi-
stage_flash_distillation...](https://en.wikipedia.org/wiki/Multi-
stage_flash_distillation>).

Steam is _not_ required for MED. Hot water < 100C is adequate. Many ocean-
going vessels already use the waste heat from their engines and MED to make
freshwater from seawater.

ORC driving RO seems to be most efficient in some situations
<[https://www.mdpi.com/1099-4300/17/11/7530>](https://www.mdpi.com/1099-4300/17/11/7530>),
but MED is a simple, no-moving-parts solution that is also efficient.

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ChuckMcM
This is a great paper, the really cool bit is that it seems fairly straight
forward to convert this to a larger process. Basically add saltwater, warm it,
take the settled part of the solution (which is now desalinated) out of the
bottom, and the salt dense water out of the top, cool it and precipitate out
the salt. Nothing has to evaporate everything runs at a normal vapor pressure
so you aren't struggling with high pressure and low pressure domains.

I'd love to see if you could use spent nuclear fuel (which is an excellent
heat source) to just run one of these things continuously for a couple of
decades :-).

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sayusyz
"The current state-of-the-art in RO desalination works very well, but the cost
of RO desalination driven by electricity is prohibitive,"

Why is taking salt out of sea water such a challenge?

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Retric
Desalination is cheap in day to day terms, it’s the scale of water we use
that’s the issue.

Sure, we can get ~1,000 gallons of drinking water for 3$ via desalination.
That works out to a monthly water bill increase of ~45$ for a family of 5.
Unfortunately, crops need incredible amounts of water. So, growing corn etc
with desalination would case food prices to ~quadruple.

~~~
amluto
Why do you think that the cost is an increase?

I pay quite a bit more than $3/kgal, and a large fraction of that goes toward
funding the SFPUC’s massive infrastructure for bringing Hetch Hetchy water to
the Peninsula. A desalination plant would be located on the coast or on the
bay and would not require this infrastructure.

As far as I’m concerned, the problem is the capital cost of the plant and the
plumbing to safely suck in saltwater and discharge brine and has essentially
nothing to do with electricity.

The situation isn’t helped by the fact that, in average and wet years, demand
for desalinated water would be nil, since the Hetch Hetchy infrastructure
already exists.

~~~
Retric
Sewage and distribution is not included in these costs. Current systems get
water for almost nothing it’s mostly distribution and sewage systems that
you’re currently paying for.

As to why this is, a single pipe carrying 1,000x as much water costs no where
near 1,000x much per foot. Each home might only need 1/10,000,000 the water,
but it’s got to be built for peak demand not average useage. On top of this,
people don’t live at sea level, so you need to pump that sea water up before
you can use it.

PS: Not to mention most distribution systems leak significantly, that 3$
assumes 100% efficiency at 50% it’s more like 6$.

~~~
amluto
None of this is at all relevant to my point. SFPUC charges over $4/ccf for
wholesale water. This does not include distribution costs, and this is paid
regardless of whether the water is sold, leaks, or is used for firefighting.
If SFPUC’s wholesale customers purchased desalinated water and managed to
escape their SFPUC contracts, they would eliminate this expense.

edit:
[http://bawsca.org/water/rates/wholesale](http://bawsca.org/water/rates/wholesale)

~~~
Retric
That’s not exclusive price just per gallon of water.

[https://sfwater.org/modules/showdocument.aspx?documentid=774...](https://sfwater.org/modules/showdocument.aspx?documentid=7742)

Wholesale untreated water is 1.02$ per 1,000 gallons. Page 16: ( 0.76 per 748
Gallons delivered). Plus a fixed fee for the size of the pipe (22.67$ for a
one inch pipe.)

~~~
amluto
Curious. I’m pretty sure my water provider pays the BAWSCA rate. I don’t know
why it’s so different.

edit: You’re looking at rate W-24. BAWSCA users seem to pay rate W-25, which
is far higher. I don’t know exactly what the difference is.

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abdullahkhalids
The paper
[https://www.nature.com/articles/s42004-019-0151-2](https://www.nature.com/articles/s42004-019-0151-2)

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amelius
How efficient and robust is the use of a membrane in these kinds of solution?
It would seem to me that the membrane is the weakest link, and also expensive.

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Overtonwindow
I don’t think the problem we should be focusing on is how to remove salt from
water, but what to do with the salt after it has been removed.

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umvi
Sell it? Put it back into the ocean?

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brokensegue
Nobody wants to buy the sludge.

Putting it back would kill ocean life in the area.

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umvi
Just dump it in the great salt lake then

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brokensegue
uhm, trucking this sludge all the way to Utah seems pretty pointless. Why not
just put it in a dump? Why does it need to be put into water?

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pixl97
Think of brine as industrial pollution. It wont stay in a dump, it will leak
out and poison an area for tens of thousands of years.

~~~
brokensegue
plenty of industrial pollutants are put into dumps/buried? what do you think
we do with them?

From wikipedia

> Other methods [of disposing of brine] include drying in evaporation ponds,
> injecting to deep wells, and storing and reusing the brine for irrigation,
> de-icing or dust control purposes.

