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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#271

Post by gameshot911 »

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…

If there's one complaint I have about PE's content, it's that he often give ambiguous answers to the central question. Sometimes it's strung throughout the 10 minute video, but I really wish he'd end all his content with a concise summary.

Re: Why is desalination so difficult?

#273

Post by ramesh31 »

mcpackieh wrote: post #236
ramesh31 wrote: post #232

Earlier quoted context omitted.

>Clean energy exists, it would require more subsidies but so what? Easy to say when your government can afford the subsidies. But the vast majority of freshwater-insecure nations will never be able to do this without a 10x technological breakthrough.

Well if they can't afford to even subsidize fresh water, their choices are move or die. What's your point then?

> Well if they can't afford to even subsidize fresh water, their choices are move or die.

Would you tell that to the Israelis if they couldn't afford it?

The point is that yes, the original article is correct. Current desalination tech is woefully inadequate to replace fresh water surface reserves without putting a massive burden on the society using it.

Re: Why is desalination so difficult?

#274

Post by credit_guy »

I think he fully gave the wrong answer. The main problem with desalination is capital cost. The Carlsbad plant cost one billion dollars to make. I bet very little of it is the cost of membranes, or the actual RO systems. It's simply a large plant, and building a large plant is expensive. The same problems that plague large nuclear power plants plague many other large construction projects, including large desal plants.

Re: Why is desalination so difficult?

#275

Post by jeffparsons »

londons_explore wrote: post #218
daveslash wrote: post #216

Earlier quoted context omitted.

I know that generally speaking, disposing of waste in the ocean and expecting it to disperse enough to be harmless is foolish and wrong. But in the case of salt, It would seem to me that the ocean can handle that amount of salt. Course, I haven't done the math. But it would seem to me that the back in == salt taken out. We'd only be changing the net salinity by the amount of water subtracted. Without having done the…

There are potentially some good reasons to just spray the brine into the air. Seawater sprayed into the air becomes tiny salt crystals, which in turn help clouds to form, and cause increased rainfall. The rain produced has negligible levels of salt. In places with dry climates, this often can turn desert land into farmland across an area hundreds of kilometers wide.

Would this be worth doing straight from seawater? I.e. place enormous solar/wind powered rigs in the ocean to spray massive volumes of seawater into the air? What would be the effect (beneficial and otherwise) of that?

Re: Why is desalination so difficult?

#276

Post by jakear »

soperj wrote: post #205
jakear wrote: post #98

Earlier quoted context omitted.

Salt overexposure is liable to dry up your skin and cause rashes/cracking leading to vectors for infection.

Is it? My parents have a salt water pool and swim in it daily through the summer.

And they never shower?

Re: Why is desalination so difficult?

#277

Post by RyEgswuCsn »

lisper wrote: post #88
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…

You've pretty much nailed it except for one minor nit: > It takes less energy but produces a concentrated liquid waste stream that must be disposed of. This implies that creating a concentrated waste stream is a problem unique to reverse osmosis. It isn't. No matter what you do you're going to end up with a bunch of salt that you have to get rid of somehow.

It sort of does. RO produces relative large amount of waste water that are harder to deal with (e.g. storage, disposal). By spending more energy, you can turn the waste water into salt solids, which can be easier to dispose of.

Re: Why is desalination so difficult?

#278

Post by rs999gti »

stouset wrote: post #128
ericlewis wrote: post #125

Earlier quoted context omitted.

good thing people need salt!

If people needed as much salt as was contained in the water to begin with, we wouldn’t need to remove it in the first place.

Can't the salt just be dumped in the desert?

Re: Why is desalination so difficult?

#279

Post by [deleted] »

dylan604 wrote: post #156
mcpackieh wrote: post #127

Earlier quoted context omitted.

Clean energy exists, it would require more subsidies but so what?

Wind powered desalination seems like a perfect combo seeing as it's pretty much always windy on the coast. California's May gray and June gloom makes me thing would keep solar from my first option.

[deleted]

Re: Why is desalination so difficult?

#280

Post by stouset »

rs999gti wrote: post #278
stouset wrote: post #128

Earlier quoted context omitted.

If people needed as much salt as was contained in the water to begin with, we wouldn’t need to remove it in the first place.

Can't the salt just be dumped in the desert?

The products of desalinization aren’t salt and fresh water. They’re somewhat saltier brine and fresh water.
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