Why is desalination so difficult? • Hacker News Board

Re: Why is desalination so difficult?

#231

Post by pas » Wed, Jul 05, 2023, 9:52 PM UTC

eropple wrote: post #225

giantrobot wrote: post #220

Earlier quoted context omitted.

The brine waste from RO is still mostly water. In order to extract the salt you'd need to evaporate the water which still takes a lot more energy. You could use evaporative ponds to let the Sun do the work but that takes a lot of space. In either case you're spending a lot more money per pound than just digging the salt out of a mine.

Is that true when we take into account environmental externalities? I am not an expert in this field; I know that many forms of mining are capital-B Bad for the environment but I don't know how salt mines impact the area around them.

well, mines are bad on the short term. the long term damage is not of the actual digging, but of the separation processes. which all can be done as clean as we wish it just costs more.

Re: Why is desalination so difficult?

#232

Post by ramesh31 » Wed, Jul 05, 2023, 9:52 PM UTC

mcpackieh wrote: post #127

ramesh31 wrote: post #114

Earlier quoted context omitted.

>Why must drinking water be a for-profit enterprise? It's about sustainability, not profit. Of course wealthy nations can (literally) burn enough money turning fossil fuels into water to make their population comfortable and happy. But most can't, and the externalized cost is unimaginable at a global scale.

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

>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.

Re: Why is desalination so difficult?

#233

Post by BeetleB » Wed, Jul 05, 2023, 9:52 PM UTC

gamegoblin wrote: post #77

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-Sta…

> That's about the same amount of water as a typical American household uses per day.

I assume you meant per year?

Re: Why is desalination so difficult?

#234

Post by mcpackieh » Wed, Jul 05, 2023, 10:02 PM UTC

coryrc wrote: post #201

mcpackieh wrote: post #106

Earlier quoted context omitted.

> It's not even remotely economical without huge government subsidies. Why shouldn't drinking water of all things receive subsidies? Why must drinking water be a for-profit enterprise?

Because the US has plenty of free water that's growing alfalfa in the desert. Subsidies are unnecessary.

Some parts of the US have too much fresh water, other parts have too little. Fresh water is a regional matter; you're talking about the American Southwest, particularly California, specifically. But this conversation is about desalination generally, and particularly Israel.

Re: Why is desalination so difficult?

#235

Post by londons_explore » Wed, Jul 05, 2023, 10:02 PM UTC

singleshot_ wrote: post #228

londons_explore wrote: post #218

Earlier quoted context omitted.

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.

Surely the salt falling on the farmland would have some kind of deleterious effects, right? I imagine you’re not trying to grow sea cucumbers in this scenario, and I’m not sure most land plants would be crazy about a ton of salt. Just ask the Carthaginians… Guessing that you know something I don’t here, though.

I believe that when every spec of salt creates a far larger water droplet - and the overall concentration in rain ends up tiny.

If anything, doing lots of this might reduce the salt concentration in soils, due to increased rainfall.

Re: Why is desalination so difficult?

#236

Post by mcpackieh » Wed, Jul 05, 2023, 10:03 PM UTC

ramesh31 wrote: post #232

mcpackieh wrote: post #127

Earlier quoted context omitted.

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

>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?

Re: Why is desalination so difficult?

#237

Post by timerol » Wed, Jul 05, 2023, 10:03 PM UTC

BeetleB wrote: post #233

gamegoblin wrote: post #77

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-Sta…

> That's about the same amount of water as a typical American household uses per day. I assume you meant per year?

No, 1000L/day is about right for a typical American household. [1] claims "The average American family uses more than 300 gallons of water per day at home." 300 gal is 1136 L.

https://www.epa.gov/watersense/how-we-use-water

Re: Why is desalination so difficult?

#238

Post by cyanydeez » Wed, Jul 05, 2023, 10:09 PM UTC

AnimalMuppet wrote: post #207

margalabargala wrote: post #190

Earlier quoted context omitted.

> 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 Salton Sea is over a hundred miles from the ocean, though, with at least one mountain range in the way. It might be a good dumping ground (or maybe not - there at least were fish in it, if there still are, this would almost certainly kill them). But it would definitely take a lot of energy to pump the water there.

Great movie: Salton Sea

Re: Why is desalination so difficult?

#239

Post by js2 » Wed, Jul 05, 2023, 10:09 PM UTC

xigency wrote: post #230

js2 wrote: post #181

Earlier quoted context omitted.

They clean the oysters before selling them: > This was the first oyster farm to feature an inventive “depuration and purification” process, which involves immersing the oysters in triple-filtered seawater once they reach full size. This ensures that the oysters are a completely safe, top-quality delicious shellfish product. https://www.usatoday.com/story/travel/10best/2022/08/04/how-... https://en.wikipedia.org/wiki/…

That’s a really convoluted way to say they rinse them off in clean water. Anyway, seafood comes from the ocean. I don’t see why they would be worse than other oysters.

> That’s a really convoluted way to say they rinse them off in clean water.

It's more than rinsing them off. Oysters are filter feeders. They need to spend enough time in clean water to pump out any contaminants. It's an FDA regulated process:

https://www.fda.gov/food/federalstate-food-programs/national...

> Anyway, seafood comes from the ocean. I don’t see why they would be worse than other oysters.

It depends on the cleanliness of the water. These oysters are raised in a lagoon surrounded by the city of Carlsbad:

https://www.google.com/maps/place/Carlsbad+Aquafarm/@33.1419...

I imagine that lagoon is subject to runoff and not nearly as clean as oysters harvested in open waters.

Re: Why is desalination so difficult?

#240

Post by strken » Wed, Jul 05, 2023, 10:12 PM UTC

Robotbeat wrote: post #199

rtkwe wrote: post #168

Earlier quoted context omitted.

It will change the salinity on the short term though at the release location and the amount a large plant will be discharging is enough to alter the local salinity so long as the plant remains operational which will negatively affect sea life in that area.

Only if the discharge is not diluted sufficiently. There is nothing about reverse osmosis that is fundamentally more toxic or harmful than the typical evaporation that takes place naturally in the ocean. And it’s pretty dishonest to claim otherwise. If there’s a problem with too high salinity of discharge, that’s an engineering problem that should be fixed with greater dilution.

If you're drawing water from a bay and not putting all that water back in, then the salinity of the bay must depend on the rate it mixes with the outside ocean, since you're removing water from the system.

I assume that in practice the amount of water taken by a desal plan is tiny and most bays have high tidal inflow and outflow, but it's obvious that more than just dilution should be considered.