Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Why is desalination so difficult?

practical.engineering

Post reply on HN
Print view View original

321–330 of 644 posts

Re: Why is desalination so difficult?

#321

Post by aerio »

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 don't think the article dodges the question at all. Did you properly read it or just skim it? It goes over multiple ways of desalinating water, distillation and osmosis - as you also cover. The most relevant paragraph to the question and a bit of a conclusion seems to be:

>And that’s the problem with desalination. It’s kind of like the nuclear power of water supply. It seems so simple on the surface, but when you add up all the practical costs and complexities, it gets really hard to justify over other alternatives. It’s also harder to compare costs between those alternatives because of desal’s unique problems. It’s just a newer technology, so it’s harder to predict hidden technical, legal, political, and environmental challenges. For example, because of the high energy demands, desalination can strongly couple water costs with electricity costs. During a drought, the cost of hydropower goes up because there’s less water available, increasing overall energy costs and thus making desalination less viable right when you need it most.

Re: Why is desalination so difficult?

#322

Post by gamegoblin »

codedokode wrote: post #314
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…

As I understand, before getting 50-cent water you need to pay a billion dollars upfront. Why so expensive?

The biggest plant in the US, near San Diego (Carlsbad), cost $1B to build and produces 200,000 m^3 per day. Israel's biggest desalination plant, Sorek, cost $400MM and produces over 600,000 m^3 per day, so 40% the cost and 300% the water. So I assume a large part of the cost of the Carlsbad plant is just all the usual reasons everything is insanely expensive to build in California.

https://en.wikipedia.org/wiki/Claude_%22Bud%22_Lewis_Carlsba...

https://www.water-technology.net/projects/sorek-desalination...

Re: Why is desalination so difficult?

#323

Post by yyyk »

ramesh31 wrote: post #70
nerbert wrote: post #34

Earlier quoted context omitted.

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...

>In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. Israel does this by burning massive quantities of fossil fuels: https://en.wikipedia.org/wiki/Energy_in_Israel#/media/File:E... It's not even remotely economical without huge government subsidies. Completely untenable with current technology for poorer countries, or anyone that cares at all about carbon e…

Note that the large increase in the graph predates seawater desalinization. If you start at 2005 (first large seawater desalinization facility per wiki) and assume all the increase is due to desalinization (definitely untrue, but simplifying), the difference isn't so large.

Re: Why is desalination so difficult?

#324

Post by xyzzyz »

cyanydeez wrote: post #241
Robotbeat wrote: post #200

Earlier quoted context omitted.

> If you dump it on a living ecosystem you tend to kill it. This is false and somewhat dishonest. This is simply a choice of not diluting it enough. There is nothing about the discharge from reverse osmosis that is any more fundamentally toxic than the natural process of evaporation. Proper dilution is essential, and treating the discharge as fundamentally toxic actually undermines the engineering to do this proper d…

This is dishonest: toxicity, dosage are fundamental properties. You can't just wave a word at it like dilution and think you're solving an engineering problem. If you got 1 gallon out of sea water, what do you think you need to dilute it to be safe? Typically, it's 99%∆.

You don’t need to dilute a gallon of sea water at all, it is perfectly safe to dump it back into sea as is.

Re: Why is desalination so difficult?

#325

Post by whiddershins »

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…

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.

Re: Why is desalination so difficult?

#326

Post by namibj »

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…

Using a fractional distillation column you can also separate filtered sea water into water vapor and brine, at equal pressure, with the brine appropriately hotter so the vapor pressure of water over the brine is the same as over the salt-free water droplets that form on the cold end.

You'd then have to compress the water vapor until it condenses barely hotter than the brine, and use both distilled water and residual brine at their approximately equal temperature (water at higher pressure than brine, though) in ofc separate counter-flow heat exchangers to pre-heat the filtered source (sea) water to the column's operating temperature (i.e., where the source water just starts boiling at the column's operating pressure (you want a decent vapor pressure to have a reasonable vapor density and thus feasible power density for capex reasons)).

Thermodynamically this should match a reverse-osmosis process with equal input/output parameters (I left out that you need pumps/turbines to "losslessly" adapt liquid between ambient pressure and internal operating pressure).

One benefit would be that you could directly heat the brine with solar thermal collectors, to get away without having to compress the vapor to condense it, essentially an open-cycle Type-1 absorption heat pump, with solar feed. (Lacking an evaporator, with the condensed output being the desired pure water, and the absorber being fed with source sea water while the return from the generator after the heat exchanger is just warm brine for discharging. If water and brine need to be sub-ambient, you'd evaporate part of the condensed water to chill both the condensate and the brine output streams. That'd be partially-open-cycle.)

Re: Why is desalination so difficult?

#327

Post by 6510 »

dylan604 wrote: post #213
sophacles wrote: post #206

Earlier quoted context omitted.

There are a few very big salt mines below the great lakes. We could totally get a desalinization plant in Michigan by pulling water from the lakes, salt from the Detroit salt mine, and combining them in the input stream to the plant!

someone has been playing in the perpetual motion machine sandbox again i see

I think it sounds quite sensible. How is it different from using all fresh water as an all you can dump free-for-all public sewage system before wondering where the drink is coming from? In India they poop in the water then take a bath in it and cook their food in it. In the west we do our pooping upstream because it looks so much cleaner that way. NO! I fail to see how adding salt before desalination is any less sensible. Lets just do it.

Re: Why is desalination so difficult?

#328

Post by superq »

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.

The Israelis ran into this problem with their mineral extraction in the Dead Sea, so they're bulldozing the dry salt waste to build a physical wall for border security (more than 10 meters high) that's apparently hard to climb:

https://www.youtube.com/watch?v=tgTheUjeDlg

Re: Why is desalination so difficult?

#329

Post by Retric »

actionfromafar wrote: post #298
pas wrote: post #231

Earlier quoted context omitted.

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.

Cost more energy

Mining salt costs vastly less energy than using slightly brackish water. Which is why it’s so much cheaper.

Re: Why is desalination so difficult?

#330

Post by dylan604 »

6510 wrote: post #327
dylan604 wrote: post #213

Earlier quoted context omitted.

someone has been playing in the perpetual motion machine sandbox again i see

I think it sounds quite sensible. How is it different from using all fresh water as an all you can dump free-for-all public sewage system before wondering where the drink is coming from? In India they poop in the water then take a bath in it and cook their food in it. In the west we do our pooping upstream because it looks so much cleaner that way. NO! I fail to see how adding salt before desalination is any less sen…

you're desalinating water to have an abundant source of fresh water. why in the world does taking fresh water and adding salt being mined from the ground together to make salt water to then desalinate make any sort of sense?
Post reply on HN