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

621–630 of 644 posts

Re: Why is desalination so difficult?

#621

Post by eru »

pvaldes wrote: post #387
pvaldes wrote: post #381

Earlier quoted context omitted.

Is not just a question of preferences, there are risks also. If the problem is that we need lots of energy to create pressure to separate salt from brine, well... I figure out that there is a lot of free "all that you can eat" pressure in the sea bottom. The problem would be to calculate if moving all this weight up and down the sea (and in open sea) would be economical or not. Or of would unavoidably lead to people…

Having all of this in mind, I would go further and propose to regulate by law changing the material of the deep sea submarine ballasts. It seems technically doable and would have some benefits 1) Lesser impact on deep sea ecosystems. No human trace left behind. Disposable loads of iron or concrete will remain forever in the bottom. If we use salt or sand instead the impact on fragile deep sea ecosystems seems reduced…

> Having all of this in mind, I would go further and propose to regulate by law changing the material of the deep sea submarine ballasts.

Which country's law? There are many of them.

> 1) Lesser impact on deep sea ecosystems. No human trace left behind.

There's barely any submarines around. This is a drop in the bucket.

> 2) Improved economics?

If it improves economics, why do you need to force people with regulation?

Re: Why is desalination so difficult?

#622

Post by sh34r »

ChiperSoft wrote: post #614
dekelpilli wrote: post #390

Earlier quoted context omitted.

Answered at 2:16 of the video - the claim is that the rain actually helps, as it makes the "mountain" steeper.

Yeah, but it runs along a river… which will now be salinated every time it rains.

I did a bit of Googling. It’s a desalination plant on the Dead Sea. That is the Jordan River. The stretch they are building the salt wall on is barren desert land.

Jericho is the nearest town, 18,000 people, 8” of rain a year, 2 miles from the border. They presumably get their water from that desalination plant. No towns that far south are getting their water from it. The river barely even makes it to the Dead Sea anymore. https://en.wikipedia.org/wiki/Jordan_River#Main_environmenta...

Adding a bit more salt into the famously-salty Dead Sea are the least of their problems in that part of the world. Where else would you put the salt, anyway? Putting it all into the Dead Sea would be even worse —- at least this reduces the amount that makes it back there. Putting it into any other watershed would cause vastly more harm.

It seems like a good enough solution to me. If you think about it, it wouldn’t be in Israel’s interests to inflame tensions with Jordan over this, if it were really an issue.

Re: Why is desalination so difficult?

#623

Post by hinkley »

aeonsky wrote: post #172
dstainer wrote: post #23

Slightly off topic, however, the post references the Carlsbad desalination facility. If you find yourself in San Diego and like oysters, I would highly recommend you checkout the Carlsbad Aquafarm. Take the tour and pick up some oysters. What's really interesting and relevant to the topic is that the oyster farm serves as a pre-filter to the desalination plant and there's an symbiotic relationship between the plant a…

Pick up some oysters, for eating? If these oysters serve as a pre-filter for the plant, would you not want to eat them as these oysters would contain all sorts of pollution?

If they were pulling disgusting water into the desalination plant, it would probably damage their equipment. If you watch the video, you'll find that macroscopic contamination is the first problem they have to solve, and the oysters should be fine for that.

Re: Why is desalination so difficult?

#624

Post by hinkley »

jtriangle wrote: post #134
MarkusWandel wrote: post #12

Some years ago I toured a maple syrup operation that has the opposite goal: Concentrate the dissolved stuff in the water. Their first stage was reverse osmosis, but only to a point. Second stage is boil, but with aggressive heat recovery from the steam to preheat the incoming liquid. All this to keep the energy cost under control.

That's more or less the correct way to run anything energy intensive, scavenge as much of the waste heat as you reasonably can. Theoretically, continuous distillation can be extremely efficient, as, you're removing as much heat as you're putting into the system. In reality, you get into diminishing returns fairly quickly, because insulation, pumps, heat exchangers, etc, are all far from free, especially at scale.

Counterflow heat exchange, baby.

Re: Why is desalination so difficult?

#625

Post by hinkley »

egberts1 wrote: post #212

MIT solar distiller in 2020 demonstrated a gallon and a half of fresh water in one hour using a square meter of close-quarter membrane distillation process. So, 10,000 sq meter of this baby could pump 150,000 gallons of fresh water over a ten-hour solar shift. Seems like the secret sauce is 1.2cm (or is that 80mm) separation between diffuser plates thus taking advantage of solar heating/condensation/collection in one…

I'm not sure what the full story is, but in 2019 MIT was also looking at one of the other costs. They claim it's common to pretreat the water with sodium hydroxide, which is purchased. Here they claim they have a way to regenerate it from the effluent:

https://news.mit.edu/2019/brine-desalianation-waste-sodium-h...

Re: Why is desalination so difficult?

#626

Post by hinkley »

samstave wrote: post #32
josephjrobison wrote: post #24

Earlier quoted context omitted.

There is talk about pumping sea water into the Saharan desert in certain parts, which gets at that. https://www.youtube.com/watch?v=V2b7ztWvFOg

Was going to say this, there are/have been many ideas to do just this - one was to build a huge canal and just then let the sea flood the area... Also - there are lots of un-earthed treasures to be found under the saharan which was once a lush environ and have been covered with sand - so prior to flooding it, we need to lidar and excavate it. What if we vaccuumed up all the sand and built a new island/continent with…

One of the more feasible ones was a linear greenhouse with a canal in it.

I'm sure someone has done the math to figure out what the proper length is for such a canal, to avoid problems with scaling. And if you've ever been near an ocean for much time, you know the salt tends to get around. So I'm not sure how in a multi-year project you keep that to a minimum.

Re: Why is desalination so difficult?

#627

Post by eru »

Ajedi32 wrote: post #408
Ajedi32 wrote: post #407

Earlier quoted context omitted.

> I figure out that there is a lot of free "all that you can eat" pressure in the sea bottom. > Things either float or sink without extra energy added. That seems intuitively wrong... where is the energy coming from in this scenario? It's like saying we could use the pressure at the bottom of the ocean to spin a turbine and get free electricity. I guess the obvious problem that sticks out in my mind is that once you'…

Here's an even more interesting physics brain teaser: Imagine you put a reverse osmosis membrane at the bottom of the ocean and connected it to a pipe filled with fresh water leading to the surface. The pressure at the bottom of the pipe would be less than the pressure at the bottom of the ocean, since fresh water weighs less than salt water. In this setup, would an endless supply of fresh water flow through the memb…

> I'm guessing not, but I'm having a hard time understanding why.

Second law of thermodynamics.

Re: Why is desalination so difficult?

#628

Post by JacobAldridge »

superq wrote: post #328
lisper wrote: post #88

Earlier quoted context omitted.

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

That’s a Lot of salt…

Re: Why is desalination so difficult?

#629

Post by oska »

refurb wrote: post #397
niemandhier wrote: post #393

Earlier quoted context omitted.

That will make the region hostile to life for 1000s of years even if climate patterns were to change.

It’s literally meters away from the Dead Sea that has salt crystallizing on the beaches. Ain’t changing anything in the environment.

They obviously are changing the environment. You'd need a proper environmental impact statement to determine what the negative aspects of these changes are. I wonder if one was done?

Re: Why is desalination so difficult?

#630

Post by refurb »

oska wrote: post #629
refurb wrote: post #397

Earlier quoted context omitted.

It’s literally meters away from the Dead Sea that has salt crystallizing on the beaches. Ain’t changing anything in the environment.

They obviously are changing the environment. You'd need a proper environmental impact statement to determine what the negative aspects of these changes are. I wonder if one was done?

By that definition, when my kid goes to the beach and digs a hole in the sand and fills it with water they are "changing the environment". Do we need a proper environmental impact statement to determine the negative aspects of these changes?

Come on. They are literally moving salt a few meters.

Post reply on HN