Why is desalination so difficult? • Hacker News Board

Re: Why is desalination so difficult?

#211

Post by rightbyte » Wed, Jul 05, 2023, 9:08 PM UTC

darkclouds wrote: post #142

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.

Its always been my understanding that any treatment to remove stuff from water is going to produce waste which needs disposing, just look at the Brita water jug filters, they need disposing. I've often wondered why dont we have more pure water pumped through the water mains in various countries, and I think after reading about Super K the Japanese Neutrino detector [1] and how the water in the tank was so pure it had…

How did they know that there was a wrench there in the first place? Maybe the mechanic forgot it somewhere else.

I thought salt and impurities in water was the driving factor in rust.

Re: Why is desalination so difficult?

#212

Post by egberts1 » Wed, Jul 05, 2023, 9:08 PM UTC

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 area.

Of course, there remains an collection issue of brine discharge which could be removed gradually instead but in same but 3-peat manner (down to 2-3 permille, or 0.2-0.3% salinity level.)

At any rate, this MIT method has leapfrogged the passive solar method ahead of reverse osmosis (RO) method by quite a bit, in terms of energy required to extra fresh water. RO still holds the insurmountable lead in base (non-fluctuating) water output rate.

https://news.mit.edu/2020/passive-solar-powered-water-desali...

Re: Why is desalination so difficult?

#213

Post by dylan604 » Wed, Jul 05, 2023, 9:08 PM UTC

sophacles wrote: post #206

dylan604 wrote: post #149

Earlier quoted context omitted.

why not? we're growing heavy water using crops in places with little water. logic is not always the deciding factor if involved at all in a lot of modern things

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

Re: Why is desalination so difficult?

#214

Post by michael_vo » Wed, Jul 05, 2023, 9:12 PM UTC

You could use simple pricing to influence behavior. 1100 litres a day for each American is so damn much. When you hike and stay in the mountain huts you are charged 3$ for a 4 minute shower.

You could probably fix the drought situations by reducing consumption.

Re: Why is desalination so difficult?

#215

Post by q845712 » Wed, Jul 05, 2023, 9:14 PM UTC

jtriangle wrote: post #178

caseyohara wrote: post #92

Earlier quoted context omitted.

Deserts are far more alive and biodiverse than people think. > The Sonoran Desert encompasses 120,000 square miles of southwestern Arizona, southeastern California, and in Mexico, northwestern Sonora and most of the Baja Peninsula. With nearly 3500 species of plants, 500 species of birds, and 1,000 species of bees, the Sonoran is the most biodiverse desert on earth.

You do have to consider though, how important are those ecosystems? Because I see this often used as a reason we can't do something, but they never qualify it with reasons why one should care. I also pose this as a completely honest question, as I don't really know if you wiped out every desert ecosystem with solar/desal/etc if it'd actually affect anything else.

it's probably a good starting position to assume that since we all the share the same closed-ish system called planet earth, there's interconnections between different systems. Certainly the border areas between desert and not-desert aren't very crisply defined. Certainly (reference in other threads) nutrients can be blown by the winds from desert into non-desert areas far away. Certainly there exist some animals who go in and out of desert regions (birds, butterflies, ...). It's a really good idea to assume that things on this planet are connected to each other.

Re: Why is desalination so difficult?

#216

Post by daveslash » Wed, Jul 05, 2023, 9:20 PM UTC

Loquebantur wrote: post #182

ericlewis wrote: post #125

Earlier quoted context omitted.

good thing people need salt!

The people who need that water tend to shed it after some time. Discarding waste water into the oceans via rivers is a huge idiocy. You essentially rely on the environment to "magically" sort it all out. Naively so and fraught with huge inefficiencies. Proper treatment of that waste in the sense of recovering usable matter streams is the logical way to go.

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 math, my gut reaction is to think that's something the ocean can handle.

The problem, as I see it, is localized concentrations. While the ocean at large might be able to absorb it, the localized concentrations can be very problematic.

Re: Why is desalination so difficult?

#217

Post by jtriangle » Wed, Jul 05, 2023, 9:23 PM UTC

q845712 wrote: post #215

jtriangle wrote: post #178

Earlier quoted context omitted.

You do have to consider though, how important are those ecosystems? Because I see this often used as a reason we can't do something, but they never qualify it with reasons why one should care. I also pose this as a completely honest question, as I don't really know if you wiped out every desert ecosystem with solar/desal/etc if it'd actually affect anything else.

it's probably a good starting position to assume that since we all the share the same closed-ish system called planet earth, there's interconnections between different systems. Certainly the border areas between desert and not-desert aren't very crisply defined. Certainly (reference in other threads) nutrients can be blown by the winds from desert into non-desert areas far away. Certainly there exist some animals who…

Sure, I mean, that's my assumption, the question is more of "how important is that connection" in a given instance. And, I suppose down that line of questioning, do we have the knowledge/systems/etc to overcome any losses?

Re: Why is desalination so difficult?

#218

Post by londons_explore » Wed, Jul 05, 2023, 9:31 PM UTC

daveslash wrote: post #216

Loquebantur wrote: post #182

Earlier quoted context omitted.

The people who need that water tend to shed it after some time. Discarding waste water into the oceans via rivers is a huge idiocy. You essentially rely on the environment to "magically" sort it all out. Naively so and fraught with huge inefficiencies. Proper treatment of that waste in the sense of recovering usable matter streams is the logical way to go.

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.

Re: Why is desalination so difficult?

#219

Post by jonny_eh » Wed, Jul 05, 2023, 9:31 PM UTC

4rt wrote: post #7

jonny_eh wrote: post #5

Earlier quoted context omitted.

Separating two things isn't necessarily expensive. Think of coin sorters that just use gravity and different sized holes.

You still have to lift all the coins.

Slide the coins off the table into the sorter, done!

Re: Why is desalination so difficult?

#220

Post by giantrobot » Wed, Jul 05, 2023, 9:32 PM UTC

suddenclarity wrote: post #145

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.

No reason to keep the 1:1 ratio. Use the salt to replace our current salt mines/outtakes and then use the water as an addition to our current freshwater usage.

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.