Why is desalination so difficult? • Hacker News Board

Re: Why is desalination so difficult?

#341

Post by Veserv » Thu, Jul 06, 2023, 2:16 AM UTC

codedokode wrote: post #316

iudqnolq wrote: post #120

Earlier quoted context omitted.

That equates to around 4,000 liters per person per day. Freshwater withdrawals is a very broad category, it also includes water released to turn hydropower turbines. But it's also unfair to compare across countries without taking into account water sent between countries in the form of produce and products.

4,000 liters per day is too much. It is 10 full bathtubs. Do Americans leave the tap open or do they take a bath 10 times per day?

It accounts for the water used in agriculture, industry, power generation, etc. Those uses completely dominate domestic water usage including for aesthetic purposes such as lawns and golf courses (e.g. around 80% of water usage in California is for agriculture and power generation if I remember correctly with only around 5-10% for all domestic usage).

As you will note, the average German consumes 1,000 liters per day by the same metric which is 2.5 baths per day which is obviously unreasonable if we were only considering direct domestic water consumption.

To be fair, the agriculture is being grown to feed people, so it is fair to include the consumed produce in each person’s water footprint. This is further exacerbated by growing animal feed for animals that are consumed which is even more “water inefficient”. The US is a major food exporter, so it is over-represented in these sorts of numbers, but it is a fair approximation to within a factor of maybe 2-3x of the embodied water consumption of the average American.

Re: Why is desalination so difficult?

#342

Post by darkclouds » Thu, Jul 06, 2023, 2:16 AM UTC

ars wrote: post #260

darkclouds wrote: post #142

Earlier quoted context omitted.

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…

> the water in the tank was so pure it had dissolved a spanner/wrench that was left in the bottom Once it dissolved a tiny bit of the metal it would no longer be so pure. So this sentence makes no sense. It takes just a minuscule amount of mineral to replicate regular well-water.

Only quoting the link. Are you suggesting the Insider website is what Trump calls fake news?

Re: Why is desalination so difficult?

#343

Post by Nifty3929 » Thu, Jul 06, 2023, 2:18 AM 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…

Residential water use is nothing.

In California, where we have persistent water shortages, residential, commercial and industrial water use ( including all landscaping, golf courses, etc) all put together still only amount to 10-20% of overall water use, depending on rainfall

Re: Why is desalination so difficult?

#344

Post by reyoz » Thu, Jul 06, 2023, 2:22 AM UTC

RO does use a lot of energy to overcome the osmotic pressure and to create flux through the membrane. An interesting concept is using the reverse process, "forward" osmosis, to extract the energy where fresh water mixes with seawater, such as a river mouth. This is called pressure-retarded osmosis (PRO) and was tried at pilot scale by Norwegian power company Statkraft. Ultimately this trial was shelved due to being uncommercial [1], perhaps future membrane development will improve the viability of FO. And yes the membranes are quite different, RO membranes are relatively thick due to the transmembrane pressures required. FO requires a much thinner support for the active layer as there is no external pressure applied to push the water through (it is drawn through by the difference in salt concentration).

[1] https://www.powermag.com/statkraft-shelves-osmotic-power-pro...

Re: Why is desalination so difficult?

#345

Post by nwallin » Thu, Jul 06, 2023, 2:25 AM UTC

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 Carlsbad plant is in Carlsbad, CA. If you look at the neighborhood it's in, the houses are 1,200-2,000 square feet and sell for $2-$5 million. Everything is expensive.

Re: Why is desalination so difficult?

#346

Post by SoftTalker » Thu, Jul 06, 2023, 2:36 AM UTC

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?

Just pump it back into the ocean. The little bit of concentrated brine from a desalination plant will be quickly diluted; the ocean contains a LOT of water.

Re: Why is desalination so difficult?

#347

Post by janalsncm » Thu, Jul 06, 2023, 2:36 AM UTC

tguvot wrote: post #317

janalsncm wrote: post #287

Earlier quoted context omitted.

Sure. So if for example your city has an average of 3400 hours of sun per year like Jerusalem does, and you know how much water you need to produce in the 8760 hours of the year, you can calculate how much to desalinate during sunny hours. Excesses can be used for excess water or sold back to the grid.

desalination plants don't serve one city. they serve country. also, in general, it's not economical to build plants that do not work 24/7 unless you are country that can spend x3 to overbuild and keep equipment idle Israel btw supplies desalinated water to Jordan and PA.

I think we’re talking past one another. Solar is definitely economical in certain circumstances, and every country overbuilds to some extent. Whether building excess solar makes sense is a question of cost of a marginal unit of energy, since the water doesn’t care what powered its desalination. The energy isn’t wasted since it will eventually need to come from somewhere.

Another consideration is political sovereignty, since solar can’t easily be turned off by a foreign adversary.

Re: Why is desalination so difficult?

#348

Post by doubleg72 » Thu, Jul 06, 2023, 2:57 AM UTC

fsckboy wrote: post #320

myshpa wrote: post #310

Earlier quoted context omitted.

Microplastics found in 90 percent of table salt https://www.nationalgeographic.com/environment/article/micro... The presence of microplastics in commercial salts from different countries [2017] https://www.nature.com/articles/srep46173 Global Pattern of Microplastics (MPs) in Commercial Food-Grade Salts: Sea Salt as an Indicator of Seawater MP Pollution [2018] https://pubs.acs.org/doi/10.1021/acs.est.8b04180 Micropla…

thank you for the information, for sure, but i was responding to the claim that sea salt "needs cleaning" to point out that that seasalt isn't being cleaned, so it puts an upper bound on the value of need as it concerns the commercial market including govt regulation. My suspicion was that where would be more to worry about from single-celled life detritus, heavy metals, forever chemicals etc, but the marketplace doe…

Well they carry all sorts of bad stuff, and recently have been found to cross blood brain barrier.

Re: Why is desalination so difficult?

#349

Post by doubleg72 » Thu, Jul 06, 2023, 3:03 AM UTC

ars wrote: post #260

darkclouds wrote: post #142

Earlier quoted context omitted.

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…

> the water in the tank was so pure it had dissolved a spanner/wrench that was left in the bottom Once it dissolved a tiny bit of the metal it would no longer be so pure. So this sentence makes no sense. It takes just a minuscule amount of mineral to replicate regular well-water.

I thought the same thing… quite the irony

Re: Why is desalination so difficult?

#350

Post by engineer_22 » Thu, Jul 06, 2023, 3:07 AM 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…

-> I've often wondered why dont we have more pure water pumped through the water mains in various countries,

It has to do with the economics of water treatment. If you supply water you're incentivised to supply it at the lowest acceptable level of treatment. That's because people just want plain old water, and they don't want to pay a lot for it.