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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#131

Post by tivert »

grogenaut wrote: post #123
tivert wrote: post #119

Earlier quoted context omitted.

> I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere? Too much salt can kill stuff (e.g. people, plants), so I suppose that makes it "toxic." May…

Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are. What's wrong with pumping 10% CO2 into your office constantly from a compressed gas plant extracting oxygen and argon next door?

> Because the brine is hyper-salty compared to ocean water and takes a while to mix back in, essentially creating a new ecosystem, brackish, where the outlets are.

How large would those brackish areas near the outlets be? It seems to be that would be a big problem in an enclosed bay, but much less so on a shore facing open ocean.

Could they run pipes out a few kilometers with small, regular holes (maybe some modification of oil pipeline technology) to spread the discharge out and mitigate the concentration problem?

Could they make the waste output less concentrated? Maybe by either running the desalination process less (would that also increase energy efficiency?) or by pre-mixing the waste with some un-desalinated intake water?

Re: Why is desalination so difficult?

#132

Post by Veserv »

iudqnolq wrote: post #120
Veserv wrote: post #112

Earlier quoted context omitted.

Freshwater withdrawals per capita in the US (which includes agricultural exports and animal feed such as alfalfa sent to Saudi Arabia) are around 1550 cubic meters per year. https://ourworldindata.org/grapher/water-withdrawals-per-cap... So that is around $775 per person per year assuming no net change in water use. In contrast, Germany uses around 410 m^3, France around 475 m^3, and Australia around 724 m^3, so the…

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.

You were quibbling about how 1,000 liters per day does not adequately account for all usage. Freshwater withdrawals is on average going to be a overestimate and the US is one of the worst outliers with significant agricultural exports which are one of the largest contributors to differential water withdrawals. Despite this, it would only be $2.00 per day for a fully desalinated water supply even if we safely overestimate usage. Despite it being 4x higher than the person you replied to said, that is still incredibly cheap.

Re: Why is desalination so difficult?

#133

Post by janalsncm »

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…

A lot of public goods are that way. What you’ve stated is practically a tautology. “Government subsidy” just means the public is paying for it. Other things that fall into that category are universal education, the military, and police.

Re: Why is desalination so difficult?

#134

Post by jtriangle »

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.

Re: Why is desalination so difficult?

#135

Post by ZeroGravitas »

mplewis wrote: post #13
aurizon wrote: post #11

several better ways, multi stage stills and reverse osmosis are leaders of the pack, https://www.veoliawatertechnologies.com/en/technologies/mult... . https://en.wikipedia.org/wiki/Reverse_osmosis

The author specifically mentions reverse osmosis.

He also covers multi-stage stills:

> My garage demo has very little going for it in terms of efficiency. It’s about as basic as distillation gets. There’s lost heat going everywhere. Modern distillation setups are much more efficient at separating liquids, especially because they can take advantage of waste heat. In fact they are often co-located with coal or gas-fired power plants for this exact reason. And there’s a lot of technology just in minimizing the energy consumption of distillation, including reuse of the heat released during condensation, using stages to evaporate liquids more efficiently, and using pumps to lower the pressure and encourage further evaporation through mechanical means.

Re: Why is desalination so difficult?

#136

Post by jtriangle »

zokier wrote: post #9

Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?

Using a desert isn't a great idea, but, using the sun to evaporate water works just fine. You're just replacing an expensive heat source with a 'free' heat source.

That's not unusual in desalination however, many facilities are combo plants, they're producing power and then using waste heat for desalination.

Re: Why is desalination so difficult?

#137

Post by dylan604 »

Timshel wrote: post #107
function_seven wrote: post #100

Earlier quoted context omitted.

It’s toxic only because of concentration. AKAIK, there aren’t any compounds in the brine that weren’t present in the seawater to begin with. The solution is dilution, but I’m sure it’s easier for me to type that than it is to achieve in a desalination plant. But, why not really-long-pipe-with-small-holes-along-the-length? That seems to me like a simple mechanism to send the brine back into the ocean without causing a…

I believe I had read just dilute it until concentration is ok then release. But might be more tricky than that ^^ : https://www.sciencedirect.com/science/article/abs/pii/S00489...

dilute it with what? the clean water you just removed the salt from?

Re: Why is desalination so difficult?

#138

Post by tguvot »

jbm wrote: post #122
ramesh31 wrote: post #102

Earlier quoted context omitted.

>Is there a practical reason why it would be difficult to do this with solar power? Same reason it's hard to do anything with solar. Grid scale storage is an unsolved problem.

I wasn't too clear about what I meant, but is storage even necessary here? For example, would there be an issue if the desalination process was left in an intermediate state for X hours / days while power is intermittent? I wonder if there are more energy-expensive desalinization processes that are better to use with intermittent power sources, like solar.

desalination plants are built to produce specific amount of water in order to cope with demand. if you going to stop desalinating while there is no solar, you need more plants in order to desalinate more water during the day. also, in general, even during day, solar not always available.

Re: Why is desalination so difficult?

#139

Post by seunosewa »

profsummergig wrote: post #60

While on the issue of water: Some countries have floods in one part, and drought in the other. The floods are so bad that large numbers of people die. Here's a great challenge that should be worked on: how to capture the flood water and use it to mitigate the droughts. Could something like Elon Musk's Boring company concept fix this?

The cost of pumping dirty water uphill ...

Re: Why is desalination so difficult?

#140

Post by ejz »

One thing that this article missed was that it was San Diego centric. In Israel, desalination is a much bigger part of the ecosystem. Over half of its domestic water comes from desalination. Quite a bit of the problem in California, as in almost every industrial application, is just that we make it hard to do anything with atoms.
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