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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#331

Post by nardi »

aerio wrote: post #321
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 a…

The article talks about why it's maybe politically or economically difficult, but not why it's physically difficult.

Re: Why is desalination so difficult?

#332

Post by reyoz »

codedokode wrote: post #312

The author have mentioned that water with more than 1-2 promille of salt is considered not drinkable. But I have a bottle of carbonated mineral water from the mountains and the label says it contains 6-9 g of salts per 1 liter. So I guess it depends on which salt it is. Also, the author mentions that people in US use 1100 liters per day (which is too much in my opinion), but not all this water needs to be drinkable,…

Having a diversity of water supplies and using water fit for purpose reduces demand for drinking water. Toilet flushing, irrigation and even washing machines do not need high quality drinking water. I have 5,000L of rainwater storage that I use for toilet flushing and irrigation. Combined with a water efficient shower head (typically the largest domestic water use in my country) we use 100 L/d/person. In some areas of my city there is dual supply plumbing that delivers highly treated wastewater for these uses.

Re: Why is desalination so difficult?

#333

Post by BenjiWiebe »

codedokode wrote: post #312

The author have mentioned that water with more than 1-2 promille of salt is considered not drinkable. But I have a bottle of carbonated mineral water from the mountains and the label says it contains 6-9 g of salts per 1 liter. So I guess it depends on which salt it is. Also, the author mentions that people in US use 1100 liters per day (which is too much in my opinion), but not all this water needs to be drinkable,…

Salts != Salt (sodium chloride)

Re: Why is desalination so difficult?

#334

Post by mcdonje »

sophacles wrote: post #209
coryrc wrote: post #201

Earlier quoted context omitted.

Because the US has plenty of free water that's growing alfalfa in the desert. Subsidies are unnecessary.

Are you sure? The alfalfa itself is subsidized. (I mean subsidies besides giving all the water to the farmers thing).

Yeah, the subsidies are actually the problem.

Re: Why is desalination so difficult?

#335

Post by markwalllberg »

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…

So basically, filters.

And also. None of this is a challenge at all. R.o. water is simple

Re: Why is desalination so difficult?

#336

Post by cyanydeez »

xyzzyz wrote: post #324
cyanydeez wrote: post #241

Earlier quoted context omitted.

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.

For you, locally, sure.

It'll kill however, plenty of things.

Re: Why is desalination so difficult?

#337

Post by Robotbeat »

cyanydeez wrote: post #336
xyzzyz wrote: post #324

Earlier quoted context omitted.

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.

For you, locally, sure. It'll kill however, plenty of things.

It won't kill anything. The surface of the ocean has natural fluctuations of salinity all the time due to evaporation, and it doesn't cause mass die offs. It's literally a necessary part of the ecosystem (and in fact discharging freshwater can cause problems to the ecosystem). But if your discharged water is within the natural 30 to 40 grams of salt per liter range, there's no ecological problem here.

Re: Why is desalination so difficult?

#338

Post by Robotbeat »

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%∆.

It's not waving a word around. There's natural variations of salinity of oceanwater, roughly 30 to 40 grams of salt per liter. So diluting the discharged brine so it's within that natural evaporation-driven band would be perfectly safe as it's in the natural variation of salinity.

Re: Why is desalination so difficult?

#339

Post by ateng »

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?

Agriculture and industry can use _a lot_ of water. Likely the 4000l figure includes that.

Re: Why is desalination so difficult?

#340

Post by Robotbeat »

8n4vidtmkvmk wrote: post #247
Robotbeat wrote: post #199

Earlier quoted context omitted.

Only if the discharge is not diluted sufficiently. There is nothing about reverse osmosis that is fundamentally more toxic or harmful than the typical evaporation that takes place naturally in the ocean. And it’s pretty dishonest to claim otherwise. If there’s a problem with too high salinity of discharge, that’s an engineering problem that should be fixed with greater dilution.

What would you dilute it with? Surely not the water we just extracted?

You dilute it with more seawater. The salinity of the ocean changes significantly just due to evaporation. The band of variability is roughly 30 to 40 grams of salts per liter for sea surface salinity. So if your discharge is within that band, it's not going to be a significant ecological problem (especially if the discharge is at the right location). It's no more a "mass ecological problem" than surface evaporation of seawater is.
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