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Why is desalination so difficult?

practical.engineering

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Re: Why is desalination so difficult?

#631

Post by pvaldes »

eru wrote: post #620
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…

> I figure out that there is a lot of free "all that you can eat" pressure in the sea bottom. Have you heard of the second law of thermodynamics? There's also lots of all-you-can-eat heat in any piece of matter, eg sea water or rock. But that doesn't mean you can get at it for anything useful, without a heat differential (or a pressure differential).

Hum, I wonder where we could find a heat differential and a pressure differential in this planet... if we except the ocean, and the land, and the atmosphere, and any place with an organism able to fart...

Re: Why is desalination so difficult?

#632

Post by tinus_hn »

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…

To me it doesn’t seem to make sense to first claim that you can’t filter for salt in water, and then talk about semi permeable membranes and osmosis. Those membranes are filters for salt!

Re: Why is desalination so difficult?

#633

Post by pvaldes »

eru wrote: post #627
Ajedi32 wrote: post #408

Earlier quoted context omitted.

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.

Laws of thermodynamics aren't really useful here. They are perfect for explaining universe, and notoriously bad for predicting local events when you zoom and focus at a fine grain level and add time. Life beings are apparently breaking or "delaying" this laws all the f*ng time.

In any case nobody is trying to make a perpetual movement machine.

The goal is to create a cheaper way to extract salt from saltwater, and use the pressure gradient at the sea to put apart brine and water could be a solution waiting for an engineer (in my opinion). It is assumed that will not be free in terms of energy. It doesn't matter as long as is slightly better than the current solution. Would be much faster than waiting for the sun so it does not need to be cheaper than that. It just needs to be able to replace the last energy-expensive phase of our current solution by another process that is cheaper or faster.

Re: Why is desalination so difficult?

#634

Post by pvaldes »

eru wrote: post #621
pvaldes wrote: post #387

Earlier quoted context omitted.

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…

If Titan proved something is that economics is not the only factor what matters here.

Re: Why is desalination so difficult?

#635

Post by DropInIn »

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

So they plan to build a wall in illegally occupied territory and salt the earth in a way that ensures thier neighbors land is useless?

JFC that country is fucking evil

Re: Why is desalination so difficult?

#636

Post by DropInIn »

refurb wrote: post #630
oska wrote: post #629

Earlier quoted context omitted.

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.

No, they are fundamentally changing the salts state and manner of interaction with the environment.

What do giant piles of salt do aside from melt and "burn" whatever they touch? That's right, they Spread.

Re: Why is desalination so difficult?

#637

Post by markhahn »

ClumsyPilot wrote: post #388
fsckboy wrote: post #320

Earlier quoted context omitted.

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…

> plastic is pretty inert which is why it remains around for so long, and while it is made from toxic things, it's generally considered safe. This is very ignorant. plastics release a wide variety of organic compounds. "Most plastic products, from sippy cups to food wraps, can release chemicals that act like the sex hormone estrogen, according to a study in Environmental Health Perspectives. The study found these che…

really unfortunate how easy it is to channel fud.

for instance, the "can release" weasel words here - technically correct, but also misleading.

it is not the case that all changes you see in plastics are because they're releasing nasty (((chemicals))) into the environment. it is routine to engineer plastics to meet arbitrary emission/contamination standards. different kinds of plastics also require different (or no) such additives. mostly, people are thinking of PVC when they worry about this.

Re: Why is desalination so difficult?

#638

Post by feoren »

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…

Have you read any of the sources you linked to? They debunk most of what you just said. "Ultra pure" water is very slightly more "watery" than regular tap water. Any effect that tap/bottled water has on anything, "ultra pure" water can at most have about 1.01x that same effect.

> water mains pipes last longer if its delivering hard water compared to soft water

This is the one thing that may actually be true, but it's because the minerals in the hard water build up on the inside of the pipes over time, creating a protective layer. This is especially great for lead pipes, as it can prevent the lead from leeching into the water.

Re: Why is desalination so difficult?

#639

Post by rvba »

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…

How are those membranesmade that they have pores + dont break?

Re: Why is desalination so difficult?

#640

Post by ClumsyPilot »

markhahn wrote: post #637
ClumsyPilot wrote: post #388

Earlier quoted context omitted.

> plastic is pretty inert which is why it remains around for so long, and while it is made from toxic things, it's generally considered safe. This is very ignorant. plastics release a wide variety of organic compounds. "Most plastic products, from sippy cups to food wraps, can release chemicals that act like the sex hormone estrogen, according to a study in Environmental Health Perspectives. The study found these che…

really unfortunate how easy it is to channel fud. for instance, the "can release" weasel words here - technically correct, but also misleading. it is not the case that all changes you see in plastics are because they're releasing nasty (((chemicals))) into the environment. it is routine to engineer plastics to meet arbitrary emission/contamination standards. different kinds of plastics also require different (or no)…

The problem is that if you are a common joe, you cannot know which is which. .So you have to assume that a random product will be leeching
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