eli5: Why is it so difficult to desalinate sea water to solve water issues?


eli5: Why is it so difficult to desalinate sea water to solve water issues?

In: 1972

It isn’t. It’s quite easy. But it’s not as cheap as just sucking it out of a hole.

Many things are “easy” but they’re just harder to make profitable.

it is extremely energy-intensive which makes it very very expensive.

the salt dissolves in the water so you cant separate them with a sieve or something similar.

The science of it is pretty easy. You can do it in your kitchen. Doing it at the scale needed to actually make it useful requires a lot of facilities, and uses up a lot of energy.

It isn’t difficult in the sense that there is no means to do it. In fact it is a fairly simple process.

The difficulty is cost and the main elements are cost of the capital (buildings, equipment) and cost of energy required to accomplish desalination.

We use a lot of water, so facilities for desalination have to be very large to make a difference. This introduces many capital cost factors – like land, disposal of brine etc.

Desalination is energy intensive and energy is expensive. Water, unfortunately, has traditionally been “sold” at a very low cost. In this sense, many parts of society consider it a “right” etc which makes it a politically and socially difficult product to sell at the price that would support desalination (ie pay for the capital and energy needed)

Having said this, there are places and countries like Saudi Arabia, Israel and Singapore who have integrated desalination into their water infrastructure. So it is not impossible, it is simply uneconomical or politically difficult to do in many cases.

One other item others haven’t mentioned is what to do with the brine (ultra salty waste). If you’re filtering enough water to make desal useful, you get quite a bit of leftover brine. If you continue throwing this back into the ocean in the same place, the spot becomes toxic to creatures around it. Similar if you try and throw it away on land. It’s always better to not create more environmental problems than you’re solving.

Is there any progress in desalination each year? Is there a cool ‘Moore’s Law’ type chart showing that the cost per Gigalitre is dropping and when it reaches $X it will be better than dams & artesian bores etc ?

It’s not difficult. It’s pretty easy really.

But it’s also super expensive to do at an industrial scale. And then exporting the water to where it’s needed is expensive too.

Energy energy energy. The absolute theoretical minimum energy needed to separate water from seawater is about 1.1 kilowatt hours per cubic meter. The practical theoretical limit for a perfect machine of reasonable size is about 2 kWh per cubic meter. At average American electricity rates that’s about $0.20 a cubic meter. Not bad, cheaper than Dasani!

The cost of actual plants being built today is not too far from this, about $0.40 or $0.50 per kWh.

But it takes about 1 cubic meter of water to grow 1 kilogram of corn, and 1 kilogram of corn sells for about $0.20 at the time I write this.

So you can see that for drinking water desalination is a practical option, but for agriculture it can’t compete with places that have natural rainfall and irrigation.





Not difficult at all. For the easiest method, all you have to do is steam the water and catch the droplets. The issue is energy.

[A person, on global average, spends 3800 liters of water a day](https://www.theworldcounts.com/challenges/planet-earth/freshwater/how-much-water-do-i-use-a-day/story). Yes, you read that right, it’s not a typo. And no, it’s not only from your daily shower. 90% of it is spent indirectly through agriculture and other products we buy.

Now imagine having to boil 3800 liters of water PR PERSON every day. You quickly see that this isn’t sustainable on any scale.

So boiling is out of the question, what other options do we have? Reverse osmosis is another simple one in theory. For the ELI5 explanation: You push the water through a membrane so fine that larger molecules like salt and other particulates are filtered out. It uses a lot less energy than boiling, so why don’t we just use this? It’s mostly down to cost and maintenance. It still requires huge pumps that are still quite energy expensive, and the membranes needs to be swapped often due to clogging up of bio and organic material. It’s likely the path forward, but we’re not quite there yet to make it properly cost effective.

There are places that have integrated desalination plants, especially some popular tourist islands, but it is done with the knowledge that it’s a money drain.

Salt really likes to dissolve in water and stay there. It’s hard to get it out, and what that means in practice is that it takes a lot of energy to do it. Which means it’s both expensive to run/build, and potentially environmentally problematic depending on your energy source.

Anyways, there’s two primary ways that it’s typically done. You can heat up the water until it evaporates. The water turns to vapor and floats away, leaving the salt behind. Then you can collect that vapor and condense it back to liquid, and you get nice fresh water. But it takes a lot of energy to evaporate a significant amount of water, especially if you want to do it in a relatively small footprint (Sunlight evaporates millions of tons of water every day, but there’s no easy way to collect most of it).

The second way is to force the water through filters that are designed to separate the salt from the water. The problem with this is that to do it at a meaningful scale, you need a lot of filters (which require fairly consistent maintenance/replacements) so operating costs are high, and you generally need to pump the water through those filters at high pressures, which means more energy use, and more costs and so on.

Unfortunately, neither of those methods get particularly great economies of scale as you size up the operations, so it’s generally just rather expensive.

One of the reasons is water is very heavy. Desalination plants are set up next to the ocean at sea level. Whatever water you make has to then be pumped, up hill , to where it’s needed. In a place like Isreal that can be a short distance and economically feasible. Trying to get that water over long distances, say from the coast of the Indian Ocean to the interior of Ethiopia would be very expensive. Most water systems try to use gravity for this reason. Dam a stream in the hills, create a reservoir and let gravity bring the water where you need it. What the Romans did. A good argument could be made that we build massive pipelines all over the world for oil, why not for water. I guess we’re willing ( in the developed world ) to pay $5 for a gallon of gas, not yet for H2O

It’s slightly more expensive. It shows you the actual scale of the water crisis. It’s not a non issue but it’s not the type of crisis that typically comes to mind when you hear that word. Agriculture is the most intensive use of water by multiple magnitudes. If the water supply runs down to the point that it’s not economical to perform agriculture they would just quit growing stuff there and grow it elsewhere and truck it in. There’s no need beyond economic benefits to grow almonds and avocados in what nearly amounts to a desert. There is other arable land in the US even. Trucking them across the Rocky Mountains may be expensive though.

Of course there’s a lot more to all of this, this is a very general overview. And it totally ignores all the environmental impacts of every step mentioned above which is not trivial by any means.

Okay there are a lot of answers which all circle back to scale ans costs–but I wanted to add my two cents because IF it could be argued enough you could make the military do it.

I was a 92W, water treatment specialist, they showed us how to run different types of equipment that can make drinkable water at differing speeds etc

You have the Light Weight and then you go to the others. But the process is easy–

Throw out the Dolphin Strainer it pulls in water and keeps out the first large debris, it gets pulled into a Raw Water bag or Tank where a High Pressure pump forces the water through strainers and what ever chemicals are needed to kill bacteria and then it is finally sent to the Clean Water bags. That’s the short and fast process.

They’d would have to go through filters faster because of the salt, but it’s cheaper then the canisters they have to use to clean a chemical attack out of water.

Then to comes back to transportation again but still you have a Trucking MOS.

All the rejected treated water is held in containers because your supposed to dispose of that downstream of where ever your pulling from and certain areas just have rules against dumping the chemically treated water.

Long Story short if anyone suggests a stupid simple idea upper management will explain why it’s not worth it–ignoring that it could fix things temporarily or in the long run.

Like Flint Michigan, nestle set up their factory and is charging them for their own water instead of being charitable.

They do it on cruise ships…. how hard can it be?

I think this is whst Gedaffi was successfully doing in Lybia before America bombed them back to the stone age

Florida has more than 130 desalinization plants. It is the most expensive water that is produced by utilities in the state. The energy needed to push the water through the filters in the reverse osmosis plants is the most expensive component, but the facilities and the filters which must be replaced frequently are also expensive. Plus, an unforeseen expense at the Tampa Bay Plant is the exotic zebra muscles grow inside the pipes, so the plant must be shut down and divers sent inside the pipes to clean the muscles out.

It isnt, but the energy cost is high. And the energy cost will remain high because thats how they maintain control…

Supply + demand. If you dont have scarcity, you make it.