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.
https://www.sciencedirect.com/science/article/pii/S1470160X14002660
https://pubs.acs.org/doi/10.1021/acs.jchemed.0c01194#
https://markets.businessinsider.com/commodities/corn-price
https://smartwatermagazine.com/blogs/carlos-cosin/evolution-rates-desalination-part-i?amp
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.
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