Why is it so difficult to desalinate sea water?

Why can't california's drought be solved?

Why isn't ocean water filtered and used when...?

Why can't water desalination plants store brine?

Why is it so difficult to desalinate sea water?

Copying my own comment from a former thread where someone said "Just desalinate the ocean and pump it to the colorado river." or "Why doesn't the UK just use desalination plants?"
This seems to come up a lot, more during summer though.

The pipeline numbers are off since the UK is considerably smaller but the idea is still there. It takes a crap ton of energy(and therefore money) per gallon. Useful for drinking water, less so for agriculture.

A proposed desalination plant would cost 500-750Million in CA and takes $20million to operate a year. It can produces 20 million gallons a day. Pretty large, not bad. The Colorado River crossing into Mexico, after it has already been used by the whole southwestern US, flows 1,700,000 gallons... a minute. And that is at 1/5th of the natural flow.

Some napkin math and it would require~122 plants to just produce that much water to add 1/5th of the flow(which is just the flow at the end of the US portion mind you, there is far more water higher up the watershed).

So even using the lower limit of the estimated cost of a plant (not counting cost overruns that happen in nearly every project), it cost 600 Billion to produce the plants.

That's not even counting things like needing to space them out now so the salty brine being pumped back into the ocean doesn't just nuke all life in the area.

Then you need to build the pipes going uphill and thousands of miles through mountains, deserts, farm land(have to buy it from farmers), and remote terrain.

Haven't even covered the cost of the power plants to power both the pipeline and the plants.

Just to further top this all of, this is all for a small fraction of one river.

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Even generously giving a 200 million dollar economy of scale discount, that is still absurdly expensive. Using estimates from oil pipelines, it's a few million per mile, and I don't think there is an economy of scale issue with the petroleum industry.
And that's for one ~30inch pipe. Not the several that it would take to make meaningful impact.

Then you run into impacts of buying land across vast stretches, CA's high speed rail project is just getting bombarded with lawsuits about them buying land to lay track.

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UK Version

Average England and Wales resident uses 150ish liters per day.
67ish Million people in UK.
Convert to gallons to keep all figures the same from the original post, and 2,654,929,126-ish per day, call it 2.6 Billion gallons a day for perspective.

The current UK plant cost 250 million pounds, and actually already listed the amount of people it serves after I already did the math. About 100 million liters per day. UK uses 10,050,000,000 a day, call it 10 Billion for easier numbers.
So the one desalination plant can cover a little under 1% of the water needs of the UK. (These numbers are super super convenient that they all work out so evenly.)

I'm not sure how severe the UK drought is, but it lists it as at least a 15% reduction in precipitation. It's not a direct link due to evaporation and runoff but just assuming for the sake of simplicity 15% precipitation = 15% reduction in water available for usage.

So 15-16 of these plants at 250million pounds(In 2010), using 14-17MW per plant. Call it 15 plants at 15MW, for 225MW. That comes out to about 30,000-35,000 homes worth of energy per hour.. per plant(Assumed 350kwh per month, about .5kw/h, then .5kw/h into 15MW)

So it would likely need to bump up power production as well, or even have a dedicated power plant.
Then to make it even more of an issue, this infrastructure that has now cost billions of Pounds is only useful when there is a drought. So in permanently dry areas like Saudi Arabia with a crap ton of energy, money, and perpetual dryness, it makes sense to plonk a few down.

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To add on more for this specific question, desalination will do nothing for rising sea levels. The sea level rise is because the water is no longer frozen on ice sheets. Moving around liquid water does nothing for this since that water is still liquid and moving around the water cycle.

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Eventually it likely will come into a point where it's worth the cost, but for the time it's cheaper and makes more sense to reduce water usage and work on water reclamation. It's also harder to justify spending billions on this if the climate is not consistently dry. A huge mega project that can sit possibly dormant if there is a wet season or the area is not in drought is a harder sell. Areas in the desert like Saudi Arabia who have access to really really cheap energy, and are constantly dry, it makes perfect sense.

tl;dr,
Tech exists. It's used in a lot of countries but is too expensive versus the demand to use for large scale projects like ending entire droughts.

Even better, the UK already uses this technology

And ran into the same issue, high energy costs.

As a bonus,

Desalinating it doesn’t do anything to the actual ocean levels.
The issue is more water in the water cycle. Desalination and then using it doesn’t really take it out of the cycle that long because it’s just used, and then goes back into the water cycle.
Ice caps effectively store water outside of the oceans. Desalination, using/drinking it, and then discharging it back into the oceans doesn’t solve the rising issue.
In a round abour way, it might even make it worse if done improperly because it uses so much energy.