Desalinization is becoming better understood, as well as cheaper, but the biggest challenge is still what to do with the leftover salt. We can't pour it back into the ocean because it will kill everything. Hopefully someday we find a way to clean and safe way to dispose of salt.
> We can't pour it back into the ocean because it will kill everything.
That's simply false. The amount of water that would need to be permanently removed from the ocean in order to measurably increase the salinity would be astounding. Let alone the amount that would need to be permanently removed to kill anything.
If local salinity was a problem (which it isn't) there's an extremely easy solution: Simply pump more water, remove less salt from it. Dump it in over a wider area of shore. Let mixing take care of the rest.
Most of the water we desalinate gets dumped right back in the ocean, add the salt back. Even the water that doesn't get into the sewage system ends up back in the ocean again via the water cycle/rainfall. (meaning we need to worry about local salinity issues but not global)
Actually most sewage systems output water that is safe to drink so we can put that water right back into our drinking water system and forget the whole problem over an over again. (If you ban lawns there might even be water left over as food is turned into water and the food probably isn't grown with city water). People generally don't like the idea of drinking sewage though so this will never happen.
Most desalinization processes don't produce "salt", but extract a certain amount of unsalted water from the in put, leaving behind more salty water. Typically reverse osmosis at best reaches 1:1, that means you need 2 units of input water and get one unit of desalinated water and one unit of salty water. For sea water desalination 1:1 is quite optimistic though. This salty water can be safely put back into the sea. With very large scale plants you might want to make sure that you don't release the salty water at a single very concentrated spot.
These 1:1 are the best numbers for demineralizing sweet water - I assumed that salt water desalination had much different proportions, which also means, that the relative change in salt content is pretty small and consequently the impact of putting it back into the sea.
It's not really a significant problem. Blending it with heated cooling water from power plants or treated effluent from waste water plants are common means of brine disposal. Provided you spread out the disposal in areas with good flow, even releasing the brine directly would have minimal impact.
There's no reason why we couldn't ship it into the middle of a desert if for some reason it couldn't go back into the sea, it wouldn't go very far once it was dumped. Conveniently, the places that rely/will rely on desalination the most happen to be desert countries.
I'd imagine the problem is the concentration of salt where the dumping occurs. There would need to be a system in place to spread the salt out over a large geographical area at concentrations that is not harmful.
It's not a global problem but a local one. Locally increased salinity and temperature can be a problem. You can drop a whole bunch of hot brine in the middle of the ocean to little effect, but the same is not true of a mangrove swamp or a bay.
In general, perform reverse osmosis (RO) until the effluent brine is up to 70 g/kg salinity. Then pump it out onto a salt pan, let the rest of the water evaporate, and sell the evaporite.
Petrochem tech comes into play here, because in steam extraction of oil-sands, wastewater comes back up contaminated with silicates, and has to be treated before it can be reused. It turns out that similar processes can be used to further concentrate desalinator effluent above the 70 g/kg that regular RO tops out at, to about 130 g/kg. From there, any solar/thermal process equipment (i.e. flash distiller) can be made much smaller.
For reference, seawater is typically 35 g/kg, and the top stratum of the Dead Sea averages 315 g/kg (with significant fluctuation due to local weather history).
So Jordan and Israel can actually do RO on water from the Red Sea (40 g/kg), and pump the effluent to the Dead Sea. They don't have to worry about the hypersaline brine killing anything, because the Dead Sea is already dead (just like it says on the tin). And the pumping is easy, because the Dead Sea is below ocean level. A pair of siphoning aqueduct pipelines (40 g/kg and 70 g/kg) can supply a RO desalinator in every town from Aqaba to Potash City.
Based on the article disposing of the double salty water "willy nilly" is the problem. We should be able to pipe a system that delivers it over a wide area. Nice info though!
I wonder if there's a good use to which the highly-salinated solution (the "leftover salt") could be put. It's a stretch, but I vaguely recall something about storing excess solar energy thermally, in dense piles of molten salt...
Part of me wonders if the water could be used for fracking as a double down on energy generation, but the other part of me says that if that were possible that people far smarter and richer than I would have already been on top of this.
An abundance of salt hardly sounds like an environmental issue. By that I mean it's not a gas that's going off into the atmosphere nor a liquid seeping anywhere (on its own)
Load salt on the huge cargo ships already crossing the oceans. Build machines that dump salt overboard slowly and continuously as the ships make their voyages.
The ships are probably full one direction (when traveling from countries that manufacture a lot) but relatively empty on the way back, so there's probably free space.
