Desalinating ocean water takes seawater, separates the fresh water from the brine, and usually pumps the brine back into the ocean. But what if we didn't pump the brine back into the ocean? (For example, if we left it out to dry in the sun and then buried the powder that's left.) If we did this long enough, would it eventually turn the world's oceans into freshwater and harm ocean life?
What does it take to reduce the salinity?
The salinity of sea water is around 35 g/kg. There are around 1,350,000,000 km³ of water, so roughly 1.3x1021 kg of seawater (1 kg/l, which is a bit off for saltwater of course, probably by 35 g). Which contains about 4.7x1019 kg of salt.
To reduce the salinity to just 34 g/kg, you need to extract 1.4x1018 kg of salt and store it in a way that it doesn't end up in the oceans again.
An estimate of the yearly produced salt is around 3.0x1011 kg/yr.
So you could supply the whole world with salt from the ocean for close to 5 million years and have decreased the salinity only by 1 g/kg. And to get freshwater oceans you need roughly 150 million years at the current rate.
I don't know how wildlife reacts to this change, I guess there are highly specialized species which cannot cope with this change, but considering the time frame it would take to desalinate all the oceans, they could evolve I guess.
How big is a mountain made out of the whole salt?
If we take sodium chloride (most of it is this) we have a volume of around 2.1x107 km3 of salt. (2.17 g/cm3 assumed)
The Mount Everest is estimated with a volume of just around 60 km3. So you need to stow away 350,000 additional Mount Everests.
If you dump it all on one spot and the cone would form in a way that the height is half of what the radius is (not sure if that is a reasonable assumption, but with a small amount of salt it looked that way). The resulting salt cone mountain would be 171 km in height.
I really hope I haven't messed up the trillion and billion things. So stuff could be of by three or six orders of magnitude :-/
The residue dry powder you refer to is salt.
Salt is toxic to most plants. The United Nations claims the world is already losing 2000 hectares per day of farm land to salt-induced degradation. This is land that is used to feed people.
In some situations, salt from affected lands can contaminate underground sources of drinking water, which will affect people, agriculture, wildlife and wilderness.
Salt affected land can be reclaimed, but it involves leaching out the salt and depositing it elsewhere; usually into rivers that transport it to the ocean. Some of the salt from reclamation is used for industrial purposes.
To store salt as you suggest would require the construction of water impervious basins, including the top, that would be unaffected by natural or human influences for perpetuity. Such influences would include: earthquake, ground movements, water inundation of the region either by rain or flooding, digging by animals or humans.
In some parts of the world sea water is used to produce salt for human consumption and industrial purposes. Evaporation ponds are created on the coast and the ponds are flooded with sea water, the water evaporates , leaving the salt which is then collected.
A smarter way to deal with the brine from desalination plants would be to send the brine to evaporation ponds and collect the salt for human or industrial purposes.
The oceans are salty because the slightly acidic rainwater dissolves minerals from ores and rocks and runs into the sea. This is a continual process, a consequence of erosion. However, the salinity of the oceans has been stable for millions of years, indicating that there is an equilibrium between processes in both directions. Salt is removed from the oceans (buried in sediments, evaporation, reaction with basalt, etc.) at the same rate it is added. So if you did manage to remove enough salt to affect the salinity (which as noted in other answers is infeasible), natural processes of erosion and sedimentation would gradually return it to normal over geological time.
Desalination of ocean water costs energy; beside fresh water, you gain salt. With some additional energy invested, you could purify this salt consisting to large extent of sodium chloride (NaCl) which may be used as table salt. In other places, you mine for rock salt from underground mines, and equally perform a purification of salt, yet without the generation of drinking water. Thus, there could be an economic and ecologic benefit to harvest and use both products at once.
If the purity of the sodium chloride does not pass food quality, nor is used to deice streets (which obviously has ecological implications on its own), then the NaCl isolated may be starting material to prepare other chemicals of large demand; e.g., sodium hydroxyde, hydrogen and chlorine gas by electrolysis (a subsequent combustion of chlorine with hydrogen yields HCl, its dissolution in water yields hydrochloric acid), or sodium carbonate in the Solvay process (used e.g., in the production of glass). Depending on the scale of operation, perhaps other compounds than NaCl may be side-products of interest, too.
Think of what happens with the fresh water that we extract via desalination. It's used for drinking, general water supply, farming and some other industries. In either case it will either eventually evaporate and eventually end up in the ocean or it is discharged back into the ocean.
Some water is of course lost in the process, but it's a relatively small amount so overall should not affect ocean's salinity, likely to have a negligible effect compared to the existing trend of increasing salinity due to anthropogenic climate change. There are more details in the linked pdf - Regime Changes in Global Sea Surface Salinity Trend
Just a rough estimation of your plan. The oceans contain approximately 1.3E9 cubic km of water --> 1,3E18 tonnes (1000kg) of salt water. In there there 3% of salt --> 3,9E16 tonnes of salt, which would require 1,8E16 m^3 of dry storage. Assuming we make the mountain 3 km high, this would still require 6 million square kilometer of storage, which is almost twice India or 60% of the USA. Good luck on your project :)