Desalination Plants: What Do They Do With Salt?

[Hoku]

I just wondered what these plants do with the salt they remove from seawater. Do they return it immediately, about where they took it from, or do they use it to infuse the fresh water that's returning to the oceans/seas with?

 

[RocketSci5KN]

Why would you use lots of energy to remove salt from seawater, and then throw it back
into a nearby freshwater river draining into the ocean? Wouldn't just using the fresh river water make infinitely more sense? I suspect they sell the salt to the commercial market.

 

[Hoku]

If our rivers were enough to meet water demands then desalinization plants wouldn't be on the discussion table. I'm looking for someone that actually knows how they work. If we desalinize water from the ocean to help supply our dwindling water resources, then I'd think we'd want to re-salinize that water as it returned to the ocean. Maybe they have a way to recycle the 'no-longer-seawater' so that, instead of returning to the sea, it just goes back to our cities. Maybe, as RocketSci5KN suspects, they let the water return to the ocean and just sell the removed salt. Whatever the case, I'd like to know how it works. Do you think I choose the wrong sub-forum??

 

[uby]

I'm no expert, but I'd imagine it goes something like this:

Seawater is pumped into the desalination plant where the dissolved minerals and impurities are removed. Fresh water is pumped via aqueducts or transported via other carriers to cities/farms/etc. for use. Removed minerals and impurities are sent to another facility where they are refined and sold as useful materials. Na/Ca/Mg salts (chlorides, carbonates, oxides mostly) are useful for a variety of mundane applications.

 

[Jobrag]

It depends on the type of plant if it uses reverse osmosis then the "waste" product is sea water with a greater concentration of minerals this would just be returned to the sea.

 

[MATLABdude]

The salinity of the seas has been stable for a few billion years:
http://en.wikipedia.org/wiki/Sea_water#Geochemical_explanations

The total amount of evaporation out of the oceans is estimated at around 434,000 cubic kilometers (km^{3}) per year (sure, most of it rains out over the oceans but a lot of it comes out over land, as well):
http://en.wikipedia.org/wiki/Water_cycle

In contrast, the estimated 12 billion gallons of water desalinized per day works out to a little under 17 cubic kilometers per year:
http://en.wikipedia.org/wiki/Desalination
http://www.google.com/search?q=12E9+gal/day+to+km^3/year

That's a tiny drop in the bucket when compared to the amount removed via evaporation (and since the water usually ends up back in the ocean anyways, salinity is maintained--assuming it doesn't pick up anything else along the way, although that's covered in the first Wikipedia article).

 

[Hoku]

Hey, dude! You've obviously used a process of reverse osmosis to extract the essential issue of this thread. From here on, the discussion probably belongs in the Earth sub-section, but I'll write this for now, anyway, and assume the thread will be moved.

I understand your arguments for the insignificance of desalinization. Here are my counter-arguments.

1) Fresh water that is evaporated from our oceans and seas has been more or less stable for the past few billion years. As a result, the sea life has adapted to the equilibrium of natures evaporation cycle. Therefore, natural sea evaporation has a net effect of zero and can effectively be dismissed since ocean life is adapted for this evaporation.

2) The amount of fresh water naturally evaporated from the oceans and seas occures over ever square centimeter of the oceans and seas. Even though there's an immense quantity of water evaporated, no single area is responsible for the bulk of the loss. Maybe there's a little more in some areas than others, but again, that's been established for a very long time and local sea life has adjusted appropriately.

In contrast, human desalination plants are very localized, and to places that are only near the shore, where significant amounts of marine life are found. So, although the net salt content in our oceans and seas is essentially unchanged, the parts that are changed are critical areas that will have an effect on the sea life there.

Currently, there aren't a whole lot of these plants around. But people are really pushing to make more as a way to offset our dwindling fresh water supplies,. This makes issues like these important.