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I have a question I've always been curious about regarding the topic.

As I understand it, the desalination process is that they take the salt water, create usable water from it and then salt sludge is pumped back out into the ocean. When I've read about it an issue from this seems to be that the waste is bad to the ecosystem they pump it back into plus that extra salt can't be that good for the ocean as a whole and its inhabitants. I understand that one plant might not be that much of an impact on the ocean as a whole but if it became more standard and over 50yrs+, especially with the changing climate and droughts maybe it would and there are greater ecological issues.

My question is: Why can't they just pump the salt water into a designated area in the desert?

I live in California and we have plenty of desert and every time I head out toward Las Vegas or travel up north I think about this.

Why couldn't some chunk of land be allocated where raw salt water can be pumped into and then let it evaporate? Wouldn't that just generate more rain which, at least in the winter, create more snow pack in the mountains and help create drinking water more naturally? The salt and stuff left behind could just stay there and become a salt flat. At a minimum couldn't they process the water and then pump the sludge into the desert (the remaining water would still evaporate) and it seems to me that would be a better solution than putting it back into the ocean for it to have more salt.

If it could be done on a reasonable scale, maybe some land that was once unusable could be terraformed to forest which would help with the climate, pull carbon out of the air, and on a 5 or 10 year cycle the trees can be cut down made into mulch and then new ones planted... Not the salt water or land where the salt is left but additional land allocation for this (we do have a LOT of desert) and I know sand would need to be cultivated but we also have a lot of cows and organic material we can use to help convert the sand into more usable soil.

Running some pipes from a coast inland while posing some logistical challenge doesn't seem unreasonable to me. Better than oil pipelines...

Thanks for the responses

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  • $\begingroup$ Hello, and welcome to Stack Exchange. A calculation on how much desalinated water might be needed to support, say, California, and how much the removed salt would affect the salinity of, say, San Francisco Bay, could be instructive. $\endgroup$ – Daniel Griscom Jul 15 '16 at 22:22
  • $\begingroup$ 13,870,000,000,000 gallons per year of water is used in CA as of 2010 according to the USGS. Lets say however we don't need all that to be processed as part of desalinization. $\endgroup$ – MadPeanut Jul 16 '16 at 1:21
  • $\begingroup$ The desalinization plant in Carlsbad is supposed to generate 100MM gallons of drinking water a day. Its a 2:1 ratio of saltwater to usable water so it needs 200MM gallons of saltwater/day or 73,000,000,000/yr. $\endgroup$ – MadPeanut Jul 16 '16 at 1:29
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Your first question, regarding salt pollution requires a back-of-an-envelope calculation. At present, about 4% of the global population relies, either wholly or in part, on desalinated water. Because this is energy intensive, it is also expensive, so such water users are parsimonious, using about 1% of the global domestic water supply. But let us, for the sake of argument, assume that the desalinated option really takes off at some time in the future, perhaps by means of less energy intensive 'forward osmosis'. We will assume that 10% of the current population (7.4 billion people) may eventually use desalinated water, yielding a daily delivery of bitterns with 2.45 x 10^^9 tonnes of salt. Compare this to the 1,5 x 10^^17 tonnes of salt already in the ocean - the salt return would be 0.0000016% of what is already in the sea. Moreover, even this trivial amount would be diluted by the hydrological cycle, because the water that is drunk, used for washing, etc, ultimately returns to the sea.

But you ask whether the local concentrated saline return would be harmful to the environment? Consider Adelaide, in South Australia, which boasts the largest desalination plant in the southern hemisphere. Very detailed studies, by both computer modelling and direct observation, show that the dispersion of the saline plume reaches concentrations undetectable against the background salinity, within a few hundred metres of the outflow. It certainly hasn't impacted the local marine ecology at all - and the South Australians are very particular about environmental issues!

Again, consider the greatest concentration of desalination plants in the world, in the Arabian Gulf. This shallow-water (<50 metres) Gulf has high natural rates of evaporation, which raises the salinity from the normal 3.6% to about 4.1%. This change is orders of magnitude more than can be attributed to the salinity change from all the desalination plants combined. The ecology and biodiversity of the Gulf is still rich, and not at all affected by local bitterns effluent. Climate change will intensify evaporation, which is much more of a worry than 'desal' plants.

As to your second question about changing the climate and increasing rainfall by pumping bitterns into the desert, there is a massive problem with that. As the salinity increases by evaporation, the vapour pressure at the lake surface decreases, which suppresses further evaporation. Since we would be dealing with hyper-saline brines to start with, we would rapidly approach an equilibrium in which the massively decreased evaporation would equal the natural deliquescence of the salt. In fact, evaporation would win, but only slightly. The loss of moisture to the atmosphere would be barely noticeable, and not sufficient to terra-form the down-wind area.

I suggest that the impacts of global warming will massively eclipse any trivial changes brought about by evaporation basins. If you don't believe me, I suggest you get some hydrologic and geochemical textbooks, and do some mass-balance calculations of your own.

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