Depending on how and where it is done large scale desalination can bring problems of its own.
From
Wiki:
Concentrate disposalRegardless of the method used, there is always a highly concentrated waste product consisting of everything that was removed from the created "fresh water". With coastal facilities, it may be possible to return it to the sea without harm if this concentrate does not exceed the normal ocean salinity gradients to which osmoregulators are accustomed. Reverse Osmosis, for instance, may remove 50% or more of the water, doubling the salinity of ocean waste. The benthic community cannot accommodate such an extreme change and many filter feeding animals are destroyed when the water is returned to the ocean. It is more of a problem as you move inland, as one needs to avoid ruining existing fresh water supplies such as ponds, rivers and aquifers. As such, proper disposal of "concentrate" needs to be investigated during the design phase.
EnvironmentalFrom an environmental point of view, in some locations geothermal desalination can be preferable to using fossil groundwater or surface water for human needs, as in many regions the available surface and groundwater resources already have long been under severe stress.
Aside from the energy costs of the process, desalination plants produce hypersaline brine that must be disposed of. These concentrates are classified by the U.S. Environmental Protection Agency as industrial wastes. The hypersaline brine has the potential to harm ecosystems, especially marine environments in regions with low turbidity and high evaporation that already have elevated salinity. Examples of such locations are the Persian Gulf, the Red Sea and, in particular, coral lagoons of atolls and other tropical islands around the world.
There ain't no such thing as a free lunch, as the saying goes.
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