Saudi Arabia.
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They can easily use PV and Wind Power to desalinate sea-water on big scale.
And put enormous amounts of briny sludge into the seas. The seas are not renewable.
https://www.theguardian.com/global-development-professionals-network/2016/sep/29/peak-salt-is-the-desalination-dream-over-for-the-gulf-states
I doubt but can't prove it, that the amount of potable water needed by all mankind would significantly change anything in the oceans as a whole. If we compare the total amount of ocean water and compare it to the total amount of potable water mankind needs and considering that the water will rain and flow back into the ocean ultimately, this does not sound reasonable.
What i could imagine that in a closed environment (see) like the Mediterranean this could be an issues but not in open says of significant size like i.e the Indian Ocean, Pacific or Atlantic Ocean.
Simply too much volume and we're not talking pumping poison back to the seas but only materials that were taken from it beforehand.
Your first paragraph is totally correct, as is your second. But I was considering Saudi Arabia.
But the oceans are mostly a desert mostly devoid of life. Life abounds in certain places, coastal regions being one. Saudi Arabia and the Gulf States are using the Red Sea and the Persian Gulf for desalination. These are enclosed bodies of water already under environmental stress. Sea life likes salt, needs salt - up to a point, above which which it effectively becomes a poison.
Saudi Arabia already (2018 estimate?) produces around 30 million m3 of brine per day, but seems to have very inefficient plant (see below). Without improved efficiency, scaling that up to meet all water needs implies brine production up by around 10 times, i.e. to 300 million m3 per day, or 100 KM3 per annum. Even with the latest and best managed technology, that's 25 km3 per annum. And add the other gulf states to the problem....
It is very possible that brine could finish off a lot of life in those waters. It probably already is.
The end of life in the oceans? NO.
Yet another kick in the teeth to the web of life? YES
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I downloaded this a couple of months ago.
file:///C:/Users/AC1/Downloads/Qadiretall2019-Stateofdesalination.pdf
The state of desalination and brine production: A global outlook Marh 2019Extract
Current global brine production stands at 141.5 million m3 /day, totaling 51.7 billion m3
/year This value is approximately 50% greater than the total volume of desalinated water produced globally.
Global brine production is concentrated in the Middle East and North Africa, which produces almost 100 million m3 /day of brine, accounting for 70.3% of global brine production. This value is approximately double the volume of desalinated water produced, indicating that desalination
plants in this region operate at an (very low) average water recovery ratio of 0.25.
Comparatively, all other regions produce substantially lower volumes of brine, with East Asia and Pacific (10.5%), Western Europe (5.9%) and North America (3.9%) having the next largest shares. Interestingly, these regions produce a substantially lower volume of brine than the amount of desalinated water they produce, indicating that recovery ratios are generally high. This is particularly apparent for North America, which produces a substantially lower volume of brine
than it does desalinated water, suggesting that desalination facilities operate at an average recovery ratio of 0.75.
In other geographical regions, brine production is approximately equivalent to desalinated water production (i.e. RR = 0.5).
As with desalinated water production, high income countries produce the vast majority of global brine (77.9%). It should be noted that ‘high income’ includes both countries from both highly developed world regions (e.g. North America, Western Europe), whose brine production tends to be smaller relative to the desalinated water production, and the oil-rich Gulf nations who typically employ thermal desalination technologies with low recovery ratios, hence high brine production. For example, Saudi Arabia alone produces 31.53 million m3 /day brine, accounting for 22.2% of the global share. The next three largest producers of brine are also oil-rich countries, with the UAE, Kuwait and Qatar having 20.2%, 6.6% and 5.8% shares in global brine production respectively. Together, these four nations produce 32% of global desalinated water and 55% of the total brine.
Comparatively, the USA produces 10.91 million m3 /day of desalinated water (11.4% global
share) but produces just 5.28 million m3 /day of brine (3.7% global share).