I'm trying to figure out a realistic setting that depends heavily on desalinating sea water for desert agriculture. Specifically, I'm trying to figure out exactly how early in the tech tree, this can be done.
The practical way to desalinate water on an agricultural scale – that part is critical – is by reverse osmosis. https://en.wikipedia.org/wiki/Reverse_osmosis#History says:
Researchers from both University of California at Los Angeles and the University of Florida successfully produced fresh water from seawater in the mid-1950s, but the flux was too low to be commercially viable until the discovery at University of California at Los Angeles by Sidney Loeb and Srinivasa Sourirajan at the National Research Council of Canada, Ottawa, of techniques for making asymmetric membranes characterized by an effectively thin "skin" layer supported atop a highly porous and much thicker substrate region of the membrane... In 1977 Cape Coral, Florida became the first municipality in the United States to use the RO process on a large scale with an initial operating capacity of 11.35 million liters (3 million US gal) per day.
So basically desalination on an agricultural scale started happening in our world in the late 1970s. But how much earlier could it happen? (Again, I'm talking a realistic setting otherwise similar to our own world, no magic or handwavium.)
Clearly, it needs to be after the Industrial Revolution; even if we imagine a medieval alchemist somehow stumbling on relevant techniques in the lab, you need industrial mass production to make it relevant to agriculture, which measures water in acre-feet.
But the above description sounds like a serendipitous discovery, or a large enough R&D budget, might have produced the relevant techniques in 1950.
What about 1925? 1900? 1875? There are lots of things that turn out to have been invented about as early as they could, because of the need for things like high precision manufacturing technology (bicycles, jet engines, computers). And there are other things that it seems really could've been invented earlier had anyone thought of them (stirrups, efficient plows, paper, Chinese wheelbarrows). Which category does reverse osmosis desalination fall into? What are the prerequisite technologies that would need to be in place before it becomes feasible?