Maine has been burned in the past by previous mining operations closing up and leaving the state to clean up the remaining mess (also in the OP article). Definitely a tough situation all around.
Regarding how much Lithium can be recovered from desalination waste:
The US currently has one operating desalination plant, Carlsbad, that processes 50 million gallons of seawater per day. If it recovered 100% of the lithium in that water, it would produce… about 16 tonnes of lithium per year.
Each day in the United States, about 27.4 billion gallons of water are withdrawn and delivered from surface water and groundwater sources for residential use
So if we supplemented 10% of our needs from Desalinated water (2.74 Billion gallons a day) and recovered the same max amount of Lithium as the example a day (50 million gallons a day for 16T of Lithium a year) then we get: