We have a 60,000 litre salt water pool that we have not added one drop of chlorine or salt in over 16 months. What do I need to do to be able to use that water to water my garden and grass?
Desalination is possible. According to Wikipedia ("Desalination"),
"Currently, approximately 1% of the world's population is dependent on desalinated water to meet daily needs .... The single largest desalination project is Ras Al-Khair in Saudi Arabia, which produced 1,025,000 cubic meters per day in 2014. Kuwait produces a higher proportion of its water than any other country, totaling 100% of its water use."
It looks like people are even working on developing very small-scale desalination processes. E.g. the "Desolenator".
I nearly got sucked down a rabbit hole investigating this, it's so fascinating.... But wow, I concur with Alephzero: big project; not really feasible. (However the Wikipedia article does have diagrams....) [j.k.]