This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
When water is deficient in some key mineral for it's intended
purpose, minerals are added by running it through a bed of salts.
Usually these beds contain sodium, calcium or magnesium bicarbonates
or sulphates; but the exact composition varies by purpose. High
quality water treatment systems have mineral cartridges downstream of
reverse osmosis units