once treated at the plant for bacteria, filtered and chlorinated. I have been using it in my cars for the past 25 years and have never had calcium, rust, corrosion and overheat problems in the cooling system. I usually change the coolant every 12 to 15 months and used a 50/50 mix. In the present cars (92 300Z and a 98 Infiniti I30) I use about a 45% AF mix that includes a bottle of Redline Water Wetter.
No doubt there are parts of the country (and world) that water quality is high in iron and calcium that would cause problems so distilled water would be a prefered choice.
