Already been done. Along with alum, tons of it used in India for decades to purify water. Various case studies that list benefits of it.

Too high doses of manganese are clearly linked to Parkinsonism. (If you want, I can list the sources.) That however pertains to acute intake, not chronic.

To get manganese ions, you'd have to acidify the water somehow too.

Are you saying this from a point of experience or a point of caution?

PP-treated water is safer than chlorine-treated water.

Are you saying the water will remain like this for a period of months or years?