Just a guess, but probably silver: http://en.wikipedia.org/wiki/Medical_uses_of_silver#Water_purification http://en.wikipedia.org/wiki/Medical_uses_of_silver#W

Just a guess, but probably silver: http://en.wikipedia.org/wiki/Medical_uses_of_silver#Water_purification http://en.wikipedia.org/wiki/Medical_uses_of_silver#Water_pu...