This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
Potassium Permanganate is no longer commonly used in the developed
world to kill pathogens. It is much weaker than the alternatives,
more expensive, and leaves a objectionable pink or brown color. If it
must be used, 1 gram per liter would probably be
sufficient against bacteria and viruses (no data is available on it
effectiveness against protozoan cysts.