Color is important with KMnO4 as it can be quickly inactivated by both
particulate and dissolved organic carbon. Most textbooks state 2 mg/L as a
dosage, but if the water to be treated contains high POC/DOC, a larger dose
will be required. If the water contains very little POC/DOC, less than 2
mg/L should be used. When KMnO4 is inactivated, it is brown in color.
It's recommended to have several buckets each containing 1 liter of the
water to be treated. One is treated with 1 mg/L, another with 2, another
with 4, etc. The one which still has a pink color after one hour and of
course the lowest dosage of any that are pink is the treatment level.
That said, I normally use 2 mg/L, but have used 4 mg/L if parasites are
present that are difficult to eliminate. At 4 mg/L there was neglible
effect on the biofilter, but what works in one system may not be the same
in another.
For more info, C. Boyd at Auburn University in Alabama has vast experience
with KMnO4.
Denise