Is the potassium permanganate used to restore oxidizing power of greensand water filters toxic or harmful?

Potassium permanganate is the chemical compound KMnO4. In this salt, manganese is in the +7 oxidation state. The salt is also known as "permanganate of potash." The permanganate ion is a strong oxidizing agent. It dissolves in water to give deep purple solutions, evaporation of which gives prismatic purplish-black glistening crystals. In 2000, worldwide production was estimated at 30,000 t/a.

Manganese greensand is another common treatment method for removing sulfur from drinking water. It is usually recommended for water that contains less than 6.0 mg/l hydrogen sulfide. A manganese greensand filter has a special coating that oxidizes hydrogen sulfide gas to solid sulfur particles, which are filtered. When all of the manganese oxide is consumed, the greensand is regenerated with potassium permanganate. Potassium permanganate is a purple oxidizing chemical that is added to the untreated water to maintain the manganese greensand filter. To work properly, the greensand must be regenerated at periodic intervals based on water usage and contaminant concentration. When greensand is used to treat high concentrations of hydrogen sulfide, more frequent regeneration is required.

As an oxidant, potassium permanganate can act as a disinfectant. For example, dilute solutions are used as a treatment for canker sores (ulcers), disinfectant for the hands and treatment for mild pompholyx, dermatitis, and fungal infections of the hands or feet. Potassium permanganate, obtainable at pool supply stores, is used in rural areas to remove iron and hydrogen sulfide (rotten egg smell) from well and waste water.

Although not highly toxic, concentrated solutions of potassium permanganate can be harmful to the skin. Permanganate is a strong oxidizer similar to chlorine, so it can cause irritation or burns with direct contact to skin. However, very little or no permanganate should reach the filtered water from a greensand system. The permanganate is only used to backwash and clean the greensand filter media. Excess permanganate is flushed out before raw water is re-routed through the distribution system. A special double rinse cycle makes sure the filter bed is free of any permanganate residual.