Cyanide (CN) is extremely toxic and is seldom found in water at significant levels. Waste water from metal finishing operations is the main source of cyanide in water.

There is no EPA classification. The EPA has not found sufficient evidence to classify Cyanide as a carcinogen.

Removal: Reverse Osmosis removes 90% to 95%, as does electrodialysis. It is also treated by chlorination, retention, and filtration.