Contaminant Level (MCL)
Cyanide (CN) is an extremely toxic and is seldom found in water at significant levels. Discharge from industrial chemical factories is the main source of cyanide in water.
According to the EPA:
Some people who drink water containing cyanide in excess of the maximum contaminant level [0.2 milligrams per Liter] over many years could experience nerve damage or problems with their thyroid.
Reverse Osmosis removes 90% to 95%, as does electrodialysis. It is also treated by chlorination, retention, and filtration. The EPA recommends granular activated carbon in combination with packed tower aeration (an air stripping method.)
Sources: EPA, Photo: WikiMedia, author: Secl
(whole house & well units)