Iron in water exists in several forms, and the form matters for treatment selection. Ferrous iron (dissolved, or 'clear water iron') is invisible in a freshly drawn glass — water looks clear but will form rusty deposits when exposed to air and oxidized. Ferric iron (particulate iron) is already oxidized and appears as red or brown particles or turbidity. Bacterial iron involves iron-reducing bacteria that form a slimy, reddish-brown deposit in pipes and fixtures and requires a different treatment approach than either ferrous or ferric iron.

A water test that characterizes which form or forms of iron are present is important before selecting treatment equipment, because filters optimized for ferric iron may not perform well on ferrous iron and vice versa.

Iron is not considered a health hazard at levels typically found in drinking water. The EPA's secondary standard for iron is 0.3 mg/L — it's a secondary standard, meaning it's based on aesthetic considerations (taste, odor, staining) rather than health effects. Most people consume adequate iron through food, and the amounts typically present in water don't represent a meaningful health risk for the general population.

Manganese is more complicated. Like iron, the EPA's current secondary standard of 0.3 mg/L is aesthetics-based. However, research has increasingly linked long-term manganese exposure above certain levels to neurological effects, particularly in infants and children. The EPA has issued a health advisory for manganese and some researchers consider the secondary standard inadequate from a health standpoint. If manganese is detected in your water, it's worth taking seriously even if the level is technically below regulatory limits.

Treatment for iron and manganese depends on the form, concentration, and what else is in your water. Oxidation filtration — using media like greensand, birm, or catalytic carbon — converts dissolved ferrous iron and manganese to their particulate forms and filters them out. Air injection systems use oxygen rather than chemical oxidants to accomplish the same thing. Water softeners can remove moderate levels of dissolved iron through ion exchange but are not ideal as a primary iron treatment at higher concentrations.