Iron Filter Types: Air Injection vs Greensand (and How to Pick by Symptom)

Updated July 2026 · Editorial team · Topic: well water / treatment

Orange staining in the toilet tank, a metallic edge to the coffee, rust smears down the bathtub — iron in well water announces itself. What it doesn't tell you is which filter to buy, and that choice hinges on details most homeowners never test for: how much iron, what kind, and whether it's traveling with manganese or a rotten-egg smell.

First, know which iron you have

The word "iron" hides two forms that behave differently at the tap. Ferrous iron is dissolved and invisible — the water runs clear from the faucet, then turns rusty in the glass or the toilet after it meets air. That's "clear-water iron." Ferric iron is already oxidized and arrives visibly rust-colored, "red-water iron." Every oxidizing filter works by converting the dissolved ferrous form into a solid it can trap, so knowing your starting form tells you how hard the filter has to work.

Two companions decide the rest. Manganese travels with iron often and leaves black rather than orange staining. Hydrogen sulfide brings the rotten-egg smell. A filter chosen for iron alone can be blindsided by either, which is why a full test beats guessing from the stain color.

How the two main systems work

Air injection oxidation (AIO) keeps a compressed pocket of air at the top of the tank. Water passes through the air, dissolved ferrous iron grabs oxygen and converts to solid ferric particles, and the media bed catches them. On its daily backwash the system rinses the trapped iron to drain and draws a fresh air pocket. No chemicals are added, ever — the oxidizer is plain air, which is the entire appeal.

Greensand (and modern equivalents like manganese-coated media) uses a chemically active surface to oxidize iron, manganese, and sulfur on contact. That coating gets consumed as it works, so it's periodically recharged with potassium permanganate — a purple oxidizer stored in a solution tank and drawn in during regeneration. The reward for that chemical upkeep is broad, reliable removal of all three troublemakers at once.

Side-by-side

Two other media worth knowing

• Birm: a low-cost media that oxidizes iron using dissolved oxygen already in the water. It's cheap but fussy — it needs pH above about 6.8 and won't touch hydrogen sulfide, so it fits only clean-smelling, mildly acidic-free water.
• Katalox Light: a modern high-capacity media that handles iron, manganese, and sulfur across a wide range, often positioned as a lower-maintenance step up. It's the newer answer where greensand used to be the only option for combined problems.

What we can't do for you

We don't run a lab, so we can't size a filter to your well from a photo of a stain. The ranges above come from how these media are specified and how oxidation chemistry behaves, both documented. Your iron concentration, its form, your pH, and any manganese or sulfide are numbers only a water test produces — and those numbers, not the color of your tub, are what a correct choice is built on.

Common mistakes

• Buying by stain color alone. Orange could be iron, black could be manganese, and both change which media works. Test before you buy.
• Putting Birm on smelly water. Birm ignores hydrogen sulfide and needs the right pH. On rotten-egg water it under-performs from day one.
• Choosing greensand and skipping the permanganate. The media only keeps oxidizing while it's recharged. Neglect the chemical and removal quietly fails.
• Ignoring pH. Oxidation slows in acidic water. Low pH may need correcting first — see the neutralizer guide before the iron stage.

FAQ

General information, not medical advice. Test your water first. Prices and specifications vary by model and region.