How iron filters work: air injection, greensand and Katalox compared

An iron filter works in two steps. First an oxidizer (air, chlorine, hydrogen peroxide, or a manganese dioxide coating on the media itself) turns dissolved iron, manganese and hydrogen sulfide into solid particles. Then a bed of filter media traps those particles, and a regular backwash flushes them to the drain. The main iron filter types differ in which oxidizer they use, and that decides how much iron, manganese and sulfur they can handle, what pH they need and what you keep buying.

This guide explains the process, compares the types and media on published numbers, and shows what your well has to supply for each one to work. If you only need to know whether a filter or a softener fits your water, the iron filter vs water softener guide answers that faster.

How an iron filter works in two steps

Iron in well water usually arrives dissolved (ferrous iron). The water looks clear at the tap, then turns orange or leaves stains once air gets to it. A filter can’t catch something that is dissolved, so the first job is to force that change on purpose, inside the tank, before the water reaches your pipes.

1. Oxidation. An oxidizer pulls electrons from dissolved iron and manganese so they become insoluble rust particles. Hydrogen sulfide is oxidized to solid sulfur or sulfate in the same way. Penn State Extension’s iron and manganese guide describes this for every oxidizing method, from greensand to chlorine.
2. Filtration. The particles lodge in the media bed as water flows down through it.
3. Backwash. On a schedule, the valve reverses the flow. Water runs up through the bed, lifts and loosens it, and carries the trapped iron out through the drain line. Then the bed settles and goes back into service.

Everything else about an iron filter follows from step 1. Air is free but weak, so air-fed filters handle less sulfur. Chlorine and peroxide are strong but have to be bought and dosed. A catalytic coating speeds the reaction up, but only within the pH range its maker gives.

Iron filter types compared

Most whole-house iron filters sold for wells fall into four types. The table shows what each uses and the limits real products publish, taken from seller pages checked on 2026-09-27. Our roundup of the best iron filters for well water has the full spec sheets.

Three things stand out. Air injection is the most common type among the filters we checked (four of five in our roundup), and it runs on nothing but power and backwash water. Peroxide leads on sulfur by a wide margin: 30 ppm of H2S against 6 to 10 ppm for the air-fed designs. And Birm is the most fussy about the water it gets; none of the complete well systems we checked is built on it.

Penn State gives a frame that fits across all of them: manganese greensand filters for combined iron and manganese of 3 to 10 mg/L, and chemical oxidation followed by filtration above 10 mg/L. (ppm and mg/L are the same thing in water.) Makers rate their air-fed filters higher than that, up to 30 ppm iron, so read those figures as the best case under good conditions.

Iron filter media types: Katalox, Birm, greensand and catalytic carbon

The media is the part inside the tank, and each one has its own rules. These come from the media makers’ own data sheets, which are stricter and more specific than most product pages.

Katalox Light. Watch Water’s Katalox Light page describes a light base mineral with a 10% manganese dioxide coating. The coating is the catalyst. The maker lists an inlet pH of 5.8 to 10.5, with iron removed best around pH 7.5 and manganese around 8.5, and says it needs no chemical to regenerate, though it works with air, chlorine, peroxide or permanganate when concentrations are higher. Its media rating is far above what any product built on it claims (up to 100 mg/L iron and 20 mg/L manganese as a media figure), so use the product’s own limit, not the media’s.

Birm. Clack’s Birm page lists the tightest conditions: pH 6.8 or more for iron, water with no oil or hydrogen sulfide, organic matter no higher than 4 to 5 ppm, and dissolved oxygen equal to at least 15% of the iron. It needs no regenerant, only backwashing. For manganese, Clack wants a pH of 8.0 to 9.0, and below 8.5 if iron is present too. Penn State puts the manganese floor at 7.5 and calls Birm’s manganese results “highly variable”.

Greensand (GreensandPlus). Inversand’s GreensandPlus data sheet describes a silica sand core with a manganese dioxide coating, rated for iron, manganese and hydrogen sulfide. For iron removal it recommends feeding chlorine or another strong oxidant ahead of the filter, sized by a simple rule: 1 mg/L of chlorine per mg/L of iron, plus 3 per mg/L of manganese and 6 per mg/L of hydrogen sulfide. Raw water below pH 6.2 has to be raised to 6.5 to 6.8 first. Older manganese greensand was regenerated with potassium permanganate, the method Penn State still describes.

Catalytic carbon. This is activated carbon made to speed up oxidation. Calgon Carbon’s Centaur page says it converts hydrogen sulfide to sulfate and breaks down hydrogen peroxide, which is why peroxide systems pair with it: the carbon finishes the reaction and removes the leftover peroxide. Air-fed filters also blend it in for sulfur.

Do iron filters remove manganese and sulfur?

Most do, but the limits spread much wider than they do for iron.

Manganese is the weak point of air-fed filters. Published limits run from 0.5 ppm (Flexx Oxi-Gen) and 1 ppm (SpringWell WS, Matrixx InFusion) up to 7 ppm (SoftPro Iron Master) and a seller-claimed 15 ppm on the Mid Atlantic Fleck 2510AIO build. Manganese needs a higher pH than iron to oxidize well: both the Katalox and Birm makers list a higher optimum pH for it. EPA’s secondary (aesthetic) standard for manganese is 0.05 mg/L, the level where black staining starts, so even a small reading on your test matters when you compare filters.

Hydrogen sulfide depends on the oxidizer. Penn State’s hydrogen sulfide guide says aeration is “normally not used” above about 2 mg/L and that oxidizing filters remove up to 2 to 3 mg/L. It calls hydrogen peroxide “an excellent oxidant” for sulfide. The makers go further: 6 to 10 ppm for air-fed designs and 30 ppm for peroxide. If your sulfur reading is high, the peroxide type is the one built for it. Our guide to sulfur smell in well water covers the cheaper checks first, such as a smell that comes only from the water heater.

What each type needs from your well

A filter that matches your test can still fail if the well can’t feed it. Three things decide that.

Backwash flow. The bed has to lift during backwash to release the iron. The media makers state this per square foot of tank: 10 to 12 gpm per ft² for Katalox Light and at least 12 gpm per ft² at 55°F for GreensandPlus. So the flow your pump must supply grows with the tank’s diameter.

Bed area is π × (diameter ÷ 2)², converted from square inches to square feet. Compare the result with the seller’s figure. The Mid Atlantic build on a 12 in tank asks for 8 gpm, inside the Katalox range. The Flexx Oxi-Gen asks for 5 gpm on a 10 in tank and 7 gpm on a 13 in tank, below the media math. The seller doesn’t explain the gap; the media is a Katalox and catalytic carbon blend, and it draws air without a full backwash. When a seller’s number sits well under the media math, ask them why before you rely on it.

To measure your pump, fill a 5 gallon bucket at an outdoor tap near the pressure tank with the pump running, and divide 5 by the minutes it took. Our iron filter roundup walks through the bucket test step by step.

pH. Every media has a floor: 5.8 for Katalox Light, 6.2 for GreensandPlus and 6.8 for Birm. Below it, the oxidation stalls and iron comes through. Acidic wells need a neutralizer ahead of the filter, or a media with a lower floor. Your lab report lists pH, and our guide to reading well water test results shows where.

A drain and power. Every type needs a drain line that can take the backwash flow, and the automatic valves need power (the Mid Atlantic build, for example, lists a 120 V outlet). Peroxide and chlorine systems also need space for a solution tank and a metering pump. Our iron filter maintenance guide covers the upkeep each type needs once it is running.

When an iron filter is not enough

Some wells need more than a standard iron filter, or something else first.

• Very high iron plus manganese. Above 10 mg/L combined, Penn State recommends chemical oxidation, typically chlorine with a contact tank, followed by filtration. A few air-fed products claim 30 ppm iron, but at that level ask the seller to confirm the unit for your full test, manganese included.
• Iron bacteria. The Minnesota Department of Health’s iron bacteria page lists the signs: sticky rust, yellow, brown or grey slime, an oily sheen and swampy or musty smells. It treats them at the well, with shock chlorination and sometimes physical cleaning by a contractor. The Flexx Oxi-Gen seller says that model is not recommended for iron or sulfur bacteria; the Matrixx InFusion seller says its peroxide system works where they are present.
• Organic matter. Clack sets a limit of 4 to 5 ppm of organic matter for Birm. If your water is tea-colored, have the lab test for tannins and ask the seller whether the filter is rated for them. We found no published tannin limit on the products we checked.
• Health contaminants. An iron filter is for iron, manganese and smell. For bacteria, arsenic, nitrate or lead, use a certified lab and equipment certified for that contaminant.

Most wells with iron are hard as well, and an iron filter does nothing for hardness. Put the filter first and the softener after it, sized on hardness alone with the water softener size calculator. To pick a specific filter for your test and your pump, go to our best iron filters for well water, which lists each model’s limits and backwash flow side by side. More well guides are in the well water treatment section.