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Air-injection iron filters for well water

Updated 2026-09-15

The default choice for iron-bearing well water: it pulls in a pocket of air, oxidises dissolved iron as the water passes through it, and traps the resulting particles in a media bed that backwashes itself. No chemicals to buy or refill.

Why
Most well iron is ferrous, meaning dissolved and invisible until it meets air. Oxidation turns it into something filterable, and air is the cheapest oxidant there is.
Watch out
It has a pH floor. Below roughly 6.8 the oxidation slows enough to matter, and manganese needs a distinctly higher pH than iron does. A filter installed on water that is too acidic will disappoint no matter how well it is sized.
Typical cost
$900–$2,600
Maintenance
Backwashes itself on a timer. The media bed lasts years rather than months, and there is no chemical to keep supplied. Budget a media change every five to ten years depending on load.

What it removes

  • Dissolved (ferrous) iron
  • Manganese
  • Hydrogen sulfide

When it does not work

Every one of these is a condition under which this equipment underperforms or fails outright, however well it is installed.

  • pH is below about 6.8. Oxidation slows as water gets more acidic, and a filter on acidic water passes iron straight through. Correcting pH has to come first, not afterwards.
  • Manganese is present and pH is below about 7.5. Manganese oxidises far more slowly than iron and needs a higher pH to do it, so a filter that clears every orange stain can leave the black ones untouched.
  • Hydrogen sulfide is above roughly 6 ppm. Injected air stops keeping up, and continuous chlorination or ozonation with filtration takes over.
  • The iron is organic or bacterial rather than simply dissolved. Organic iron resists plain oxidation and fouls the media.
  • Tannins are present. They coat oxidising media and blunt it, and they are not removed by it.
  • The well cannot deliver the backwash flow the tank needs. A bed that cannot be lifted and rinsed will channel, then pack, then stop working. This is the constraint that most often decides tank size.

What happens inside the tank

Iron in groundwater is usually dissolved. Water comes out of the tap clear, sits in a glass for twenty minutes, and turns orange. That delay is both the problem and the solution: the iron was there all along, invisible, because dissolved ferrous iron is clear. Exposed to oxygen it becomes ferric iron, which is a rust-coloured particle.

An air-injection filter forces that reaction to happen inside a tank instead of inside your washing machine. A pocket of compressed air sits in the top of the vessel. Water enters, passes through the air, oxidises, and the particles are caught by the media bed underneath. On a schedule the valve reverses flow, lifts the bed, flushes the accumulated iron to drain, and rebuilds the air pocket.

Nothing is consumed except water during the backwash, so this arrangement has largely displaced chemical feed for ordinary iron levels.

pH and oxidation

Oxidation is a chemical reaction and its speed depends on pH. As water gets more acidic, iron oxidises more slowly, and below roughly pH 6.8 it can be slow enough that iron passes through the bed and out the other side. The filter is working as designed; the water is not letting it.

Manganese is worse. It oxidises far more slowly than iron and generally wants pH around 7.5 or above. This produces a correctly sized, correctly installed filter that clears every orange stain in the house and leaves the black staining on the shower floor exactly where it was.

If pH is low and manganese is present, pH correction is the stage that makes the filter work at all, and it goes first.

A calcite neutralising bed raises pH by dissolving calcium into the water, so it raises hardness on the way through. Size the softener for the hardness after that stage, not the number on the lab report.

The media is not interchangeable

“Air-injection filter” describes how the oxidant gets in, not what is in the tank, and the media inside determines what the unit tolerates. Two of the common choices want opposite conditions, which is a good way to end up with an expensive tank that does very little.

Greensand is built to work with an oxidant dosed ahead of it. Chlorine is part of the design rather than a threat to it.

Birm is the mirror image. Its manufacturer specifies free chlorine below 0.5 ppm, because chlorination sharply reduces its activity, and it needs dissolved oxygen in the water to function at all. It also cannot deal with hydrogen sulfide, which has to be removed before the water reaches it.

So a filter installed behind a chlorine feed, with Birm in the tank, is being slowly destroyed by the stage in front of it. Nothing about the installation looks wrong, and the symptom is that iron comes back.

Ask what media is in the tank before buying, and check its specification against the rest of the train rather than against the water alone.

Sizing, and the constraint that usually decides it

Two flow rates matter, and they pull in opposite directions.

Service flow is what the house needs at peak, which rises with bathroom count. Backwash flow is what the tank needs to lift and clean its bed, and it rises with tank diameter. A bigger tank handles more iron and needs more backwash flow to stay healthy.

That second number ruins equipment when it is ignored. A tank whose backwash requirement exceeds what the pump can deliver never gets properly cleaned. The bed channels, then compacts, then stops filtering, and the symptom is iron reappearing months after an expensive installation.

Measure what the well actually delivers before choosing a tank. On a weak well the answer is sometimes a storage tank and a booster pump ahead of the treatment, which is a designed installation rather than a catalogue purchase.

Read next

  • Treatment

    Water softeners for well water

    Trades the calcium and magnesium that cause scale for sodium, on a resin bed that regenerates itself with brine. It is the right answer for hard water and the wrong answer for an iron problem, though it is sold for both.

  • Treatment

    Chlorine and peroxide injection systems

    A metering pump injects chlorine or hydrogen peroxide, a retention tank gives the reaction time to finish, and a backwashing filter removes what it produced. It handles water that defeats air injection.

  • Treatment

    Acid neutralizers for low-pH well water

    Passes water through a bed of calcite, or calcite blended with magnesia, which dissolves slowly and raises pH. It is the stage that makes an oxidising filter downstream work at all on acidic water.

  • Comparison

    Air injection vs chemical injection

    Air injection is the default and the right answer for most well iron. Chemical injection is for the specific cases air cannot reach: sulfide above about 6 ppm, very heavy iron, organic iron and iron bacteria.

  • Comparison

    Water softener vs iron filter: which your well needs

    They are not alternatives. An iron filter oxidises and removes iron; a softener exchanges calcium and magnesium. Water with both problems needs both, in that order, and water with only one needs only one.

  • Contaminant

    Hard water from a well

    Dissolved calcium and magnesium. It scales water heaters, coats fixtures, stops soap working, and shortens the life of every appliance that heats water.

Not sure what is in your water yet?See which tests to order, thenplan the treatment from your results.

Questions

How is this different from a greensand filter?
Greensand is a media rather than a delivery method, and the traditional kind is regenerated with potassium permanganate, which means keeping a chemical topped up. Air injection oxidises with air alone, so it is the lower-maintenance arrangement and the more common residential choice now.
Does it matter which media is in the tank?
More than almost anything else, and two of the common ones want opposite things. Greensand is designed to run with an oxidant fed ahead of it. Birm is degraded by chlorine, its manufacturer specifying free chlorine below 0.5 ppm, and it also needs dissolved oxygen present to work at all. Putting Birm behind a chlorine feed, or greensand behind nothing, produces a filter that underperforms for reasons the owner will never see. Birm also cannot handle hydrogen sulfide, which has to be removed before the water reaches it.
Will it remove the rotten egg smell too?
Up to a point. Minnesota's health department puts oxidising media filtration as effective for hydrogen sulfide up to about 6 mg/L. Past that, continuous chlorination or ozonation with filtration is the answer.
Does it soften the water?
No. Hardness is calcium and magnesium, and removing it is ion exchange, which is a different process in a different tank. Iron-bearing hard water usually needs both, with the filter first.
Can I put it after the softener instead?
It would work for a while and cost you the softener. Iron reaching the resin coats it and permanently reduces capacity, and resin is the more expensive thing to replace. Filter first.

Sources