How Does an Iron Filter Work? (Air Injection Oxidation Explained)

An AIO (air injection oxidation) filter is one tank that takes iron out of well water without chemicals. Air at the top of the tank turns the dissolved iron into specks, the Katalox Light filter media below catches them, and every night the filter rinses the specks down the drain and draws in fresh air.

Some callers think the air does the work. It helps, but the Katalox Light does most of it. On its own it removes up to 30 parts per million of iron; the air pocket just gets some of the iron out before the water reaches it.

The 4-Stage AIO Process

In our AIO filters, air turns the dissolved iron into specks and the Katalox Light catches them. It happens in four stages, in this order, and the backwash rinses everything out each night.

How the air pocket gets there

Katalox Light fills most of the tank, and a pocket of air sits on top of it. There is no compressor and no air pump. A small Venturi nozzle on the back of the valve (a narrow passage that pulls in air as water rushes through it) draws air in every time the filter cleans itself. It is the same step a softener uses to pull in salt water, except this valve pulls in air.

What the air does to the iron

Well water comes in at the top and passes through the air first. Iron that is dissolved in the water comes out of the tap clear and turns orange after it sits. In the air pocket that happens right away: the oxygen turns the dissolved iron into solid specks before the water reaches the media. Manganese does the same thing. For more on clear and red water, see the types of iron in well water.

What the Katalox Light filter media does

The water then runs down through the Katalox Light. It catches the specks and takes out the iron the air did not get to, down to 3 microns. Clean water rises up the center tube and goes to the house. As I put it to a caller in July, "The actual media itself is what removes most of the iron."

How the filter cleans itself

As you use water, the air pocket shrinks and the media fills up with iron. So the filter reverses the flow, washes the iron down the drain, and draws in a new air pocket. The next section shows each step and how long it takes.

In the video I walk through the same tank. I mention customers running it at 50 ppm of iron; the rating is 30 ppm, and above 30 I put two 2.5 tanks in a row.

What happens during the nightly backwash

Our AIO filters clean themselves every night at 2 AM. That is how they ship, and the cycle takes about an hour. The valve runs three steps, in this order:

An owner called me a few months after his install, wondering why his filter ran every night. I told him, "In order for it to have an air pocket every day, it's gotta cycle every night." The 1 on the display is the number of days until the next backwash, so it always shows 1. If your household uses little water, it can be set to every other day.

Each backwash uses about 70 gallons on the 1.5 cubic foot size and about 140 gallons on the 2.0 and 2.5. For where that water goes and whether a septic system can take it, see why an iron filter backwashes.

Will an air injection filter work for you?

One air injection filter handles iron up to 30 ppm. It also takes out manganese up to 15 ppm and the rotten egg smell (hydrogen sulfide) up to 10 ppm. If you do not have a report yet, our $199 well water test kit shows iron, manganese, pH and hardness from one sample; it does not measure hydrogen sulfide.

Above 30 ppm, I put two 2.5 tanks in a row, and the second tank takes what the first one leaves. Amy H. had over 20 ppm of dissolved iron and 7 ppm of rust particles, plus manganese and some sulfur, and local water companies had told her they could not help. She bought two 2.5 tanks, and her review, a verified purchase, says it in one line: "Solved my very high iron issues!" Verified buyer

Here is what each line of a lab report means for this filter.

Have your water test results?

Send me your test before you order

Text a photo of your lab report. I will tell you whether one tank handles it, which size, and whether anything else goes in with it.

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What an air injection filter needs to work

It needs a drain, an outlet and enough water from your well. Here is the full list:

• A drain for the backwash water. A floor drain, a utility sink or a standpipe. Drain tubing is not included. Your plumber sizes it for the backwash flow and the length of the run, with an air gap where it empties.
• An outlet. A standard 110 to 120 volt grounded outlet within reach of the valve.
• Enough flow from the well. Each size needs a certain flow to backwash, shown in the table below.
• Normal water pressure. Between 20 and 80 psi. Pentair caps the valve at 80 psi because of the oxygen in the tank.
• The right spot in the line. After the pressure tank and any sediment filter, and ahead of the softener. A UV light goes last.

On a weaker well I lengthen the backwash instead of ruling out the size. How many people live there, your iron and your well's flow decide the size; the iron filter sizing guide walks through it.

What ships: the empty Vortech mineral tank, the Fleck 2510AIO valve with the stainless steel bypass valve on it, a fill funnel, and the Katalox Light in separate bags, poured into the tank at install. We recommend a licensed plumber, who typically finishes in 2 to 3 hours for about $800, and I support your plumber by phone at 800-460-5810. The steps are on how to install an iron filter.

Why Katalox Light matters more than the air

The Katalox Light does most of the work, and the air helps it. Air injection on its own is not something we use. Callers tell me their old air injection unit "lasts for about a year, and then I have to get another one." Those older systems had a separate air chamber ahead of a separate filter, and they only worked well on lower iron.

In ours, the air pocket and the Katalox Light share one tank. Without the air, the Katalox Light still removes up to 30 ppm of iron. The air just makes it more effective.

Other filters work differently. Birm and greensand take out lower levels of iron. Birm needs oxygen in the water and a pH of at least 6.8, per Penn State Extension, and greensand has to be regenerated with potassium permanganate. I do not recommend chlorine or potassium permanganate feed systems for iron: if a backwash or rinse does not run right, chemical residue can get into your water. A water softener is not a substitute either. Softeners remove only small amounts of iron, and the iron shortens the resin's life; see can a water softener remove iron. Every method side by side, with costs, is on how to remove iron from well water.

What upkeep an air injection filter needs

There is no yearly maintenance. You replace the Katalox Light every 6 to 8 years. In the video I put it this way: "you just set the controls, set the time of day, and it does its own thing."

• Every 6 to 8 years. The old media comes out (outside, or with a wet vac) and new Katalox Light goes in. A 2.5 takes 2.5 cubic feet of Katalox Light (two 1 cubic foot orders and one half cubic foot), about $875 at today's prices.
• Over 10 to 15 years. The pistons inside the valve can wear. They are fairly simple to replace.

Two signs mean something needs a look. If the water comes out clear and turns red after it sits, the valve may not be drawing enough air, or the backwash is not running often enough. If the water comes out red, the media is loaded with iron, from too little backwash flow or backwashes that do not run often enough. Those are Pentair's own fixes for this valve; text me what you see and I will walk you through it. The rest of the upkeep is on iron filter maintenance.

Frequently asked questions