Why Does an Iron Filter Backwash? (Water Use, Drain & Septic Answers)

Aidan Walsh 32 years in water treatment Updated October 27, 2026

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Every iron filter has to backwash. The Katalox Light inside the tank catches the iron and holds it, and on a schedule (every day on ours) the valve sends water up through the tank to flush that iron down a drain and refill the air pocket at the top. On our sizes that is about 70 or 140 gallons each time. To do it, the filter needs 5 to 15 gallons a minute from your well depending on the size, a standard outlet, and a drain that can take a strong flow. A septic system in good working condition can take the water.

Fleck 2510 AiQ control valve on top of a black iron filter tank

Caller "It's the smaller one, so that does like 70 gallons at a pop? And so I can put that in my septic system?"

Aidan "Yeah, it's about 70 gallons on the money. Here in Maryland and Southern PA, we always put them in the septic system."

A caller in June with 2 people in the house, looking at the 1.5 cubic foot filter. This page is the long version: how much water, where it can go, and when septic is the wrong place for it.

Quick Answer

An iron filter backwashes because its media catches the iron and holds it: the valve reverses the water to lift the media, flush the trapped iron down a drain and draw in a fresh air pocket, and without that the iron fouls the tank.

How often. Ours ship set to backwash every day. Each cycle is a 14-minute backwash, about 40 minutes of drawing air and a 1-minute rinse. See why it has to backwash.
How much water. About 70 gallons on the 1.5 cubic foot and about 140 on the 2.0 and 2.5, each time. See how much water each backwash uses.
Well flow. From about 5 gallons a minute for the 1.5 to about 15 for the 3.5. A weaker well gets a longer backwash. See how much well flow the backwash needs.
Where the water goes. A floor drain, a laundry drain or tub, a standpipe, outside, or a dry well. No drain line ships with the filter. See where the backwash drain line can go.
Septic. Yes, if your septic system is in good working condition. See the septic answer.
Power. A standard grounded outlet near the tank. See the outlet answer.

Why an iron filter has to backwash

An iron filter has to backwash because it gathers the iron, holds it in the tank, and has to get rid of it before the media fouls. There is no non-backwashing iron filter. The air pocket at the top of the tank turns the dissolved iron into specks, and the Katalox Light (the filter media) catches them. That iron stays in the tank until the backwash flushes it out.

During the backwash the valve sends the water the other way: up through the media, lifting it and rinsing the trapped iron out the drain line. Then it draws in fresh air to rebuild the air pocket and rinses the media back down. Ours ship set to do that every day, because the daily backwash keeps the media clean, especially at high iron, and it rebuilds the air pocket. You can set it to run less often; how far, and how, is in how often your iron filter backwashes.

People ask why we sell carbon filters and acid neutralizers that do not backwash but no iron filter like that. Katalox Light catches solid iron, and if it didn't backwash every day or every other day the iron would foul the tank. Once it's fouled, you have to take the tank outside, dump it, clean it out and fill it with new Katalox Light. How the air pocket and the media do the work is in how air injection works.

In the video I'm standing beside the older Fleck 2510 valve. The filters we ship now have the Fleck 2510 AiQ, so the button I push at the end is different on yours; the manual backwash for both valves is in how to clean an iron filter.

How much water each backwash uses

Each backwash uses about 70 gallons on the 1.5 cubic foot filter and about 140 gallons on the 2.0 and the 2.5. At one backwash a day, that is about 490 gallons a week on the 1.5 cubic foot and about 980 on the 2.0 or 2.5. The weekly numbers are just the gallons times 7, and they come down if you set it to backwash less often.

Filter size	Water per backwash	Per week, every day	Per week, every 3 days	Well flow the backwash needs
1.5 cubic foot	About 70 gallons	About 490 gallons	About 160 gallons	About 5 to 6 gallons a minute
2.0 cubic foot	About 140 gallons	About 980 gallons	About 330 gallons	About 7 to 8 gallons a minute
2.5 cubic foot	About 140 gallons	About 980 gallons	About 330 gallons	About 10 gallons a minute
3.5 cubic foot	Not listed yet. Text me about your well	Not listed yet	Not listed yet	About 12 to 15 gallons a minute

All of that water is your own well water carrying the iron it took out. There is no salt and nothing added, which is why it can go to a septic system or outside.

How much well flow the backwash needs

The backwash only cleans the media if your well can push enough water through the tank to lift it, and the wider the tank, the more water it takes. The table above has the flow for each size.

A weaker well does not rule out a bigger tank. Ours come set for a 14-minute backwash, and on a well that can't give the full flow I extend it to 20 or 25 minutes. As I told a caller in Idaho: "we backwash it for a longer period of time and rinse it for a longer period of time to compensate for the lack of flow rate, and it works just fine." What I would not do is put the 3.5 on a 5 GPM well. How to find your well's flow and which size it can clean is in whether your well can backwash the tank.

Where the backwash drain line can go

The backwash can go to any drain that can take a strong flow, or outside. No drain line ships with the filter: your plumber sizes it for the backwash flow and the length of the run, and ends it above the drain with an air gap. The iron filter has a much stronger backwash than a softener, and you can run the line 30 to 40 feet and it still has power coming out the other end, so the drain doesn't have to be right beside the tank.

Where it drains	Will it work?	What to watch
Furnace condensate drain	No	It's made for a slow trickle. A caller ran the backwash into one and ended up with media on the floor.
Floor drain	Yes	Tie the end of the line down. The backwash is strong enough to throw a loose line out of the drain.
Washing machine drain	Yes	Secure the line in the standpipe so it can't jump out.
Laundry or utility tub	Yes	Run the line down the side of the tub and secure it. My own system backwashes into a stainless steel utility sink.
A vertical PVC drain pipe	Yes, with a standpipe	Your plumber cuts into the pipe and adds a standpipe (a short open pipe the drain line drops into) with a P-trap, just like the drain for a shower or a tub.
Outside, through the wall	Yes	Cut the pipe through the wall on a 30 to 45 degree angle so it drains empty in winter. It won't hurt the grass, but the iron turns the ground orange where it runs.
A dry well	Yes	A pit about 2 feet deep outside the wall, lined with gravel and sand, for when the septic is not an option.
Your septic system	Yes, if it is in good working condition	See the septic answer.

Pentair's manual for the AiQ valve says the line has to be semi-rigid or rigid, because a floppy hose can whip around when the backwash starts, and never pushed down into the drain itself. The air gap size, the flow restrictor the bigger units need and the rest of the drain step are in end the drain line above the drain. Local code has the last word on the air gap, and your plumber knows yours.

I recommend having a plumber put it in: about 2 to 3 hours, and the drain line is the part that takes planning. I'm a phone call away for your plumber, and if you'd rather do it yourself, text me with any questions.

Is iron filter backwash safe for septic?

Yes, if your septic system is in good working condition. The discharge is your own well water carrying oxidized iron, with no chemicals or salt. When we did installations in Maryland, Pennsylvania, Virginia and Delaware, the backwash went into the septic system, and the plumbing inspectors in Maryland wanted it that way. If you can run it into the septic, that's better than running it on the ground.

A caller in South Carolina told me his septic person said not to backwash into the septic tank, and didn't say why. I told him it has no adverse effect on the septic system unless the system is failing and the field isn't perking well. If yours is like that, or you'd rather go by what your septic person says, send it outside or into a dry well instead (the drain table has both). In a cold climate, the septic is the better place when an outside line could freeze.

No drain near your well tank?

Text Aidan a photo of your well tank

Text a picture of your well tank and the nearest drain, plus what your well puts out, and Aidan will tell you where the backwash line can go and which size your well can clean.

Text Aidan443-277-2204 Call800-460-5810Aidan

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Does an iron filter need an outlet?

Yes. Every automatic iron filter, ours and everyone else's, has a mechanical or digital valve that needs electricity to run the backwash, and a line to drain the water. Ours plug into a standard 110 to 120 volt grounded outlet within reach of the valve. If there is no outlet near your well tank, an electrician can usually run one off the electric that already feeds your pressure tank.

What goes wrong with the backwash

Backwash trouble shows up as iron at the faucets, media at the drain, or a filter that has fouled. Each one has a plain cause.

What you see	Why	What to do
Iron at the faucets between backwashes	The media is holding more iron than the schedule clears, often after someone set it to backwash less	Run a manual backwash (how to clean an iron filter), set it back to every day, and text me if it keeps happening
Iron coming through on a weak well	The backwash can't lift and rinse the media at that flow	Lengthen the backwash to 20 or 25 minutes. Text me your well's flow first
Black granules at the drain	The flow restrictor (a small part that limits how hard the backwash runs) that comes with the bigger units was left out, or the tank was filled too full	Put the filter in bypass and text me before it backwashes again
Milky water right after a backwash	Tiny air bubbles from the air pocket	Nothing. It clears from the bottom of the glass up
Iron that no backwash clears	The media has fouled from too few backwashes	A backwash won't fix it. The tank comes out, gets dumped and cleaned, and gets new Katalox Light (how to replace iron filter media)

What buyers wrote after installing one

These are verified purchases of our 2.5 cubic foot filter, newest first.

Buyer	Bought	What they wrote
Derek C., May 2026 Verified buyer	2.5 cubic foot iron filter	"Aiden was great help! You got to be to code on the backwash line air gaps."
A buyer, Dec 2025 Verified buyer	2.5 cubic foot iron filter	"No more stains/orange toilet bowls. Milky bubbles after backwash and impacts to pH were as noted."
Dustin H., Jun 2024 Verified buyer	2.5 cubic foot iron filter	"In, out, and the drain. Ran a back wash cycle then put it online and it works GREAT!"

Frequently asked questions

How long do I backwash my filter?

On ours you don't time it yourself. The valve runs a 14-minute backwash on its own, then draws air for about 40 minutes and finishes with a 1-minute rinse. On a weaker well we set the backwash longer, 20 or 25 minutes.

Should I put a filter before or after my water softener?

The iron filter goes before the softener. If both backwash, their drain lines can tee into one, as long as they are set to backwash at different times. The full order on a well is in the correct order of well water treatment.

How much is an iron filter for well water?

Ours run from $2,495 for the 1.5 cubic foot to $3,495 for the 3.5, and the 2.5 at $2,895 is the size we ship most. Every size is on our iron filters page.

About the author

Aidan Walsh

Co-owner & Residential Water Treatment Specialist, Mid Atlantic Water

Aidan Walsh has spent 32 years in residential water treatment, the first stretch of it installing and servicing well systems before Mid Atlantic Water became an online supplier. Mid Atlantic Water is a family company; every recommendation on this site is based on what actually failed in the field and what fixed it. If you have a question this page did not answer, call 800-460-5810 or text him at 443-277-2204.