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Iron Bacteria vs. Dissolved Iron: Why It Matters for Treatment

Well owners treat iron problems and fail because they’re treating the wrong type of iron. There are three distinct iron problems in well water. They cause different symptoms. They need different solutions. Buy a greensand filter for iron bacteria, and you’ll have a fouled filter in six months and the same orange staining you started with.

The Three Types of Iron

Ferrous iron is dissolved iron. The water comes out of your tap looking completely clear. Then it sits in a white sink for a few minutes and turns orange. That color change happens because dissolved ferrous iron oxidizes when it hits air, converting to ferric iron particles. This is the most common type in private wells.

Ferric iron is already oxidized before it reaches your tap. Your water runs orange or rusty from the moment you open the faucet. The particles are visible. You might see them settle to the bottom of a glass. These particles stain fixtures and laundry immediately.

Iron bacteria are living organisms. They’re not a mineral at all. Iron bacteria feed on dissolved iron and produce a thick, reddish-brown slime as a byproduct. You’ll find this slime coating the inside of your toilet tank, clogging your well screen, building up around faucet aerators, and lining your pipes. The water might look orange or brownish, but the signature is the slime, not just discoloration.

You can have more than one type at the same time. Iron bacteria thrive in iron-rich water because iron is their food source.

How to Tell Which Type You Have

Start with a simple visual test. Collect a glass of water directly from your tap. Let it sit for 5-10 minutes. If it starts clear and turns orange, you’re dealing primarily with ferrous iron. If it runs orange immediately and particles settle to the bottom, you have ferric iron, likely from oxidation happening in your well or pipes.

Open your toilet tank. Lift the lid and look inside. Healthy water leaves no residue or maybe a small amount of mineral scale. Iron bacteria leave a distinct brownish, reddish, or orange slime that often has a slight sheen and a stringy or gelatinous texture. It doesn’t look like rust particles. It looks biological, because it is.

Smell your water. Iron bacteria sometimes produce a swampy, oily, or petroleum-like odor alongside the orange color. This isn’t hydrogen sulfide (rotten eggs). It’s different, and it’s another indicator pointing toward bacteria rather than simple mineral iron.

Visual inspection gets you most of the way there. But the only way to confirm iron bacteria and measure dissolved vs. particulate iron is a lab test. A standard water chemistry test measures total iron and can distinguish ferrous from ferric. Iron bacteria require a separate biological test. These are different tests and you’ll need to ask for both if you want the full picture.

See how to test your well water for iron for which tests to order and how to read results.

Why Treatment Differs by Type

This is where most people go wrong. They buy an iron filter without knowing which type of iron they have.

Ferrous iron responds well to oxidation followed by filtration. An air injection system, a greensand filter with an oxidizing agent like potassium permanganate, or a birm filter will oxidize dissolved iron and then trap the resulting particles. Ion exchange in a water softener also removes ferrous iron to some degree, though softeners aren’t designed as iron filters.

Ferric iron is already oxidized, so you skip the oxidation step. A quality sediment filter will capture the particles. The catch is sizing it correctly and replacing cartridges before they clog. High ferric iron loading can overwhelm an undersized sediment filter quickly.

Iron bacteria need something completely different. No filter removes iron bacteria by physically trapping them. You need a biocide. Chlorine is the standard approach. The well and plumbing need to be shock chlorinated to kill the bacterial colonies, including the ones living in the slime layers on your well casing, screen, and pipes.

Here’s the critical mistake: if you install a filter before shock chlorinating, you trap iron bacteria inside the filter media. The media becomes a breeding ground. The bacteria colonize the filter and spread back into your plumbing whenever water flows through. Installing filters before eliminating the bacteria makes the problem worse.

The Order of Treatment Matters

If you have iron bacteria, the sequence is non-negotiable.

First, shock chlorinate your well. This means introducing a high concentration of chlorine into the well, letting it sit, and then flushing the system thoroughly. Done correctly, it kills iron bacteria colonies throughout the well and plumbing system. See the step-by-step shock chlorination guide for the full process.

Second, test your water again after chlorination. Confirm the iron bacteria are gone before spending money on filtration. If they return quickly, the source is still present and you may need continuous chlorination or UV disinfection alongside filtration.

Third, install filtration matched to your remaining iron type. If you also have dissolved ferrous iron, an air injection or oxidizing filter addresses that. If you have ferric particles, a sediment filter handles them.

For ongoing iron bacteria protection, some well owners add a continuous chlorination system before their filtration train. Low-level chlorination keeps bacteria from re-establishing while the carbon or catalytic filter downstream removes residual chlorine before it reaches your taps.

What to Do Next

If you haven’t tested yet, that’s the first step. Get a lab test that covers total iron, dissolved iron, and a separate iron bacteria test. Don’t guess based on symptoms alone, because the overlap between types is common and treatment costs differ significantly.

Start with the iron contaminant overview for a full picture of the problem. If you’re ready to look at treatment options, how to remove iron from well water walks through the decision process by iron type. Once you know what you have, the best iron filters for well water compares systems matched to different iron profiles.

Sources

Frequently Asked Questions

What does iron bacteria look like in a toilet tank?
Iron bacteria create a reddish-brown, slimy buildup in toilet tanks, water heater tanks, and around fixtures. It often looks stringy or gelatinous rather than like rust particles settling to the bottom. If you open your toilet tank and see a brownish slime that has a slight sheen, iron bacteria are the most likely cause. Simple rust or ferric iron deposits look more like reddish powder or sediment.
Can iron bacteria make you sick?
Iron bacteria themselves aren't typically harmful to humans at the concentrations found in well water. They don't produce toxins and aren't pathogens. But they create conditions that support other bacterial growth, including potential pathogens. And the slime they produce clogs pipes and filters faster than simple dissolved iron would. Test for bacteria separately if you suspect iron bacteria, since a bacteria test tells you about pathogens, not iron bacteria.
Does a water softener remove iron bacteria?
No. A water softener removes dissolved ferrous iron to some extent through ion exchange, but it doesn't address iron bacteria. The slime produced by iron bacteria will actually foul a softener resin bed over time, reducing its effectiveness. If you have iron bacteria, you need shock chlorination before or alongside any filter installation.
How do I test for iron bacteria vs. dissolved iron?
Dissolved iron shows up on a standard water chemistry test. Iron bacteria require a separate biological test, or you can use visual inspection as a first indicator (slime in toilet tank, slimy well casing, gelatinous deposits in pipes). Many state labs and certified mail-in services offer an iron bacteria test alongside their standard well panels. The two are different tests.
Can I have both dissolved iron and iron bacteria?
Yes, and it's common. Iron bacteria thrive in iron-rich water because iron is their food source. If your well has high dissolved iron, the conditions are favorable for iron bacteria. You may need a combined approach: shock chlorination to address the bacteria, then ongoing filtration for dissolved iron.