If your toilet tank has a reddish-brown slime you can wipe off with a finger, you do not have an iron problem. You have a living one.
Iron bacteria are naturally occurring organisms that make their living oxidising dissolved iron. The slime is their by-product, and it is the reason the usual iron advice — fit an iron filter — fails so reliably on these wells.
Telling it apart from plain iron
The two get confused constantly, and the fix is completely different. The distinguishing feature is texture.
| Dissolved iron | Iron bacteria | |
|---|---|---|
| Deposit | Hard, gritty, orange | Soft slime, stringy or fluffy |
| In the toilet tank | Orange ring, scrubs off hard | Brown-red slime, wipes off easily |
| Smell | Metallic | Swampy, oily, cucumber or sewage-like |
| Standing water | Clear, then rusty sediment | Rainbow or oily sheen on the surface |
| Filters | Gradual staining | Clog unexpectedly fast |
That rainbow sheen is a useful tell, and it causes a lot of unnecessary alarm about fuel contamination. An oil sheen breaks into rigid plates when you disturb it. A bacterial sheen swirls and reforms.
Why it matters even though it is "just a nuisance"
There is no drinking water standard for iron bacteria, and public health agencies treat it as an aesthetic and operational problem rather than a health one. That is fair. What it costs you is still real:
- Clogged everything. Well screen, drop pipe, pressure tank, aerators, and especially filter cartridges. A sediment filter that should last three months lasts three weeks.
- Lost yield. Biofilm on the well screen restricts flow, and owners routinely misread that as the well going dry.
- Pump wear, from working against the restriction.
- Disinfection interference. This is the indirect health point worth taking seriously: biofilm physically shelters organisms inside it from chlorine and from UV light.
So a well with iron bacteria that also tests positive for coliform is harder to clear than one without, and a clean bacteria result is slightly less reassuring than it would otherwise be. Keep testing for coliform and E. coli on schedule.
Where it comes from
Iron bacteria need iron and oxygen, and they are introduced rather than spontaneous. The common entry routes are all human:
- Drilling, pump installation or repair with equipment that was not disinfected
- A well opened and left open
- Surface water reaching the casing through a bad seal or a well pit
- A new pump laid on the ground before going down the hole
That last one is why conscientious contractors disinfect tools and drop pipe. If your problem appeared shortly after well work, this is almost certainly the reason.
Treatment
Shock chlorination is the main tool, but iron bacteria need considerably more than a routine disinfection:
- Higher concentration, longer contact. Routine shock aims at roughly 200 ppm for a few hours. Iron bacteria want stronger solution and overnight contact — often 12 to 24 hours.
- Physically disturb the biofilm first. Brushing or surging the casing breaks the film so chlorine can reach what is underneath. A slime layer is remarkably good at protecting itself.
- Circulate. Run chlorinated water back down the well and through every fixture, so the whole system is treated and not just the water column.
- Expect discharge. The dead biofilm has to come out somewhere. Water runs dirty for a while and filters clog — budget for cartridges.
Our shock chlorination guide has the dosing arithmetic. For iron bacteria, treat those volumes as a floor rather than a target.
Continuous chlorination — a small injection pump and a contact tank — is the usual answer once repeated shocking stops being tolerable. It costs more to install and needs carbon filtration downstream to take the chlorine taste back out, but it holds the population down permanently instead of resetting it twice a year.
What does not work: an iron filter alone, a softener, UV on its own (the film coats the sleeve and shields the organisms behind it), or any filtration at all without dealing with the biofilm upstream first.
Setting expectations honestly
This is the part most articles skip. Iron bacteria are rarely eradicated from a private well. The organism lives in the aquifer around the borehole as well as inside it, and nobody can disinfect an aquifer.
What you can realistically achieve is control: knock it back hard, then either repeat annually or install continuous treatment. Owners who know that going in are far happier than owners who shock once, watch it return in four months, and conclude they did it wrong.
Keep a record of when you shock, what you used, and how long before the slime comes back. That interval is the only number that tells you whether your routine is holding — or whether it is time to stop shocking and start treating continuously.
Frequently Asked Questions
What is iron bacteria in well water? Naturally occurring bacteria that feed on dissolved iron and produce a reddish-brown slime as a by-product. They are not a direct health threat, but the biofilm clogs pipes, pumps, screens and filters, and it can shelter other organisms from disinfection.
How do I know if I have iron bacteria or just iron? Dissolved iron leaves hard orange staining, and water that is clear at the tap then throws a deposit. Iron bacteria produce a slimy, stringy or fluffy mass you can feel — classically in the toilet tank — along with a swampy or oily smell and a rainbow sheen on standing water.
Is iron bacteria dangerous to drink? It is generally considered a nuisance rather than a health hazard, and there is no drinking water standard for it. The caution is indirect: biofilm can harbour other bacteria and shield them from chlorine, so a well with iron bacteria should still be tested for coliform and E. coli on schedule.
Will an iron filter remove iron bacteria? No. An iron filter removes dissolved iron; it does nothing about the organism, and the filter media itself becomes another surface for the biofilm to colonise. The treatment is physical and chemical removal of the biofilm, usually shock chlorination at higher strength and longer contact than a routine disinfection.
Does iron bacteria come back after shock chlorination? Often, yes. A single shock knocks the population back but rarely eradicates it, because biofilm in the aquifer around the borehole cannot be reached. Many owners settle into shocking once or twice a year, or move to continuous chlorination, rather than expecting a permanent fix.