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Slime or sludge in well water

Updated 2026-09-15

Iron bacteria, most likely. They are living organisms that metabolise iron and produce a slime as they go, and that slime is why this problem does not respond to the equipment that handles ordinary iron.

Why
Ordinary iron is a mineral to be oxidised and filtered. Iron bacteria are a biological colony living in the well and the plumbing, and filtering their output does nothing about the colony.
Watch out
This is the case where an oxidising filter and a softener both fail, and fail expensively, because the slime blinds media and fouls resin. The usual answer starts with shock chlorinating the well rather than with a purchase.

Where it shows up

  • Reddish-brown slime in the toilet tank
  • Gelatinous film inside the pressure tank
  • Clogged fixtures and filter housings
  • Smell like fuel, sewage or damp earth
  • Water quality that varies week to week

Most likely causes

In the order worth ruling out.

  1. 1

    Iron in well water

    The most common well water complaint in the country. Above 0.3 ppm it stains everything water touches, and the form it arrives in decides whether a filter or a softener is the right answer.

  2. 2

    Coliform and E. coli in well water

    The one result that is about the well's physical condition rather than the water's chemistry. Bacteria in a well mean something is getting in, and disinfection treats the consequence rather than the cause.

Biology rather than chemistry

Nearly everything else on a well water report is chemistry: a mineral dissolved in water at some concentration, waiting to be oxidised, exchanged or filtered out. The quantity is stable, the treatment is deterministic, and once the right equipment is installed the problem stops.

Iron bacteria are biology. They are a living colony established in the well, the casing, the pressure tank and the pipework, metabolising iron and secreting a gelatinous sheath as they do it. That changes the problem in three ways.

They are upstream and downstream of any filter, because they live in the plumbing rather than passing through it.

They vary, because populations grow and die back with temperature, use patterns and how long water stands. Water quality that changes week to week without explanation is characteristic.

And they foul equipment actively. The slime coats media beds and resin, and a coated bed does not filter. This is the case where buying the standard iron filter makes things worse, because the money is spent and the symptom continues.

Telling it apart from ordinary iron

Reach into the toilet tank, where this shows up first.

Ordinary iron staining is hard and dry. It marks the porcelain, it resists bleach, and it comes off with an oxalic or citric acid cleaner.

Iron bacteria leave something you can feel: wet, slippery, gelatinous, often more red-brown than orange, sometimes with a rainbow sheen on the surface of standing water. It reappears within days of being cleaned.

The smell is another clue. Iron bacteria are often accompanied by an odour people describe as fuel, sewage, cucumber or damp earth rather than the sharp rotten-egg smell of hydrogen sulfide alone.

What the treatment involves

The sequence is different from every other problem on this site, because the target is a colony rather than a concentration.

Shock chlorination first, to knock the population back throughout the well and the plumbing. Done properly this means a calculated dose held for a period of hours and then flushed, and it needs to reach the whole system rather than just the water in it. Concentration, contact time and thorough distribution all matter, and getting them wrong wastes the effort.

Continuous chlorination with filtration after that, because shock treatment is rarely permanent. A colony established in a well casing tends to recover, and the ongoing feed is what prevents it.

This is a case for a well contractor rather than a self-directed purchase. The diagnosis is confident enough from the symptoms, but the execution depends on the specific well, and shock chlorination done at the wrong concentration is either ineffective or hard on the pump and plumbing.

Test for total coliform and E. coli at the same time. Iron bacteria are not considered a direct health hazard, but a well that supports them can support organisms that are.

Read next

  • Symptom

    Orange or rust stains from well water

    Iron, in essentially every case. The useful question is not whether it is iron but which form, because dissolved iron and already-oxidised iron need different equipment.

  • Symptom

    Well water smells like rotten eggs

    Hydrogen sulfide. Run the hot and cold taps separately before doing anything else, because if only the hot water smells, the water heater is manufacturing it and the repair costs a fraction of a treatment system.

  • 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.

  • Comparison

    UV vs chlorination for well water

    UV where the water is clear and the contamination is at the source. Chlorination where the water is not clear, where the plumbing itself is suspect, or where iron and sulfide need oxidising anyway.

  • 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 ordinary iron staining?
Texture and consistency. Ordinary iron leaves a hard, dry stain you clean off with an acid cleaner. Iron bacteria leave something wet, slippery and gelatinous, often more red-brown than orange, and it comes back quickly. Ordinary iron is also steady, while iron bacteria produce water that varies noticeably from week to week.
Are iron bacteria a health risk?
They are not considered a direct health threat in themselves. The practical problems are the slime, the smell, the clogging and the damage to equipment. Their presence does mean the well is hospitable to organisms, though, so it is worth testing for total coliform and E. coli at the same time.
Why did my new iron filter not help?
Because the colony is upstream of it and inside it. Iron bacteria coat media with biological slime that blocks the surface doing the work, so the filter blinds rather than filters. This is one of the specific cases where air injection is the wrong technology and continuous chlorination is the right one.
What treats it?
Shock chlorinating the well to knock the colony back, then continuous chlorination with filtration to stop it re-establishing. Because the bacteria live in the well and the plumbing rather than simply passing through, a single treatment is rarely permanent, and this is a good case for a well contractor rather than a catalogue purchase.

Sources