Skip to content
WellWise
Treatment plannerSymptomsContaminantsEquipmentTesting

Chlorine and peroxide injection systems

Updated 2026-09-15

A metering pump injects chlorine or hydrogen peroxide, a retention tank gives the reaction time to finish, and a backwashing filter removes what it produced. It handles water that defeats air injection.

Why
Air is a weak oxidant. Where sulfide is above roughly 6 ppm, iron is very high, or bacteria are involved, a stronger oxidant with a proper contact time is the only arrangement that keeps up.
Watch out
It is the only common residential train with a consumable chemical and a pump. Both need attention, and the chlorine it adds has to be removed downstream by carbon.
Typical cost
$1,400–$3,800
Maintenance
A chemical supply kept topped up, a metering pump that needs periodic attention, and a filter that backwashes itself. This is the arrangement that punishes neglect.

What it removes

  • Heavy iron loads
  • High hydrogen sulfide
  • Iron and sulfur bacteria
  • Organic iron

When it does not work

Every one of these is a condition under which this equipment underperforms or fails outright, however well it is installed.

  • The chemical runs out. Unlike a media bed that degrades gradually, a dry solution tank means no treatment at all, immediately.
  • Retention time is too short. The reaction is not instant, and a tank sized for average flow rather than peak flow passes partly treated water during a shower.
  • Nothing removes the chlorine downstream. Injecting chlorine and not following it with carbon trades an iron problem for a taste and odour problem.
  • The metering pump drifts out of calibration. Too little does nothing; too much wastes chemical and loads the carbon.
  • It is installed where air injection would have been adequate. The capability comes with real ongoing maintenance, and taking that on unnecessarily is a poor trade.

When the simpler answer stops being enough

Air injection covers most well iron, and where it works it is the better choice: nothing to buy, nothing to refill, nothing to calibrate. The reasons to step up to chemical oxidation are specific rather than general.

Minnesota’s health department puts oxidising media filtration as effective for hydrogen sulfide up to about 6 mg/L. Above that, the reaction needs a stronger oxidant and a proper contact time.

Very high iron is the second reason. There is a load beyond which a bed cannot keep up between backwashes regardless of how well it is sized.

Organic and bacterial iron is the third. Iron bacteria produce a slime that coats and blinds media and resin, and it is not something air will oxidise away. Water like this usually needs the well shock chlorinated first and then continuously chlorinated, and no amount of filtration alone will resolve it.

The three parts, and why all three are needed

The metering pump doses the chemical in proportion to flow. Too little achieves nothing, too much wastes chemical and overloads the carbon downstream.

The retention tank exists purely to buy time. Oxidation is fast but not instantaneous, and a tank that provides adequate contact at average flow can fall short during peak demand, which is when someone will notice.

The backwashing filter removes what the reaction produced. Oxidising iron without filtering it converts dissolved iron into visible orange particles, which is a worse outcome than doing nothing.

Skip any one of the three and the train does not work.

Read next

  • Treatment

    Air-injection iron filters for well water

    The default choice for iron-bearing well water: it pulls in a pocket of air, oxidises dissolved iron as the water passes through it, and traps the resulting particles in a media bed that backwashes itself. No chemicals to buy or refill.

  • Treatment

    Activated carbon filters for well water

    Adsorbs organic compounds, chlorine and the things that make water taste and smell wrong, on a bed of carbon with an enormous internal surface area. It is the polishing stage, and it is mandatory behind any chemical injection.

  • 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

    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.

Not sure what is in your water yet?See which tests to order, thenplan the treatment from your results.

Questions

Chlorine or peroxide?
Chlorine is cheaper, leaves a residual that keeps working through the plumbing, and is the right answer where bacteria are involved. Peroxide oxidises without leaving chloride behind or creating chlorination by-products, and tends to be preferred where the problem is sulfide and taste rather than biology. Peroxide costs more to run.
How big does the retention tank need to be?
It is sized for contact time at peak flow, not average flow. The failure mode is specifically during high demand, which is also when someone is standing in the shower noticing the smell.
Will this deal with iron bacteria?
This is the arrangement that can. Iron bacteria form a biological slime that ordinary oxidising filters and softeners cannot clear, and continuous chlorination plus filtration is the standard response, usually after shock chlorinating the well itself.
Is it safe to drink chlorinated well water?
Municipal systems chlorinate as a matter of course, so the principle is well established. The practical concerns are getting the dose right and removing the residual before the tap with carbon, the stage after this one.

Sources