Skip to content
WellWise
Treatment plannerSymptomsContaminantsEquipmentTesting

UV disinfection for well water

Updated 2026-09-15

Passes water along a quartz sleeve containing an ultraviolet lamp, which scrambles the DNA of anything living in it. No chemicals, no taste change, nothing added to the water.

Why
It is effective against the full range of waterborne pathogens, including protozoa that resist chlorine, and it does it continuously with no contact tank.
Watch out
It goes last in the train and it depends on everything before it. Light cannot reach an organism hiding behind an iron particle or a clouded sleeve, and a UV lamp on raw well water is an expensive way to accomplish very little.
Typical cost
$400–$1,200
Maintenance
An annual lamp replacement whether or not it has failed, and a quartz sleeve cleaned on a schedule. Output falls long before the lamp goes dark, which is why replacement is by calendar rather than by symptom.

What it removes

  • Bacteria
  • Viruses
  • Protozoa including cryptosporidium and giardia

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.

  • Turbidity is above about 1 NTU. Suspended particles shadow organisms from the light, and a shadowed organism is an unaffected one.
  • Iron is above about 0.3 ppm. Iron both absorbs ultraviolet light and deposits on the sleeve.
  • Manganese is above about 0.05 ppm, for the same reasons, and manganese deposits are harder to clean off.
  • Hardness is above about 7 grains per gallon. Scale forms on the quartz sleeve and blocks the light that does the work.
  • The lamp is past its service life. Output decays gradually, so a lamp that still glows can be well below the dose needed. There is no visible symptom of failure.
  • It is being used to compensate for a well that is open to the surface. Disinfection treats water on the way to the tap; it does not seal the opening that let contamination in.

Why UV goes last

Every other stage can tolerate being slightly out of order. This one cannot.

Ultraviolet disinfection works by delivering a dose of light to an organism. Dose is intensity multiplied by exposure time, and intensity falls off with anything in the water that absorbs or scatters light. A particle of oxidised iron is, from the perspective of a bacterium sheltering behind it, a wall.

So UV goes last, downstream of sediment filtration, iron removal and softening, where the water is as clear as the train can make it. Installed anywhere else it produces a number on an invoice and very little else.

The operating limits above are equipment specifications rather than published health standards, and they vary between manufacturers. Check the figures for the unit being bought. Light has to reach the organism, whatever the unit.

The failure mode with no symptom

Lamps do not usually fail by going dark. They fade. Output drops steadily over months, and a lamp at the end of its life can be well below the dose required while still looking perfectly lit.

There is no residual to test at the tap, so nothing downstream reveals the problem. Replace the lamp on the calendar, and a unit with an intensity sensor and an audible alarm earns its premium.

The quartz sleeve is the other half of the same problem. It fouls gradually with whatever the water carries, and a clouded sleeve reduces dose exactly as a tired lamp does.

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

    Sediment filters for well water

    Catches the physical debris a well produces before it reaches anything expensive. On sandy wells this is the difference between a treatment train that lasts a decade and one that fails in a season.

  • 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.

  • Contaminant

    Nitrate in well water

    A genuine health standard rather than a staining threshold, and the population at risk is infants under six months. It is also a signal about the well itself, because nitrate is not naturally abundant in deep groundwater.

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

Questions

How do I know it is working?
You do not, from the tap. UV leaves no residual, so there is nothing downstream to measure the way you can measure chlorine. That is the tradeoff for adding nothing to the water, and it is why lamp replacement is scheduled rather than reactive. Units with a UV intensity sensor and alarm are worth the extra.
Is it better than chlorination?
For different things. UV handles protozoa that resist chlorine and changes nothing about taste. Chlorination leaves a residual that keeps working through the plumbing and the water heater, which matters when the contamination is in the distribution system rather than the source.
My test came back with coliform. Is UV the fix?
It is part of a fix, not the fix. Coliform means the well is open to the surface somewhere, so the wellhead, casing and cap need inspecting and the well needs disinfecting. Treating the water while leaving the opening is managing a symptom indefinitely.

Sources