Water filter type
Ultraviolet disinfection
A lamp that inactivates bacteria, viruses, and protozoa as water passes through. Standard practice on private wells with a positive bacteria result.
What ultraviolet does to a microorganism
A UV lamp emits light at a wavelength that damages the genetic material of bacteria, viruses, and protozoa as they pass. They are not filtered out and they are not killed in the usual sense. They are left unable to reproduce, which is what makes them harmless to drink.
Because nothing is removed, there is no cartridge filling up and no chemical residual left in the water. The trade is that protection exists only at the lamp. Anything that enters the plumbing downstream is untouched.
Dose is what matters, and dose is a function of lamp output and how long the water is exposed. Run water through faster than the unit is rated for and the dose falls below what the certification assumed.
Class A and Class B, which is a real distinction
NSF/ANSI 55 defines two classes and they are not interchangeable. Class A systems are validated to treat water of unknown microbiological quality, which is the case that matters on a private well with a positive coliform result.
Class B systems are supplemental only. They are intended for water that is already known to be microbiologically safe, as an extra measure. A Class B unit is not an adequate response to a positive bacteria test.
If someone is selling you a UV system for a well that has tested positive, ask which class it is certified to. The answer should be A.
Pre-filtration and the annual lamp change
Suspended particles shield organisms from the light. A microbe travelling inside a speck of silt can pass through unharmed, which is why a sediment pre-filter is a requirement rather than an upgrade. Iron and manganese also coat the quartz sleeve and block output.
The lamp is replaced every twelve months whether or not it still lights. Output falls well before the lamp visibly fails, so a glowing lamp past its service life is the most common way these systems quietly stop protecting anyone.
A lamp that is off, whether from a power cut or a failed ballast, means no treatment at all. On a well with a known bacteria problem, a solenoid valve that shuts off flow when the lamp fails is worth the extra cost.
What it reduces, and under which standard
Each row names the NSF/ANSI standard the category is tested under. A standard describes what the test covers. It does not mean every unit on the market carries that certification, so check the specific model against its listing before you buy.
| Contaminant | Standard | Note |
|---|---|---|
| Bacteria, viruses, and protozoa | NSF/ANSI 55 | Class A systems are for water of unknown microbiological quality. Class B is supplemental only, for water already known to be safe. |
What it does not do
Every filter has a boundary. These are the things this one will not fix, listed so you can rule it out quickly if your problem is on the list.
- Anything chemical. Lead, nitrates, arsenic, PFAS, and hardness all pass through unchanged.
- Turbidity. Cloudy water shields organisms from the lamp, so a sediment pre-filter is required, not optional.
- Recontamination downstream of the lamp.
- Disinfection during a power cut.
When it fits
- A private well has tested positive for total coliform or E. coli
- You want continuous disinfection without adding chemicals
- Water is already clear or can be filtered clear first
When it does not fit
- Water is turbid or high in iron, until that is treated upstream
- The property loses power often and there is no failsafe valve
- The concern is chemical rather than microbiological
Space, services, and upkeep
A horizontal or vertical run of about 3 feet, with clearance to withdraw the lamp.
No drain connection is required. It needs a power outlet within reach. This is not a rental option, because it fixes to the building and stays there.
Replacement interval. Lamp replaced every 12 months regardless of whether it still lights. Sleeve cleaned at the same time.
Problems this is used for
We recommend this equipment against these confirmed problems. Each page explains how to confirm the problem before spending anything, and when treatment is not warranted.
- Bacteria in well water. Private wells are not regulated the way public supplies are, and no one tests them but the owner.
Property types where this comes up
Common questions
How do I know the lamp is still working?
You largely cannot by looking, which is the problem. Ultraviolet output falls well before a lamp stops emitting visible light, so a lamp that still glows can be delivering too low a dose to be effective. That is why replacement is annual regardless of appearance. Units with a UV intensity sensor measure actual output rather than assuming it, and on a well with a known bacteria problem that sensor is worth paying for.
Does it leave anything in the water?
No. There is no chemical residual and nothing is added. That is an advantage for taste and an important limitation for protection, because there is nothing left in the water to deal with contamination entering the plumbing after the lamp. UV protects at the point it is installed and nowhere downstream of it.
Why do I need a sediment filter as well?
Because a microorganism travelling inside a particle of silt is shielded from the light and passes through unaffected. Cloudy water and iron both reduce effective dose, and iron also coats the quartz sleeve so less light reaches the water each month. Pre-filtration is part of the system rather than an optional upgrade, and the certification assumes it is there.
Other filter types to compare
If the notes above rule this out, these are the neighbouring options worth reading before you decide.
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