Ultraviolet water treatment passes water along a quartz sleeve containing a lamp that emits light at a wavelength which damages the genetic material of bacteria, viruses and protozoa, leaving them unable to reproduce. It adds nothing to the water, changes nothing about its taste, and leaves no residual protection downstream. It is the standard answer to a positive bacteria test on a private well.
Why wells are where UV belongs
The EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, and that around 15 percent of the US population, over 43 million people in more than 23 million households, depend on them. No utility is disinfecting that water, so a well that draws in surface water through a compromised casing or cap has nothing standing between the contamination and the tap. UV is well suited to this because it is continuous, needs no chemical handling and no contact tank, and its only consumables are a lamp and a sleeve. The wellhead fault still has to be fixed, though: UV treats the symptom, not the cause.
Clear water is a hard requirement
UV works by light reaching the organism. Anything that blocks, scatters or absorbs that light defeats it. Turbidity shelters organisms behind suspended particles. Iron, manganese and hardness deposit a film on the quartz sleeve that steadily reduces transmission until the unit is doing very little while still appearing to run. That is why UV always sits last in a treatment train, after sediment filtration and after iron and hardness removal, and why manufacturers specify limits for iron, manganese, hardness and turbidity in the feed water. A UV unit fitted without adequate prefiltration is the most common way this technology disappoints.
Dose, flow rate and sizing
The effectiveness of a UV system depends on dose, which is intensity multiplied by exposure time. Exposure time falls as flow rises, so a unit sized for a modest flow provides much less dose when several fixtures run at once. Size the unit for peak simultaneous household flow rather than average use, and understand that the rated flow on the box assumes a stated water clarity your supply may not meet. Better units carry an intensity sensor that measures light actually reaching the far side of the chamber and alarms when it falls, which is worth having because there is no other way to observe a UV system failing.
Maintenance and its limits
The lamp is replaced annually even though it still glows, because output falls well before the lamp fails visibly. The quartz sleeve is cleaned on the same schedule and replaced periodically, since it clouds with age and mineral film. Leave the system powered continuously: cycling a UV lamp shortens its life considerably. Understand the limits too. UV does nothing to chemical contaminants, nitrate, lead, hardness or taste, and it leaves no residual, so plumbing downstream can recontaminate treated water. After any well disinfection or plumbing work, that downstream section needs attention as well.
Questions people ask about ultraviolet water treatment
Does UV remove anything from the water?
No. It inactivates microorganisms so they cannot reproduce, but nothing is physically removed. Chemical contaminants, nitrate, hardness and taste are entirely unaffected.
How often does the lamp need changing?
Annually, regardless of whether it still lights. Output declines steadily long before the lamp visibly fails, so replacement runs on the calendar rather than on appearance.
Do I need a filter before a UV system?
Yes. UV needs clear water, so sediment filtration comes first, along with iron and hardness removal where the test calls for it, or the quartz sleeve fouls and transmission falls.
Should I turn the UV off when away?
No. Cycling the lamp shortens its life significantly, and water standing in an unpowered chamber is untreated. Leave it powered and flush the system on return.