What Is UV Water Disinfection? What It Does and Doesn't Do

UV is a fix for germs, most often in a private well. It doesn’t change anything dissolved in your water, so it’s usually one part of a treatment setup, not the whole thing.

What does UV water treatment remove?

UV doesn’t strain anything out; it inactivates germs. A lamp shines through the water as it flows past, and the light damages the DNA of microorganisms so they can’t reproduce, the Ohio Department of Health explains. The CDC lists parasites, bacteria and viruses as what UV systems with pre-filtration deal with, and chemicals as what they don’t. The EPA’s treatment guide adds that UV isn’t effective against heavy metals or organic chemicals, and that it uses no extra water.

So UV won’t touch lead, nitrate, arsenic or PFAS. If a test shows those, you need treatment certified for them, such as reverse osmosis or a carbon filter. The upside, per the Minnesota Department of Health, is that UV adds no chemicals to the water.

What do NSF/ANSI 55 Class A and Class B mean?

Ohio’s fact sheet summarizes NSF’s two classes. Class A systems deliver a UV dose of at least 40 mJ/cm² and are designed to inactivate bacteria, viruses, Cryptosporidium and Giardia in contaminated water, with an alarm or shut-off if the dose drops too low. Class B systems deliver at least 16 mJ/cm² and are meant only to reduce nuisance, non-disease-causing microbes, not to disinfect water that may be unsafe. Ohio accepts only Class A as the main disinfection for a private water system. NSF’s listings show which class each certified model holds, so check before you buy.

Do you need UV for well water?

Only if germs are a real problem. If a test finds bacteria, start with your health department and our well water testing guide: retest, find the cause and fix the well first. If UV becomes part of the answer, Ohio advises sizing it to treat all household water at peak flow.

Clear water matters. Minnesota recommends a pre-filter if turbidity is above 1 NTU, and Ohio notes that iron and hardness can coat the quartz sleeve around the lamp and block the light. Ohio also says a lamp loses much of its strength after about a year even if it still glows, so replace it yearly.

On city water, germs are your utility’s job, and the EPA notes that conventional treatment is highly successful at removing them. Most city households don’t need UV at home.