What Is a UV Water Filter and How Does It Work? Dose, Limits and Maintenance

A UV water filter passes water past a 254 nm UV-C lamp inside a quartz sleeve, which attacks the DNA of bacteria, viruses and protozoa so they can't reproduce, without adding chemicals. UV doesn't remove chlorine, lead, hardness, taste or odor, and it leaves no residual disinfectant in the plumbing. It works only on clear water, with a pre-filter, the right dose and a yearly lamp change. If you're comparing systems, see our UV water purifier collection.

How UV Water Disinfection Works

UV water disinfection works by exposing water to germicidal UV-C light at 254 nm as it flows through a stainless-steel chamber. The light damages the DNA of microorganisms in the water. Nothing is added to the water, and nothing is filtered out.

UV-C at 254 nm and what it does to microorganisms

UV-C light at the germicidal 254 nm wavelength attacks the DNA of microorganisms and eliminates their ability to reproduce, according to Viqua. An organism that can't reproduce can't multiply in the water. The Greene County, New York Department of Health also identifies 254 nm as the germicidal wavelength.

Inside a UV system: lamp, quartz sleeve, chamber, controller and sensor

A UV system has a UV lamp inside a quartz sleeve, a stainless-steel chamber the water flows through, a controller (ballast) and, on many models, a UV sensor. Viqua says the controller manages lamp output, and some controllers include lamp-change timers and low-UV alarms. Table 1 shows what each part does.

Table 1. Parts of a UV System

Dose = intensity × contact time

UV dose, measured in mJ/cm², equals the lamp's UV intensity multiplied by the time the water spends in the chamber. When water flows faster, it spends less time in the chamber and receives a lower dose. That's why every UV system is rated for a maximum flow at a given dose.

What UV Treats and What It Doesn't

UV treats bacteria, viruses and protozoa at the rated dose, but it doesn't remove chemicals, metals, minerals or taste. UV is a disinfection stage only. Other water problems need other treatment.

Bacteria, viruses and protozoa

UV inactivates bacteria, including E. coli, viruses and protozoa such as Cryptosporidium and Giardia, at the rated dose. The US EPA's dose table shows that viruses need a much higher dose than Cryptosporidium or Giardia for the same log inactivation. That's covered in the dose section below.

What UV does not remove: chlorine, lead, hardness, taste, TDS

UV does not remove chlorine, lead, hardness, dissolved solids, or taste and odor, because it only acts on microorganisms. The Connecticut Department of Public Health notes that UV does not improve taste, odor or clarity. Table 2 lists what UV does and doesn't treat.

Table 2. What UV Does and Doesn't Treat

Downsides to plan for: no residual, power outages, clear water needed

The main downsides of UV are that it leaves no residual disinfectant, stops working without power, and needs clear water, according to the Connecticut Department of Public Health.

• Water is treated only as it passes the lamp, with no protection downstream.

• After a power outage, run water to waste until the system is back at full intensity.

• Cloudy or stained water blocks UV light; see the limits below.

UV Dose Ratings: NSF/ANSI 55 Class A vs Class B

NSF/ANSI 55 rates UV systems in two classes: Class A at a minimum dose of 40 mJ/cm² and Class B at a minimum of 16 mJ/cm². Viqua says the accepted standard for most applications is 30 mJ/cm², and 40 mJ/cm² for NSF 55.

Class A (40 mJ/cm²) vs Class B (16 mJ/cm²)

An NSF/ANSI 55 Class A system delivers at least 40 mJ/cm² at its alarm set point, while a Class B system delivers at least 16 mJ/cm², per the Greene County, New York Department of Health. Check which class your system is certified to before you rely on it. Your local health department can tell you which class it requires.

Table 3. NSF/ANSI 55 Classes and Viqua's Dose Guidance

Flow rate changes the dose

A UV system's dose drops as flow rises, so the same system has a different rated flow at each dose. Viqua's specifications show this for its Viqua VH200 and VH410. Size a system by its flow at the dose you need, not its top flow.

Table 4. Flow at Dose (Viqua Specifications)

EPA log-inactivation doses for Cryptosporidium, Giardia and viruses

The US EPA's UV dose table (40 CFR 141.720) sets doses for public water systems using 254 nm low-pressure lamps: 12 mJ/cm² for 3-log Cryptosporidium, 11 for Giardia and 143 for viruses. For 4-log inactivation, the doses are 22, 22 and 186 mJ/cm². These are public water system values, not household ratings, but they show how much harder viruses are to inactivate.

Will UV Work on Your Water? Pre-Treatment Limits

UV works on your water only if the water meets the system maker's limits for iron, manganese, hardness, tannins, turbidity and UV transmittance. Test your water before you choose a system. Viqua also recommends a 5-micron pre-filter.

Viqua water-quality limits

Viqua lists these limits for water entering a UV system: iron under 0.3 ppm, hardness under 7 gpg, turbidity under 1 NTU and UV transmittance above 75% at 254 nm. Table 5 shows the full list. Local health departments can set different limits.

Table 5. Will UV Work on My Water? (Viqua Limits)

Why a 5-micron pre-filter comes first

Viqua recommends placing a 5-micron filter ahead of a UV system so particles don't block the UV light. The pre-filter sits upstream, before water reaches the chamber. Replacement cartridges are in our whole house sediment water filter collection.

Well water: test before you size

If you're on well water, test it against Table 5 before sizing a UV system. The Connecticut Department of Public Health recommends testing raw and treated water every year. If a result is over a limit, treat that problem upstream of the UV system first.

UV Maintenance at a Glance

UV maintenance comes down to a yearly lamp change, cleaning the quartz sleeve at the same time, resetting the timer and testing your water. After any installation or service, the Connecticut Department of Public Health advises disinfecting the plumbing with chlorine.

Lamp and sleeve schedule

Replace the UV lamp every year of continuous use, about 9,000 hours for the Viqua VH410, and clean the quartz sleeve whenever the lamp is changed, per Viqua.

• Reset the lamp timer after each change.

• Test for coliform monthly for 6 months after installation (CT DPH).

• Test raw and treated water every year (CT DPH).

Finding the right replacement lamp

The right replacement lamp depends on your system's model number, shown on the silver label on the UV chamber. DFS stocks lamps for many systems in our UV bulb replacement collection. Questions about which system or lamp fits your home? Call the DFS team at 1-800-277-3458.

Frequently Asked Questions

Does a UV water filter remove chlorine?

No, a UV water filter does not remove chlorine. UV attacks the DNA of microorganisms and doesn't remove dissolved chemicals. If chlorine taste or odor is the problem, you'll need a separate treatment stage, because the Connecticut Department of Public Health notes UV does not improve taste or odor.

Is UV-treated water safe to drink?

UV-treated water has been exposed to UV-C light that inactivates bacteria, viruses and protozoa at the system's rated dose. UV adds no chemicals and leaves no residual. Test your water yearly, and test for coliform monthly for 6 months after installation, as the Connecticut Department of Public Health recommends.

Does UV water treatment change the taste?

No, UV water treatment does not change the taste of water, and the Connecticut Department of Public Health says it doesn't improve taste, odor or clarity either. UV adds nothing to the water and removes nothing from it. Taste problems need a different treatment.

Can UV treat well water on its own?

UV can disinfect well water only if the water already meets Viqua's limits for iron, hardness, turbidity and UV transmittance. Viqua recommends a 5-micron pre-filter ahead of the UV system. Test your well water first, and treat any problem over the limits upstream of the UV.

What happens to UV disinfection during a power outage?

UV disinfection stops during a power outage because the lamp needs electricity to produce UV-C light. After power returns, the Connecticut Department of Public Health says to run water to waste until the system reaches full intensity. UV leaves no residual to protect water during the outage.

What is the difference between a UV filter and a carbon filter?

A UV filter disinfects water by inactivating microorganisms with UV-C light, while a carbon filter is a separate treatment stage that physically treats water as it passes through. UV removes nothing from the water. Check any carbon filter's NSF certification to see which contaminants it is certified to reduce.

Sources

• Viqua, UV overview (viqua.com/uv/); How UV Works; Pretreatment; UV System Components

• Viqua VH200 and VH410 product pages (flow at dose, lamp life)

• US EPA, 40 CFR 141.720 (UV dose table)

• Greene County, NY Department of Health, UV fact sheet (NSF/ANSI 55 Class A and B)

• Connecticut Department of Public Health, UV fact sheet

• Chautauqua County, NY, Private UV Light Disinfection System Requirements