They treat different problems. Ultraviolet light disinfects germs but removes no chemicals. Activated carbon improves taste and smell and, when certified, reduces some chemicals such as lead. Reverse osmosis removes germs and many dissolved chemicals but wastes water. Pick by what your water test shows, and read the label.
What does ultraviolet treatment do?
Ultraviolet treatment systems use UV light to disinfect water or reduce the number of germs in it. NSF's standard for these systems, NSF/ANSI 55, describes UV units that inactivate or kill bacteria, viruses and cysts. The CDC says UV systems with pre-filtration remove parasites, bacteria and viruses, and that they work better than systems that do not filter the water first. The CDC also states plainly that UV systems do not remove chemicals.
That last point matters. UV treats germs and nothing else. It does not soften water, reduce chlorine taste, or remove arsenic or lead. A UV lamp also has no effect on hardness scale.
So where does UV fit? Most often it belongs on water that may carry germs, such as a private well, or in a setup where a person has a medical reason to avoid germs in water. Phoenix's water is disinfected with chlorine before delivery, and the city's report says its entire supply is safely disinfected that way, so a typical city home does not need UV for the tap. If you are on a well, test the water at least once a year, as the CDC recommends, and let the results decide. If you have a weakened immune system, the CDC advises talking to your health care provider and a water disinfection specialist about whether to get a system.
What does activated carbon do?
Activated carbon is the material inside most pitcher, faucet, refrigerator and under-sink filters. The EPA describes granular activated carbon as extremely porous, with small pores that trap contaminants as water passes through. The CDC says pitcher and refrigerator filters often use activated carbon, and that their main purpose is to improve taste and smell rather than the safety of water. It also notes that these filters typically use a carbon filter to make water taste better or to remove heavy metals like lead.
Two NSF standards cover carbon filters, and the difference is the whole story.
- NSF/ANSI 42 covers filters certified to reduce aesthetic impurities such as chlorine and taste and odor.
- NSF/ANSI 53 covers filters certified to reduce a contaminant with a health effect. Lead is a common claim.
The EPA also reports that carbon point-of-use filters it studied can greatly reduce PFAS levels, provided the filter is maintained. It cautions that current certification standards, as of April 2024, do not yet show that a filter removes PFAS down to the levels the EPA has set for a drinking water standard.
What carbon does not do is disinfect. The CDC says most home filters, like pitcher or fridge filters, are not designed to remove germs. Carbon also does not soften water. Every carbon filter has a limited capacity and needs regular replacement, a point we cover in how often to change a whole-house water filter.
There is one more catch. The CDC warns that filters remove beneficial chemicals too, such as chlorine, which kills germs, and fluoride, which prevents cavities. It adds that a whole-home filter that removes chlorine or other disinfectants can let more germs grow in your plumbing. This is why we would not add a whole-house carbon filter for taste alone without thinking about it. A filter at the kitchen tap gives you the taste improvement without stripping disinfectant from every pipe. If your main complaint is taste, see why Phoenix water can taste like chlorine.
What does reverse osmosis do?
The CDC explains that reverse osmosis reverses the natural flow of water. The water is pushed from a solution with more substances through a filter into a solution with fewer, and it often passes through additional filters before and after the membrane. NSF/ANSI 58 covers these systems, and NSF notes that most include one or more extra filters on either side of the membrane.
RO is the broadest of the three for chemicals. The CDC says it removes parasites, bacteria and viruses, and will remove some chemicals including lead, copper, chromium, chloride and sodium. It may also reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate and phosphorus. The CDC's instruction is to check the label for the specific chemicals the system removes.
The trade-offs are real.
- Water waste. The EPA says an RO system wastes about one gallon of water for every gallon it treats, sending the untreated stream down the drain.
- Maintenance. The EPA notes that you must change filter cartridges and the RO membrane. Our page on reverse osmosis filter replacement explains the schedule.
- Minerals. RO may reduce calcium and magnesium, which is one reason the water can taste flat. See whether reverse osmosis removes healthy minerals.
- Incoming water quality. The EPA notes that additional water treatment might be needed depending on your incoming water quality. In hard water areas, a prefilter or softener can protect the membrane.
For a broader look at the decision, read whether reverse osmosis is worth it.
How do you choose between UV, carbon and RO?
Start from what you need, not from the product. Here is a simple way to match a problem to a tool.
| Your concern | Tool that fits | Why |
|---|---|---|
| Germs in well or rainwater | UV with pre-filtration, or a certified microbial filter | The CDC says UV with pre-filtration removes parasites, bacteria and viruses |
| Chlorine taste and smell | Carbon filter certified to NSF/ANSI 42 | NSF 42 covers chlorine, taste and odor |
| Lead at the kitchen tap | Carbon filter certified to NSF/ANSI 53 for lead, or NSF/ANSI 58 RO | Certification must name lead |
| Chromium, sodium, arsenic, nitrate | RO certified to NSF/ANSI 58, checked against its label | The CDC says RO removes some chemicals, including chromium and sodium, and may also reduce arsenic and nitrate |
| Scale and spotting | A softener, not UV, carbon or RO | NSF/ANSI 44 covers softeners for hardness |
| Everything at once | Combine systems | The CDC says you can use multiple types of systems to remove germs or chemicals more effectively |
The combination point is worth stressing. The CDC notes that if needed you can use multiple types of systems, and many under-sink RO units already include carbon stages before and after the membrane. A well owner might run a sediment filter, a UV lamp and an RO tap together.
Then follow four rules that apply to all three.
- 1Test first. Phoenix reports its results each year, and your city does too. For well water, the CDC says test at least once a year. Use the result to pick.
- 2Read the certification claim. NSF says certification does not mean a filter reduces every contaminant, so check the standard and the specific claim on the label.
- 3Maintain it. Phoenix's report warns that not following manufacturer instructions for home treatment can result in potentially unsafe water, and the CDC advises that people with weakened immune systems avoid changing filters themselves.
- 4Do not treat what you do not have. The CDC says that if your tap water has no harmful germs or chemicals you may not need a system at all.
If you want help matching a system to your house, we can look at your plumbing and your water report and give an upfront price before any work starts. The estimate is free.
Quick answers
Here are the short versions.
Does UV remove chlorine or metals? No. The CDC says UV systems do not remove chemicals.
Does carbon make water safe from germs? No. Most carbon filters are not designed to remove germs.
Is RO the best of the three? It covers the most chemicals, but it wastes water and needs upkeep. Better is a matter of your test result.
Do I need any of them on city water? Not always. The CDC says you may not need a system if your tap water has no harmful germs or chemicals, and many people choose one for taste.