UV and reverse osmosis don’t compete. They treat completely different types of contamination.
UV inactivates living organisms. RO removes dissolved matter. If you pick the wrong one for your problem, you’ve spent money on something that doesn’t help.
What UV Does
Ultraviolet light at 254 nm damages the DNA and RNA of microorganisms. Bacteria, viruses, protozoa, and cysts that pass through the UV chamber can’t reproduce after exposure. The effect is called inactivation, not removal. The organisms are still present in the water, but they’re biologically dead and can’t cause infection.
The water itself isn’t changed. No chemicals added. No taste difference. No byproducts.
UV does absolutely nothing for lead, PFAS, nitrates, fluoride, arsenic, or any dissolved chemical.
What RO Does
A semipermeable membrane with 0.0001-micron pores removes contaminants through physical exclusion. Dissolved salts, heavy metals, chemicals, and even most biological particles can’t pass through the membrane. They concentrate in a reject stream that exits to drain.
RO removes PFAS (90-99%), fluoride (85-96%), arsenic (94-99%), nitrates (70-80%), and lead (95-99%). It also achieves 99.99%+ reduction of bacteria and high reduction of viruses through size exclusion.
But RO membrane integrity is the key variable for biological protection. A membrane operating correctly blocks bacteria. A damaged or aging membrane might not. That’s why well water with serious biological contamination warrants UV in addition to RO, not just one or the other.
The NSF 55 Class A vs. Class B Distinction
Not all UV systems are equal. NSF 55 sets two classifications.
Class A: 40 mJ/cm² UV dose minimum. Rated for purification, treating water from sources with unknown or suspected microbial contamination. This is what well owners with bacterial concerns need.
Class B: lower UV dose. Rated for supplemental disinfection of already-safe water. Not appropriate for primary biological treatment.
When shopping for UV systems, check the NSF 55 classification listed in the product specs. “NSF certified” without specifying Class A doesn’t mean it’s rated for purification.
When UV Alone Is Enough
Well water with confirmed bacterial contamination and no chemical contamination. If your well test shows coliform bacteria but acceptable levels of nitrates, arsenic, and other chemicals, a Class A UV system addresses the health risk.
UV is also the ongoing preventive measure after an acute bacterial event. If you’ve shock-chlorinated a well after flooding or bacterial intrusion, UV installed after the chlorine has dissipated provides continuous biological protection going forward.
When RO Alone Is Enough
City water with PFAS, fluoride, or lead concerns. Municipal water treatment has already handled biological contamination. Your water is microbiologically safe when it leaves the treatment plant. What you’re filtering for is dissolved chemical residue.
An NSF 58 certified under-sink RO at the kitchen tap addresses the primary exposure pathway at reasonable cost. See best under-sink RO systems for specific options.
When Both Make Sense
Well water with both biological risk and chemical contamination. This is common in agricultural areas where wells may carry both bacteria from surface infiltration and nitrates from fertilizer runoff. Or in the northeast where arsenic in bedrock coexists with seasonal coliform spikes.
For these situations, a combined system or sequential treatment addresses the full range. Several under-sink systems integrate UV after the RO membrane stage. The sequence matters.
Required order: sediment pre-filter, carbon pre-filter, RO membrane, UV chamber, carbon post-filter.
UV must come after the RO membrane. Turbid or chemically complex water reduces UV effectiveness by blocking or scattering the UV light. The membrane does the heavy lifting first. UV adds biological inactivation as a final stage.
Systems like the Aquasana OptimH2O and iSpring RCC1UP-AK combine both in one under-sink unit. Combined systems run $250-450, which is more than either technology alone but less than buying and installing them separately.
Cost
Under-sink RO only: $150-400. Filter replacement $50-100 per year.
UV add-on (inline lamp for existing under-sink system): $50-150. Lamp replacement roughly every 12 months.
Combined RO + UV under-sink system: $250-450. Filter and lamp replacement $80-150 per year.
The Recommendation
City water households dealing with PFAS, lead, or chemical contamination don’t need UV. RO handles what matters.
Well water households with bacterial concerns need UV Class A, either standalone or as part of a combined system. UV protects at every stage of biological exposure. RO doesn’t.
If you have well water with both categories of risk, the Aquasana OptimH2O or a similar combined system is worth the investment. It’s the only way to get documented protection against biological contamination and dissolved chemical contamination from a single under-sink installation.
See best UV water purifiers for Class A options, or check the filter decision guide to work through your specific water quality situation.