Guides · Disinfection
When UV Belongs After Filtration
UV does not filter anything — it disinfects. Why it goes last, what it needs upstream, and the maintenance people forget.
What UV actually does
An ultraviolet system exposes water to UV-C light, which damages the DNA of bacteria, viruses, and protozoa so they cannot reproduce. It removes nothing: no sediment, no lead, no chlorine, no nitrate.
That is why a UV unit is a disinfection stage, and why it belongs in a train with filtration rather than instead of it.
Why it goes last
UV light has to reach the organisms. Sediment, iron, and humic colour shield them, so a UV unit downstream of dirty water quietly stops working even though the lamp is on.
Standard order on a well: sediment pre-filter, then iron/sulphur treatment if needed, then a 5-micron cartridge directly before the UV, then the UV, then point-of-use drinking filtration.
Many manufacturers specify a maximum turbidity and a pre-filter rating; follow theirs rather than a general rule.
What UV is good for
A private well with a history of coliform hits, or a spring or cistern supply that is not chlorinated.
A belt-and-braces stage after a softener or carbon system where the water will sit in a tank.
It does not replace a boil-water notice. During a notice, follow the utility's instruction.
Sizing and power
UV dose depends on lamp intensity and contact time. Oversizing the flow rate through a small unit reduces the dose, so match the unit to the peak flow of the house.
The lamp needs power continuously, including during holidays. A UV system on a switched outlet is a common and invisible failure.
Some units have a solenoid that shuts the water off if the lamp fails — worth the extra cost if the water is otherwise untreated.
Maintenance people forget
Lamps lose intensity over time even while glowing. Replace on the manufacturer's hour or month figure, typically annually.
Clean the quartz sleeve when you change the lamp. Iron and hardness film on the sleeve blocks the light.
Retest bacteria annually. A UV system that has silently lost dose looks identical to one that is working.
FAQ
Quick answers
+Does UV remove lead or PFAS?
No. UV disinfects only. Lead, PFAS, nitrate, and other dissolved contaminants need filtration or RO downstream or upstream of the UV.
+Can I put UV before my carbon filter?
It is better after. Carbon fines and sediment can shield organisms from the light, and most manufacturers specify a particulate filter immediately before the UV.
+How often do UV lamps need replacing?
Typically once a year, or per the manufacturer's hour rating. The lamp still glows when it is past its useful dose, which is why the schedule matters more than appearances.