UV disinfection dose (NSF/ANSI 55)
A UV lamp only disinfects if the water actually receives enough dose — and flow rate decides that.
UV disinfects microbes but removes nothing — no sediment, no chemicals, no color. It must follow adequate filtration, and it is not a remedy for water of unknown safety. Test your water and follow EPA and local guidance.
A UV disinfection system inactivates bacteria, viruses, and protozoa by hitting them with ultraviolet light. Whether it works comes down to dose — the UV energy each microbe actually receives — measured in millijoules per square centimeter (mJ/cm²). Dose is intensity multiplied by exposure time, so anything that lowers either one lowers your protection.
The NSF/ANSI 55 classes
| Class | Validated dose | Intended use |
|---|---|---|
| Class A | 40 mJ/cm² | Disinfection of water that may be microbiologically unsafe |
| Class B | 16 mJ/cm² | Supplemental treatment of already-safe water only |
For a private well used as drinking water, a Class A system (40 mJ/cm²) is the relevant standard. Class B is not intended to make unsafe water safe.
Flow rate and water clarity set the real dose
Two things erode the dose a system delivers:
- Flow rate. Faster flow means less time in front of the lamp — less contact time, less dose. A system rated for 40 mJ/cm² at 9 GPM delivers far less if you push 15 GPM through it.
- UV transmittance (UVT). Cloudy or tinted water absorbs UV before it reaches the microbes. Low UVT can collapse the effective dose even at the rated flow — which is why pre-filtration and clarity matter so much.
Size to your peak flow, not your average. The system must still deliver its rated dose when several fixtures run at once. Use the highest simultaneous flow your household can draw as the design flow.
Gear this guide uses
UV only works on clear water at adequate dose — so pre-filtration is part of the system.
- An NSF/ANSI 55 Class A UV system sized to your peak flow is the relevant standard for a private well.
- Sediment pre-filters clear the water first so it actually transmits UV.
- A replacement UV lamp keeps dose up as the bulb ages past its rated hours.
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Sources
UV requires clear water and adequate dose; it does not remove chemicals or sediment. Test your water and consult a treatment professional.