UV disinfection dose (NSF/ANSI 55)

A UV lamp only disinfects if the water actually receives enough dose — and flow rate decides that.

UV is for clear, already-tested water.

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.

Well checks your UV dose against the flow

Enter your lamp rating, design flow, and water clarity and Well checks whether you're clearing the NSF/ANSI 55 Class A 40 mJ/cm² target — alongside filtration-stage sizing. Free to download.

Get Well on the App Store

Sources

NSF/ANSI 55 (ultraviolet microbiological water treatment systems) — Class A / Class B doses
US EPA private-well and drinking-water guidance; Penn State Extension water-treatment resources

UV requires clear water and adequate dose; it does not remove chemicals or sediment. Test your water and consult a treatment professional.