Short definition
UV disinfection is a chamber with a UV lamp that inactivates bacteria, viruses, and protozoa in water flowing through it. It’s a common WA private well treatment because it handles cryptosporidium and giardia (which chlorine struggles with) and adds no chemicals. Lamp replacement is required annually regardless of usage.
What it is
A UV disinfection unit consists of a stainless or PVC chamber with a UV lamp inside a quartz sleeve. Water flows through the chamber and the UV light passes through the water — inactivating any pathogens that get the right dose of UV.
Key specifications:
- Wavelength: 254 nm (low-pressure mercury lamp; standard residential UV).
- Effective dose for disinfection: ≥40 mJ/cm² (NSF/ANSI 55 Class A standard for known-pathogen water).
- Lamp life: ~9,000 hours; replace annually regardless of operation. Effective UV output drops over time even when the lamp still appears to glow.
What UV inactivates:
- Bacteria (E. coli, total coliform).
- Viruses.
- Protozoa (cryptosporidium, giardia) — including chlorine-resistant pathogens.
What UV doesn’t do:
- Remove dissolved chemicals or metals.
- Remove sediment or particulates.
- Provide residual disinfection in plumbing — UV only inactivates pathogens at the moment of exposure; it doesn’t stay in the water.
Position in the filter train: after all filtration — sediment, iron, softener, carbon — and as the last stage before the water reaches fixtures. Cloudy water blocks UV penetration, so pre-filtration is mandatory.
Sizing: by GPM. Typical residential rating is 8–14 GPM.
Why it matters to a homeowner
For private well homes in WA — particularly Olympic Peninsula, Mason, Jefferson, Skagit, and other rural counties — UV disinfection is the standard biological treatment for bacterial contamination. Chlorination is the alternative, but UV has practical advantages:
- No chemicals added — no chlorine taste or smell, no disinfection by-products.
- Effective on cryptosporidium and giardia — protozoa that are chlorine-resistant and that can show up in surface-influenced wells (springs, shallow wells near waterways).
- Continuous operation — runs whenever water flows; no dosing schedule.
- Low operating cost — about $50–$100 per year for the annual lamp replacement plus electricity.
Cost: $400–$1,500 for the unit and install. Annual lamp replacement: $30–$80.
The catch is the annual lamp replacement. UV lamps lose intensity over time even when they appear to be working. By 9,000 hours (about a year of typical use), output drops below the disinfection threshold. The water still flows; the UV lamp still glows; but the pathogens aren’t being killed effectively. Set a calendar reminder.
When a contractor installs UV, they should also configure (and you should ensure) a UV intensity sensor with an alarm — this tells you when the lamp is dropping below the disinfection threshold before annual replacement comes due.
Common failure modes
- Lamp burnout (annual replacement skipped) — no disinfection, but water still flows. Homeowner unaware.
- Quartz sleeve scaling in hard water — reduced UV penetration; clean periodically.
- Inadequate pre-filtration (cloudy water blocks UV) — no disinfection.
- Power outage — no disinfection during the outage; flush after restoration.
- Iron and manganese coating quartz sleeve in well water — must address upstream of UV with iron filter.
Common variants
- Low-pressure UV (254 nm; standard residential) vs. medium-pressure UV (broader spectrum; commercial).
- NSF/ANSI 55 Class A (40 mJ/cm²; for known pathogens) vs. Class B (16 mJ/cm²; for already-disinfected water as added safety).
- UV (no chemicals, no taste change, no residual) vs. chlorination (residual disinfection through plumbing) vs. ozone (high-end, no residual).
Washington note
UV disinfection is widely used on WA private wells and small community systems:
Olympic Peninsula well home with positive coliform test — install UV after sediment and iron filters; verify intensity sensor is functional.
Mason County rainwater catchment with potable use — UV is essential; combine with sediment and carbon filtration. Verify county approval for potable rainwater use.
Pre-purchase well-water lab test reveals coliform — factor UV install ($400–$1,500 plus annual lamp) into negotiation.
Annual lamp replacement — schedule alongside spring plumbing checklist. Some lamps have hour-meters that count down; some don’t. Calendar reminder is reliable.
Cabin or second-home with UV — bypass during winter shutoff to prevent freeze damage to the chamber.
For Group A and Group B small community water systems regulated by WA DOH under WAC 246-290, UV is one approved disinfection method. The system operator handles maintenance; homeowners on these systems benefit without managing the equipment themselves.