Well Water Filtration: Do You Need UV, RO, or Both?

On city water, a municipal plant disinfects before the tap. On a private well, you are the treatment plant — and the two technologies people ask about solve completely different problems.

What UV does (and doesn't)

Ultraviolet purification kills or neutralizes living things: bacteria like E. coli and coliform, viruses, and parasites such as giardia. It is the gold standard for microbial safety on wells. What it does not do: remove anything. Nitrates, arsenic, iron, lead, sediment — UV leaves all of it in the water.

What RO does (and doesn't)

Reverse osmosis physically removes dissolved contaminants — nitrates (common near agriculture), arsenic (common in certain regions' groundwater), lead, PFAS, sodium, and most everything else. RO membranes also block microbes, but most manufacturers don't certify RO alone as microbiologically safe, because a membrane defect is invisible until someone gets sick.

So which do you need?

• Test showed bacteria only: UV — either whole-house or built into an under-sink unit.
• Test showed nitrates, arsenic, or metals only: RO at the kitchen tap for drinking and cooking water.
• Both, or you want belt-and-suspenders drinking water: RO + UV together. Several modern tankless under-sink systems combine them in one unit.

The step before any of this: test the water

Everything above is conditional on knowing what's in your well, and wells vary house to house — your neighbor's results don't transfer. A certified mail-in well test panel costs $30–150 depending on depth of analysis, and many county health departments test for bacteria cheaply or free. Annual testing is the standing recommendation for any private well; buying treatment before testing is how people spend $600 solving a problem they don't have while missing the one they do.