Does tap water have bacteria in it?

Treated municipal water is required to be virtually free of disease-causing bacteria. The EPA's MCL for total coliform is zero detections per month for systems serving over 1,000 people. However, biofilms can form in distribution pipes and household plumbing, and bacterial regrowth can occur when disinfectant residual drops below effective levels. Homes with private wells have no federal testing requirements — well water bacteria contamination is common, particularly coliform bacteria, E. coli, and Giardia.

What filter removes bacteria and viruses from water?

To claim "purification" (vs. simple filtration), a filter must demonstrate log reductions of bacteria, viruses, and protozoa per NSF/ANSI 58 (for RO) or NSF P231 (the purifier standard). UV purification achieves 99.99% reduction (4-log) of bacteria and viruses without adding chemicals. Reverse osmosis removes bacteria and protozoa (>99.9%) but is less effective against viruses alone — most RO systems combine the membrane with a UV stage. For private wells or emergency water purification, a UV system (like Viqua or TrojanUV) plus RO is the gold standard.

I have a private well — should I test for bacteria?

Yes — annually at minimum, and after any of these events: flooding, drought, nearby construction, new pump installation, or any time your water changes color, odor, or taste. The EPA recommends testing well water every year for total coliform bacteria and E. coli. Both tests are available through state health department labs for $20–$50, or through Tap Score's Well Water test ($239) which includes bacteria plus dozens of chemical contaminants in a single comprehensive panel.

Can UV light replace a water filter?

No. UV light deactivates bacteria, viruses, and protozoa by damaging their DNA — but it doesn't remove chemical contaminants, particulates, sediment, heavy metals, or PFAS. UV purification and chemical filtration serve complementary purposes. For well water, the correct setup is: sediment pre-filter → chemical filtration (carbon or RO) → UV purification. The UV stage comes last because turbidity (particles in water) shields microorganisms from UV light.

How to Remove Bacteria & Viruses from Drinking Water

Bacteria and Viruses in Water: Municipal vs. Well Water Are Different Problems

If you're on city water, biological contamination is largely managed by your utility. Chlorination or chloramination kills the vast majority of bacteria and viruses in treatment. The EPA's regulations require utilities to maintain disinfectant residual through the distribution system precisely to prevent bacterial regrowth. Boil-water advisories are issued when that system fails — and they do fail occasionally, as residents of Jackson, Mississippi experienced in 2022 during their multi-month water crisis.

If you're on a private well, you are entirely responsible for your own biological safety. No federal or state agency monitors your well's bacteria levels. In a survey of 10 states, the USGS found coliform bacteria in 20–50% of untested private wells depending on region. E. coli (indicating fecal contamination) was found in 1–4% of wells in the same studies. Shallow wells and wells near livestock operations, septic systems, or flood-prone areas are at highest risk.

Well Users: Test Annually, Filter Accordingly

Unlike chemical contaminants that change slowly, bacterial contamination can appear suddenly — after a flood, a heavy rain event, or a small crack in your well casing. Annual testing is the minimum. If your well has ever tested positive for E. coli, install a UV purification system before re-using the water — and investigate the source of contamination (usually a failing well seal or nearby septic system).

Biological Contaminants: The Major Categories

Bacteria

Examples: E. coli O157:H7, Salmonella, Legionella pneumophila, Campylobacter
Size: 0.2–10 microns
Symptoms: Gastroenteritis (nausea, diarrhea, vomiting), Legionnaires disease (Legionella), can be fatal in immunocompromised individuals
Removal: UV (99.99%), RO membrane (99.9%), 0.2-micron hollow-fiber ultrafiltration, boiling

Viruses

Examples: Norovirus, Hepatitis A, Rotavirus, Enteroviruses
Size: 0.02–0.2 microns
Symptoms: Gastroenteritis, hepatitis, respiratory illness — viruses cause the most severe waterborne illness per infection event
Removal: UV (99.99%), RO membrane (partial, 99%), boiling. Standard filtration often inadequate — size is smaller than many filter pores.

Protozoa

Examples: Giardia lamblia, Cryptosporidium parvum
Size: 2–15 microns (cysts)
Symptoms: Giardiasis (severe diarrhea lasting weeks), cryptosporidiosis (profuse watery diarrhea, dangerous for immunocompromised)
Removal: RO membrane (>99.9%), 1-micron filters, UV (Cryptosporidium requires higher UV dose than bacteria), boiling

UV Purification: The Most Reliable Biological Treatment for Wells

Ultraviolet light at 254 nanometers penetrates the cells of bacteria, viruses, and protozoa and permanently damages their DNA, preventing reproduction. At the correct UV dose (measured in mJ/cm²), UV achieves 4-log (99.99%) inactivation of bacteria and viruses without adding any chemicals to the water or creating disinfection byproducts.

For residential well water, a UV system is typically installed as a point-of-entry (whole-house) system after a sediment and chemical filter. The correct sequence matters:

1Sediment pre-filter (5–10 micron) — removes particles that would shield microorganisms from UV light
2Chemical filtration (carbon block or RO) — removes PFAS, lead, chlorine, and chemical contaminants
3UV purification — final disinfection stage, inactivates any remaining bacteria/viruses/protozoa

Our Recommended UV Systems

Viqua D4 ($350) — 12 GPM whole-house flow rate, NSF/ANSI 55 Class A certified (the highest UV performance standard), 1-year UV lamp replacement. Best for families of 4–6 on well water.

SpringWell WS Whole-House System ($1,297) — Includes sediment filter, carbon stage, and UV purification in an integrated system specifically designed for well water. Single-vendor solution with phone support. Best for homeowners who want a turnkey package.