How to Purify Tap Water: Every Method Compared
The most reliable way to purify tap water is a rolling boil for one minute (three minutes above 2,000 meters), which kills bacteria, viruses, and protozoa. Filtration, UV light, chemical tablets, and solar disinfection each work too — but every method has blind spots, and the right choice depends on what you're trying to remove.
Boiling: The Gold Standard for Pathogens
Boiling is the oldest purification method and still the most dependable against microorganisms. The CDC recommends bringing water to a full rolling boil for one minute — and three minutes at elevations above 2,000 meters (about 6,500 feet), where water boils at a lower temperature. That treatment reliably kills bacteria such as E. coli and Salmonella, viruses like norovirus and hepatitis A, and hardy protozoa including Giardia and Cryptosporidium, which resist chlorine.
What boiling does not do is remove anything that isn't alive. Heavy metals like lead and arsenic, nitrates, pesticides, and salts all stay in the water — in fact, boiling slightly concentrates them as water evaporates. If your concern is chemical contamination rather than germs, boiling is the wrong tool. Let boiled water cool covered, and store it in a clean, closed container.
Filtration: Know What Your Filter Actually Removes
Filters vary enormously, and the pore size and media determine what they catch.
- Activated carbon (pitcher and faucet filters) improves taste and removes chlorine, many pesticides, and some organic chemicals. Standard carbon filters do not reliably remove bacteria, viruses, or dissolved metals unless certified for them.
- Ceramic filters physically block bacteria and protozoa down to roughly 0.2–0.5 microns, but viruses (around 0.02–0.3 microns) can slip through.
- Hollow fiber membranes, common in travel filters, remove bacteria and protozoa; only the tightest (0.02 micron) versions also catch viruses.
- Reverse osmosis (RO) forces water through a semi-permeable membrane and removes nearly everything: pathogens, lead, arsenic, nitrates, fluoride, and most dissolved solids. It is the most complete treatment available for home use, at the cost of wasted water and slower flow.
Whatever you buy, check for NSF/ANSI certification (Standard 53 for health contaminants, Standard 58 for RO, P231 for microbiological purifiers) rather than trusting marketing claims.
UV Light: Effective, With Caveats
Ultraviolet (UV-C) treatment scrambles the DNA of bacteria, viruses, and protozoa so they can't reproduce. Home UV systems and portable UV pens both work well against all three pathogen classes — including chlorine-resistant Cryptosporidium — which makes UV one of the few methods that matches boiling on microbes.
The caveats: UV only works in clear water, because particles shield microorganisms from the light. Cloudy water must be pre-filtered. UV also removes nothing chemical — no metals, no salts, no pesticides — and battery-powered pens can fail without warning, so travelers should carry a chemical backup. See our travel filter and purifier guide for how UV fits into a travel kit.
Chemical Purification: Chlorine Dioxide and Iodine
Purification tablets and drops are cheap, weightless, and never break — which is why they're the standard backup method. Chlorine dioxide is the better modern choice: it kills bacteria and viruses within about 30 minutes and, unlike plain chlorine or iodine, will also inactivate Cryptosporidium if you wait the full contact time (up to 4 hours for cold or dirty water).
Iodine tablets work against bacteria, viruses, and Giardia, but not Cryptosporidium, and they leave a medicinal taste. Iodine is not recommended for pregnant women, people with thyroid conditions, or continuous use beyond a few weeks. With either chemical, cold water needs a longer contact time, and neither removes chemical contaminants — they only address the biological side.
SODIS: Solar Disinfection When You Have Nothing Else
Solar disinfection (SODIS) is a WHO-recognized last-resort method used widely in low-income regions. Fill a clear PET plastic bottle (2 liters or smaller) with clear water, and lay it in full, direct sunlight for at least 6 hours — or two full days if the sky is more than half cloudy. The combination of UV-A radiation and heat inactivates most bacteria, viruses, and protozoa.
SODIS costs nothing and requires no equipment beyond a bottle, but it is slow, weather-dependent, works only with clear water, and provides no chemical removal. Treat it as an emergency technique, not a plan.
Which Method Should You Use?
Match the method to the threat and the setting:
- Travel in a country rated Caution or Not Safe (check the world safety map): a purifier bottle or boiling for drinking water; bottled water as the convenient default.
- Microbial worry at home (boil water advisory, well contamination): boiling first, then a certified purifier — see our boil water advisory guide.
- Chemical worry at home (lead, nitrates, arsenic, PFAS): reverse osmosis or a carbon filter specifically certified for that contaminant. Boiling and UV do nothing here.
- Taste and chlorine smell: a simple activated carbon pitcher is enough.
- Backcountry or emergencies: hollow fiber filter plus chlorine dioxide tablets covers essentially everything biological.