Water — Track 1: Treat
Boiling, bleach, tablets, filters, and UV each remove different threats from different sources. Knowing which method to reach for — and what it cannot do — is the actual skill.
Why this matters
A household that stores two weeks of water and knows nothing about treatment has one strategy: wait for the tap to come back on. A household that also knows how to treat water from any local source — a creek, a rain barrel, a swimming pool — has a strategy that doesn't have an expiration date.
Treatment skills don't replace storage. They extend it. When stored water runs low, the ability to safely use water from other sources is what keeps options open. That ability requires knowing which method to use, how to use it correctly, and what it cannot handle.
The most important thing to understand before reading this page: no single treatment method removes everything. Boiling kills biological threats but leaves chemicals. Filters remove bacteria and protozoa but most don't touch viruses. Bleach disinfects biological threats but not sediment or chemicals. Knowing the limits of each method is as important as knowing how to use it.
What water can contain
Biological: bacteria, protozoa, viruses
E. coli, Giardia, Cryptosporidium, norovirus. Treatable with multiple methods.
Chemical: industrial, agricultural, household
Pesticides, solvents, nitrates, petroleum. Most treatment methods do not remove these.
Heavy metals: lead, arsenic, chromium
Boiling concentrates these. Requires certified carbon or reverse osmosis filtration.
Physical: sediment, turbidity, debris
Not a health threat on its own, but clouds the water and reduces effectiveness of chemical and UV treatments.
The critical rule
If water may be contaminated with chemicals, heavy metals, or industrial waste — do not treat and drink it. No standard emergency treatment method makes chemically contaminated water safe. Follow official do-not-use advisories. Use stored water only.
Critical distinction
This is the most important distinction in water treatment and the most commonly misunderstood. Most people calling something a "filter" own a filter — not a purifier. The difference determines whether viruses survive.
Filter
Standard filters work by physical pore size — anything larger than the pore diameter is blocked. Bacteria (0.2–10 microns) and protozoa (1–300 microns) are caught. Viruses (0.02–0.3 microns) pass through most filter media.[1]
In the US and most developed countries, viruses in backcountry water sources are rare. Filters are adequate for most domestic emergency use cases. In areas with compromised sewage infrastructure — post-flood, post-earthquake, developing countries — virus risk increases significantly.
Examples
Sawyer Squeeze, LifeStraw, Berkey (most models), Waterdrop gravity systems, Brita
Purifier
Purifiers use a combination of very fine filtration, chemical treatment, UV, or ion exchange to address all three biological threat categories. NSF/ANSI Standard 58 and 62, or EPA Guide Standard Protocol, define the certification threshold for a purifier claim.[2]
Purifiers are the right choice when the water source is unknown, when sewage contamination is possible, when traveling internationally, or when operating in post-disaster conditions where infrastructure has failed.
Examples
Grayl Geopress, MSR Guardian, SteriPEN + filter combo, boiling, chlorine dioxide tablets
The methods
The most reliable biological treatment
Boiling is the most reliable and universally available treatment method. It requires no chemicals, no filters, and no equipment beyond a heat source and a pot. The CDC recommends bringing water to a rolling boil for at least one minute — at elevations above 6,500 feet, boil for three minutes because the lower boiling point at altitude reduces effectiveness.[3]
Let boiled water cool in a covered container. It will taste flat from dissolved oxygen being driven off — pouring it back and forth between two clean containers restores some of the taste. Store in a sealed container and use within 24 hours if kept at room temperature.
What boiling does not do
Boiling does not remove chemicals, heavy metals, nitrates, or salts. In the presence of lead pipes or chemical contamination, boiling actually concentrates these contaminants by reducing water volume. If chemical contamination is suspected, do not boil and drink. Use stored water or follow official guidance.
Fuel consideration: Extended boiling consumes fuel. In a situation where fuel is limited, bring water to a full rolling boil for one minute rather than simmering for longer. One minute at a full boil is sufficient at sea level.
Cheap, widely available, widely misused
Unscented household bleach is a reliable and inexpensive disinfection option. The EPA and CDC both document the protocol: use regular, unscented bleach with 6–8.25% sodium hypochlorite concentration. Do not use scented bleach, splashless bleach, color-safe bleach, or bleach with added cleaners — these contain additives that make them unsafe for water disinfection.[4]
EPA dosage protocol
Clear water
8 drops
per gallon
(~1/8 tsp)
Cloudy water
16 drops
per gallon
(~1/4 tsp)
Contact time
30 min
minimum
then check smell
After 30 minutes the water should have a faint chlorine smell. If it does not, repeat the dose and wait 15 more minutes. If it still has no chlorine smell after the second dose, do not use the water — it may contain organic matter that is consuming the chlorine faster than it can disinfect.
Bleach loses potency over time — commercially sold bleach degrades about 20% per year.[5] Buy fresh bleach annually. The full bleach disinfection protocol, including the cloudy-water pre-filtering step, is on the Bleach Water Disinfection page.
Cryptosporidium resistance
Chlorine bleach is not effective against Cryptosporidium, a protozoan parasite resistant to standard chlorine concentrations. In floodwater, post-hurricane conditions, or any situation with potential Cryptosporidium contamination, use boiling or a certified filter instead of bleach alone.
The lightest backup — every kit should have these
Chemical tablets are the most portable and shelf-stable treatment option. A small packet weighs almost nothing and can treat hundreds of gallons. The main tablet types are chlorine dioxide, iodine, and sodium dichloroisocyanurate (NaDCC). Chlorine dioxide is currently the gold standard — it is effective against bacteria, viruses, Giardia, and Cryptosporidium at the extended contact time. The full comparison is on the Water Purification Tablets page.
Tablet type comparison
Pre-filtering turbid water: All chemical treatments are less effective in cloudy or turbid water — organic matter and sediment consume the chemical before it can disinfect. Pre-filter cloudy water through a clean cloth, coffee filter, or by letting sediment settle before treating.
Removes bacteria and protozoa — most don't touch viruses
Filtration is the most practical ongoing treatment method for household use. A gravity filter on a counter can process a family's daily water needs without fuel, chemicals, or hands-on attention. The key limitation is virus removal — most consumer filters are not purifiers and do not remove viruses.
For domestic US use from treated municipal sources or clean backcountry water, this limitation is rarely consequential. In post-disaster situations where sewage and water infrastructure have both failed, pairing a filter with a chemical treatment step (chlorine dioxide tablets) addresses viruses as well.
Carbon filters (found in gravity systems and Brita-style pitchers) also improve taste and reduce chlorine, some heavy metals, and volatile organic compounds — but they are not rated as primary treatment for biological threats on their own.
The product guides covering filtration: Gravity Water Filters, Portable Water Filters, and Build a Gravity Filter.
Purifies all biologicals — requires clear water and battery
UV light disrupts the DNA of microorganisms, rendering them unable to reproduce — effectively inactivating bacteria, protozoa, and viruses without chemicals. UV devices treat all three biological threat categories, which is the key advantage over most filters.
The critical limitation is turbidity. Particles in cloudy water shield microorganisms from UV exposure. For UV treatment to be effective, water must be visually clear first — pre-filter or let sediment settle before using a UV device. This makes UV a poor standalone solution for murky surface water.
The second limitation is battery dependence. In an extended power outage, a handheld UV device (SteriPEN) eventually runs out of charge. It works well as one layer in a multi-method system, less well as a sole backup. The full UV product comparison is on the UV Water Purifiers page.
The most thorough method — also the most resource-intensive
Distillation collects water vapor from boiling water and condenses it into a separate clean container. Because only water molecules evaporate (not salts, metals, or most contaminants), distillation produces the cleanest water of any household method. It is the recommended approach when chemical contamination is suspected and no other safe water source exists.
The limitation is fuel and output. Boiling water to produce steam and collecting it into a second container takes significant fuel and produces water slowly. In a practical emergency, distillation is used as a last resort or supplement — not as a primary day-to-day method.
Note on volatile chemicals
Some volatile organic compounds (VOCs) have a lower boiling point than water and can carry over into the distillate. Distillation removes most contaminants but is not a perfect solution for all chemical contamination scenarios. In industrial contamination events, follow official guidance rather than relying on home distillation.
Side by side
No single method scores best in every column. The right choice depends on what your water contains, what equipment you have, and how much time and fuel you can spend.
Sources: CDC Emergency Water Treatment guidelines; EPA Emergency Disinfection of Drinking Water; NSF/ANSI Standards 53, 58, 62. "Crypto" = Cryptosporidium parvum. Bleach is ineffective against Crypto at standard emergency dosages.
Pre-filtering
Turbid or cloudy water reduces the effectiveness of every chemical and UV treatment method. Sediment, organic matter, and particles physically shield microorganisms from contact with disinfectants, and consume chlorine before it can do its work.
Pre-filtering is not treatment — it does not make water safe to drink. It is preparation that makes subsequent treatment more effective. Always treat after pre-filtering.
Pre-filtering options in order of effectiveness
The layered approach
The most resilient water treatment strategy combines methods in sequence. Each layer addresses what the previous one misses. The standard layered approach for emergency household treatment:
Pre-filter
Remove sediment and turbidity to improve subsequent treatment effectiveness.
Treat biologically
Boil, disinfect with chlorine dioxide tablets, or pass through a certified filter to address bacteria, protozoa, and viruses.
Store in a sealed container
Treated water can be recontaminated by unclean containers or hands. Store in a sealed, clean container and use within 24 hours at room temperature.
None of the methods on this page — boiling, bleach, tablets, filters, UV, or distillation — reliably makes water safe when the contamination is chemical. Industrial spills, agricultural runoff, petroleum products, and unknown toxic sources require official guidance, not improvised treatment. When in doubt, use stored water only and follow local emergency management instructions.
See the Water Contamination guide for when treatment is insufficient and the Boil Water Advisory page for what different advisory types require.
Go deeper
This page covers how every method works. These pages go deeper into specific products, protocols, and skills.
The full EPA protocol with exact dosages, the cloudy-water procedure, and a printable quick-reference card.
Full protocol →
Chlorine dioxide vs. iodine vs. NaDCC compared — with dosing charts, shelf life, and the Aquatabs vs. Micropur question.
Tablet guide →
How gravity filters work, what certifications mean, and how to size a system for your household.
Filter guide →
Straw, squeeze, pump, and gravity bag filters for emergency kits and evacuation — what each format handles.
Portable guide →
When UV makes sense, its clear-water limitation, and how handheld vs. inline UV systems differ.
UV guide →
Set up a two-bucket gravity filter at home. What it handles, what it doesn't, and how to verify it's working.
Build guide →
Next steps
Treatment extends your options — stored water is still the foundation. Know your household target first.
Calculate →
Container types, placement, and rotation schedules. Build the supply that treatment expands.
Storage guide →
The step-by-step first-response protocol for when your tap stops working and you need to act quickly.
Get the plan →