A water filter strains out protozoa and bacteria, while a purifier also deals with viruses, using a much finer filter, a chemical disinfectant or ultraviolet light. No portable filter or purifier makes water contaminated with fuel or chemicals safe to drink, and boiling remains the most reliable way to kill germs.
The words filter and purifier are used loosely on packaging, and the difference matters most in exactly the situations a home kit is for: a flood, a boil-water advisory, or a water main break.
This guide sets out what each treatment method removes, where each one fails, and how to check a claim before you trust it.
Four kinds of contamination
- Protozoa, such as Giardia and Cryptosporidium. Relatively large, and resistant to some chemical disinfectants.
- Bacteria, such as E. coli. Smaller, but still caught by a fine filter.
- Viruses. Far smaller than bacteria, and small enough to pass through most portable filters.
- Chemicals and metals, including fuel, pesticides and lead. These are dissolved in the water and are a separate problem from germs.
In everyday use, a filter is a device that removes protozoa and bacteria. A purifier is one that also deals with viruses. The US Centers for Disease Control and Prevention (CDC) puts the practical point this way: most portable filters remove parasites and, depending on pore size, bacteria, but not viruses, so filtration should be combined with disinfection, or the water should be boiled.
What each method removes
| Method | Bacteria | Protozoa | Viruses | Chemicals, metals, taste | Notes |
|---|---|---|---|---|---|
| Boiling | Yes | Yes | Yes | No, and it can concentrate them | The most reliable method for germs. CDC: a rolling boil for 1 minute, and longer at high altitude. |
| Hollow-fibre filter (0.1 to 0.2 micron) | Yes | Yes | No | No, unless a carbon stage is added, and then mainly taste | Light, fast and needs no power. Ruined by freezing once wet. Flow drops in silty water. |
| Purifier (bottle or pump) | Yes | Yes | Yes, if the maker's claim is backed by testing | Some models reduce some chemicals with carbon; many do not | Check which test protocol the claim rests on. Cartridges have a limited life. |
| Chlorine dioxide tablets or drops | Yes | Yes. Cryptosporidium needs prolonged contact, up to 4 hours, and longer in cold or cloudy water | Yes | No | The CDC's current travel health guidance describes chlorine dioxide as effective against all waterborne pathogens, including Cryptosporidium with prolonged contact. |
| Chlorine tablets or household bleach | Yes | Giardia with long contact. Not Cryptosporidium | Yes | No | Widely available. Follow your public health unit's instructions for amounts. |
| UV pen | Yes | Yes | Yes | No | Needs clear water and working batteries. |
Iodine tablets behave much like chlorine and share its weakness against Cryptosporidium. The CDC notes that iodine is not suitable during pregnancy, for people with thyroid conditions, or for long-term use.
No single portable method covers everything. The dependable approach is two steps: filter first to remove sediment, protozoa and bacteria, then disinfect to deal with viruses. Or boil.
Viruses and floods
In clear backcountry lakes and streams in Canada, the main risks are protozoa and bacteria, which is why a simple filter serves most hikers well. Viruses become the concern when water is contaminated with human sewage.
Flooding is how that happens at home. Floodwater overwhelms sewers and septic systems and carries their contents into wells, basements and sometimes the municipal supply. Public Safety Canada describes floods as the most frequent natural hazard in Canada and the most costly in terms of property damage.
For a flood or a boil-water advisory, a filter alone is the wrong tool. Boil the water, or use a disinfectant or a true purifier. If your local authority issues a boil-water advisory or a do-not-consume order, follow its instructions, which are specific to what is in the water. Our flood guide and water disruption guide cover the wider plan.
Freezing destroys wet hollow-fibre filters
A hollow-fibre filter is a bundle of tiny tubes with microscopic pores. Once it has been used, water stays inside the fibres. If that water freezes, it expands and can crack them, and the damage leaves no visible sign. A cracked filter still flows, and it lets germs through.
This is a common problem in Canada, where kits are kept in cars, garages and cabins.
- A filter that has never been wet can be stored in the cold. One that has been used should be kept above freezing from then on.
- In cold weather, carry a used filter inside your coat and sleep with it in your sleeping bag.
- If a wet filter may have frozen, replace it. You cannot test it at home.
- For a vehicle kit, chemical tablets are the safer choice, because they are unaffected by freezing. See the winter car kit checklist.
Gravity filters: ask for the standard and the claim
Countertop gravity filters are popular for home kits because they treat several litres at a time with no power. Their performance depends entirely on the filter elements inside, and marketing claims in this category range from independently certified to unverified.
Before you buy one, ask two questions:
- Which NSF/ANSI standard is the element certified to? Standard 42 covers aesthetic effects such as chlorine taste. Standard 53 covers health-related contaminants. Protocol P231 covers microbiological purifiers, meaning bacteria, viruses and cysts.
- For which specific claim? Certification is granted claim by claim. A statement such as "certified to NSF/ANSI 53" tells you little until it names the contaminant, for example lead or cysts.
Then look the model up in the certifier's public listings, which are linked in the sources below. "Tested to" a standard by the manufacturer is a weaker statement than "certified to" it by an independent body. If a seller cannot tell you the standard and the claim, treat the element as a sediment and taste filter and disinfect the water separately.
A sensible home setup
Treatment is the second line. Stored water is the first, because it works with no effort on a bad day. The Government of Canada's guideline is about four litres per person per day, and the CDC describes unopened commercially bottled water as the safest emergency supply. Replace stored kit water at least once a year.
Beyond that, our suggestion is a simple layered setup:
- Stored water for at least 72 hours. See how much water per person per day.
- A way to boil it, if you have a safe one. Never use outdoor fuel-burning stoves indoors.
- A filter for sediment, protozoa and bacteria.
- Chlorine dioxide tablets for viruses, and as a backup that survives freezing.
- A clean container kept only for treated water.
Our home water kit combines storage, a gravity filter and chlorine dioxide tablets along those lines. The same result can be assembled from separate parts.
Frequently asked questions
What is the difference between a water filter and a water purifier?
A filter removes protozoa and bacteria by straining them out. A purifier also deals with viruses, using a much finer filter, a chemical disinfectant or ultraviolet light. Neither one makes water contaminated with fuel or chemicals safe to drink.
Do water filters remove viruses?
Most portable filters do not. The CDC says most portable filters remove parasites and, depending on pore size, bacteria, but not viruses. To deal with viruses, boil the water or combine filtration with a disinfectant such as chlorine dioxide.
Can a water filter freeze?
A hollow-fibre filter that has been used can be destroyed by freezing, because water left inside the fibres expands and cracks them. The damage is not visible and the filter will still flow. If a wet filter may have frozen, replace it.
Sources
- CDC: water treatment when hiking, camping and travelling
- CDC Yellow Book: water disinfection for travellers
- CDC: a guide to drinking water treatment for backcountry and travel use
- CDC: how to create and store an emergency water supply
- NSF: certified drinking water treatment units listings
- Public Safety Canada: floods
General information only. Follow instructions from local emergency officials, and call 911 in a life-threatening emergency.