Filter, Purify, or Distill: What Each One Actually Removes

Water treatment marketing uses filter, purifier, and purification interchangeably. They are not the same thing, they remove different classes of contaminant, and buying the wrong one for your situation is the most common expensive mistake in this whole category.

There are four things that can be wrong with water, and no single method addresses all four.

The four contaminant classes

Biological. Bacteria, protozoa, and viruses. This is what makes people acutely ill within hours to days, and it is the concern in almost every short term emergency, boil water notice, or flooded well situation.

Particulate. Sediment, silt, organic debris. Rarely dangerous by itself, but it clogs filters and it shields pathogens from chemical disinfection, so it has to go first.

Chemical. Industrial contaminants, agricultural runoff, fuel, solvents, chlorination byproducts, and the fluorinated compounds usually called PFAS. Chronic rather than acute, and largely invisible.

Dissolved solids. Salts and minerals. Seawater is the extreme case. Nothing you own removes these except distillation or reverse osmosis.

Every method below is good at some of these and useless against others.

What each method actually does

Three things in that table are worth dwelling on, because they are where people go wrong.

Filters do not remove viruses

This is the most consequential misunderstanding in the field. A standard backpacking or gravity filter is rated at around 0.2 microns, which is small enough to physically block bacteria and protozoa including cryptosporidium and giardia. Viruses are an order of magnitude smaller and pass straight through.

In the North American backcountry this matters less, because the realistic pathogens are protozoan and bacterial. In a municipal failure, a flood, or anywhere sewage may have entered the supply, viruses are exactly the risk, and a filter alone does not address it.

The Environmental Protection Agency draws this distinction formally. A device may only be labelled a water purifier in the United States if it removes bacteria, protozoa, and viruses. Anything labelled a filter is making a narrower claim, and the label is meaningful rather than marketing.

The practical answer is to combine methods: filter first for particulate and protozoa, then disinfect chemically or with UV for viruses. Or use a device with a genuine virus rating, which usually means a much finer hollow fibre membrane or an integrated chemical stage.

Chlorine struggles with cryptosporidium

Household bleach and standard chlorine tablets are effective against bacteria and viruses, and they are cheap, shelf stable, and easy. Their weakness is cryptosporidium, a protozoan parasite with a tough oocyst wall that resists chlorine at practical concentrations and contact times.

Chlorine dioxide, sold as tablets or a two part liquid, does handle cryptosporidium, but it needs a substantially longer contact time to do it, often four hours rather than thirty minutes. Read the label, because the short time on the front of the packet is usually the bacterial figure.

Filters handle cryptosporidium easily. This is the clearest case for combining a filter with a chemical rather than choosing between them.

Carbon removes taste, not pathogens

Activated carbon is excellent at chemicals, chlorine taste, and many organic compounds. It does essentially nothing against biological contamination, and a warm wet carbon block will happily grow bacteria if left standing.

A pitcher filter on your counter is a taste and chemistry device. It is not emergency water treatment, and treating it as such is a genuine risk.

Boiling is still the best emergency method

For a boil water notice or an unknown biological risk, boiling remains the most reliable option available to a household, and it requires no purchased equipment.

Bring water to a rolling boil and hold it for one minute, or three minutes above about 6,500 feet where the lower boiling point reduces the thermal dose. That kills every biological pathogen of concern. The Centers for Disease Control has published this guidance consistently and it has not changed.

What boiling cannot do is remove chemicals, and it concentrates them slightly as water evaporates. If the concern is industrial contamination rather than pathogens, boiling makes the problem marginally worse rather than better.

The obvious limitation is fuel and power. In a winter outage where the grid is down and the gas range needs an electric igniter, boiling may not be available, which is why stored water matters more than any treatment method. The arithmetic for how much is in the water storage planner.

Bleach, done correctly

Unscented household bleach at 5 to 6 percent sodium hypochlorite, at roughly eight drops per gallon, stirred, and left to stand thirty minutes, is the standard emergency disinfection dose. The water should smell faintly of chlorine afterwards. If it does not, repeat the dose once and wait another fifteen minutes.

Three failure modes are common.

The bleach must be unscented, with sodium hypochlorite as the only active ingredient. Colour safe, scented, and splashless formulations contain surfactants and other additives that are not for drinking.

Bleach degrades. It loses potency over roughly six to twelve months from manufacture, faster if stored warm. A bottle in the back of a cupboard from three years ago may be substantially weaker than the label says, and there is no way to tell by looking. Rotate it, or store calcium hypochlorite granules instead, which keep far longer.

Cloudy water needs to be filtered or settled first. Suspended particles shield pathogens from contact with the chlorine, and organic matter consumes the chlorine before it can do its job.

What no consumer device does

No filter or purifier makes seawater drinkable. Desalination requires reverse osmosis at high pressure or distillation. Hand operated RO desalinators exist for marine survival and cost hundreds to thousands of dollars, and they produce water slowly. Nothing in a backpacking kit will do this.

Nothing consumer grade removes radiological contamination reliably. Some reduction is possible with reverse osmosis and specific ion exchange media, but this is not a scenario a household filter addresses.

Most filters do not remove PFAS. The fluorinated compounds now found in a great many water supplies pass through standard filtration. Specific activated carbon and ion exchange systems certified for PFAS reduction do work, and the certification is what to look for rather than the marketing. NSF/ANSI standard 53 covers health effects reduction and NSF/ANSI 58 covers reverse osmosis systems, and a genuine certification will name the standard and the specific contaminants.

Building a sensible household setup

The layered answer costs less than most single purchases and covers more.

Stored water first. This is the highest value item and the one people skip in favour of gear. Treatment extends a source. Storage covers you when there is no source, which is the more common situation in a municipal failure.

A gravity filter as the workhorse. Something in the 0.2 micron class with a carbon stage, capable of processing several thousand gallons over its life. Handles particulate, protozoa, bacteria, and improves taste and chemistry. Needs no power, which matters.

Chemical treatment as the virus stage and the backup. Chlorine dioxide tablets cover what the filter misses, weigh nothing, store for years, and give you a second method if the filter fails or clogs.

Boiling as the always available fallback, provided you have a heat source that does not depend on the grid. That connects directly to the heating without power problem, since the same fuel source solves both.

Know your actual local risk. Municipal utilities publish annual water quality reports, and your state or county health department can tell you what contaminants are documented in your area. Well owners should test rather than guess, and most county extension offices or health departments will point you at certified labs. Treating for a contaminant you do not have is wasted money, and not treating for one you do is worse.

The short version

Filters block things by size and miss viruses. Chemicals kill things by chemistry and struggle with cryptosporidium. Carbon handles chemistry and no biology. Boiling handles all biology and no chemistry. Distillation and reverse osmosis handle nearly everything and are slow, expensive, or both.

No single method covers the field, the marketing rarely says which one a product actually is, and the EPA labelling distinction between filter and purifier is the fastest way to tell.

Combine a filter with a chemical, store water in advance, and find out what is actually in your local supply before buying anything specialised.

Related: The Colorado River shortage explained covers whether regional scarcity is genuinely your problem. Can you really make drinking water from air covers the energy arithmetic behind atmospheric water generation.

Last reviewed: July 2026