Water Purification Methods Compared: Filters, Tablets, Boiling, and UV

Water storage is what you rely on when everything is functioning normally. Water purification is what you rely on when it is not.

The distinction matters. Stored water runs out. Treated tap water from a 55-gallon tank covers you for a week or two, which handles most domestic emergencies comfortably. But longer outages, evacuation scenarios, or situations where the mains supply itself is compromised require the ability to make water safe from other sources: streams, rivers, collected rainwater, or a supply you are not certain about.

This guide covers every meaningful purification method available to a household in 2026: how each works, what it removes, what it does not remove, and when to use it.

The Four Methods and What They Actually Do

Every water purification approach falls into one of four categories:

Boiling. Uses heat to kill biological threats: bacteria, viruses, protozoa. Does not remove chemicals or sediment. The oldest and most reliable method. Requires a heat source and fuel.

Filtration. Uses physical barriers (ceramic, hollow fibre, activated carbon) to remove particles, bacteria, and protozoa from water. Most portable filters do not remove viruses. Gravity filters with the right elements also remove chemicals and heavy metals.

Chemical treatment. Chlorine or iodine-based tablets or drops that kill biological threats in clear water. Does not remove sediment or chemicals. Works on bacteria and viruses (with the right product and enough contact time).

UV light. Destroys the DNA of bacteria, viruses, and protozoa without chemicals or heat. Does not remove sediment, chemicals, or heavy metals. Fast and leaves no taste. Requires battery power.

Each method has different applications. The practical household approach is not to pick one and rely on it: it is to understand when each is appropriate and have more than one available.

Boiling: The Failsafe

Boiling is the baseline. Every other method fails in some scenario. Boiling does not, provided you have heat and a container.

The guidance: bring water to a rolling boil for at least one minute. At elevations above 6,500 feet, boil for three minutes (lower atmospheric pressure reduces boiling temperature). You do not need to boil longer than this: everything harmful is dead within the first minute of a rolling boil.

What boiling kills: all biological threats: bacteria, viruses, protozoa, fungi, parasites. 100% reliable on biological contamination.

What boiling does not do: remove sediment, chemicals, heavy metals, or dissolved minerals. If your water smells of petrol, has a strong chemical odour, or is visibly silty, boiling addresses only the biological element of the problem.

Practical note on cloudiness: if water is turbid (visibly cloudy from sediment), strain it through a clean cloth or coffee filter before boiling. Sediment can shield microorganisms from the heat. Straining first means the boil is fully effective.

Heat sources that work for boiling during power outages: a gas hob (gas supply is typically maintained during power cuts), a camping stove with butane or propane canisters, a wood fire or fire pit, a solid fuel stove. If you have a gas hob, you have boiling capability in almost any domestic outage scenario.

The practical limitation of boiling is fuel and time. Boiling a day's water supply for a family of four (7-8 litres at the practical planning rate) requires significant fuel and time if done daily. For short outages, fine. For extended periods, a filtration system is more sustainable.

Portable Filtration: LifeStraw and Sawyer

Portable filters (straw-style and squeeze-style) are the most practical tool for treating small volumes of water from uncertain sources. They remove bacteria and protozoa to 99.9999% and 99.9% respectively. They are lightweight, require no power, and last for hundreds of litres.

The important limitation shared by all portable mechanical filters: they do not remove viruses. In the UK and US, this is generally not a concern: domestic water sources and most freshwater in developed countries does not carry significant viral contamination. For international travel or post-disaster urban flooding where sewage overflow is possible, the absence of viral removal matters.

The LifeStraw Personal is the most widely carried emergency water filter in the world. Why? At around $20, it is affordable enough to put one in every bag in the house: school bags, car glovebox, 72-hour kit, hiking pack. The straw-through design means you can drink directly from any freshwater source. You do not need a container.

The limitation of the straw design is that you cannot use it to fill containers easily. For filtering larger volumes: filling a water bottle for the day, treating a litre for cooking: you need to hold the straw inverted from a full container and squeeze, which is awkward. The Sawyer Squeeze solves this.

The Sawyer Squeeze is the more capable option. It does everything the LifeStraw does, plus it can be used in three additional ways: as a squeeze filter (fill the included pouch, screw the filter on, squeeze into a bottle), as an inline filter on a hydration pack, or set up as a primitive gravity filter by hanging the pouch above a container and letting gravity do the work. That last configuration means it functions as a simple gravity filter when you do not want to manually squeeze.

The other advantage of the Sawyer: it is backflushable with an included syringe that reverses water through the filter to clear any blockage and restore flow rate. Properly maintained, the Sawyer Squeeze is rated for 100,000 gallons of lifetime use: essentially unlimited for household emergency purposes.

If you are choosing one portable filter for the 72-hour kit or the car emergency bag: the Sawyer Squeeze. If you are buying something affordable for every member of the household to carry individually: the LifeStraw.

Home Gravity Filtration

For home use (treating larger volumes consistently, at a counter) a gravity filter is the right tool. This is a different category from portable filters, and a different guide covers it in full.

The short version: a gravity filter (British Berkefeld, Alexapure Pro, ProOne Big+) sits on your counter and filters water continuously without electricity. You pour tap water or collected rainwater into the top chamber, it filters through ceramic or ceramic-carbon elements, and the treated water sits in the bottom chamber ready to drink. Flow rate is slow (1-3 litres per hour depending on the filter) but a full filter holds 8-10 litres of clean water ready to pour at any time.

The gravity filter is your long-term home water treatment system, while portable filters above serve as your on-the-go or emergency-bag tools. They are complementary.

See the gravity water filter guide for full coverage of home gravity filtration.

Chemical Treatment: When and How

Chemical treatment (specifically chlorine-based tablets) is the lightweight, zero-power backup method that belongs in every 72-hour kit and car emergency bag.

Aquatabs are the standard. One 49mg tablet treats one litre of clear water, kills 99.9999% of bacteria and 99.99% of viruses, and costs approximately 10-15 cents per tablet. They work. FEMA, the Red Cross, and UNICEF recommend them. You should have a tube of them.

The correct process: fill a container with the clearest water you can find. Add one Aquatab per litre. Stir. Wait 30 minutes before drinking. If the water is cold (below 10°C/50°F), wait 60 minutes. If the water is turbid, pre-filter it first: sediment reduces the effectiveness of chemical treatment by shielding microorganisms.

What chemical treatment does not do: remove sediment, heavy metals, chemical contamination, or agricultural runoff. It is a disinfection tool, not a filtration tool. Clear water from a clean source treated with Aquatabs is safe to drink. Silty, chemically-smelling water treated with Aquatabs is safer, but still has issues that chemical treatment alone cannot resolve.

Bleach is the household alternative to tablets. Standard unscented household bleach (5-8.25% sodium hypochlorite) can be used to treat water at 8 drops per gallon (approximately 0.5ml per 4.5 litres). It works on the same principle as Aquatabs. The practical disadvantage is measuring bleach accurately in an emergency, and bleach has a shelf life: it degrades to 50% effectiveness within 6 months of opening. Aquatabs have a 5-year shelf life and need no measuring.

UV Light Purification

The SteriPen is the one tool on this list that kills viruses. It does this by exposing water to UV-C light, which destroys the DNA of any microorganism present. One litre in 90 seconds. The optical sensor on the Adventurer Opti confirms when the treatment cycle is complete.

This is meaningful in two scenarios: international travel, and post-flood urban situations where sewage overflow is possible. For domestic UK or US emergency use with water drawn from rivers and lakes, virus contamination is a low concern, but it exists, and the SteriPen addresses it where chemical treatment might feel excessive or slow.

The limitations are practical rather than technical: it requires batteries (two CR123s, keep spares), the UV bulb is glass and fragile, and it does not work effectively on turbid water (sediment and cloudiness reduce UV penetration, so pre-filtering is necessary). And it leaves the water exactly as it was for taste, sediment, and chemical content: it addresses only the biological threat.

The correct use of a SteriPen is as part of a combined approach: strain or gravity-filter the water first to remove sediment, then treat with UV. This sequence is fast, effective against all biological threats, and leaves no chemical taste.

Combining Methods: What Kate's Household Actually Does

The practical approach is not to choose a single purification method. It is to have options appropriate to different scenarios, and to understand which to reach for in each.

Power cut, mains water still running: Use the British Berkefeld gravity filter as normal. No change to routine. The filter runs on no power and treats mains water continuously. This covers the majority of UK and US outage scenarios.

Power cut, mains water interrupted or suspect: Draw water before pressure drops if possible (fill the gravity filter top chamber, fill any large containers). Treat drawn water with Aquatabs for drinking if the supply is from a potentially compromised source. Boil for cooking: the gas hob still works.

Flood or natural disaster, water supply uncertain: Do not use flood water without full treatment. Flood water may contain sewage, chemical runoff, and a high biological load. Pre-filter through a clean cloth or coffee filter to remove gross sediment. Treat with Aquatabs (kills bacteria and viruses) or boil (kills all biology). If using the Sawyer Squeeze, remember it does not remove viruses: in a flood scenario, combine with Aquatabs. The SteriPen is the most complete single-tool solution for flood water: pre-filter for sediment, then UV treat.

Away from home, on the move: The LifeStraw or Sawyer Squeeze covers drinking from any freshwater source: rivers, streams, lakes. For a 72-hour kit or car bag, the Sawyer Squeeze gives more options. Aquatabs as backup. A bottle of treated water as starting point.

The household setup I recommend: a gravity filter as the home base system, a Sawyer Squeeze per adult in emergency bags, a tube of Aquatabs in each bag, and a SteriPen if international travel or flood risk is a realistic concern.

Pre-Filtering: The Step Everyone Skips

Every purification method works better on clear water, and some require it to work properly.

Pre-filtering means removing gross sediment and turbidity before applying your main purification method. The tools for this are simple: a clean cloth, a bandana, a coffee filter, or purpose-made mesh bags. Pour the water through the cloth into a clean container. Repeat if necessary until the water is at least visually clearer.

Pre-filtering does not make water safe. It is a preparation step, not a purification step. After pre-filtering, apply boiling, chemical treatment, UV, or a proper filter. The pre-filtering step removes the sediment that would otherwise block your filter faster, reduce the effectiveness of chemical treatment, or scatter UV light.

If you have a Sawyer Squeeze and the source water is very turbid, use the hollow-fibre filter in a pour-through mode rather than squeeze mode: gravity will pull clear water through more slowly but without forcing sediment through the membrane. Backflush afterwards.

Related Guides

For the home gravity filter that serves as your primary household filtration system, see the gravity water filter guide.

For building and maintaining your water storage reserve: the foundation that means you rarely need emergency purification, see the long-term water storage guide.

Frequently Asked Questions

Is boiling enough for all emergencies?

For biological contamination, yes. Boiling is definitive against all biological threats. The situations where boiling is insufficient are chemical contamination (agricultural runoff, industrial spills, petrol or solvent contamination) and situations where you lack heat or fuel. For the vast majority of domestic UK and US outage scenarios, a working gas hob plus boiling capability is sufficient.

Do I need a UV purifier in the UK?

For most UK emergency scenarios, no: viral contamination of UK freshwater sources is very low. A gravity filter plus Aquatabs covers the realistic threat profile. A UV purifier becomes relevant if you are travelling internationally or if a major flood event has compromised the local water supply with sewage overflow.

Can I drink from a stream or river if I have a LifeStraw?

Yes, with caveats. The LifeStraw removes bacteria and protozoa effectively. It does not remove viruses, chemical contamination, or heavy metals. For UK and US backcountry use, it is generally sufficient. Do not use it downstream of industrial sites, agricultural land that may use heavy pesticides, or in flood conditions where sewage overflow is possible.

How long do filter elements last?

LifeStraw: rated 1,000 litres. Sawyer Squeeze: rated 100,000 gallons (essentially unlimited for emergency use if backflushed regularly). Gravity filter elements (British Berkefeld Sterasyl): approximately 2,000 litres per pair, or 6-12 months of household use. Replace based on flow rate deterioration rather than a fixed schedule: a filter slowing to a crawl even after cleaning is telling you the element is spent.

What about UV systems for the home?

Whole-house UV systems exist and are excellent for off-grid homes with a well supply. They are an infrastructure investment: plumbed in, typically running on a transformer, and sized for the flow rate of the house supply. For most homes connected to mains supply, they are unnecessary. For rural homes with private well water, they are worth serious consideration.