Water Purification: A Practical Guide to Making Water Safe to Drink

Water is not optional. A human can survive three weeks without food but only three days without water. In any situation where the mains supply is interrupted or suspect, finding and treating water becomes the most urgent priority.

The good news: clean water is all around you. Rain, rivers, streams, puddles, condensation, water trapped in pipes, water stored in cisterns. The challenge is not finding water — it’s making what you find safe to drink. This guide covers the three stages of water treatment: filtering debris, killing biological contaminants, and understanding what each method can and cannot do.

One important principle first: all water should be considered potentially unsafe until you have treated it. Clear, cold water from a mountain stream may look pristine and still contain microscopic parasites. Treat everything.

Stage One: Filtering — Removing Debris and Particles

Before you can disinfect water, you need to remove the bits floating in it: soil, plant material, insect debris, rust flakes, sediment. This step doesn’t kill anything — it just makes the water clearer and allows the next stage (disinfection) to work properly. Dirty water also tastes terrible and will clog other filters quickly.

The Canvas or Cotton Filter (Millbank Bag Principle)

Military surplus and camping shops sell ‘Millbank’ or canvas water bags — essentially a canvas or heavy cotton sack that you pour water through. Water passes slowly through the fibres, trapping sediment and organic material. It’s a centuries-old principle and it works.

You can make a functional equivalent with what you have:

• Take a pair of jeans or heavy canvas trousers. Stand them upright in a bucket, stuff the legs with grass, gravel, or a stone at the bottom to keep the shape, and pour water through the waist. The fabric acts as the filter medium.
• A heavy canvas or linen pillowcase, stretched over a frame or just gathered at the top and suspended, works for smaller quantities.
• The key material is cotton or canvas — synthetic fabrics behave differently and aren’t as effective.

Pour water through slowly. What comes out the bottom won’t be clear, but it will be free of the larger debris that would clog a filter or make disinfection unreliable. Pass it through twice if the water is very dirty.

When the fabric starts to clog, rinse it thoroughly and squeeze out as much water as possible. The filter will dry and can be reused. Replace fabric that has started to rot or has holes.

Pocket Tube Filters

These are small, portable water filters — roughly the size and shape of a large pen — that you fill and squeeze water through. They contain a ceramic or fibreglass filter element with a pore size typically around 0.2 microns. This is small enough to trap bacteria (e.g. E. coli, Salmonella) and protozoa (e.g. Giardia, Cryptosporidium) which are the most common causes of waterborne illness in the UK.

They do not remove viruses (e.g. Hepatitis A, Rotavirus) — viruses are much smaller than the filter pores and pass through. This is a critical limitation. You still need to disinfect the water after filtering.

Cost and availability: Basic models are available from outdoor retailers and online from around £15–£20. LifeStraw and similar brands are widely stocked. They are worth having as a routine part of a home emergency kit alongside candles and torches. Check before you buy: replacement filter cartridges cost nearly as much as the unit itself, so check availability before relying on one.

How to use:

• Fill the container at the top — a dirty stream, a rainwater butt, a puddle.
• Squeeze or suck from the outlet. Water is pushed or drawn through the filter element.
• Filtered water comes out the bottom. Collect it in a clean container.
• Don’t let the dirty end touch the filtered water — this is obvious but easy to get wrong in the dark or under stress.

Maintenance: Backflush the filter regularly — run clean water through it in reverse to clear trapped particles. Replace the filter element when flow slows significantly or the unit is past its rated life. Read the manufacturer’s instructions.

Improvised Pre-Filters

If you have no commercial filter and no canvas or jeans, even a improvised setup helps:

• T-shirt or cotton clothing (clean, not heavily soiled) stretched over a container — pour water through it
• A plastic bottle with the bottom cut off, packed tightly with grass, sand, and gravel in layers — a crude but functional biosand filter
• Layers of cloth — just something to trap the worst of the sediment before you move to disinfection

Stage Two: Disinfection — Killing Biological Contaminants

Filtering removes debris. Disinfection kills or inactivates the living organisms in water: bacteria, viruses, and protozoa. You need both stages. Filtering alone leaves organisms. Disinfection alone is unreliable on very dirty water — sediment protects organisms from the disinfectant.

Bleach (Chlorine Disinfection)

Ordinary household bleach (not scented, not gel, not coloured) contains sodium hypochlorite, typically 5–6% chlorine concentration. This is the most widely available and reliable emergency water disinfectant when used correctly.

Dosage:

• For water with 5–6% sodium hypochlorite (standard household bleach): add 2 drops per litre (4 drops per US quart) of clear water
• For very dirty or cold water: double it — 4 drops per litre
• Stir or shake, leave for 30 minutes

The water should have a slight chlorine smell after 30 minutes. If it doesn’t, add the same dose again and wait another 15 minutes. If the smell is very strong, too much bleach has been added — let it stand longer before drinking or add a tiny pinch of sodium thiosulphate (available from aquarium shops) to neutralise the excess.

What bleach kills: bacteria, viruses, most protozoa. It does NOT reliably kill Cryptosporidium oocysts, which have a chlorine-resistant life stage. If you suspect Cryptosporidium (water from a source with livestock or human waste nearby), filter first, then disinfect, and consider boiling as well if possible.

Storage: Bleach degrades over time, especially in heat and sunlight. Keep a bottle in your emergency kit, replaced annually. Write the purchase date on the bottle. Check the lid is tight.

Water Purification Tablets

Chlorine dioxide tablets are the standard for outdoor and emergency use. They are widely available from camping shops and outdoor retailers, inexpensive, and very effective.

They work more slowly than liquid bleach but are more effective against a wider range of organisms, including Cryptosporidium:

• Drop one tablet into 1 litre of water (check the packet — dosing varies by brand and tablet size)
• Wait 15–30 minutes depending on water clarity and temperature — cold, turbid water needs longer
• The water is ready when the tablet has dissolved and the water looks clear

Clarity is important: if the water is still visibly dirty after the tablet has dissolved, filter it first and treat again. Tablets are wasted on muddy water because the active ingredient reacts with the organic matter rather than the organisms.

Brands: Polar Pure (iodine-based), Potable Aqua (chlorine dioxide), and generic camping water treatment tablets are all effective. Check the packet for the active ingredient and dosing. Iodine-based tablets should not be used by people with thyroid conditions or by pregnant women — check the warnings on the packet.

Boiling

Boiling is the most reliable disinfection method. It kills everything — bacteria, viruses, protozoa, and their eggs — with no chemical residue and no uncertainty about dosage.

• Bring water to a rolling boil (not just warm)
• Maintain the boil for 1 minute (adding 1 minute for every 1,000 feet of altitude above sea level — at UK altitudes the difference is negligible)
• If you have no thermometer, a rolling boil at sea level is fine
• Let it cool before drinking. Use a clean container.

Boiling does not remove chemical contaminants, sediment, or debris. Filter first if needed. But where boiling is possible — over a fire, a gas hob, a camping stove — it is the gold standard of disinfection.

Stage Three: Understanding What Each Method Does and Doesn’t Do

This is the part most guides skip. It matters.

Filtration (canvas, pillowcase, DIY cotton): Removes debris and large particles. Does not kill organisms. Reduces sediment that interferes with chemical disinfection. Extends the life of pocket filters by preventing them clogging.

Pocket tube filters (ceramic/glass fibre): Removes bacteria and protozoa. Does NOT remove viruses. Flow slows as the filter loads with particles. Requires backflushing and eventual cartridge replacement.

Bleach: Kills most bacteria and viruses. Less effective against protozoa cysts (particularly Cryptosporidium). Requires correct dosage. Water must be relatively clear to work. Does not remove chemical contaminants.

Purification tablets (chlorine dioxide): Kills bacteria, viruses, and protozoa including Cryptosporidium. Slower than bleach. Requires clear water to work reliably. Less affected by temperature than some methods.

Boiling: Kills everything. Does not remove chemicals, sediment, or debris. Requires fuel and a container. Best available method where feasible.

Sunlight (SODIS): See below.

The Sunlight Method: SODIS

Solar water disinfection (SODIS) uses ultraviolet radiation from sunlight to kill organisms in water. It is effective, free, and requires no consumables beyond a plastic bottle. It is particularly useful in sunny climates and hot weather.

Method:

• Fill clear plastic bottles (PET, the standard water bottle type) with filtered or settled water
• Place in direct sunlight — on a roof, a rock, a sunny windowsill
• Leave for at least two full sunny days (48 hours minimum). In cloudy weather, leave for three to four days
• The UV radiation and thermal effect together kill bacteria, viruses, and protozoa

Critical limitations:

• Does not remove chemical contaminants. If the water contains pesticides, heavy metals, or other chemicals, sunlight does nothing to address them.
• Less effective on turbid (cloudy) water. Particles in the water scatter UV light, preventing it reaching organisms throughout the volume. Filter or settle the water first.
• Not effective against all viruses in practice. Research shows SODIS works well for bacteria and protozoa. Viral inactivation is less consistent in real-world conditions compared to controlled laboratory tests.
• Requires plastic bottles. Glass blocks the necessary UV wavelengths.
• Temperature matters. Water above 50°C accelerates the disinfection process. Warmer water in direct sunlight works faster than cold.

SODIS is a useful technique when electricity and fuel are not available, the water source is relatively clean (e.g. rainwater, stream water with minimal contamination), and you have time — it takes two days minimum. Do not rely on it as your sole method in a genuine emergency without backup disinfection.

Where to Find Water When the Taps Run Dry

If your mains supply stops, there are still multiple sources of water available in most UK settings.

Stored Water in Your Home

• Water tanks in the loft (header tanks) — these supply your taps and will gravity-feed while there is water in them. Don’t let them run dry.
• The hot water cylinder — a full cylinder holds a useful amount of water, and the water is generally clean.
• Water in pipes — if you turn off the mains stopcock, the water already in your pipes is available. Open the lowest tap in the house to drain it.
• Baths, sinks, and washing-up bowls — fill these immediately when you suspect the supply is at risk. This is your reserve.

Rainwater

Rainwater is one of the cleanest natural water sources. Collect it by putting any clean container outside — a bucket, a bowl, a tarpaulin draped to channel water into a container. Do not collect the first heavy rain after a dry spell, which washes significant particulate from the air. After a brief rinse of the collection surface, subsequent rain is cleaner.

Rainwater should still be filtered and disinfected before drinking, particularly if you are collecting from a roof that may have bird or animal contamination.

Ground Water: Springs and Seeps

Natural springs occur where groundwater naturally emerges at the surface. They range from obvious, flowing streams to subtle damp patches in a hillside bank. A spring that has been flowing consistently over time is generally a reliable source — if the flow has stopped during a drought, it is unlikely to resume quickly.

How to identify: Look for persistently damp ground in a hillside or bank, particularly where the land changes slope. Moss and water-loving vegetation growing in a specific area can indicate a seep. During dry weather, dig a small hole in such an area — if water collects, you have found groundwater.

Natural springs are usually cleaner than water from drains, puddles, or surface collection points, but should still be filtered and disinfected. Spring water can pick up contamination from the ground, including agricultural runoff, septic systems, or natural mineral contamination.

Flowing Water: Streams and Rivers

Flowing water is generally cleaner than still water, because it is constantly replenished and aerated. However:

• Upstream matters enormously. A stream that looks clean near your house may flow through farmland, sewage works, or industrial sites further up. Follow the water upstream if possible to understand its source.
• Flowing water can still contain organisms, particularly in slow-moving or stagnant sections.
• In summer, streams can reduce to a trickle or disappear underground. A dry riverbed does not mean water is not present underground — dig into the gravel.

The best approach is to filter then disinfect any natural water source, regardless of how clean it appears. Assume contamination. Treat everything.

Still Water: Ponds, Lakes, and Reservoirs

Standing water is more likely to harbour biological contaminants than flowing water, because organisms concentrate as the water evaporates and the sun warms it. Algae blooms in particular can make still water dangerous to drink even after treatment.

If using still water:

• Avoid water with obvious algae (green, smelly, or thick with plant growth)
• Avoid water that is murky or has obvious animal contamination
• Filter before disinfecting — the sediment load in pond water will consume disinfectant
• Boil if at all possible

What About Sea Water?

Do not drink seawater. It will kill you faster than drinking nothing at all.

Seawater contains approximately 3.5% salt. The human kidney can only produce urine that is less salty than seawater. To process the salt in seawater, your body would have to produce urine that is saltier than the sea — which requires more water than you are drinking. The result is accelerated dehydration, kidney failure, and death.

This is not a matter of filtering, boiling, or treatment. Desalination requires distillation (evaporating water and condensing it) or a reverse osmosis membrane. None of these are achievable in the field without specialist equipment.

If you are near the coast and short of water, the water found inland in streams and groundwater is almost always fresher than the sea. Find a stream or spring. Do not drink from the sea.

Water From Unusual Sources: A Quick Reference

• Toilet cistern (not the bowl): If the cistern has been refilled from the mains, the water inside is generally clean. Do not drink from the toilet bowl.
• Hot water cylinder: Clean. Do not drink water from a system that uses a water softener if you can avoid it — the softened water contains high sodium levels from the regeneration process.
• Header tank in loft: Generally clean, but can accumulate debris and insects over time. Filter and disinfect.
• Swimming pool or hot tub: Contains chlorine or other chemical treatment. Filter and further disinfect before drinking. Not ideal but not usually dangerous if chlorine levels were maintained.
• Car windscreen washer bottle: Contains screenwash fluid (often methanol-based). Not drinkable.
• Ice cubes: If made from clean water, they are fine. If from an unknown source, treat as water.

Storing Emergency Water

If you want to keep a reserve of water for emergencies:

• Use food-grade containers (Jerry cans, old water bottles — not old chemical containers even if they look clean)
• Rotate stored water every 6–12 months — use it and replace it
• Keep containers in a cool, dark place. Light and heat cause plastic to degrade and algae to grow
• Do not store water directly on concrete floors for long periods — place on a mat or shelf
• Three litres per person per day is the absolute minimum for drinking only. Include water for cooking and basic hygiene. A week’s supply for one person is roughly 25–30 litres

Summary: The Treatment Sequence

For most water you find in a UK emergency situation, work through this sequence:

1. Filter — Remove debris using canvas, a cotton cloth, or improvised filter. This protects the next stage.
2. Disinfect — Add bleach (2 drops per litre, double for dirty water) or water purification tablets. Wait 30 minutes.
3. Boil if possible — If you have heat, boil for 1 minute. This is the most reliable step and removes chemical contaminants that bleach cannot.
4. SODIS as backup — If you have clear plastic bottles and sunlight, and no other option, leave in direct sunlight for 48 hours minimum. Filter first.

If you have a pocket filter: filter first, then disinfect. The filter removes the organisms the disinfectant might miss. The disinfectant kills what the filter lets through.

What You Need in Your Emergency Kit

• A pocket tube water filter (and replacement cartridge)
• A small bottle of household bleach (5%, replaced annually)
• A strip of water purification tablets (or several doses)
• A length of clean cotton cloth or small square of canvas for pre-filtering
• One or two 5-litre food-grade water containers
• A folding water container or jerry can

None of this is expensive. All of it is simple. The skill of using it takes a few minutes to learn and could save your life.