1 Basics
How much water: the numbers
You can last about three days without water, less in heat or with diarrhoea. Plan in litres per person per day.
| Use | Litres / person / day | Notes |
|---|---|---|
| Drinking, survival minimum | 3–4 | More in heat, hard work, fever, diarrhoea, breastfeeding. Up to 6–8 L in summer field work. |
| Cooking | 1–2 | Dry beans, rice and pasta need water; reuse pasta water in soup. |
| Basic hygiene | 2–6 | Hand washing, a strip wash, dishes. This is what stops disease. |
| Total, emergency standard | ~15 | The humanitarian minimum for drinking, cooking and hygiene. |
| Garden, animals, laundry | extra | A laying hen drinks ~0.3 L, a goat 4–10 L, a vegetable bed ~20 L/m² per week in dry weather. |
Dark yellow urine, peeing fewer than four times a day, headache, dizziness when standing up. Aim for pale straw-coloured urine. Treatment of dehydration is on the Medical page.
Work out your household's storage and daily need on the Calculators page.
2 Sources
Finding water: best to worst
Treat every source except sealed bottled water. The order below is roughly from cleanest to dirtiest.
Hidden water inside a house
- Hot water tank / boiler cylinder: 50–300 L. Switch off power or gas first, close the cold inlet valve, open a hot tap upstairs to let air in, then drain from the valve at the bottom. The first litres may be rusty and full of sediment; let it settle.
- Pipes: open the highest tap in the house, then collect from the lowest one. Several litres.
- Toilet cistern (the tank, not the bowl): clean enough to treat and drink unless a colour or chemical block is in it.
- Ice cubes, the liquid in canned vegetables and fruit, a filled bathtub (treat before drinking).
Outside
Springs and seeps
Often the best natural source. Collect where water comes out of the ground, not downstream. Still treat: the ground above may be grazed.
Streams and rivers
Collect from fast-flowing water upstream of villages, farms, roads and animals. Take from the middle, below the surface, not the scum or the bottom.
Lakes and ponds
Collect away from the shore. Avoid water with blue-green scum or a paint-like film: toxic algae survive boiling and filtering.
Rain, snow, dew
Rain is the cleanest free water (collect it). Melt snow before drinking; eating it cools you down. Dew: drag a cloth through wet grass at dawn and wring it out.
Digging
In a dry stream bed dig on the outside of a bend, where the last water stood. Wait for the hole to fill, let it settle, scoop from the top.
Plant bags
A clear plastic bag tied over a leafy branch in sun gives 100–300 mL a day. Avoid poisonous plants. A dug "solar still" costs more sweat than it gives.
- Sea water or urine: the salt makes you lose more water than you drink.
- Car radiator or heating circuit water: antifreeze (glycol) is poisonous; heating water contains inhibitors.
- Flood water: sewage plus fuel, pesticides and industrial chemicals. Treatment does not remove chemicals.
- Water near mines, factories, landfill, fields just sprayed, or with an oily sheen, strange smell or colour.
- Swimming-pool water as regular drinking water: fine for washing and flushing, but it carries stabilisers and other pool chemicals.
- Alcohol instead of water: it dehydrates.
3 Collect
Rainwater: a roof is a water source
1 mm of rain on 1 m² of roof is 1 litre. An ordinary house roof collects tens of thousands of litres a year.
How much you can collect
Litres per year = roof area (m², measured flat from above, not along the slope) × annual rainfall (mm) × 0.8 (losses).
| Roof (flat area) | 500 mm / yr | 750 mm / yr | 1000 mm / yr |
|---|---|---|---|
| Shed, 10 m² | 4,000 L | 6,000 L | 8,000 L |
| Small house, 60 m² | 24,000 L | 36,000 L | 48,000 L |
| Farmhouse or barn, 150 m² | 60,000 L | 90,000 L | 120,000 L |
Four people at 15 L a day need about 22,000 L a year. Rain is uneven, so the tank has to carry you through the driest month or two: a 60 m² roof with a 3,000–5,000 L tank is a realistic household setup.
Setting it up
Check the roof
Good: clay or concrete tiles, glazed metal, slate. Avoid for drinking: bitumen/tar felt, treated wood shingles, lead flashing, copper, fresh paint, damaged asbestos-cement sheets, anything near a chimney that burns rubbish.
Screen the gutters
Mesh over the gutter or a leaf trap at the top of the downpipe. Clean it before the rainy season and after storms.
Fit a first-flush diverter
A capped vertical pipe on the downpipe that fills first and holds the bird droppings and dust. Size it for about 1 L per m² of roof. A small hole or loose cap in the bottom lets it empty slowly between rains.
Use a sealed, dark tank
Opaque food-grade plastic, concrete or IBC totes in a cover. Light grows algae; open water breeds mosquitoes in a week. Fine mesh on every inlet and overflow.
Draw from above the bottom
Tap 5–10 cm above the floor of the tank. Overflow pipe to a soakaway or a second tank, away from the house foundations.
Treat before drinking
Rainwater is usually clean of chemicals but not of germs from birds and animals. Filter and boil or chlorinate drinking water.
No gutters? Stretch a tarp at an angle with a low corner over a bucket. Any clean surface works: a car bonnet, a plastic sheet, a table turned over.
4 Purify
Making water safe: which method kills what
Dirty water carries three kinds of germs (bacteria, viruses, parasites) and sometimes chemicals. No single method removes everything.
| Method | Bacteria | Viruses | Protozoa (Giardia, Crypto) | Chemicals, metals, salt |
|---|---|---|---|---|
| Boiling | Yes | Yes | Yes | No (concentrates them) |
| Chlorine | Yes | Yes | Giardia mostly; Crypto no | No |
| SODIS (sunlight) | Yes | Mostly | Mostly | No |
| Hollow-fibre / ceramic filter | Yes | Usually no | Yes | No |
| Sand / biosand filter | Mostly | Partly | Mostly | No |
| Activated carbon | No | No | No | Some pesticides, taste, smell |
| Distilling | Yes | Yes | Yes | Salt and metals yes; some solvents carry over |
Settle → filter → disinfect → store covered. Let muddy water stand overnight, pour off the clear top through a cloth or filter, then boil or chlorinate. Clear water makes every disinfection method work much better.
5 Clarify
Filters: from a T-shirt to a biosand filter
Filtering removes mud and most parasites and makes chlorine and sunlight work. Except for a proper microfilter, it does not make water safe alone.
Step 1: settle and pre-filter
- Settle: stand muddy water in a bucket for 12–24 h and pour off the top two thirds without stirring.
- Cloth: pour through four layers of clean cotton cloth (an old sari or T-shirt folded). This removes a surprising amount of debris and plankton.
- Clump the mud (optional): a pinch of alum (from the pharmacy or pickling aisle) per 10 L, stirred for a minute and left for an hour, drops fine clay to the bottom.
Step 2: a bucket sand filter
Container
A 60–100 L barrel or a tall bucket. Outlet at the bottom under the gravel, protected by mesh, leading up the outside to just above the sand level.
Wash the sand and gravel
Rinse until the water runs clear. Fine sand (like builder's plastering sand) does the work; coarse sand stops it washing into the gravel.
Fill in layers
Gravel, coarse sand, then fine sand. Keep 5 cm of water standing above it. A plate or lid with holes breaks the fall of the poured water so it does not dig into the sand.
Use it every day
Pour one batch daily; the slime layer needs food and oxygen. Flow slows over months: stir the top 2 cm of sand, scoop off the dirty water, restart.
Still disinfect
A mature filter removes 90–99 % of germs. Boil, chlorinate or SODIS the output for drinking.
Charcoal
A layer of crushed charcoal from your own fire (no ash, no lighter fluid) improves smell and taste. It is not real activated carbon and does not remove heavy metals or reliably remove chemicals. Replace it every few weeks or it becomes a breeding ground for bacteria.
Bought filters worth having
Hollow-fibre squeeze and gravity filters (0.1–0.2 micron) and ceramic candle filters remove bacteria and parasites, not viruses. Backflush them as the maker says, and never let a hollow-fibre filter freeze once wet: ice cracks the fibres invisibly. Stock spare candles and O-rings. Where human sewage may be upstream, add chlorine or boiling to cover viruses.
6 Disinfect
Boiling: the method that always works
A rolling boil kills bacteria, viruses and parasites. It does nothing against chemicals.
Clear it first
Settle and filter through cloth. Boiling kills germs hidden in mud too, but clear water is easier to judge and tastes better.
Bring to a rolling boil
Big bubbles that keep coming when you stir. Tiny bubbles on the bottom are not enough.
Keep it boiling for 1 minute
3 minutes above 2000 m altitude, where water boils at a lower temperature.
Cool covered, store covered
Cool in the same pot with a lid, then pour into clean containers. Pour back and forth between two clean jugs to put air back in and fix the flat taste.
Boiling 1 L takes roughly 0.3–0.5 kg of dry wood on an open fire, far less on a rocket stove (see Fire & Heat). Always use a lid. Boil only drinking and cooking water, and use SODIS or chlorine for the rest. Water held at 70 °C for a few minutes (pasteurised) is also safe; a wax-pellet indicator (WAPI) shows when you reach it.
7 Disinfect
Chlorine: household bleach and pool shock
Cheap, light, and it keeps protecting the water in storage. Use plain bleach: no scent, no "thick" or "splash-less" gel, no detergent added.
Dose with liquid bleach
Read the label for the sodium hypochlorite percentage, then use this table. 1 drop ≈ 0.05 mL; a standard medicine dropper or a syringe without needle is ideal.
| Bleach strength | Per 1 L clear water | Per 10 L | Per 20 L jerrycan |
|---|---|---|---|
| 1 % | 10 drops | 5 mL (1 tsp) | 10 mL |
| 2.5–3 % | 4 drops | 2 mL | 4 mL |
| 4–6 % | 2 drops | 1 mL (20 drops) | 2 mL |
| 8–10 % | 1 drop | 0.6 mL (12 drops) | 1.2 mL |
Filter and dose
Cloudy, very cold or green-tinged water: double the dose.
Mix and wait 30 minutes
An hour if the water is cold (below 10 °C).
Smell it
It should smell faintly of chlorine. If not, dose again and wait another 15 minutes. If it still does not smell, the water is too dirty or the bleach is dead: find another source or boil.
Store covered
The small leftover chlorine protects stored water from recontamination for weeks.
Pool shock (calcium hypochlorite): years of chlorine in a small tub
Granular calcium hypochlorite, 65–73 % (sold as pool "shock", no algaecide or clarifier added), keeps far longer than liquid bleach. 1 kg treats millions of litres.
Make a stock solution
1 heaped teaspoon (about 7 g) in 7.5 L of water. Stir, let the chalky sediment settle, use the clear liquid. This stock is roughly the strength of household bleach.
Dose
1 part stock to 100 parts water (100 mL stock per 10 L). Wait 30 minutes, same smell test.
Label it
Mark the stock "POISON – water chlorine" and make fresh stock every month or two.
- Dry pool shock is a strong oxidiser: kept next to fuel, oil, brake fluid or organic matter it can start a fire. Store it sealed, dry, cool, alone.
- Never mix chlorine with acids (vinegar, toilet cleaner, rust remover) or ammonia: toxic gas.
- Liquid bleach loses strength with heat and age, roughly a fifth per year; old bleach may do nothing. Rely on the smell test.
- Chlorine does not kill Cryptosporidium. If people get watery diarrhoea from chlorinated water, boil or microfilter instead.
Iodine
2 % tincture of iodine: 5 drops per litre, 10 for cloudy water, wait 30 minutes. Not for pregnant women, people with thyroid problems, or daily use beyond a few weeks. Iodine tablets: follow the packet.
8 Disinfect
SODIS: clean water from sunlight and a bottle
UV light and heat in a clear bottle kill most germs in six hours of sun. No fuel, no chemicals, just time.
Solar water disinfection
- Use clear, colourless PET bottles up to 2 L (plastic drink bottles marked PET or 1). Glass works but lets less UV through.
- Water must be clear: filter it until you can read a newspaper headline through the full bottle from behind. Fill three quarters, shake 20 seconds for oxygen, top up.
- Lay bottles flat on a sunny roof, ideally corrugated metal. 6 hours of full sun, or two full days if the sky is more than half cloudy. Rainy days do not work: use another method.
- Drink directly from the bottle or pour into a clean cup. Do not store SODIS water for long; it has no residual protection.
- Replace bottles when they are scratched or cloudy, roughly every 6–12 months.
- In winter at high latitudes (northern Europe November–February) the sun is too weak; use boiling or chlorine.
- If the water gets above 50 °C (a black backing helps) it works about three times faster.
9 Salt & chemicals
Distilling: sea water and chemical risk
The only home method that removes salt and heavy metals. It costs a lot of fuel.
Boil water in a pot with a lid that is tipped or shaped so condensed steam runs into a separate clean cup. A pressure cooker with a tube pushed onto the steam vent and run through cold water makes a simple still. Throw away the first splash of condensate; do not boil the pot dry.
- Expect roughly 1 L of fresh water for 1–2 kg of wood. Worth it for sea water near the coast, not as a daily source.
- Volatile chemicals (petrol, solvents) boil off and end up in the output. Distilling is not a fix for fuel-contaminated water.
- Distilled water has no minerals; that is fine when you eat normal food.
10 Long term
Wells and springs: water that does not depend on rain
Groundwater is steady through droughts and usually cleaner than surface water. Protecting it is as important as digging it.
Where
- At least 30 m from any latrine, septic tank, manure heap or animal pen, and uphill of them.
- Look for old wells on historic maps and at old farmhouses; many were only capped, not filled. Ask the oldest neighbours.
- Green vegetation in dry weather, rushes and willows point to shallow water.
Protect it
Seal the top
A raised concrete or stone collar 50 cm above ground and a lid, so surface water, animals and children cannot fall in.
Apron and drainage
A sloping concrete apron 1–2 m around, draining spilt water away. Mud around a well is how dirt gets in.
One clean bucket
A dedicated bucket and rope that never touches the ground, or better a hand pump.
Shock-chlorinate
After building, flooding or repair: mix about 1 L of 5 % bleach per 1,000 L of water in the well, wash down the walls, leave 12–24 h, pump until the smell is gone.
A surface suction pump (pitcher pump, garden pump) can only lift water about 7 m, a physical limit set by air pressure. Deeper water needs a pump down in the well (piston or submersible) or a bucket. A 12 V submersible pump runs from a small solar system (see Electricity).
Never climb into a well or deep hole: carbon dioxide and other gases collect at the bottom and kill quickly, often the rescuer as well. Lower a candle on a string; if it goes out, the air is bad. Hand-dug wells collapse: they need a lining of concrete rings or brick.
11 Store
Storing water: for months
Clean water in a clean, sealed, dark container stays safe for a long time. Most stored water goes bad through dirty hands and cups.
Good containers
- Food-grade HDPE (recycling code 2) jerrycans and barrels, blue or opaque
- 20 L jerrycans: 20 kg is the most one person can carry
- Glass bottles for small amounts
- IBC totes that held food (ask what was in them)
Bad containers
- Anything that held chemicals, fuel, pesticide or detergent
- Milk jugs (thin plastic that breaks down, milk residue)
- Clear containers in light (algae)
- Open buckets without a lid
Rules
- Store in the cool and dark, off concrete, away from fuels and solvents (plastic absorbs smells)
- Pour out, never dip cups in
- Chlorinated tap or treated water: rotate every 6 months
- Label the fill date
Cleaning a container
Wash with soap and water and rinse. Then sanitise: 1 teaspoon (5 mL) of 5 % bleach per litre of water, swirl over every surface, leave 30 seconds, pour out, air dry or fill directly.
Water stock checklist
Before you need it
A separate container for drinking water with a tap, washed hands before touching it, and a toilet away from the water source prevent more illness than any filter. See hand washing and latrines.