Make water safe
Kill what is in the water before it kills you.
Everything for this step
- Field guide
- Fire → 8 sections Drinking water → 8 sections Micro-organisms → 6 sections Poisons → 2 sections The electromagnetic spectrum → 2 sections
- Builds
- Slow sand filter
- Reference sheets
- 1 sheet on this page ↓ · 1 sheet in Drinking water · 1 sheet in The electromagnetic spectrum
- Sciences it rests on
- Microbiology · Chemistry
- Track
- Make water safe, level 1 of 3 · next: Make chlorine disinfectant
Why this matters
Diarrhoeal disease is the single largest killer in every collapse, every refugee camp and every siege on record. It is also the most completely solved problem on this entire site — the fix is heat, and heat is free.
The shape of it
- Clarify first, disinfect second. Turbid water resists every disinfection method. Let it stand until the silt drops, then pour off the clear top, or strain through tightly woven cloth folded several times.
- Boil. Bring to a rolling boil. That is sufficient — the killing is done on the way up, well before 100 °C. Add roughly a minute of boiling for every 1,000 m of altitude if you want margin.
- If you cannot boil, use the sun. Fill a clear PET bottle no larger than 3 litres, shake it to aerate, and lay it on a reflective or dark surface in full sun for 6 hours — two full days if overcast. This is SODIS; it works on UV plus heat and needs the water already clear.
- Build a slow sand filter for volume. A barrel with, bottom to top: coarse gravel, fine gravel, then 60+ cm of clean fine sand. Run water in slowly and keep the sand permanently wet. After a few weeks a biological layer forms on the sand surface which does the real pathogen removal.
- Charcoal improves taste and removes some chemistry. It does not reliably remove pathogens. Never treat a charcoal filter as disinfection.
- Store treated water in a covered, narrow-necked vessel with a dedicated dipper. Never put hands in the store.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Clarify the water before you try to disinfect it
Suspended particles physically shield microbes from heat, from chlorine and from ultraviolet, and organic matter consumes chlorine before it reaches anything alive. Clarifying is not cosmetic and it is not optional.
- Settling is free: fill a vessel, leave it, decant the clear top. Three pots in rotation makes it continuous — today's fills the first, yesterday's decants into the second, and only the third goes to the fire.
- Cloth, folded four to eight times, removes a surprising amount including the copepods that carry guinea worm. Fold the same face down every time, and wash and sun-dry it between uses.
- Alum, if you have it: roughly a teaspoon to 20 litres — stir hard for a minute, slowly for five, then leave an hour.
- Crushed Moringa oleifera seed does the same job where the tree grows: about one seed per litre, ground.
The target is water you can read newsprint through. That is not clean and it is not safe; it is merely ready to be made safe.
2 · Boil — the method with nothing to measure and nothing to get wrong
Bring the water to a rolling boil: a full, tumbling boil, not the first bubbles. One minute at the boil is enough, and above about 2,000 m make it three minutes, because water boils cooler at altitude.
Most of the killing is already done on the way up — pathogens die progressively from about 60 °C — which is why the boil itself is a visible indicator rather than a timed dose. Cover the pot while it heats and while it cools: an uncovered pot re-contaminates from the air, the fire and hands, and it wastes fuel.
Boiling is the most fuel-hungry method on this page by a wide margin. Use it when someone is already ill, when the water is doubtful, and for infants — and use sunlight or chlorine to spare fuel on ordinary days.
3 · If you cannot boil, use the sun
Solar disinfection needs only clear bottles and a day:
- Clear PET bottles, no larger than 3 litres. Scratched, coloured or glass bottles block the ultraviolet that does the work.
- Fill two-thirds, shake hard for 20 seconds to aerate, then fill to the top and cap it.
- Lay them flat in full sun on a dark or reflective surface — a metal roof is ideal — where nothing will shade them as the sun moves.
- Six hours of strong sun. If it is cloudy for more than half the day, leave them out for two consecutive days.
- The water must already be clear — the newsprint test from entry 1. Turbid water is not disinfected by sunlight.
4 · Chlorinate when you have bleach, and know the dose
Household bleach is the cheapest reliable disinfection there is, and the dose is small enough to get badly wrong.
- Two drops of 5% unscented household bleach per litre of clear water. Double it for water that is cool or slightly cloudy.
- Stir, then wait 30 minutes before drinking.
- It should smell faintly of chlorine afterwards. No smell means the dose was consumed by organic matter — add the same again and wait another 30 minutes.
- Bleach strength varies from about 3% to 6% — read the label and scale. Unscented only: no thickeners, no perfume, no detergents.
Chlorine does not kill Cryptosporidium and is slow against Giardia. Where those are the worry, filter or boil.
5 · Build a slow sand filter for volume
For a household or a group, a slow sand filter turns a daily chore into a tap. The dimensions and plumbing are in the build; the numbers that decide whether it works:
- Fine sand, effective size 0.15–0.35 mm, washed until the rinse water runs clear. Coarser sand passes pathogens; finer clogs.
- Bed depth at least 60 cm, on graded gravel.
- Flow 0.1–0.4 cubic metres per square metre of bed per hour — slow is the entire principle. Faster does not filter.
- 5–10 cm of standing water above the sand, always. The outlet is plumbed above bed level so the bed physically cannot be drained.
- It must ripen. The biological layer that does most of the work — the schmutzdecke — takes 2 to 3 weeks to establish. Water from a new filter is not treated water.
6 · Clean the filter without killing it
When the flow falls too low, scrape off the top 1–2 cm of sand only — the clogged layer — and no more. Do not dig, stir or wash the bed below it: that is the living layer you spent three weeks growing.
Rinse the scraped sand, dry it in the sun and keep it; return it to the bed after several cleanings, and top up with fresh washed sand when the bed drops below about 50 cm. After every scrape the filter needs roughly a week to recover, so plan a second filter or a store of treated water before you clean the first.
7 · Know what none of this fixes
Every method above is aimed at living contamination. None of them removes:
- Salt — no filter, no boiling. Only distillation.
- Arsenic, fluoride, nitrate — invisible, tasteless, and serious. They are regional: arsenic in deltas and tube-well country, fluoride in volcanic and granite regions, nitrate near latrines and fertilised fields. Boiling concentrates all three.
- Algal toxins — heat-stable.
- Industrial and mining contamination.
Charcoal improves taste and removes some chemistry; it is not disinfection and must never be treated as such. The testing table in the wells guide says what to suspect where, and how to test cheaply.
8 · Store it so the treatment is not undone in the last metre
Treated water recontaminates faster than people expect, and always by the same routes: hands, cups dipped into the store, an uncovered vessel, and a container that once held untreated water.
- Narrow-necked, covered vessel with a dedicated dipper or a tap.
- Never a hand or a personal cup in the store.
- Wash the store weekly with treated water, and let it dry in the sun.
- Different, clearly marked containers for treated and untreated water.
- Keep it cool and dark — warm water in the light grows what little survived.
Reference sheets
One drawn sheet covering the whole subject at once — the breadth a photograph cannot give, bought at the cost of accuracy. Read them for range and vocabulary, never as the thing you identify from.
Build it
Dimensioned drawings, bills of materials and construction procedures for the things this step asks for.