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🚰 Water Purification — With What You Have

How to make creek, spring, rain or well water safe to drink when there's no store and no power: settling and straining, homemade sand, biosand and clay-pot filters, charcoal (what it does and doesn't do), boiling, bleach and pool shock, sunlight, solar stills, simple tests, and safe storage. The manuals are in Water-Purification\ (Filters-Sand-Charcoal-Ceramic, Disinfection, Settling-Coagulation, Overviews-Testing), Water\ and Homestead-Skills\Sanitation-Water. Getting the water out of the ground is in the Wells & Pumps guide.

The four rules of safe water

1. Clear it, then kill the germs. Settle or filter muddy water first; then boil or disinfect.
2. Charcoal and sand filters alone do NOT make water safe. They clear water and improve taste; charcoal beds even grow bacteria and shield them from chlorine. Charcoal goes last, after disinfection (WHO, pp. 51–52; Wilderness Medical Society, p. 6).
3. Boiling kills germs but removes no chemicals — not fuel, metals, salt or fire-retardant runoff.
4. Keep treated water clean: covered container with a spout, never dip a cup into it.

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🧭 Which method does what

From CAWST's comparison of household methods (CAWST Fact Sheets):

Method	Germs (bacteria / viruses / Giardia-Crypto)	Muddy water	Chemicals	Notes
Settling, three-pot storage	some	very good	no	First step for muddy water
Cloth straining	larger organisms only	some	no	Quick first step
Alum, wood ash, Moringa seed	some (lab good, field ~50%)	very good	no	Clears water fast
Biosand / slow sand filter	good / fair / very good	very good	no (some iron)	Needs a living top layer
Ceramic pot filter	very good / fair / very good	very good	no	Needs silver coating for best results
Boiling	kills all	no	no	Uses fuel
Chlorine (bleach, pool shock)	kills bacteria & viruses; weak on Crypto	no	no	Cheap, protects stored water
Sunlight (SODIS)	good	needs clear water	no	Free; sunny months only
Solar still / distilling	kills all	yes	removes salts & metals	Slow, ~1 gallon/day per 12 sq ft
Charcoal / "activated" carbon	poor — not a germ barrier	some	taste, odor, many organic chemicals	Use last

WHO's bar for a "protective" method is removing about 99% of bacteria and protozoa and 99.9% of viruses; most homemade methods reach that only in combination (WHO Evaluating Household Water Treatment, p. 11). Best simple combination: settle or filter → boil or chlorinate → (optional) charcoal for taste (the household methods drawn, WHO/WEDC pp. 19–21; overview with photos).

Clear it, kill the germs, then charcoal if you want it, never instead of boiling or bleach (a diagram drawn for this guide).

📚 Go deeper: CAWST Household Water Treatment Fact Sheets — every method with removal data; WHO Managing Water in the Home; Practical Action Household Water Treatment.

🫙 Step 1 — Clear the water

• Let it settle. Three-pot method: fill pot 1 from the source; each day pour pot 1 gently into pot 2, and pot 2 into pot 3 (your drinking pot) — water sits at least 2 days and much of the dirt and some germs settle out. Still disinfect it (CAWST, p. 4).
• Strain through cloth: old, soft cotton folded 7–8 times catches particles bigger than about 20 microns — silt, worms, the plankton cholera rides on — but not bacteria or viruses. Wash the cloth after each use (CAWST, pp. 14–15; photo of a folded-cloth filter, p. 14).
• Make the mud drop (coagulants):
  • Alum (pickling alum): about ⅛ tsp per gallon; stir fast 1 minute, slowly 5 minutes, settle at least 30 minutes, pour off through cloth (WMS, p. 7; how the mud clumps).
  • Wood ash, lime, crushed beans also work as rough coagulants (same source).
  • Moringa seed: about 1 crushed kernel per liter of muddy water — shake the paste in a cup of water, strain it in, stir 1 minute fast and 5–10 minutes slow, settle 1–2 hours (ECHO note; WEDC, p. 3). Moringa won't grow here (frost) — stockpile seed if you want it.
  • Coagulants clear water; they don't make it safe. Always disinfect afterward.
• Is it clear enough? Look down through a clear 2-liter bottle of the water at large print — if you can read it, it's under about 50 NTU, clear enough for filters. For chlorine, clearer is better: if the water is still cloudy, use double the bleach dose (CAWST Testing, p. 81). SODIS needs clearer water — under 30 NTU (EAWAG SODIS manual, p. 26), so settle or filter it first unless it is truly clear.

📚 Go deeper: CAWST — settling, coagulants and cloth, pp. 4–15; WEDC Moringa seed coagulant.

🏺 Step 2 — Homemade filters

Biosand filter (best household filter you can build)

A concrete or plastic box of sand with a living layer on top (the "schmutzdecke") that eats germs (CAWST Biosand Construction Manual; labeled cut-away, p. 17):

• Layers, bottom to top: 2 in of drainage gravel (¼–½ in), 2 in of small gravel, then fine washed sand (0.7 mm or finer) filled with water already in the box so no air gets trapped (pp. 59, 92; filling it, step by step, p. 92).
• About 2 in of water must stand above the sand when it stops flowing — more starves the living layer of air; letting it dry out kills it. A diffuser plate keeps poured water from digging up the top (pp. 18–19; the standing-water level drawn, p. 24).
• Use it every day, one bucket (12–18 L) at a time, 6–12 hours apart. The living layer takes up to 30 days to grow — disinfect the output until then and always for babies and the sick. When flow gets slow, stir the top inch and scoop off the dirty water ("swirl and dump") (CAWST Fact Sheets, pp. 18–19; swirl-and-dump drawings).
• Results once mature: bacteria about 88–98% removed in the field, Giardia-type parasites over 99.9%, viruses less reliable (p. 19).
• Cold winter water slows the living layer — filter slower and disinfect (EPA slow sand, p. 9).

The outlet tube's height keeps about 2 in of water standing over the sand, so the living layer never dries out (CAWST Biosand Manual, p. 17).

"Swirl and dump" when the flow gets slow: repeat steps 2–4 a few times, then put the diffuser back and wash your hands (CAWST Biosand Manual, p. 104).

Slow sand filter (for a group or homestead)

The same idea, bigger: water trickles down through about 3 ft of fine sand at 0.1–0.2 m per hour; build two beds so one can be cleaned (scrape off the top inch). A mature filter removes 95–100% of fecal bacteria and nearly all viruses and cysts. Takes about 3 weeks to "ripen" when new (IRC Slow Sand Filtration, pp. 38, 48; parts of a slow sand filter, p. 32).

The bucket "layer filter" — gravel, sand, charcoal

A bucket of gravel, sand and charcoal clears and improves water but is not a germ barrier. The right way to use charcoal is the three-stage system: gravel roughing filter → slow sand biofilter (germs) → crushed charcoal (chemicals and taste) → covered storage; replace the charcoal yearly (Aqueous Solutions Biochar Water Filter, pp. 2–9; the three-drum layout, p. 2; what goes in each drum, p. 9).

Clay pot (ceramic) filter

Clay mixed with sawdust or rice husk (about 50:50 by volume to start) is pressed into a pot and fired to about 900 °C (1,650 °F) — the sawdust burns out and leaves tiny pores (CMWG Ceramic Pot Filters, p. 62; RDIC handbook). It sits in a covered bucket with a tap (drawing; Potters for Peace filter) and gives 1–3 liters an hour. Test local clay first (clay testing); flow-test each pot (at least 1 liter in the first hour, not too fast — too fast means cracks) (CMWG, p. 114). They work best with a colloidal silver coating, which you can't make from raw materials — without it, still disinfect. Replace every 1–2 years or when cracked. Kiln plans: Mani kiln (cut-away drawing, p. 3).

📚 Go deeper: CAWST Biosand Filter Construction Manual; IRC Slow Sand Filtration; CMWG ceramic filter manual; Wikipedia: biosand filter · slow sand filter · ceramic water filter.

⚫ Charcoal: making it and using it right

• What it does: soaks up taste, odor and many organic chemicals (pesticides, some fuel residues). What it doesn't: reliably remove germs — WHO doesn't recommend it for germs, and bacteria grow in the bed (WHO, pp. 51–52).
• Plain vs. activated: ordinary charcoal has its pores clogged with tar, so it adsorbs poorly. Factories "activate" it by heating to about 800 °C in steam, which opens the pores (FAO Industrial Charcoal Making, p. 28).
• Homemade water-treatment charcoal (Aqueous Solutions Char Drum Oven):
  1. Pack dry wood pieces (1–2 in thick) tightly in a steel drum, set a chimney drum on top, and light it from the top (the stacked drums). It burns about 2 hours down to faint blue flames (flame colors).
  2. Quench with plenty of water (photo) — the steam partly opens the pores (crude activation, not true activated carbon).
  3. Crush and sieve to 1–5 mm pieces; wash out the dust.
• Use it as the LAST step after the water is filtered and disinfected, or in a column after a sand filter; replace it yearly. Hardwood (oak, madrone) makes good charcoal. Chemical activation (acids, zinc chloride) isn't a safe home method (rule of thumb).
• Charcoal-making in bulk: Practical Action brief (a portable metal kiln, p. 3) and the Peace Corps charcoal manual.

📚 Go deeper: FAO — what activated charcoal is, p. 28; Aqueous Solutions — charcoal oven; Wikipedia: activated carbon.

🔥 Step 3 — Kill the germs

Boiling — the surest method. A rolling boil for 1 minute (3 minutes above 6,500 ft — we're far below that) kills bacteria, viruses and parasites even in cloudy water. Let it cool covered; don't add ice (WHO Boil Water, p. 1; OSU, p. 5). Fuel cost: about 0.2 kg (½ lb) of wood per liter — a family's 4 gallons a day is about 7 lb of wood a day (WHO Bulletin, p. 4) — see the Firewood guide.

Bleach (plain unscented, 6–8.25% sodium hypochlorite) — EPA, p. 1:

Water	6% bleach	8.25% bleach
1 quart, clear	2 drops	2 drops
1 gallon, clear	8 drops	6 drops
1 gallon, cloudy or very cold	16 drops	12 drops

Stir, wait 30 minutes. It should smell faintly of chlorine; if not, dose again and wait 15 more minutes. Cold water needs longer: 1 hour at 50–65 °F, 2+ hours below 50 °F — winter creek water (CAWST, p. 59). Bleach loses strength — use it within a year. Chlorine is weak against Cryptosporidium: settle or filter first.

Pool shock (calcium hypochlorite) — stores for years as dry granules. Dissolve 1 heaping teaspoon in 2 gallons of water to make a stock solution; then use 1 part stock to 100 parts water (1 pint per 12½ gallons), wait 30 minutes (EPA, p. 2). Use plain granules (no algaecide or stabilizer), mix outdoors with eye protection, and make stock in small batches — it lasts about a month in heat, longer cool and dark (CDC Safe Water Systems, p. 62).

Iodine: 5 drops of 2% tincture per quart (10 if cloudy), wait 30 minutes (EPA, p. 2) — fine short-term (a month at most), never for pregnant women or anyone with thyroid disease or an iodine allergy (WMS, p. 14).

Sunlight (SODIS): clear plastic (PET) bottles of clear water, lying flat in full sun 6 hours (2 days if more than half cloudy). Works well here March–September, poorly in the winter rains (EAWAG SODIS manual, p. 23; how-to drawings, p. 26; the home poster, p. 76).

Clear bottles, clear water, lying flat in full sun for 6 hours (a diagram drawn for this guide).

Solar pasteurizing: water held at 65 °C (150 °F) is pasteurized — a solar box cooker can do several gallons on a summer day (Ciochetti & Metcalf). A lump of beeswax in a sealed tube melts at about 62 °C — just under the target, so melted wax means "nearly there": keep it hot a while longer, or boil it, to be sure (CAWST, p. 75).

📚 Go deeper: EPA Emergency Disinfection of Drinking Water; CDC Safe Water Systems handbook — making and storing chlorine; Wilderness Medical Society guidelines; Wikipedia: SODIS.

☀️ Salty or chemical water: the solar still

A shallow black basin under sloped glass or clear plastic: the sun evaporates the water, it condenses on the cover and runs into a gutter (photos of stills, CAWST p. 71) — leaving salt, metals and germs behind. Expect about 1 gallon a day per 10–12 sq ft of glass (CAWST, p. 71; Aprovecho, p. 43). Some fuel-type chemicals can carry over (rule of thumb).

Salt, metals and germs stay in the basin; only the water that condenses on the glass reaches the jar (a diagram drawn for this guide).

📚 Go deeper: CAWST — solar distillation; Wikipedia: solar still.

🔬 Simple tests

• Turbidity tube: a clear tube with a black-and-white disk at the bottom — pour water in until the disk disappears; the depth tells cloudiness (disk gone at 18 cm ≈ 50 NTU; at 85 cm ≈ 5 NTU) (Myre & Shaw, pp. 11–16; the parts, p. 7; finished tubes, p. 10; Secchi disk — the same idea in a lake).
• H₂S bacteria test: a paper strip with a special medium in a bottle of water turns black in 1–2 days if fecal bacteria are present — keep it warm (above 77 °F). Agreed with lab tests about 90% of the time; sulfur well water can give false alarms (WHO, pp. 16, 33; a test vial, CAWST; coliform bacteria). The test bottles must be bought ahead.
• Chlorine check: a faint chlorine smell 30 minutes after dosing means enough was added (checking it with a color comparator).
• Sanitary check: look at the source — animals, outhouses, runoff upstream? Fix the source first (CAWST Testing; how springs and wells get polluted).

📚 Go deeper: CAWST Introduction to Drinking Water Quality Testing; Turbidity tube.

🪣 Storing water safely

• Covered container with a tap or narrow spout, opaque, easy to clean; never put untreated water in it; clean it with soap or chlorine; keep it off the ground in shade; pour, don't dip (CAWST, pp. 83–84; safe storage containers; a tap fitted to a bucket; jerrycan).
• Don't reuse milk jugs — they can't be cleaned well (OSU, p. 2).
• Chlorinated water keeps best; boiled water can be recontaminated — drink it within a day or two.

📚 Go deeper: CAWST — safe storage and handling, p. 83; CDC Safe Water Systems — choosing a storage vessel, p. 69; CAWST Biosand Manual — cleaning a storage container, p. 106.

🔥 After a wildfire

Burned slopes send ash, metals, nitrate and organic chemicals into creeks after storms — levels jumped 10 to 10,000 times after a Colorado fire (USGS, p. 1). Boiling, chlorine, sunlight and sand or clay filters do not remove these. Fire-damaged plastic pipes can leach benzene, and boiling doesn't remove it (WQA). After a fire, prefer springs or wells outside the burn, or rainwater; distilling and charcoal are the only partial chemical barriers here.

📚 Go deeper: USGS Wildfire Effects on Source-Water Quality; OWRD — your well after wildfire.

🖼️ Pictures & diagrams

Look before you try it. 🖼️ = drawings in the books on this drive; 📷 = photos in the offline Wikipedia.

Clearing muddy water

• Household methods at a glance (shaking, sand and candle filters, chlorine, SODIS) — WHO/WEDC, pp. 19–21; Portable water purification
• Straining through folded cloth — CAWST, p. 14
• Alum and Moringa seed — Alum; Moringa; Flocculation

Sand, clay and charcoal filters

• Biosand filter, every part labeled — CAWST manual, p. 17; CAWST fact sheet, p. 17; Biosand filter
• The 2-inch standing-water level — CAWST, p. 24; Schmutzdecke
• Putting in the gravel and sand — CAWST, p. 92
• Swirl-and-dump cleaning — CAWST, p. 104
• Slow sand filter, outlet- and inlet-controlled — IRC, p. 32; IRC, p. 33; Slow sand filter
• Gravel → sand → charcoal drum system — Aqueous Solutions, p. 2; the roughing filter, p. 6; Biochar
• Clay pot filter in its bucket — RDIC, p. 5; CMWG, p. 15; Ceramic water filter
• Ceramic candle filter — WHO/WEDC, p. 20
• Kiln for firing filters — Mani kiln, p. 3
• Char drum oven, step by step — Aqueous Solutions, p. 6; flames, p. 7; quenching, p. 8; Activated carbon
• Charcoal kilns — Practical Action, p. 3; Charcoal

Killing the germs

• Chlorine tablets, SODIS and a tapped bucket — WHO/WEDC, p. 21; Water chlorination
• Bleach and pool shock — Sodium hypochlorite; Calcium hypochlorite
• SODIS: right and wrong bottles, how to lay them out — EAWAG, pp. 26–27; home poster, p. 76; Solar water disinfection
• Solar box cooker for pasteurizing — Solar cooker
• Solar stills — CAWST, pp. 71–72; Solar still

Testing and storing

• Turbidity tube — parts, p. 7; finished tubes, p. 10; Turbidity
• H₂S bacteria test — CAWST, p. 180; Coliform bacteria
• Chlorine color comparator — WHO/WEDC, p. 45
• Safe storage containers and cleaning them — CAWST, p. 105; CAWST, p. 106; Jerrycan
• Covered water pots — WHO/WEDC, p. 39

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🔗 Related guides

• Water Wells, Pumps, Springs & Cisterns
• Firewood — fuel for boiling; charcoal from hardwood
• Make-Do Household Guide — sanitation, SODIS, containers
• Medical — diarrhea and rehydration
• Illinois Valley — Start Here — local water facts
• Wildfire: defending your place with no grid — clearing, hardening, water with no power, stay or go, and after the fire.