Purification

Overview

Use a multi‑barrier approach: pre‑filter to remove sediment, then disinfect by boiling, chemicals, UV, or filtering to an appropriate pore size. No single method is perfect for all threats; choose based on source and context.

Skill Level: Basic

Boiling

Robust and widely effective for microbes.

Bring water to a rolling boil for at least 1 minute. At elevations above 2,000 m / 6,500 ft, boil for 3 minutes.
Let cool naturally; protect from re‑contamination (pour into clean, covered container).
Fuel cost: High. Plan fuel budget; boil larger batches to save time.

📝 Note: Boiling inactivates bacteria, viruses, and protozoa (including Giardia and Cryptosporidium) but does not remove chemicals/heavy metals.

Chemical Disinfection

Fast, light, and works well on clear water. Less effective in very cold or turbid water; extend contact time.

Unscented household bleach (sodium hypochlorite)

Strength varies. Common: 6% or 8.25%.
Clear water: 2 drops/L (about 8 drops/gal) for 6%; 1–2 drops/L (6 drops/gal) for 8.25%.
Cloudy water: Double the dose. Mix well; wait 30 minutes; a slight chlorine smell should be present. If not, add the same dose again and wait 15 more minutes.

📝 Note: Bleach degrades with age, heat, and sunlight. Use plain, unscented, recently purchased bleach when possible. If there is no chlorine smell after the contact time, re‑dose and wait again before drinking.

Chlorine dioxide tablets/drops

Effective against bacteria, viruses, and Giardia; needs up to 4 hours for Cryptosporidium (follow manufacturer directions).
Better taste than bleach; works over a wider pH range.

Iodine (tincture or tablets)

Clear water: typically 5 drops/L of 2% tincture (or per tablet instructions); 30 minutes contact; double dose/time for cold/cloudy.
Avoid in pregnancy, thyroid disease, or prolonged use; poor efficacy against Cryptosporidium.

⚠️ Caution: Chemical methods require clear water. Pre‑filter turbid water and extend contact times in cold conditions.

Calcium Hypochlorite (HTH) Stock Solution (Pool Shock)

For situations where you must store a compact disinfectant and mix as needed.

Make a stock solution: In a ventilated area with eye protection, dissolve one heaping teaspoon (~1/4 oz) of high‑test granular calcium hypochlorite (68–73% HTH; no additives) in 2 gallons (7.6 L) of water. This yields ~500 mg/L available chlorine.
Dose for drinking water: Add 1 part of this chlorine stock solution to 100 parts of water being treated (about 16 fl oz of stock to 12.5 gallons of water). Mix, wait 30 minutes, and verify a faint chlorine smell; if absent, repeat the dose and wait 15 more minutes.
Use only plain HTH without algaecides or other pool additives. Store chemicals safely and label clearly.

Reference: EPA Emergency Disinfection of Drinking Water (EPA 816‑F‑15‑003).

Filters

Mechanical removal of pathogens and particulates.

Pore size: 0.1–0.2 µm hollow fiber filters remove bacteria and protozoa (Giardia/Crypto). They do not remove viruses; use a purifier (adds virus barrier) or follow with chemicals/UV when viral risk exists (crowded camps, floods).
Carbon: Improves taste/odor; reduces some chemicals; does not “purify” alone.
Freezing: Don’t use if a hollow fiber filter has frozen; internal fibers may crack (invisible failure).
Maintenance: Backflush as instructed; protect from biofilm growth; store dry when possible.

UV Pens

DNA/RNA disruption in clear water.

Use only with low‑turbidity water (pre‑filter first). Stir per instructions to cover all sides; treat container mouth threads.
Battery dependent; cold reduces output—keep warm in a pocket.
Repeat the cycle for larger volumes or cold water.

Solar Disinfection (SODIS)

Passive, low‑resource method for sunny conditions.

Use clear PET bottles (≤2 L), fill with clear water; place on reflective surface in full sun for 6 hours (or 2 consecutive sunny days in partial cloud). Shake to oxygenate before exposure.
Works best in equatorial/sunny climates; unreliable in high latitudes/winter.

💡 Tip: PET bottles pass UV‑A better than many hard plastics (e.g., some polycarbonates) and heat rapidly when placed on dark or reflective surfaces. Painting the lower half of a dedicated bottle black (outside) or placing bottles on dark metal can raise temperature and speed treatment—mark and reserve such bottles for SODIS only.

⚠️ Caution: SODIS needs clear water. Pre‑filter turbidity first. Avoid tinted or UV‑blocking containers.

Field-Expedient Containers

Improvise when standard containers fail.

Rigid: Bottles/cans; remove sharp edges; boil only in metal containers.
Flexible: Heavy freezer bags can hold treated water; double‑bag for redundancy.
Liners: Clean food‑grade liners inside non‑potable shells (e.g., a trash can) to transport water.

Pre-Filtering

Reduce turbidity before disinfection.

Cloth: Bandana/coffee filter to remove large particles.
Settle: Let water sit; decant the clearer top layer.
Improvised sand/charcoal column: Layer cloth → sand → cloth → crushed charcoal → cloth. Run water slowly; then disinfect chemically or by boiling.

☑️ Checklist — Multi‑Barrier Treatment

Source assessed; no chemical sheen/odor; cyanobacteria avoided
Pre‑filter cloth/settling for turbid water
Choose method: boil (fuel), filter (pore size), chemical (time), UV (clarity)
Protect from re‑contamination (clean container, lids)
Mark treated vs untreated containers clearly

Examples

Snowmelt camp: Melt and boil large batch; store in covered, clean bottles; avoid eating snow directly.
Murky desert pool: Settle overnight, decant, filter (0.1 µm), add chlorine dioxide; wait full Crypto time.
Post‑flood urban: Prefer bottled/packaged water. If none, filter + activated carbon + chlorine; avoid water with chemical odors.

Narrative — The Boil‑Only Kitchen The power was out, and the faucet sputtered. You found a stockpot, two clean jars with lids, and half a fuel canister. Rather than chase cups at a time, you settled silt from a bucket, poured the clear top into the pot, and brought it to a rolling boil—three minutes, because of the mountain sticker on the fridge. One jar got a strip of blue tape—“TREATED”—and the other “RAW.” The rule was simple: raw water only touches the big pot and the “RAW” jar; hands and cups touch the blue‑taped jar. Fuel stayed enough for breakfast because you boiled once, not five times.

Common Mistakes

Cross‑contamination: Using the same lid/container for raw and treated water.
Under‑treating Crypto: Not waiting the full time for chlorine dioxide in cold water.
Relying on filters after they’ve frozen; hollow fibers may be cracked.
Assuming filters remove chemicals/heavy metals; they don’t without specific media.
Skipping rolling boil or elevation adjustment; short boils risk survival of pathogens.
Using iodine in pregnancy/thyroid disease or for prolonged periods.

Key Takeaways

Combine steps: pre‑filter + disinfect. Match the method to the threat profile.
Boiling is broadly effective but fuel‑intensive; filters don’t remove viruses; chemicals need clear water and time.
Prevent re‑contamination by separating clean/dirty gear and labeling containers.

Scenario

🧭 Scenario (Desert pool, murky): Sun drops in 90 minutes.
🔍 Decisions: Boil muddy water vs settle/filter/chemical.
✅ Outcome: You settle, decant, cloth pre‑filter, filter 0.1 µm, then chlorine dioxide for Crypto overnight; you sip your reserve with salt while waiting.
🧠 Lessons: Multi‑barrier tailored to turbidity
🏋️ Drill: Time how much clearer water gets after 30 minutes of settling.