Water Treatment
Part of Water Systems
How to make water safe to drink using settling, filtration, and disinfection — without chemicals or electricity when necessary.
Why This Matters
Waterborne disease is one of the leading killers in any population that lacks reliable water treatment. Cholera, typhoid, dysentery, and hepatitis A spread primarily through contaminated water. In a rebuilding scenario — when infrastructure is disrupted, population is displaced, and sanitation systems are compromised — waterborne disease can kill faster than almost any other threat.
The good news is that effective water treatment requires neither sophisticated technology nor industrial chemicals. The combination of physical settling, slow sand filtration, and solar disinfection (SODIS) has been proven to reduce waterborne disease by 50–90% in field conditions. Adding even basic chlorination raises protection to 99%+. These methods require minimal resources and can be implemented at the household level.
Understanding water treatment means understanding what makes water unsafe, which treatment step addresses which risk, and how to verify that treatment is working.
What Makes Water Unsafe
Pathogens: Microorganisms that cause disease.
- Bacteria: Vibrio cholerae (cholera), Salmonella typhi (typhoid), E. coli (diarrhea). Killed by: boiling, chlorination, UV light, slow sand filtration.
- Viruses: Hepatitis A, rotavirus, norovirus. Killed by: boiling, chlorination, UV. Not reliably removed by slow sand filtration alone.
- Protozoa: Giardia, Cryptosporidium. Killed by: boiling. Resistant to chlorination at normal doses. Removed by slow sand filtration.
Turbidity: Suspended particles. Not directly harmful but shields pathogens from disinfection and indicates contamination pathways. Must be removed before chemical disinfection.
Dissolved contaminants: Nitrates (from agricultural runoff), heavy metals (arsenic, lead, fluoride in certain geologies), industrial chemicals. These are NOT removed by any of the biological/physical treatments described here. Require specialist treatment or source change.
Treatment Train: Layered Defense
No single treatment removes all risks. Use multiple steps:
- Settling — remove large particles and most turbidity
- Roughing filtration — remove remaining coarse particles
- Slow sand filtration — remove bacteria, protozoa, and fine turbidity
- Disinfection — kill remaining pathogens (especially viruses)
- Safe storage — prevent recontamination after treatment
Each step makes the next more effective. Disinfection on turbid water fails because particles shield pathogens. Sand filtration on highly turbid water clogs quickly. The order matters.
Step 1: Settling
Allow water to stand undisturbed for 2–24 hours in a covered container. Sand and silt settle to the bottom; decant the clear top 70–80% of the water to the next treatment step. Discard the turbid bottom portion.
Improvement — coagulation: Add a natural coagulant to make fine particles clump and settle faster:
- Moringa seeds: Crush 2–3 seeds per 10 liters to a powder, stir vigorously for 5 minutes, allow to stand 30 minutes. The proteins in the seeds carry a positive charge that neutralizes the negative charge on clay particles, causing flocculation.
- Cactus mucilage: Slice a nopal (prickly pear) pad, stir into turbid water (one slice per 20 liters). The mucilage works as a coagulant. Used in Mexico and Central America.
- Alum: 1 gram per 10 liters (a pinch). Stir rapidly 1 minute, gently 10 minutes. Stand 30 minutes. Standard municipal coagulant, widely available, highly effective.
After coagulation, the settled floc at the bottom must be discarded — it contains concentrated pathogens.
Step 2: Sand Filtration
See the Sand Filters article for full construction details. A properly matured slow sand filter (2–4 weeks of continuous operation) removes:
- Bacteria: 99.9%+
- Protozoa: 99.9–100%
- Turbidity: 90–99%
- Viruses: 90–99% (less reliable than for bacteria)
The filter effluent should be clear and odorless. Cloudiness indicates the filter is not performing — check loading rate (too fast?) and whether the schmutzdecke has matured.
Step 3: Disinfection
Boiling (most reliable)
Bringing water to a rolling boil for 1 minute kills all pathogens at altitudes below 2,000 m. At 2,000–5,000 m altitude (lower boiling point), boil 3 minutes.
Practical tips:
- A rolling boil — large bubbles breaking the surface — is sufficient. A full 10-minute boil wastes fuel and adds no protection.
- Boiled water tastes flat because dissolved oxygen has been driven off. Improve taste by pouring between containers several times to re-oxygenate.
- Boiled water must be stored in a clean, covered container. Recontamination occurs easily from hands, insects, or dirty vessels.
Fuel cost: Boiling is fuel-intensive for a community. At a household level, bring a full pot to boil once daily for drinking and cooking water.
Chlorination
Chlorine kills bacteria and viruses reliably. It is less effective against Cryptosporidium (which slow sand filtration should have removed).
Sources of chlorine:
- Household bleach (sodium hypochlorite): Typically 3–8% available chlorine. Widely available from laundry supply.
- High-test hypochlorite (HTH, pool shock): 65–70% available chlorine. Concentrated — use very small amounts. Stores much longer than bleach.
- Calcium hypochlorite granules: 65–70%. Same as HTH.
Dosing for clear water (turbidity < 1 NTU):
- 5% bleach: 2 drops per liter, or 8 drops per US gallon
- HTH 65%: dissolve 1 gram in 10 liters first to make a stock solution, then add 1 mL of stock per liter
Contact time: After adding chlorine, wait 30 minutes before drinking. This allows the chlorine to react with pathogens.
Residual test: After 30 minutes, a faint smell of chlorine should remain. If there is no smell, the water was too turbid or contaminated — add more chlorine and wait again. If the smell is very strong (bleachy), dilute with more treated water or allow to stand open for 30 minutes.
Sodium thiosulfate neutralization: If treated water tastes too strongly of chlorine, add a small amount of sodium thiosulfate to neutralize (available as photography chemical “fixer”). Do not use photographic-grade chemicals for drinking water unless no alternative exists.
Solar Disinfection (SODIS)
Fill a clear PET plastic bottle (1–2 L) with water. Place in direct sunlight for:
- 6 hours in full sun
- 2 days in cloudy conditions
- Does not work when very turbid — water must be clear
UV radiation and thermal heating together inactivate bacteria and viruses. Tested in field conditions across many countries with consistent 2–3 log reduction in E. coli.
Effectiveness improvements:
- Place bottle on a reflective surface (aluminum foil) to double UV exposure
- Use a solar cooker or parabolic reflector to heat water to 55°C+ — pasteurization occurs (milk pasteurization is 72°C for 15 seconds; 63°C for 30 minutes; even 55°C for 1 hour inactivates most pathogens)
SODIS requires no fuel and no chemicals. The bottle is the only equipment. It is the most accessible disinfection method for dispersed rural households.
Wood ash and lime
A traditional method with some scientific basis: adding wood ash to water raises pH above 11, which kills many bacteria. Lime (calcium hydroxide) achieves the same effect more reliably.
Dose: 1 teaspoon of hydrated lime per 10 liters, stir, allow 1 hour. Then add vinegar or allow to settle — high pH water is unpleasant to drink. This method is less reliable than chlorination and should be a last resort.
Safe Storage
Treated water is recontaminated if stored improperly.
Safe storage vessel requirements:
- Narrow opening (less than 50 mm) — cannot introduce a dirty hand
- Fitted lid or cover
- A tap or spigot for dispensing — never pour from the vessel
- Washed with dilute bleach weekly
The “safe water chain”: Protect water at every step from source to mouth. Treat it, store it safely, dispense it hygienically, and wash hands before drinking. Breaking any link in the chain can negate all treatment steps. The clean cup matters as much as the clean water.
Simple field test: If you have no testing equipment, take the treated water source and allow it to stand in a clear vessel for 24 hours. Clear, no sediment, no color, no odor = likely safe. Cloudy, colored, or foul-smelling = treat further. This is not a substitute for pathogen testing but catches gross problems.