Water Purification Methods

Contaminated water kills faster than almost any other environmental hazard, primarily through diarrheal diseases. Multiple purification methods exist, each with different equipment requirements, effectiveness, and fuel costs.

Key Concepts

• —Boiling: bringing water to a rolling boil for one minute (three minutes above 2000 m elevation) kills all pathogens reliably; it addresses biological but not chemical contamination.
• —Solar disinfection (SODIS): filling clear PET bottles and leaving them in direct sunlight for six hours (two days if cloudy) inactivates most pathogens through UV-A radiation and heat.
• —Slow sand filtration: biological activity in the top layer of a slow sand filter (schmutzdecke) degrades pathogens over time; more effective than simple physical filtration alone.
• —Chemical treatment: household bleach (5-8% sodium hypochlorite) at 2 drops per liter, or iodine tablets, disinfect water but leave taste and do not remove turbidity or chemicals.
• —Turbidity removal: settling suspended particles for 30 minutes, or pre-filtering through cloth or sand, dramatically improves the effectiveness of all subsequent purification steps.

Practical Guide

1. 1.Pre-filter all turbid water through several layers of tightly woven cloth before applying any other method; turbidity protects pathogens from UV and chemical disinfectants.
2. 2.To boil at scale, build a rocket stove from bricks or clay to reduce fuel use by 60-70% compared to an open fire; use a lid to reach boiling faster.
3. 3.Set up a SODIS station using salvaged clear PET bottles filled with settled water, laid on a reflective surface (corrugated metal roof) angled toward the sun.
4. 4.Construct a slow sand filter in a 200-liter barrel: 10 cm gravel at the bottom, 60 cm coarse sand, then 60 cm fine sand; allow two to four weeks for the biological layer to mature before trusting output.
5. 5.Add 2 drops of 5% bleach per liter of clear water, stir, and wait 30 minutes before drinking; double the dose for turbid water and wait an hour.
6. 6.Test treated water quality by smell (chlorinated water should have a faint bleach smell) and by observing community health; diarrhea rates are a direct proxy for water safety.
7. 7.Store treated water only in clean, covered containers to prevent recontamination; label containers and establish a community protocol against dipping hands into storage.