★☆☆☆☆ Beginner 6 hours (mostly passive)

Solar Water Disinfection (SODIS)

Use the sun's UV rays to kill pathogens in water. Requires nothing but a clear bottle and 6 hours of sunshine — endorsed by WHO.

What You'll Need

Clear PET plastic bottle 2-liter soda bottle works best, must be clear, not colored
Pre-filtered water Filter through cloth first to remove sediment

01

Pre-filter the water

Filter water through a cloth, shirt, or coffee filter to remove visible particles. SODIS only works if the water is relatively clear — turbid water blocks UV penetration. If you can't read text through the bottle, it's too cloudy.
02

Fill the bottle three-quarters full

Leave a quarter of the bottle empty. Cap it and shake vigorously for 20 seconds. This dissolves oxygen into the water, which combines with UV radiation to create reactive molecules that destroy pathogens more effectively.
03

Top off and cap

Fill the bottle completely and screw the cap on tight. Lay the bottle on its side on a reflective surface — a sheet of corrugated metal, aluminum foil, or even a light-colored rock works.
04

Expose to direct sunlight

Place in full sun for at least 6 hours on a sunny day, or 2 full days if partly cloudy. UV-A radiation and heat work together to kill bacteria, viruses, and parasites. The bottle should be horizontal, not vertical, to maximize UV exposure.
05

Check temperature for extra safety

If the water reaches 50°C (122°F), the process is 3x more effective. On hot days with the bottle on a dark surface, this happens naturally. You can feel the bottle — if it's uncomfortably hot to hold, you're above 50°C.

This method is used by 6+ million people globally and is WHO-recommended for emergency water treatment.
PET bottles are labeled with recycling symbol #1. Don't use bottles that are scratched, cloudy, or colored.
In a group, set up multiple bottles in rotation: expose new ones each morning while drinking yesterday's.
SODIS does not remove chemical contaminants. Only use on biologically contaminated water.