SODIS Solar Water Purification

SODIS (Solar Disinfection) uses UV radiation from direct sunlight to kill pathogens in water. A clear plastic bottle in direct sun for 6+ hours achieves a 3-4 log reduction of bacteria, viruses, and protozoa — enough to make contaminated water drinkable.

SODIS is WHO-endorsed, proven effective in field studies, and requires zero gear beyond clear plastic bottles. It is not as fast as boiling and does not remove chemicals, but it works when fuel is unavailable, filters are broken, and chemicals are exhausted.

How SODIS works

• ☐ UV-A radiation (320-400 nm) from sunlight damages pathogen DNA
• ☐ Heat from solar warming boosts effectiveness (synergy effect above 45°C water temp)
• ☐ Requires CLEAR plastic (PET bottles only — PVC and glass block UV)
• ☐ Water must be clear enough to read newspaper print through at the bottle top
• ☐ Works for bacteria, viruses, protozoa (bacteria and viruses = 99.9%+, protozoa = 99%+ with longer exposure)
• ☐ Does NOT remove chemicals, heavy metals, or sediment
• ☐ WHO-tested and field-validated in 30+ countries with proven disease reduction

SODIS is not the best method in most situations — boiling, filtering, or chemical treatment is usually faster and more reliable. But SODIS excels in specific scenarios where other methods fail.

Best use cases

• ☐ No fuel available (cannot boil)
• ☐ Filter is broken or clogged
• ☐ Chemical treatment is exhausted
• ☐ Long-term sustained scenarios (weeks+) where resupply is impossible
• ☐ Tropical/subtropical climates with abundant sun
• ☐ Refugee/disaster scenarios with broad distribution needs
• ☐ NOT for: cold weather, winter, northern latitudes, cloudy climates, contaminated water sources
• ☐ NOT for: water containing industrial chemicals or heavy metals

SODIS is simple: fill, aerate, expose, consume. The whole process takes 6+ hours in full sun but is fully passive — no attention needed.

Step-by-step SODIS

• ☐ STEP 1 — Use only clear PET plastic bottles, 2 liters or smaller (larger bottles have too much depth for UV penetration)
• ☐ STEP 2 — Pre-filter water through a cloth if cloudy or contains sediment
• ☐ STEP 3 — Fill bottle 3/4 full, cap, shake for 20 seconds to aerate (boosts effectiveness)
• ☐ STEP 4 — Top off the bottle completely to displace any remaining air pockets
• ☐ STEP 5 — Lay bottles on their side (maximizes UV exposure surface) on a dark surface if possible
• ☐ STEP 6 — Expose to DIRECT sunlight for 6 hours minimum (full sun day)
• ☐ STEP 7 — For cloudy weather, extend to 2 full days of exposure
• ☐ STEP 8 — Drink directly from the bottle to avoid recontamination from dirty hands

SODIS has real limits. Understanding them prevents dangerous mistakes.

SODIS limitations

• ☐ Does NOT work through glass, PVC, or colored plastic (only clear PET)
• ☐ Does NOT remove chemicals, heavy metals, pesticides, or sediment
• ☐ Cloudy water blocks UV — pre-filter or settle before use
• ☐ Cold water reduces effectiveness — water should reach at least 20°C ideally
• ☐ Cyanobacteria and some algae toxins are NOT destroyed by UV
• ☐ Cryptosporidium is resistant — needs longer exposure (10+ hours)
• ☐ Fails in winter / northern latitudes / cloud cover without extended time
• ☐ Single-use: do not reuse the same PET bottle more than ~12 times (plastic degrades)

SODIS requires almost nothing. A few clear bottles and a sunny spot is all you need.

Kit essentials

• ☐ 5+ clear 2L PET bottles (label removed for maximum UV)
• ☐ Dark cloth or sheet for setup (absorbs heat, boosts effectiveness)
• ☐ Cotton bandana or cloth for pre-filtering cloudy water
• ☐ Small notebook to track exposure time
• ☐ Aluminum foil for solar reflector setup (amplifies UV)
• ☐ Backup chemical treatment (Aquatabs) for cloudy days  → Buy
• ☐ Backup filter (Sawyer Mini) for pre-treatment of very dirty water  → Buy