Guide · Water

Solar disinfection (SODIS)

If you have clear plastic bottles and sunshine, you can make water safer to drink without fuel or chemicals. This method, often called SODIS, uses sunlight's UV rays and heat to kill most germs.

What you need

• Clean, clear plastic bottles, ideally PET (the type used for fizzy drinks and bottled water). Remove labels.
• Bottles of 2 litres or less. Larger bottles let less light through to the middle.
• Clear water. Cloudy water blocks the sunlight.
• A sunny spot, preferably on a dark or reflective surface like a metal sheet or roof.

How to do it

1. Clear the water first. Let it settle, then pour off the top, or filter it through a clean cloth. A good test: stand the full bottle on a page of large print. If you can't read the words through it, the water is too cloudy and needs more settling and filtering.
2. Fill the bottle three-quarters full, screw on the lid and shake for about 20 seconds to add oxygen. Then fill it to the top.
3. Lay the bottle on its side in full sun, so the light passes through the shallowest depth of water. Make sure nothing shades it.
4. Leave it for at least 6 hours in bright sunshine.
5. If more than half the sky is cloudy, leave it for two full days.
6. Drink straight from the bottle, or pour into a clean cup. The water is best used within a day or two.

When SODIS won't work

• During continuous rain or heavy overcast — collect rainwater instead.
• With cloudy or muddy water that you can't clear.
• In glass bottles or coloured, scratched or very old plastic bottles. Glass blocks much of the UV.
• At night, obviously. Start early in the day.

What it doesn't do

• It doesn't remove chemicals, fuel, heavy metals or salt.
• It's not as reliable as boiling. If you have the fuel and a pot, boiling is better.
• It's less effective against some parasites, so pre-filtering helps.

Tips

• Run two sets of bottles: one in the sun, one ready to drink.
• Replace bottles when they become scratched or cloudy.
• Mark treated and untreated bottles so you don't mix them up.

SODIS is a useful fallback, especially in sunny places. Used carefully, it greatly cuts the risk of illness from contaminated water.

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