UV disinfection and solar water disinfection (SODIS)

Ultraviolet light has more energy than visible light. When it hits microorganisms, it damages their genetic material, their DNA or RNA. The pathogens are not necessarily destroyed at once, but they can no longer multiply and can no longer cause disease.

UV-C devices work with a lamp or LEDs that emit very high-energy UV-C light. This light practically does not reach the Earth's surface naturally, because the atmosphere absorbs it. The devices therefore deliver a high dose in a short time. SODIS, on the other hand, uses the sun's weaker UV-A light plus some warming of the water over many hours.

Both methods share the same weak spot: light must reach the pathogens. Particles absorb and scatter the light and cast tiny shadows in which germs can survive. That is why UV and SODIS only work with clear or well pre-clarified water.

SODIS was developed by the Swiss research institute Eawag for households in sunny regions. The manual specifies clear, undamaged PET bottles of at most 2 litres. Coloured, badly scratched or larger bottles let too little light through or are too thick for the sun to reach all of the water equally well. Labels are removed.

Check clarity beforehand with the newspaper test: stand the filled bottle on a headline and look down through it. If the text is not readable, the water is too cloudy and must first settle or be pre-filtered. Then lay the closed bottles flat on a reflective surface such as corrugated metal or an emergency blanket – flat, so that the layer of water is thin.

The exposure time is 6 hours of full sun. When heavily overcast, two consecutive days are needed. SODIS does not work during continuous rain. Afterwards, drink straight from the bottle or pour into a clean vessel without mixing the water with raw water.

Portable UV-C devices come as wands that you dip into the water or as bottle caps with built-in LEDs. According to manufacturers and the US Centers for Disease Control and Prevention (CDC), they work in clear water against bacteria, viruses and protozoa. The volume, duration and method are set by the manufacturer – you must follow them exactly.

Typically you fill the specified volume into a vessel with a suitable opening, immerse the lamp and move it during the cycle so that all the water is irradiated. The device indicates when the cycle is complete. If it is interrupted, for example because the battery is flat, the water counts as untreated.

The weak spot is energy: without charged batteries the device does not work, and cold weakens many batteries. Spare batteries or a second method as a backup are therefore sensible. Do not look into the lit UV-C lamp – UV-C light can damage eyes and skin.

Water can look clear and still be inadequately treated if fine turbidity shades the light. If in doubt, pre-clarify or choose another method, such as boiling. With SODIS, follow the conditions exactly: the effect on bacteria is the best documented, while viruses and parasites vary in sensitivity by species.

UV and SODIS leave no residual protection. Keep treated water in the same clean vessel and drink it soon. If it comes into contact with dirty hands, vessels or raw water, it is contaminated again.

UV light does not work against chemicals, heavy metals and salt. Water from contaminated sources, flood water suspected of carrying fuel, or seawater does not become drinkable this way.

SODIS is ideal when it is sunny, the water is clear and time does not matter – for example at a fixed camp or at home in a sunny region when fuel is short. It costs almost nothing, needs no chemicals and does not change the taste. Rotating several bottles can cover a small daily requirement.

UV-C devices suit trips with clear water from springs and streams, when things need to be quick and no waiting time is possible. They are light but depend on batteries and an intact device.

Both are unsuitable for cloudy water, for large volumes and whenever chemical contamination is suspected. SODIS is also ruled out in rain, persistent overcast, in the shade and with glass or coloured bottles. Then boiling or chemical disinfection are the better options.