It is the cheapest water treatment there is: a clear plastic bottle and a day of sunshine. Fill a transparent PET bottle with clear water, lay it on its side in full sun, and the sun’s ultraviolet light, helped by the warmth, kills the microbes behind most diarrhoeal disease. It costs nothing but bottles and sun — and because it depends on clear water, strong sun and a daily habit, that is exactly where its limits lie.
At a glance Origin: that sunlight disinfects water was shown by Aftim Acra at the American University of Beirut (early 1980s); the household SODIS method was developed and spread by Martin Wegelin and the Sandec group at Eawag, Switzerland (from 1991) · Maturity: Scaled — used by millions, especially across Latin America · Effort: free (bottles and sun), but the daily habit is the real work · Licence: open — Eawag publishes the method free.
How it works. Use a clear, colourless PET bottle no bigger than about two litres (2 L), clean and not scratched — deeper water stops the sunlight reaching the bottom. Check the water is clear enough to read newspaper print through the full bottle (below about 30 NTU); if it is cloudy, clarify it first — let it settle, strain it through clean cloth, or clarify with crushed moringa seeds — then check again. Fill the bottle to the top, cap it, and lay it on its side in full sun — a roof or a sheet of corrugated metal is ideal, because it reflects heat back — for at least six (6) hours of strong sun, or two (2) full days if more than half the sky is clouded over. Drink it fresh, poured into a clean cup just before drinking, and treat a new batch each day.
What to expect — and what it does NOT do. It does NOT work on cloudy or muddy water — the particles shield the microbes from the light, so turbid water must be cleared first; that is the honest two-step, clarify then SODIS, and one without the other is not enough. It needs strong sun: in a long overcast or rainy spell, or a high-latitude winter, the water may never get enough, and another method (boiling, filtering) is needed. The hardest limit is human, not technical — it only helps if it is actually used, every day. In a careful trial in Bolivia (Mäusezahl and colleagues, 2009), despite heavy promotion only about a third of households used it consistently, and childhood diarrhoea did not fall significantly; where families do keep it up, trials have shown real reductions. It also does NOT remove chemical contamination — arsenic, fluoride, salt and industrial chemicals pass straight through; it is for microbial safety only. And disinfected water poured into a dirty cup, or stored badly, is unsafe again.
About the plastic bottle itself. Does sun on a plastic bottle put something into the water? Two separate questions. The chemicals long studied — the phthalate DEHP, and antimony from the PET — have been measured, even in heavily reused bottles left in strong sun, well below WHO drinking-water limits (DEHP around 0.1 to 0.7 micrograms per litre, against a WHO guideline of eight (8)), close to the trace amounts found in water kept in glass. Microplastics are the newer, less-settled concern, and here the mechanism cuts both ways: the same sunlight that disinfects the water also breaks down the plastic surface and sheds tiny particles into it — recent work measured the buildup at roughly fourteen to twenty (14–20) micrograms per litre over a few weeks of sun, somewhat above ordinary tap water, with the science still developing. The honest balance: where the alternative is drinking water carrying the microbes behind diarrhoeal disease — a real and immediate danger, above all to children — killing those microbes is the larger, proven benefit, and it outweighs the small, uncertain plastic risk. Reduce that risk by replacing bottles as they get scratched, cloudy or brittle — a degraded bottle sheds more — rather than reusing one indefinitely.
Who made it. Named together: Aftim Acra and colleagues at the American University of Beirut, who showed in the early 1980s that sunlight disinfects water; and Martin Wegelin and the Sandec group at Eawag, who turned that into the simple household SODIS method from 1991 and carried it to communities.
Used SODIS — clear bottles in the sun? Tell us how it went — what you used it on, how it performed, whether it worked where you are, whether you kept it up day to day, what you’d change, and roughly where and when. We want the honest result, including what didn’t work. Your contact details stay private; we read and check every report before anything is published, and we’ll never publish a detail you’ve asked us to keep private.
Sources: Eawag/Sandec’s free SODIS guide and method pages (sodis.ch; eawag.ch), including the Spanish-language edition read in-language; the field-effectiveness trial — Mäusezahl et al., “Solar drinking water disinfection (SODIS) to reduce childhood diarrhoea in rural Bolivia,” PLoS Medicine (2009), doi.org/10.1371/journal.pmed.1000125; on the plastic itself — the phthalate/antimony migration measurements (Eawag; summarised at hip.fhi360.org) and the sunlight-driven microplastic-release study (RSC Soft Matter, 2025, doi.org/10.1039/d5sm00469a).
