From the slums of Nairobi to the most remote corners of the Andes, clean water is a necessity.  But without significant investment in sanitization infrastructure, this resource is incredibly difficult to provide. But after years of research and development, and cheap, reusable, and nearly universally effective system of water treatment has been developed.

It’s called SODIS, or Solar Water Decontamination, and requires only a source of fresh water, a clear plastic bottle, and sunlight. It sounds impossible, but the science behind it is solid: sunlight contains a great deal of ultraviolet radiation, which disrupts the genetic processes of bacterial reproduction and can destroy the microorganisms cell walls. Ultraviolet radiation also has an ionizing effect on the water itself, creating highly oxidative compounds like hydrogen peroxide, a common household antiseptic. While it may be difficult to believe, all it takes is six hours sunlight to destroy the microbial inhabitants in a bottle of water, rendering it fit for human consumption.

While the physical and psychological benefits of an essentially inexhaustible, reliable water source are obvious, clean water also carries an economic and political advantage. In most city slums, public utilities do not provide water service. Instead, private firms generally redistribute public water at a significant mark up, and with severely compromised quality. But with the ability to store and purify their own rainwater, SODIS gives slum residents a safer, and far cheaper alternative, allowing them to concentrate their income for other necessities.

The method is not without its detractors, though. Some urban residents refuse to employ the practice, either out of distrust of outsiders, the fact that it seems too easy to be true, or because they’ve been able to get by on their current water supply without significant trouble in the past. But the data are overwhelmingly in favor of the practice, with diarrhea cases reduced by anywhere from 30 to 80 percent in areas where SODIS has become widespread.

Still, the practice is not a universal solution. Though SODIS creates lethal conditions for many microbes, it does little to treat chemically contaminated water. While some larger toxins do break down in the presence of ultraviolet light, their byproducts can still be unhealthy to ingest.  The amount of available sunlight can also theoretically limiting the effectiveness of SODIS, but because a vast majority of the developing world lies in close proximity to the equator, this is unlike to be a significant concern.

Additional questions have been raised recently about the possible health risks posed by the repeated use of the PETE plastics that SODIS employes (glass containers block some UV light, as do many other clear plastics) to purify water. Studies indicate chemical levels after a single SODIS use are well within safe limits, but some questions remain on the safety of long-term use.  Fortunately a far safer plastic, LDPE, has been proven effective in practice, though it is not nearly as ubiquitous as its potentially phthalate-leaching counterpart.