Solar water disinfection, also known as SODIS is a free method of disinfecting water using only sunlight and plastic PET bottles. SODIS is recommended by the World Health Organization (not my favorite people) as a viable method for household water treatment and safe storage. SODIS is in use in many third world countries.
To use Sodis you fill Pet plastic bottles with contaminated water, and then lay them in the sun. The best place is a corrugated metal roof. Be sure to shake your bottles before placing in sun. Pet plastic bottles can be identified by the recycling number 1, usually located on the bottom. The full bottles need to be exposed to sun for at least six hours. Sometimes this may require that you put them out a second day.
Sodis will not work if the water is overly turbid, you should be able to read a medium print though the full bottle. All labels must be removed from the bottles and the bottle should be in good shape, not a lot of scratches.
Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms:
UV- light interferes directly with the metabolism and destroys cell structures of bacteria.
UV- light (wavelength 320-400 nm) reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens.
Cumulative solar energy heats the water. If the water temperature rises above 122°f, the disinfection process is three times faster.
The following issues should also be considered:
Bottle material: Some glass or PVC materials may prevent ultraviolet light from reaching the water. Commercially available bottles made of PET are recommended. Polycarbonate blocks all UV light, and therefore should not be used.
Aging of plastic bottles: SODIS efficiency depends on the physical condition of the plastic bottles. Heavily scratched or old, damaged bottles should be replaced.
Shape of containers: the intensity of the UV light decreases rapidly with increasing water depth. PET soft drink bottles are often easily available and thus most practical for the SODIS application.
Leaching of bottle material: There has been some concern over the question whether plastic drinking containers can release chemicals or toxic components into water, a process possibly accelerated by heat. The Swiss Federal Laboratories for Materials Testing and Research have examined the diffusion of adipates and phthalates (DEHA and DEHP) from new and reused PET-bottles in the water during solar exposure. The levels of concentrations found in the water after a solar exposure of 17 hours in 140°F water were far below WHO guidelines for drinking water and in the same magnitude as the concentrations of phthalate and adipate generally found in high quality tap water.
Concerns about the general use of PET-bottles were also expressed after a report published by researchers from the University of Heidelberg on antimony being released from PET-bottles for soft drinks and mineral water stored over several months in supermarkets. However, the antimony concentrations found in the bottles are orders of magnitude below WHO and national guidelines for antimony concentrations in drinking water. Furthermore, SODIS water is not stored over such extended periods in the bottles.