Solar water disinfection

Solar Water Disinfection, or SODIS, is a method of disinfecting water using only sunlight and plastic polyethylene terephthalate bottles.

Principle
By filling a polyethylene bottle with contaminated water and leaving it in bright sunlight, it is treated through three synergistic radiation mechanisms.


 * UV-A (wavelength 320-400nm) which react with oxygen dissolved in the water produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive forms of oxygen kill the microorganisms.
 * UV-A also interferes with the reproduction cycle of bacteria by damaging their DNA
 * Infrared heating the water. If the water temperatures raises above 50°C, the disinfection process is three times faster.

The combined effect of all three mechanisms is greater than that of each of the individual components.

Cautions
If the water bottles are not left in the sun for the proper length of time, the water may not be safe to drink and could cause illness.

There is concern over whether plastic drinking containers can leach 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 60°C 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. The antimony concentrations found in the bottles however are orders of magnitude below WHO and national guidelines for antimony concentrations in drinking water.

Applications
SODIS is valuable for boiling water, where fuel or cookers are unavailable or prohibitively expensive. Even where fuel is available, however, SODIS is a more economical and environmentally friendly option.

In theory, the process could be used in disaster relief or refugee camps. However, supplying bottles may be more difficult than providing equivalent disinfecting tablets containing chlorine, bromine, or iodine. Additionally, in some circumstances, it may be difficult to guarantee that the water will be left in the sun for the necessary time.

SODIS has been applied in several communities in Brazil, one of them being Prainha do Canto Verde north of Fortaleza. There, the villagers have been purifying their water with the SODIS method. It is quite successful, especially since the temperature during the day can go beyond the 40ºC (100ºF) and there is a limited amount of shade.