This process - generally referred to as SODIS - harnesses solar energy to disinfect drinking water, using only sunlight and plastic PET bottles. SODIS is cost effective, efficient and recommended by the World Health organisation as an effective method for decentralised water treatment. It is used as a viable method for household water treatment and storage and is already being applied in many developing countries.

When polluted drinking water is exposed to sunlight organisms which cause sickness are destroyed. Solar radiation has three main effects which can inactivate pathogenic organisms. UV-A interferes directly with the metabolism and destroys cell structures of bacteria. UV-A (wavelength 320-400nm) reacts with the oxygen in the water, creating highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens. Infrared radiation heats the water. If the water temperatures raises above 50°C, the disinfection process becomes up to three times as fast.

For SODIS to work effectively it requires a threshold solar radiation intensity of at least 500 W/m2 at a water temperature of about 30°C, for approximately 5 hours. This would create energy of 555 Wh/m2 in the range of UV-A and violet light, 350nm – 450nm, the equivalent to about 6 hours of European midday summer sunshine. When the water temperature exceeds 45°C synergetic effects of UV radiation and temperature create a more efficient disinfection. The process is very simple and can be carried out in a domestic environment. The contaminated water is put into the transparent bottles, to increase oxygen saturation the bottles can be filled up three quarters, shaken for 20 seconds then filled completely. Highly cloudy water (turbidity higher than 30 NTU) should originally be filtered. The full bottles are then exposed to the sun, after which they are ready for consumption. To prevent re-contamination the water should be stored in the same bottles. SODIS is particularly useful when fuel or cookers are not available and even when these can be obtained, is more economical and environmentally friendly. SODIS is only negated by a limitation of bottles or exceptionally turbid water and of course it must be insured that the water is left in the sun for long enough. SODIS is a good option for water disinfection in disaster areas, however the distribution of disinfectant tablets may at times be more practical. Certain cautions must be taken in regards to ensure safe and successful disinfection. 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. If the sunlight is less strong, due to overcast weather or a less sunny climate, a longer exposure time in the sun is necessary. Other things that should be considered are the bottle material; Some glass or PVC materials may prevent ultraviolet light from reaching the water. Commercially available bottles made of glass or PET are recommended. The handling is much more convenient in the case of PET bottles. Polycarbonate blocks all UVA and UVB rays, and therefore should not be used. The Aging of plastic bottles, can affect SODIS efficiency. Heavily scratched or old, blind bottles should be replaced. The Shape of Containers is a factor. The intensity of the UV radiation decreases rapidly with increasing water depth. At a water depth of 10cm and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. PET soft drink bottles are often easily available and thus most practical for the SODIS application. Oxygen; Sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute in the destruction process of the microorganisms. Under normal conditions (rivers, creeks, wells, ponds, tap) water contains sufficient oxygen (more than 3 mg Oxygen per litre) and does not have to be aerated before the application of SODIS.

Posted under House & Home, Renewable Energy