SODIS Clean Water From The Sun

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Solar radiation from the sun can purify (disinfect) water and make it safe for drinking from harmful bacteria.This method is called SODIS

SODIS is an effective method for treating water where heating fuel or cookers are unavailable. Even where fuel is available, SODIS is a more economical and environmentally friendly option on the larger scale.

The application of SODIS is limited if enough bottles are not available, or if the water is highly cloudy. In fact, if the water is highly turbid, SODIS cannot be used alone; additional filtering is then necessary.

Exposure to sunlight has been shown to deactivate diarrhea-causing organisms in polluted drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms:

· UV-A interferes directly with the metabolism and destroys cell structures of bacteria.
· UV-A (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 (including the infrared radiation component) heats the water. If the water temperatures rises above 50 °C (122 °F), the disinfection process is three times faster.

At a water temperature of about 30 °C (86 °F), a threshold solar irradiance of at least 500 W/m2 (all spectral sunshine light) is required for about 5 hours for SODIS to be efficient. This dose contains energy of 555 Wh/m2 in the range of UV-A and violet light, 350–450 nm, corresponding to about 6 hours of mid-latitude (European) midday summer sunshine.

At water temperatures higher than 45 °C (113 °F), synergistic effects of UV radiation and temperature further enhance the disinfection efficiency. Recent work has shown that common table salt (NaCl) is an effective flocculation agent for decreasing turbidity for the SODIS method as well.
SODIS Process For Emergency Household Application
· Use colourless, transparent PET water or soda pop bottles (2 litre or smaller size) with few surface scratches are chosen for use. The labels are removed and the bottles are washed before the first use.

· Water from contaminated sources is filled into the bottles. To improve oxygen saturation, bottles can be filled three-quarters, shaken for 20 seconds (with the cap on), then filled completely and recapped. Very cloudy water with a turbidity higher than 30 NTU must be filtered prior to exposure to the sunlight.
· Filled bottles are then exposed to the sun. Bottles will heat faster and to higher temperatures if they are placed on a sloped sun-facing corrugated metal roof .
· The treated water can be consumed directly from the bottle or poured into clean drinking cups. The risk of re-contamination is minimized if the water is stored in the bottles. Refilling and storage in other containers increases the risk of contamination.

Suggested treatment schedule

Weather conditions Minimum treatment duration
Sunny (less than 50% cloud cover) 6 hours
Cloudy (50–100% cloudy, little to no rain) 2 days
Continuous rainfall Unsatisfactory performance

Tips & Warnings For Purifying Water Via SODIS
The PET recycling mark shows that a bottle is made from polyethylene terephthalate, making it suitable for solar water 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.
Some glass or PVC materials may prevent ultraviolet light from reaching the water. Commercially available bottles made of 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.

SODIS efficiency depends on the physical condition of the plastic bottles, with scratches and other signs of wear reducing the efficiency of SODIS. Heavily scratched or old, blind bottles should be replaced.

The intensity of the UV radiation decreases rapidly with increasing water depth. At a water depth of 10 cm (4 inches) and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. PET soft drink bottles are often easily available and most practical for the SODIS application.

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/L of oxygen) and does not have to be aerated before the application of SODIS.

There has been some concern over the question of 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 in 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 (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

Once removed from sunlight, remaining bacteria may again reproduce in the dark. Adding just 10 parts per million of hydrogen peroxide is effective in preventing the regrowth of wild Salmonella.

Solar water disinfection does not remove toxic chemicals that may be present in the water, such as factory waste.

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