Concerns that bottle scratches negatively influence the quality of solar water disinfection (SODIS) are unfounded, a new NGO study reveals. Ambient temperature and bottle diameter, however, do greatly influence disinfection effectiveness, the study finds.
The NGO Nature Healing Nature has been promoting SODIS for many years in Africa and South America. While working in Togo, staff heard that a respected nonprofit organization had stopped promoting SODIS because they noticed many of the plastic bottles being used for SODIS were noticeably scratched after six months of use. They hypothesized the scratches would interfere with UV transmission and thus the effectiveness of purification. Rather than ignore their hypothesis, Nature Healing Nature decided to investigate if increased bottle scratch density results in decreased solar disinfection quality.
Test bottles (three scratched). Photo: Nature Healing Nature
Three tests were conducted using plastic bottles of varying size and density of scratches. The first two tests were conducted in the summer when ambient temperatures exceeded 35⁰C. Pre-SODIS water e-coli contamination exceeded 30,000 cu/100ml (much greater than typically found in remote villages and urban slums). Disinfection was found to be complete after six hours of exposure to direct sunlight regardless of scratch density. However, the bottle water temperatures in the summer tests exceeded 50⁰C for several hours of the six hour exposure to sunlight. In such circumstances, pasteurization would have been the driving force for disinfection regardless of scratch intensity.
A third test was conducted in cold weather with ambient temperatures above freezing and below 20⁰C. Bottle water temperatures never exceeded 27⁰C. Complicating the third test however, was a transition in sky conditions from clear to hazy, three hours into the initial six hours of sunlight exposure. Although samples were drawn for analysis after six hours of sunlight exposure, the bottles were left outside for 30 consecutive hours. The last six hours of sun exposure were under clear skies. E-coli and other fecal coliforms were found to be present in all bottles regardless of scratch density after the initial six hours of exposure. Although E-coli were no longer present after thirty hours, other fecal coliforms were still present in large, however reduced, quantities.
Nature Healing Nature recommends several independent winter studies be done to confirm their results.
Based on the study, Nature Healing Nature developed the following SODIS field-practice recommendations:
- Keep the bottles as clean as possible, but it is “OK” if the bottles are scratched.
- Use the newest and cleanest bottles for SODIS and older, heavily scratched bottles for storage and daily use.
- Use bottles less than 10 cm in diameter (typically bottles of 1.5 litres or less).
- Do whatever possible to increase the bottled water temperature during SODIS (orientation, placement on a hot surface, painting one side of the bottle black, etc.).
- In winter time, increase the exposure to two days (as on cloudy days).
- If the beneficiaries can not find enough bottles to supply their daily needs, suggest they store drinking water for a full three days before consumption
Read the full report:
Illian, M., Cikhart, M. and Henri, A. (2011). The effect of bottle scratches on SODIS water disinfection : a field test on how bottle scratches affect the quality of solar water disinfection. Houston, Texas, Nature Healing Nature. 17 p. : 12 fig., 4 graphs. Includes references. Download the full report.
For more information contact:
Alex Henri, Project Assistant, Nature Healing Nature, USA, alex_henri@yahoo.com, http://www.naturehealingnature.org/
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