Back at University [in 2004] we did a project investigating water purification systems for developing countries. I have summarised some of the interesting sections that I worked on here…
It is estimated that over a quarter of the world’s population do not have access to clean drinking water(1). Even in water-rich countries contamination of drinking water supplies promotes the often easily preventable spread of waterborne diseases. Additionally, the communities affected are frequently unaware of the link between water quality, health and sanitation. The need is simple, but the solution is not necessarily as straight forward, with hundreds of researchers around the world working towards technically, culturally and socially appropriate methods of water purification.
Solar pasteurisation
Pasteurisation involves the purification of water by heating it to a certain temperature for a specific length of time. Contrary to what many people believe, it is not necessary to boil water to effectively purify it. Extensive research has found that heating water to 65ºC for 6 minutes, or to a higher temperature for a shorter time, will kill all germs, viruses, and parasites(2).
Even more surprising is the effectiveness of lower temperatures at making water safe to drink. Some research suggests that samples with a maximum temperature of only 55ºC became completely disinfected within 7 hours(3).
Solar disinfection (also called ‘SODIS’ or ‘Solar UV’)
Ultraviolet (UV) filters have been successfully used for many years in the developed world, however, their relative costly nature and need for a reliable electricity source has prevented a transition to use in developing countries. The doses of UV created in these filters are much higher than those produced by the sun, but the time spent in the filter is quite short. This fact led researchers to experiment with the prolonged exposure of water to the sun’s UV light as an effective method of purification.
The advantage of the technology is its simplicity as it only requires relatively inexpensive 1-2 litre clear plastic (PET) bottles, or something similar, to hold the water. Field testing of the technology has indicated its appropriateness to climates with average solar intensity of above 500W/m2 for 3-5 hours of the day. Testing in Haiti showed a 52% effectiveness for one day exposure and a 100% effectiveness for a two day exposure(4).
Combined solar thermal/solar disinfection
Interestingly, the treatment of water using solar disinfection is enhanced as water temperature increases. A group of researchers from the Nestle Product Technology Centre experimented with this idea using relatively cheap food packaging material to make two types of containers(5). The ‘pouches’ had a UV transmitting upper layer, with the back layers made from either metallised plastic to reflect light or from black plastic to increase temperature. The pouches were protected from the wind with small three-sided reflective boxes.
Although the experiments were conducted in October (going into winter) in Connecticut, Ohio, the samples showed positive results after six hours of exposure to sunlight. Their results indicated that light reflection (in the reflective packages) was more important than a modest temperature increase (in the packages with a black backing).
Experiments
To experiment with some of the above ideas I constructed several ‘low tech’ solar water purifiers. Pasteurisation temperatures were reached relatively easily with simple flat plate and parabolic collectors (below left)… in fact I got up to 80 degrees fairly quickly! Other techniques using the combined method can be constructed using insulated reflective boxes with water containers inside (below right).
(1) Murcott, S, Clean water for 1.7 billion people?, Submission for ‘Development by Design’ Workshop, July 22, 2001.
(2) Ciochetti, D. A., Metcalf, R. H., Pasteurization of Naturally Contaminated Water with Solar Energy, Applied and Environmental Microbiology, 47:223-228, 1984
(3) Joyce, T.M., McGuigan, M., Elmore-Megan, M., Conroy, R.M., Inactivation of fecal bacteria in drinking water by solar heating, Applied and Environmental Microbiology, 62:399-402, 1996
(4) Oates, P.M., Shanahan, P., Polz, M.F., Solar disinfection (SODIS): simulation of solar radiation for global assessment and application for point-of-use water treatment in Haiti, Water Research, 37:42-54, 2003
(5) Walker, D.C., Len, S., Sheehan, B., Development and evaluation of a reflective solar disinfection pouch for treatment of drinking water, Applied and Environmental Microbiology, 70:2545-2550, 2004
