Geoscience Reference
In-Depth Information
The treatment of water with solar radiation was practiced in ancient India more than four
thousand years ago, and the modern practice of SODIS using clear water in plastic bottles
placed in the sun was developed in Lebanon in the 1980s. The SODIS process uses a combina-
tion of UV-A radiation and heat from sunlight to kill pathogens in drinking water. The standard
procedure is to fill used, clean, clear PET soda bottles with water and to lay them flat in full
sunlight for six hours. Cloudy skies require two days of daylight exposure and the process does
not work at all during rainy weather, when rainwater collection is recommended instead.
SODIS requires clear water and works best at water depths of 4 inches or less, so it is not
suitable for large containers of water. If the water is murky enough to prevent you from reading
newsprint through the filled PET bottle, it won't work for SODIS. Clear plastic bags and glass
bottles also work well for SODIS, but window glass does not. Ninety-nine percent of glass
bottles let most of the sun's UV radiation pass through the glass, but typical window glass fil-
ters out the UV-A radiation, so it won't work for SODIS.
In hotter climates, painting one half of the SODIS bottle black (to act as a solar collector)
or placing the bottles on a black or reflective surface will heat the water to higher temperatures,
increasing the pasteurization effect. Clear bottles in hot sun can reach temperatures of 131˚F
(55˚C), which kills many pathogenic bacteria over a period of hours. With a black painted sur-
face or a metal container in hot sun, the water temperature can reach 140˚F (60˚C) or more,
where true pasteurization can occur, rapidly deactivating most enteric viruses, bacteria, and
parasites (Sobsey 2002, 14). In places like Nepal, where temperatures are cooler but UV-A ra-
diation is higher due to the altitude, it has been found that SODIS treatment is more effective
without any paint on the bottles. For an excellent source of both basic information on SODIS
and links to numerous technical papers, see
www.sodis.ch
.
The Bangladesh Sari Filter
In Bangladesh, where annual flooding often contaminates nearly all surface waters and where
over half of the tube wells are severely contaminated with arsenic and fluoride, villagers have
been trained to make a reasonably effective filter out of a minimum of eight layers of a folded
sari. Actual scientific controlled tests have verified the efficacy of these low-tech “sari filters.”
It turns out that older saris work better than ones made of new cloth, because after several laun-
derings the thread fibers tend to become soft and loose, reducing the pore size compared with
pore size on new sari cloth. The folded sari filters are held snugly over the neck of a large water
jug as it is dipped in a river or pond until filled. After each use, the sari is first rinsed out in the
