Solar water disinfection can be used to reduce bacteria counts in small volumes of drinking water at the individual household level. The simple technique of putting water in plastic bags or other containers and exposing it to the sun for 2-6 hours inactivated up to 100% of the bacteria in contaminated water. Scientists at Brace Research Institute in Quebec, Canada have been working with solar water disinfection since 1988. Several of their recent research reports are summarized below.
It was found that near ultraviolet (UV) radiation of 300-400 nm had a bactericidal effect apart from temperature: “water temperatures from 12-43°C did not affect the inactivation of bacteria.” The bactericidal effect of the radiation is affected by the turbidity of the water, the materials of the container, and the climate. Highly turbid and contaminated water is not easily disinfected by the sun, so it is best to settle and/or filter the water before putting it in the sun. On cloud cover: “The time for complete elimination of pathogenic bacteria was found to vary from 2 hours in hot arid areas to 5 hours in humid tropical regions or when clouds partly obscured the sun.” “An average solar intensity of about 600 W/m2 over 4-6 hours must be maintained in order to permit the complete elimination of all bacteria in a water sample.” Obviously, best results can be achieved around midday when the sun is strongest.
What is the best container to use? Many tests have been done with various materials. In general, clear containers are better than tinted ones. Transparent plastic bottles and glass jars, while they may be used, have many formulations and they can give inconsistent results and may transmit poorly in the required UV range. The two best containers were transparent plastic bags (disinfecting “to a level of zero coliforms on almost all occasions when the water was exposed for 6 hours or more”) and open metal (aluminum) pans. The advantages of the pans over plastic bags are their greater capacity and durability. Containers should be capped or sealed with thin plastic wrap (usually with UV transmission levels >90%) to prevent contamination, especially in dusty areas.
ECHO Staff 1996. Solar Water Disinfection. ECHO Development Notes no. 53