77. Water disinfection using solar radiation

Obtaining clean drinking water is a continual challenge in many countries. Often the only water available is rife with disease-causing bacteria, and must be disinfected to make it safe. Conventional methods for disinfecting drinking water, including boiling and the addition of chlorine compounds, can be time-consuming and expensive. There may be procurement and distribution problems for chlorine compounds can also give the water an unpleasant taste. In many areas there is no fuel for boiling water.

The main part of the solar reactor is a transparent serpentine tube.

Researchers in Lebanon have developed a low-cost, practical means to provide safe drinking water to rural and urban areas, using the ability of sunlight to destroy bacteria. Solar radiation is a form of renewable energy that is abundant and accessible in most Southern countries.

The researchers began by exposing water to sunlight in batches, in plastic bags and clear or blue-tinted pyrex glass containers. The bacteria in the water were destroyed in about 75 minutes. Water in regular glass containers required 175 minutes. They also tested a process called the halosol technique, where relatively high doses of sodium hypochlorite (chlorine) are used to disinfect small volumes of heavily polluted water. The water is then dechlorinated through exposure to solar radiation, which removes the bad taste and smell. In pyrex containers, 99% of the chlorine was removed after 110 minutes of exposure. In plastic bags, only 35 minutes of exposure was required.

After confirming the success of exposing water to the sun in batches, the researchers designed a flow-through system for continuous disinfection of larger quantities of water. In this system, a storage reservoir is connected to a tank, from which the water is directed by gravity at the desired rate through a solar reactor. In one prototype, the reactor is a serpentine pyrex tube resting on a metallic surface angled at 35° facing South, for maximum exposure to the sun (this angle may vary according to the location). The second prototype uses a solar reactor made up of four pyrex glass containers and one pyrex glass helix, which increases its total capacity. The disinfected water is then conducted to a distribution tank.

The flow-through system can also be used with the halosol technique, to remove excess chlorine from heavily-treated water. The researchers continue to work on improvements to the system. These improvements may include increasing the system’s capacity by increasing the diameter of the pyrex tubes, and installing a radiometric device to synchronize the water flow with variations in the intensity of sunlight.

Prerequisites

The system can be made locally from pyrex, which is a good transmitter of solar radiation, durable, and locally available in Lebanon. In areas where pyrex is unavailable, other durable plastics that are relatively inexpensive and can be locally molded can also be used. In the first prototype, called type I, the glass tubing is 13.4 m in length, 22 mm in diameter, and 1.5 mm thick. In the type II prototype, the glass helix is 10.5 m long, 12 mm in diameter, and 1 mm thick.

Potential users

Primary health care workers and technical people working in the areas of solar energy and water disinfection. The system can potentially be used in small communities, refugee camps, institutions, and in disaster situations where water supply is interrupted.

Contact

Dr Aftim Acra
Acting Chairman
Faculty of Health Sciences
American University of Beirut
Beirut, Lebanon
Telex: AMUNOB 20801 LE

Resources and publications

· Water Disinfection by Solar Radiation, Assessment and Application, by A. Acra, M. Jurdi, H. Mu’allem, Y. Karahagopian, Z. Raffoul, IDRC 1989, 86 pp.